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Research Collection
Doctoral Thesis
Low-P tolerance of various maize cultivarsthe contribution of the root exudation
Author(s): Gaume, Alain
Publication Date: 2000
Permanent Link: https://doi.org/10.3929/ethz-a-003877645
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Diss. ETH Nr. 13529
Low-P tolerance of various maize cultivars :
the contribution of the root exudation
A dissertation submitted to the
SWISS FEDERAL INSTITUTE OF TECHNOLOGY
ZURICH
For the degree of
DOCTOR OF NATURAL SCIENCES
Presented by
ALAIN GAUME
Dipl. Ing. Agr. ETH-Zurich
Born March 03, 1970
Citizen of Epiquerez (JU)
Accepted on the recommendation of
Prof. Dr. E. Frossard, examiner
Prof. Dr. H. Sticher, co-examiner
Prof. Dr. A. Guckert, co-examiner
Prof. Dr. W. Horst, co-examiner
Zurich, 2000
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Table of contents
LIST OF ABBREVIATIONS
ABSTRACT 1
RESUME 3
GENERAL INTRODUCTION 6
Importance of phosphorus (P) in crop production 6
P deficiency and availability : the problem 6
Strategies to alleviate P deficiency and low-P availability 7
Plant P efficiency 8
P nutrition: the contribution of root exudates 9
Objectives, hypothesizes and progress of this present research work 10
CHAPTER I: Phosphate acquisition by Zea mays L. in 13
sand-ferrihydrite-phosphate systems.
Keywords 13
Abstract 13
Introduction 14
Materials and Methods 15
Results and Discussion 20
Conclusion 48
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Table of contents
CHAPTER II: Low-P tolerance of maize cultivars (Zea mays L.): 50
Significance of root growth, and organic acids
and acid phosphatase root exudation.
Preamble 50
Keywords 51
Abstract 51
Introduction 52
Materials and Methods 54
Results and Discussion 58
Conclusion 76
CHAPTER III: Aluminum resistance in two cultivars ofZea mays L.: 78
Root exudation of organic acids and influence of
phosphorus nutrition.
Preamble 78
Keywords 79
Abstract 79
Introduction 80
Materials and Methods 82
Results and Discussion 85
Conclusion 93
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Table of contents
CHAPTER IV: Effect of some organic acids on P sorption, 94
desorption and exchangeability on a
synthetic ferrihydrite.
Preamble 94
Keywords 95
Abstract 95
Introduction 96
Materials and Methods 97
Results and Discussion 102
Conclusion 117
CHAPTER V: Effect of maize root mucilage on P 120
adsorption and exchangeability on
a synthetic ferrihydrite.
Preamble 120
Keywords 121
Abstract 121
Introduction 122
Materials and Methods 123
Results and Discussion 128
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Table of
GENERAL CONCLUSION 143
P and Fe mobilization in soil: 144
the influence of soil mineral properties
Low-P tolerance of maize plants: 144
the contribution of some putative mechanisms
Root exudation 144
Root biomass, root length and other morphological traits 145
Efficiency of some organic root exudates, of the root system 146
and plant development in P mobilization
Tropical acid soils: 147
the Al and P dilemma
Fe acquisition by maize plants: 148
the contribution of some root exudates
The rhizosphere: 148
roots can affect mineral properties and P chemistry
Outlook 149
LITERATURE CITED 151
REMERCIEMENTS
CURRICULUM VITAE
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List of abbreviations
ANOVA Analysis of variance
CEC Cation exchange capacity
CIAT International Center for Tropical Agriculture
CIMMYT International Maize and Wheat Improvement Center
Cp Phosphate concentration in solution
DNA Deoxyribonucleic acid
EAWAG Swiss Federal Institute for Environmental Science and Technology
E(t) Quantity of isotopically exchanged P at time t
ETH Swiss Federal Institute of Technology
Fed Dithionite-citrate-bicarbonate-extractable Fe
Fe0 Oxalate-extractable Fe
GA Galacturonic acid
ICRAF International Center for Research in Agroforestry
ITÖ Institute for Terrestrial Ecology
K Affinity constant of the compound for the ferrihydrite
MW Molecular weight
n Decrease with time of 33P activity remaining in the solution
NST NST90201 (S) CO-422-2-3-1-7 maize cultivar
PEP Phosphoerco/pyruvate
PEPC Phosphoefto/pyruvate carboxylase
PAE Phosphate acquisition efficiency
PGA Polygalacturonic acid
pNP p-nitrophenol
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List of abbreviations
/;NPP p-nitrophenylphosphate
PUE Phosphate utilization efficiency
PZC Point of zero charge
Qa Amount of adsorbed compound
Qa max Maximal amount of adsorbed compound
R Introduced amount of radioactivity at time 0
r(t) Amount of radioactivity remaining in the solution after t minutes of
isotopic exchange
RM Root mucilage
RM dry Treatment root mucilage with dry ferrihydrite
RM wet Treatment root mucilage with ferrihydrite dispersed in deionized water
prior to mucilage adsorption
RNA Ribonucleic acid
SA3 SA3-C4HC (16x25)-2-4-9-7-B-B-B-B-l maize cultivar
SD Standard deviation
SE Standard error
SSA Specific surface area
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Abstract -1
Phosphorus (P) deficiency is, with aluminum (Al) toxicity, one of the main limiting
factors for crop production in acid tropical soils. The strong P sorption on Fe and Al
oxides has been shown to be responsible for the low plant availability of both applied P
and soil P in these soils. Scientists have been able to identify maize genotypes that are
adapted to such unfavourable plant growth conditions. The major aims of this work were
to determine i) the influence of some iron oxide properties on plant P and Fe acquisition,
and ii) the contribution of root exudates in the low-P tolerance and Al resistance of some
maize cultivars. A low-P tolerant {Zea mays L. NST), a low-P, Al-resistant and acid soil
tolerant {Zea mays L. Sikuani) and a low-P susceptible maize cultivar {Zea mays L. SA3)
were compared with a Swiss cultivar {Zea mays L. Corso).
In a sand-ferrihydrite-P system, with Sikuani and Corso and two different ferrihydrites,
this work underlined the importance of some surface properties of soil minerals, such as
the specific surface area and the porosity, on P and Fe acquisition by maize. The effect of
the presence of roots on some properties of poorly crystallized minerals, such as
ferrihydrite, was demonstrated. The ferrihydrite and the P bound to the mineral
represented the only source of Fe and P for the plants. The exchangeability and the
acquisition of P by the plant decreased, while Fe acquisition increased with increasing
specific surface area and porosity of the ferrihydrite. Using the isotopic exchange kinetic
method, our results suggested that the main source of P for maize was the P isotopically
exchangeable in a week. After 12 weeks of plant growth, P and Fe acquisition by plants
were positively related to the higher root dry weight of Sikuani than of Corso. The
observed decrease of the specific surface area, of the porosity, and of the oxalate-
extractable Fe (Fe0), and the sorption of organic matter in the ferrihydrite adhering to the
roots were positively correlated with the decreased ferrihydrite P-sorption capacity during
plant growth. The presence of organic matter in the mineral adhering to the roots
demonstrated the release of organic compounds from maize roots.
Under hydroponic sterile conditions and after 18 days of plant growth, the contribution of
root exudation to the low-P tolerance of the four selected maize cultivars was confirmed.
Under P deficiency organic acid contents in roots, in phloem sap and in root exudates
increased. Differences between genotypes in the organic acids content of roots and
phloem were not related to their low-P tolerance. However, root exudation increased
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Abstract - 2
more strongly for the cultivar NST, in particular, and Sikuani, than SA3 and Corso. There
was a difference between genotypes in the organic acid composition of phloem and root
contents, and root exudates. Trarcs-aconitic acid and malic acid were predominant. Root
acid phosphatase activity was higher in the cultivars tolerant to low-P conditions. The
release of protons from maize roots was low and was not related to the low-P tolerance of
maize cultivars. Under P deficiency root length, root dry weight, and anthocyanidin
coloration of leaves were higher in tolerant to low-P soils (NST and Sikuani), than in
susceptible cultivars (SA3 and Corso) and might contribute to the low-P tolerance of
maize plants. Our research demonstrated that the lower Al-related inhibition of root
growth in Sikuani, than in Corso was associated with a higher Al precipitation in the
presence of P in root tissues, higher contents and concomitantly higher exudation of
citric, malic and succinic acids, all known to complex Al, for Sikuani than for Corso.
The efficiency of some organic compounds, present in maize root exudates, to mobilize P
bound to ferrihydrite was studied. In competitive adsorption treatments with P on a
synthetic ferrihydrite, organic acids decreased P adsorption and increased P mobilization
and exchangeability of sorbed P. The effect of organic acids decreased in the order citric,
malic, trans-aconitic, succinic and formic acid and was higher when sorbed prior to P
addition, than the other way around. Nevertheless at the rate determined in our study, root
exudation of organic acids by maize might not contribute to a significant enhancement in
the P mobilization of P adsorbed on ferrihydrite in our conditions. Citric, malic and trans-
aconitic acids to some extent solubilized ferrihydrite. Similarly, the preliminary addition
of a high-molecular-weight root exudate, mucilage, collected on the nodal roots of Corso,
strongly decreased the subsequent P adsorption. This effect was mainly due to the
flocculation of ferrihydrite aggregates in the presence of mucilage, which limited the
transport of P from the solution to the adsorption sites. The mobilization by the root
mucilage of P sorbed on the ferrihydrite was low and the solubilization of ferrihydrite not
detected.
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Résumé - 3
La déficience en phosphore (P) représente, avec la toxicité à l'aluminium (Al), un des
principaux facteurs limitant la production végétale dans les sols tropicaux acides. La forte
fixation de P sur les oxydes de fer et d'aluminium contenus dans ces sols a été démontrée
comme étant responsable de la faible disponibilité pour les plantes du P appliqué sous
forme d'engrais et du P du sol. Des génotypes de maïs adaptés à ces difficiles conditions
de croissance ont été identifiés. Les principaux buts de ce travail étaient de déterminer i)
l'influence de quelques propriétées des oxydes de fer pour l'acquisition de P et de Fe par
les plantes, et ii) la contribution des exsudats racinaires dans la tolérance de quelques
cultivars de maïs aux sols pauvres en P et à la toxicité à l'aluminium. Un génotype
tolérant aux sols pauvres en P {Zea mays L. NST), un génotype tolérant aux sols acides,
pauvres en P et présentant une haute teneur en Al {Zea mays L. Sikuani), et un génotype
non adapté aux sols pauvres en P {Zea mays L. SA3) furent comparés à un génotype
cultivé en Suisse {Zea mays L. Corso).
Dans des systèmes expérimentaux sable-ferrihydrite-P, avec Sikuani et Corso et deux
différentes ferrihydrites, ce travail souligne l'importance de quelques propriétés des
minéraux du sol, telles que la surface spécifique et la porosité, pour le prélèvement du P
et du Fe par le maïs. L'action racinaire sur certaines propriétés de minéraux faiblement
cristallisés, tels que la ferrihydrite, est démontrée. La ferrihydrite et le P lié à cette
dernière représentaient les seules sources en P et Fe pour la plante. L'échangeabilité et le
prélèvement du P par la plante diminue, alors que le prélèvement du Fe augmente avec
l'augmentation de la surface spécifique et de la porosité de la ferrihydrite. A l'aide de la
méthode des cinétiques d' échanges isotopiques, nos résultats suggèrent que la source
principale de P pour le maïs est le P isotopiquement échangeable en une semaine. Après
12 semaines de croisssance, le prélèvement de P et de Fe par la plante est positivement
corrélé à la matière sèche racinaire plus élevée chez Sikuani que Corso. La diminution de
la suface spécifique, de la porosité et de la fraction de Fe extractable par l'oxalate (Fe0),
et la présence de matière organique dans la fraction de ferrihydrite adhérante aux racines
sont positivement corrélés à la diminution de la capacité d'adsorption en P de la
ferrihydrite pendant la croissance de la plante. La présence de matière organique dans la
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Résumé - 4
fraction de ferrihydrite adhérante aux racines démontre la libération de composés
organiques par les racines de maïs.
Dans des conditions hydroponiques stériles et après 18 jours de croissance, la
contribution de 1' exsudation racinaire à la tolérance aux sols pauvres en P des quatre
génotypes de maïs fut confirmée. Dans des conditions de déficience en P, la teneur en
acides organiques des racines, du phloème exsudé par la tige et des exsudats racinaires
augmentent. Les différences mesurées entre génotypes dans la teneur des racines et du
phloem en acides organiques ne sont pas liées à leur tolérance aux sols pauvres en P.
Cependant, l'exsudation racinaire de ces composés augmente pour les cultivars NST, en
particulier, et Sikuani plus fortement que pour SA3 et Corso. Des différences entre
génotypes existent dans la composition en acides organiques du phloème, des racines et
des exsudats racinaires. Les acides trans-aconiûque et malique sont prédominants.
L'activité racinaire en phosphatase est plus élevée dans les cultivars tolérants aux sols
pauvres en P. La libération par les racines de maïs de protons est faible et n'est pas
corrélée à la tolérance des cultivars aux sols pauvres en P. Lors de déficience en P, la
longueur racinaire, la matière sèche racinaire et la présence d'anthocyanidine dans les
feuilles sont plus élevés pour les cultivars tolérants (NST et Sikuani) que susceptibles
(SA3 et Corso) aux sols pauvres en P et pourraient contribuer à cette tolérance. Nos
recherches démontrent que la plus faible inhibition par Al de la croissance racinaire pour
Sikuani que Corso, est associée à une précipitation plus importante de l'aluminium dans
les tissues racinaires en présence de P, à une plus haute teneur et plus forte exsudation
des acides citrique, malique et succinique, connus comme pouvant complexé
l'aluminium, chez Sikuani que chez Corso.
L'efficacité de quelques composés organiques, présents dans les exsudats racinaires de
maïs, de mobiliser du P lié à la ferrihydrite, fut étudiée. Dans des expériences
d'adsorptions compétitives avec P sur une ferrihydrite synthétique, les acides organiques
testés diminuent l'adsorption du P, et augmentent la mobilisation et l'échangeabilité du P
adsorbé. L'influence des acides organiques diminue dans l'ordre citrique, malique, trans-
aconitique, succinique et formique et est plus importante lorsque leur asdorption se fait
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Résumé - 5
avant celle du P, qu'après. Toutefois aux concentrations déterminées dans notre travail,
l'exsudation racinaire d'acides organiques par le maïs ne pourraient pas contribuer dans
une large mesure à la mobilisation de P adsorbé sur la ferrihydrite. La solubilisation à
différents degrés de la ferrihydrite par les acides citrique, malique et îra/w-aconitique est
démontrée. Dans des expériences d'adsorptions semblables, l'addition préliminaire d'un
exsudât racinaire de haut poids moléculaire, le mucilage, collecté sur les racines nodales
du cultivar Corso, diminue fortement 1'adsorption subséquente de P. Cet effet est
principalement dû à la flocculation des aggrégats de ferrihydrite en présence de mucilage.
Le transfert de P de la solution vers les sites d'adsorption étant ainsi limité. La
mobilisation par le mucilage du P lié à la ferrihydrite est faible. La solubilisation de la
ferrihydrite par le mucilage racinaire ne fut pas détectée.
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General Introduction - 6
Importance of phosphorus (P) in crop production
Phosphorus, as phosphate, is an integral component of a number of important compounds
present in plant cells, such as the sugar-phosphates used in respiration and photosynthesis
and the phospholipids that make up plant membranes. It is also a component of
nucleotides used in plant energy metabolism and in the DNA and RNA molecules. P
requirement for optimal growth is in the range of 3-5 mg g" plant dry matter during the
vegetative stage of growth. P deficiency results in reduced plant growth, delayed crop
maturity, and a reduction in crop yield.
P deficiency and availability: the problem
Soil P deficiency is one of the most limiting factors affecting plant growth on a world¬
wide basis (Fairhurst et al., 1999). About 5.7 billion ha of soils do not contain sufficient
available P for optimum crop production (Sanchez and Salinas, 1981; World Bank, 1994;
Batjes, 1997). Von Uexkiill and Mutert (1995) estimate that 95% of the acid soils located
in tropical Africa, America, Asia, and the Pacific and Australia are deficient in P. In
many of these soils the extent of the P deficiency is so high that plant growth ceases once
the P stored in the seed has been exhausted. Phosphorus deficiency affects crop
production not only directly, but also soil fertility as a whole, rendering low-P soils prone
to degradation (Sanchez et al., 1997).
Soil P is derived from the weathering of the primary P minerals (mostly apatite).
Geochemical and biological processes of pedogenesis affect the total amount of P in soil
and its availability for plant (Walker and Syers, 1976). In strongly weathered soils there
tends to be less P, due to leaching and erosion. In the tropics and subtropics (Oxisols,
Ultisols and Alfisols), strong weathering is also correlated with an increase in the amount
of sesquioxides, which exhibit high P-sorption properties (Parfitt, 1978; Frossard et al.,
1995; Torrent, 1997), and a decrease in the proportion in primary calcium minerals
(Sanchez and Logan, 1992). While it is suggested (Tiessen et al., 1992) that in highly
weathered soils most of the plant-available P is derived from the mineralisation of
organic P forms, in less weathered soils plant-available P is derived predominantly from
inorganic P fractions. As a result of the inherent characteristics of soil parent material and
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General Introduction - 7
of weathering, the limited availability of P in tropical soils may be due to low content
and/or severe P sorption.
Strategies to alleviate P deficiency and low-P availability
The most obvious strategy to alleviate P deficiency is to add large quantities of phosphate
fertilizers to P-deficient soils, either as water soluble P fertilizer or as rock phosphate
(Roche et al., 1980; Von Uexkiill and Mutert, 1995; Sanchez et al., 1997). The alleviation
of soil P deficiency, with large applications of P fertilizer during vegetative crop growth,
has been an essential step in the development of large and sustainable yields of the crops
grown in low-P acid soils (Mutert and Sri Adiningsih, 1998). This strategy faces however
two problems: i) in tropical areas very little P fertilizers are usually available, either
because of the low purchasing power of the indigenous small-scale farmers or because of
the lack of infrastructure for distribution, and ii) phosphate resources are limited and not
renewable. For all these reasons the development of innovative local-scale farming
systems based on the use of renewable, and indigenous and economically available
source of P, such as recaptured urban residual P, waste recycled P, or animal and green
manure P is important. However, alternative P sources might not be able to cover the
severe P-deficiency conditions in the tropical acid soils and to sufficiently increase the
agricultural production in many developing countries.
Another possibility is to identify plant species that are efficient in term of P uptake, and
to include them in crop rotation in combination with relatively low levels of P inputs.
This can result in an increased accumulation of P in the biomass in the upper horizon,
leading in the long term to an increase in soil P fertility and to an increased production at
the agrosystem level. Successful examples of such strategies include the use of legumes
or other plants as green manure in pastures or cereal cropping systems (Friesen et al.,
1997; Gijsman et al., 1997; Rao et al., 1997; Sanchez et al., 1997; Oberson et al., 1999),
or the implementation of agroforestry systems (Hands et al., 1995; Sanchez et al., 1997).
The rotation of maize with Sesbania sesban used as green manure rather than continuous
maize cropping on P-deficient soil has been shown to increase maize yields (Maroko et
al., 1999). Such strategies are highly promising, but they encounter a resistance from
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General Introduction - 8
farmers' side, because for instance of the difficulty of managing legumes or because of
the long time needed for their installation (e.g. in the case of agroforestry systems).
Another strategy is to improve the P efficiency of crops and forage plants, i.e. either to
increase the acquisition of P by plants or to decrease the amount of P needed by plant to
obtain an optimum yield. This aim can be achieved either by breeding programs or by
genetic engineering. Breeding programs have been successful for instance in producing
maize cultivars tolerant to soil acidity (Granados et al., 1993; Pandey et al., 1994). Plant
biotechnology methods could be used to improve P-acquisition efficiency by identifying
genes responsible for the adaptation of given plants to low-P soils and to transfer them in
agricultural crops that exhibit a low P efficiency. But the use of P-efficient genotypes is
not an alternative to P fertilizer application. Improved plants will have to be included in
cropping systems in combination with relatively low levels of P inputs.
None of these research directions can be neglected in the search for an increased food
production through sustainable agriculture in the tropics. However, as suggested by Rao
et al. (1999a), to optimize the use of strategic P inputs and native soil P in P-limited soils,
research needs to focus on identifying genotypic differences and on understanding the
specific mechanisms involved in the acquisition of P from different P sources.
Plant P efficiency
Large differences in P efficiency exist between plant species and between cultivars within
species. Genotypes that can acquire and use scarce P resources more efficiently from
low-P soils could improve and stabilize agricultural production (Friesen et al., 1997; Rao
et al., 1999b). Genotypical differences in nutrient efficiency are related to differences in
efficiency of acquisition by the roots, or in utilization by the plant, or both (Sattelmacher
et al., 1994; Horst et al., 1996b; Rao et al., 1999a). Phosphate acquisition efficiency
(PAE) is defined as the total amount of P taken up by the plant, or as the amount of P
taken up per unit of root length. Phosphate utilization efficiency (PUE) is defined as the
dry matter production per unit nutrient in the dry matter (Marschner, 1995a).
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General Introduction - 9
Since plants take up their P by the roots as orthophosphate from the soil solution,
important parameters in determining PAE are: root length, diameter, branching pattern,
abundance and length of root hairs (Fredeen et al. 1989; Lynch et al. 1991; Mollier and
Pellerin, 1999). Other processes important for the PAE are the rate of orthophosphate
uptake from the solution (Km), the minimum concentration at which uptake occurs
(Cmin) (Waisel et al., 1996), the exudation of protons, of complexing or chelating
substances or of enzymes in the rhizosphere (Uren and Reisenauer, 1988; Raghothama,
1999; Jones and Farrar, 1999), and the symbiosis with mycorrhizal fungi (Wilcox, 1991;
Tarafdar and Marschner, 1994; Marschner, 1995a). The PUE on the other side includes,
besides P acquisition, P demand within the cell, P transport within the shoot and within
the root, compartmentation within the aerial parts (including the seeds) and within the
root, and shoot-root transport (Marschner, 1995a).
P nutrition: the contribution of root exudates
The zone of soil surrounding the root, the rhizosphere, is chemically, physically and
biologically different from that of the bulk soil. Typically the rhizosphere is characterized
by elevated microbial population and lower nutrient availability than the bulk soil
(Marschner, 1995a). The creation of the rhizosphere is driven by the release of a diverse
range of organic compounds from the root to the adjacent soil, fuelling microbial growth
and proliferation. The extent of this zone depends upon the quantity and diffusion
characteristics of the root exudates and soil properties such as water content, CEC, and
texture; typically the rhizosphere extends for 1 to 2 mm away from the root surface
(Waisel et al., 1996). Root exudates are known to contain both high (mucilage, proteins,
sloughed cells) and low (sugars, amino acids, organic acids) molecular weight
components (Uren and Reisenauer, 1988). Root exudates can facilitate the induction of
nodulation and the establishment of mycorrhizal association (Marschner, 1995a;
Kapulnik et al., 1993), they can contribute to metal detoxification (Horst et al., 1982;
Delhaize et al., 1993b), they can increase water availability (Guinel and McCully, 1986)
and root movement (Ray et al., 1988) or/and can attract root pathogens such as
nematodes (Cohn et al., 1996). There is now overwhelming evidence that some plants
directly modify the rhizosphere in order to gain access to previously unavailable reserves
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General Introduction -10
of soil nutrients, e.g. Fe (Römheld, 1987) and P (Bar-Yosef, 1996b). While differences in
root morphological characteristics (growth and distribution, root diameter, and root hair
length) explain to a large extent the differences among cultivars in P acquisition
(Sattelmacher et al., 1994), there is still limited evidence that root exudates actually play
a significant role in plant P acquisition. This is mainly due to the poor experimental
techniques used for quantifying the rate of exudation from roots (Jones and Farrar, 1999).
Some root exudates can induce the release of orthophosphate to the soil solution and then
enhance P acquisition by plants. Root exudates relevant for plant P nutrition include
phosphatase enzymes, organic acids, and H+. Phosphatase can increase the hydrolysis of
soil organic P (Tarafdar and Jungk, 1987). Some organic acids, such as citric acid, malic
acid or oxalic acid, are known i) to compete with P for similar adsorption sites, whereby
organic acids directly replace P by ligand exchange (on crystalline Al- and Fe-oxides)
(Earl et al., 1979; Parfitt, 1979; Gerke, 1992; Hue, 1991; Staunton and Leprince, 1996),
and ii) to complex metal ions in the exchange matrix holding the P (Ca in calcium
phosphate or Fe3+ and Al3+ in Fe- and Al-oxides) (Zhang et al., 1985; Otani et al., 1996).
Protons released from plant roots can solubilize inorganic P (Grinsted et al., 1982;
Hedley et al., 1982; Moorby et al., 1988).
Objectives, hypotheses and progress of this present research work
The general objective of this research is to understand some processes governing P
acquisition efficiency in a model crop: maize, in order to increase on a long-term basis
plant yields in the tropical low-P acid soils. The specific aim of this present work is to
study the effect of P deficient conditions on the growth and the root exudation of various
maize cultivars known to be more or less tolerant to low-P conditions. And then to
determine under controlled conditions the efficiency of some selected root exudates on P
mobilization.
In collaboration with the International Maize and Wheat Improvement Center
(CIMMYT, Mexico) which has the world mandate for developing maize cultivars for
developing countries, some genotypes selected for different tolerance to low-P soils were
compared. We hypothesized that the tolerance of the cultivars tolerant to low-P
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General Introduction -11
conditions might be due to a higher P-acquisition efficiency (PAE) through the root
exudation of organic acids or phosphatase enzymes, than cultivars susceptible to low-P
soils. Since proton release from roots and root architecture can also strongly affect the P
acquisition efficiency (Mollier and Pellerin, 1999; Bertrand, 1998) they will also be
considered.
The different sections of this work are organized as follows. First, in order to better
understand the ability of maize to mobilize the phosphate adsorbed on soil particles and
to investigate plant growth under phosphate stress conditions, a pot experiment will be
carried out (chapter I). We will use two synthetic ferrihydrites as a model for a mineral
highly reactive towards P. The acquisition of P by the plant from the rhizospheric and
non-rhizospheric portions of the soil will be determined. The efficiency of two maize
cultivars, with a priori different tolerance to low-P conditions, in acquiring P adsorbed on
the ferrihydrite will be studied. The effect of maize roots on the properties of the
ferrihydrite will complete this research.
In the second chapter of this research we will develop a system for measuring root
exudation under sterile conditions. We will then attempt to determine for four maize
cultivars, at a seedling growth stadium, if the selected differences in maize plant
tolerance to low-P conditions could be explained by the exudation of specific organic
compounds from roots and/or by different root growth. Our research will focus on the
release of organic acids, acid phosphatase enzymes, and protons.
In acid soils, crop production is often not only affected by P deficiency but also by the
presence of plant-toxic Al species. Some organic acids present in the roots and released
in the rhizosphere are known to complex Al (Delhaize et al., 1993b; Lan et al., 1995). In
chapter III we will determine if the selected resistance of a maize cultivar to acid soils
could be related to the synthesis and/or release of organic acids. We will also ascertain if
resistance to Al is related to the P nutritional status of the plant.
-
General Introduction -12
Organic acids can modify P sorption onto metallic oxides. The effect of some organic
acids released by maize roots on P sorption onto a ferrihydrite, and desorption, and on the
proportion of sorbed P remaining isotopically exchangeable will be determined (chapter
IV).
Seminal and nodal roots surfaces of maize, particularly apical zones, are covered by high-
molecular-weight mucilage, which consists mainly of polysaccharides (Cortez and Bill,
1982). In the last part of this research (chapter V) we will study the effect of mucilage on
P sorption and desorption on the surface of a ferrihydrite and on the proportion of sorbed
P remaining isotopically exchangeable.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems -13
Keywords: Iron acquisition, Iron oxide porosity, Isotopic exchange kinetic, Phosphorus
acquisition, Zea mays. L, Ferrihydrite.
Abstract
Iron oxide as a major compound in the sorption of phosphate in soils is responsible for
the low availability of both applied P and soil P and represents a main source of Fe for
plants.
This work underlined the importance of some surface properties of poorly crystallized
minerals, such as ferrihydrite, on the P and Fe acquisition by two maize cultivars, Corso
and Sikuani, in sand-ferrihydrite-phosphate systems. Sikuani was selected as tolerant to
low-P acid soils. Two ferrihydrites with different specific surface area (SSA = 172.9 and
317.9 m2/g) and porosity were compared. Using the isotopic exchange kinetic method we
defined the nature of the P taken up by the plant. The effect of the maize roots on the
properties of the ferrihydrite present in the rhizosphere was also determined.
P exchangeability and acquisition by the plant decreased with increasing SSA and
porosity of the ferrihydrite. In the ferrihydrite sample with a high SSA incubated without
plant the amount of isotopically exchangeable P decreased during the experiment, while
no change was observed for the ferrihydrite with the lower SSA. Fe acquisition by plant
increased with increasing SSA and porosity. P acquisition was higher in the fraction of
ferrihydrite adhering to the roots. Differences between Corso and Sikuani cultivars in P
and Fe mobilization was demonstrated. The observed larger root system of the cultivar
Sikuani than of Corso explained the higher acquisition of P and Fe by Sikuani than by
Corso after 12 weeks of plant growth. Our results suggested that the main sources of P
for both maize cultivars were the P in the solution and the P isotopically exchangeable
within one week.
The SSA, the porosity and the amount of oxalate-extractable Fe (Fe0) of the fraction of
ferrihydrite adhering to the roots decreased during plant growth, while no change to a
more crystallized form of iron oxide was observed. These modifications and the presence
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems -14
of organic compounds in the iron oxide decreased ferrihydrite P-sorption capacity during
plant growth.
Introduction
Phosphorus (P) deficiency is one of the main limiting factor for crop production in acid
tropical soils (Granados et al., 1993). Aluminum and iron oxides and oxyhydroxides
(named afterwards oxides) are the main minerals governing P sorption in acid soils (He et
al., 1992; Parfitt, 1978; Barrow, 1985; Torrent et al., 1990, 1992). The reaction of P with
metallic oxides involve a rapid, strong ligand exchange with reactive hydroxyl groups
located on the oxide surface. The degree of crystallinity or porosity of iron oxides also
strongly affects the extent of phosphate sorption (Willett et al., 1988).
The strong P sorption on metallic oxides has been shown to be responsible for the low
plant availability of both applied P and soil P in acid soils (He et al. 1991; Parfitt 1978;
Menon et al. 1995). Laboratory and greenhouse studies indicate that P sorbed onto
metallic oxides is potentially available to plants although it can be only very slowly
released (Parfitt 1979; Soltan et al., 1993; He et al. 1994). This observation is in
agreement with the long-term residual effect of P fertilizers determined in variable-
charge soils (Barrow 1985). Most of these studies have been done, however, with well
crystallized minerals, such as goethite and hematite. A lack of information exists on the
effect of poorly crystallized iron oxides, such as ferrihydrite, on the P nutrition of plants.
Because of its high specific surface area, ferrihydrite can profoundly influence soil
properties, even if present in low concentrations (Childs, 1992). Ferrihydrite can
represent most of the specific surface area and P adsorption-sites in soils (Childs, 1992).
The solubility of Fe in soils is largely controlled by Fe oxides (Lindsay, 1991). The low
solubility of these minerals is the main cause of Fe deficiency in plants.
To cope with low P or Fe availability in soils plants have evolved various strategies:
increase their nutrient acquisition efficiency (total amount of nutrient taken up by the
plant, or amount of nutrient taken up per unit of root length) and/or increase their nutrient
use efficiency (dry matter production per unit nutrient in the dry matter) (Marschner and
Römheld, 1994). Large differences in P and Fe acquisition-efficiency or use-efficiency
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems -15
exist between different species and also between genotypes of a single species
(Marschner 1995a; Sattelmacher et al., 1994; Horst et al., 1996a). More specifically, there
is now ample evidence that some plants can profoundly modify the physico-chemical and
biological properties of their rhizosphere in order to take up P and Fe present in sparingly
soluble minerals (Marschner, 1995 a).
The aim of this work was to assess the importance of some physicochemical properties
(specific surface area, porosity, amount of P adsorbed) of a poorly crystallized iron oxide,
ferrihydrite, on the P and Fe acquisition by maize. Two maize cultivars with a priori
different tolerance to soils with a low P-availability were considered. In a first step the
uptake of P and Fe by maize from P-ferrihydrite complex was studied in a pot
experiment. Then the changes in isotopically exchangeable P remaining on the oxide
after different periods of plant growth was investigated in order to obtain some
information on the nature of the P taken up by the plant. And in the last part the changes
in ferrihydrite properties (specific surface area, porosity, oxalate-extractable Fe) upon
plant growth was studied. All along this work the fraction of ferrihydrite adhering to the
roots was distinguished from the fraction of ferrihydrite non-adhering to the roots so as to
check if the presence of maize roots has had an effect on ferrihydrite properties and
mobilization of P and Fe.
Materials and Methods
Substrate.
The substrate used in this work was a sand-ferrihydrite mixture with in a weightweight
ratio of 100:1. Quartz sand (0.7-1.2 mm and 5-8 mm) was first washed into a 1.5 M HCl
solution for 72 h, thoroughly rinsed in distilled water and dried at room temperature. Two
2-line ferrihydrites, called hereafter ferrihydrite I and II, were used after having been
ground (labor planet mill type Pulverisette 5 by Fritsch) into particles with a diameter
between 20 and 200 jim. Ferrihydrite I was purchased from Aldrich (N° 37,125-4), while
ferrihydrite II was synthesised as described by Schwertmann and Cornell (1991). The
preparation of ferrihydrite II is presented in the chapters JV and V. The properties of both
ferrihydrites are presented in the Table 1.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems -16
Table 1. Ferrihydrite properties. Fe0 : oxalate-extractable Fe (Schwertmann, 1964); Fej :
dithionite-citrate-bicarbonate- extractable Fe (Mehra and Jackson, 1960). Fe0 / Fe^: ratio
oxalate-extractable Fe / dithionite-citrate-bicarbonate-extractable Fe.
Ferrihydrite I Ferrihydrite II
2 -1
Specific surface area (SSA) (m g ) 172.9 317.9
Porosity (%) 44.8 96.3
Pore diameter (nm) 5.9 2.7
Single particles diameter (nm) 10.0 5.0
Point of zero of charge (PZC) 7.3 7.6
Fe0 (mg g" ) 435.4 487.7
Fe0 / Fed 1.0 1.0
Preparation ofsand-ferrihydrite-P substrates.
Increasing amounts of P in the form of KH2P04 were mixed to 17.5 g of ferrihydrite in
300 ml of 50% Hoagland nutrient solution (Hoagland and Arnon, 1938), without P and
Fe. The four following concentrations of P were added to the mixture: Pi= 6.6 10~3, P2=
24.8 10"3, P3= 33.0 10"3 and P4= 66.0 10"3 tig P m"2 ferrihydrite. These concentrations
represented a total amount of 20.0, 75.0, 100.0 and 200.0 mg P per pot within the
ferrihydrite I treatment, and a total amount of 36.8, 137.9, 183.9 and 367.7 mg P pro pot
within the ferrihydrite II. The mixture was shaken for 36 h on an end-over-end shaker and
the P concentration remaining in the solution was determined (Table 2). The ferrihydrite -
P-Hoagland suspension was then mixed with 1.45 kg of quartz sand (0.7-1.2 mm) and
poured into 2 1 PVC cylindrical pots (10.5 cm diameter, 22.5 cm height). At the bottom of
each pot 300 g of the 5-8 mm diameter sand were placed in order to prevent anaerobiosis.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems -17
Table 2. P in solution (300 ml) and adsorbed after P adsorption on 17.5 g ferrihydrite
(mg P l"1 and |ig P nT2 ferrihydrite). P1= 6.6 u,g P m~2 ; P2= 24.8 u,g P m"2 ; P3= 33.0 |ig
P rn2
; P4= 66.0 (ig P m"2; n.d. : not detected; mean value ± SE; n = 6.
P P in solution (mg PI") P adsorbed (jig P m"2 ferrihydrite)
Ferrihydrite I Ferrihydrite II Ferrihydrite I Ferrihydrite II
Pi 0.02 ±0.002 n.d. 6.60 ±0.093 6.60 ±0.031
p2 0.21 ±0.018 0.13 ±0.010 24.78 ±0.217 24.79 ±0.189
p3 0.84 ±0.024 0.69 ±0.021 32.92 ±0.234 32.96 ±0.218
P4 87.64 ±0.834 24.13 ±0.616 57.32 ±0.615 64.70 ±0.583
Vegetal material
Two maize cultivars, Zea mays L. Corso and Zea mays L. Sikuani, were compared. Corso
is a Swiss silo maize with fast growth during early development. ICA V-110 Sikuani
(called hereafter Sikuani) is an open pollinated variety developed in Colombia by
recombining selected acid soil-tolerant lines derived from Population SA3 (Friesen et al.,
1997). It yields 4.0 t grain/ha under normal soil conditions and 2.1 t grain/ha under acid
soil conditions (pH = 4.2 and 60% Al saturation).
Kernels were selected based on their weight (between 0.28-0.30 g) in order to limit the
variability between the plants. The seeds were surface sterilized using 18 M H2SO4 for
30 s. After rinsing, they were dipped into 95° alcohol for 5 min, subsequently washed
with sterile water and dipped into 10% P-free H2O2 for 30 min, and finally rinsed
thoroughly with sterile water.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems -18
Plant growth
Three pre-germinated seeds were transplanted per pot. At the three leaves vegetative
growth stage, the pots were thinned to one plant per pot. Controls were carried out with
the same treatments but without plant. During the entire experiment the pots were kept
near the water holding capacity by weighing once and watering once or twice a day with
the modified Hoagland and Arnon nutrient solution (1938). This nutrient solution
consisted of MgS04 (1 mM); Ca(N03)2 (2.5 mM); K2S04 (1.25 mM); H3B03 (0.01 mM);
MnS04 (0.001 mM); ZnS04 (0.001 mM); CuS04 (0.0005 mM); Na2Mo04 (0.0005 mM).
The pH was adjusted to 5.5. The pots were placed in a growth chamber at 25/18 °C,
75/90% relative humidity, 16/8 h day/night regime and a light intensity of 250 |imol
quanta m" sec" . The position of the pots was changed within and between the treatments
twice a week.
Sampling and analysis
The sampling periods were fixed at 3, 6, 9 and 12 weeks plant growth. At each sampling
date four pots of each P concentrations with plants, and three control treatment pots
without plants, were selected. The plants were cut at 3 to 5 cm above substrate level in
order to simplify the following work steps. After weighting, the pot content was
transferred with care into a 5 1 Pyrex Becher. This receptacle was slowly shaken and
some milliliters of the substrate suspension were collected and filtered (0.025 urn). After
determining the pH, the solution was analyzed for total P (Bowman, 1989) and inorganic
P (John, 1970). Roots were shaken to separate the sand-ferrihydrite fraction non-adhering
to the roots. To collect the fraction of substrate adhering to the roots, roots were briefly
rinsed with a few ml of deionised water. The ferrihydrite (20-200 |im) was properly
separated from the sand (0.7-1.2 and 5-8 mm) in both sand-ferrihydrite fractions with
distilled water using a sieve of 0.3 mm diameter mesh and afterwards air-dried.
Ferrihydrite was analyzed for total P (HC104 digestion method; Dick and Tabatabai,
1977; Cade-Menun and Lavkulich 1997), isotopically exchangeable P (Fardeau et al.,
1985), inorganic C (determination of Ca and Mg carbonates; El Mahi et al., 1987), total C
(CNS-Analyser Carlo Erba ANA 1500), organic C (total C-inorganic C), porosity,
specific surface area (SSA), and oxalate-extractable Fe (Fe0) (amorphous iron extraction;
Schwertmann, 1964).
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems -19
The different plant parts, roots, leaves, shoot and spikes were thoroughly rinsed with
deionized water and fresh weight was recorded. Dry weight was determined after drying
in an oven (8 h, 80°C). Dry plant material was cut and ground in an agate ball mill
(Schwingmühle type MM-2 by Retsch) and analyzed for total Fe (ICP, Varian Liberty
220) and total P by spectrophotometry (Kontron Uvikon 800), according to John (1970).
Because of the impossibility to totally remove iron oxide adhering at the root surface, Fe
content was only determined in the aerial parts of the plants.
Isotopic exchange kinetic method
A 1/100 ratio ferrihydrite/water suspension is shaken for 24 h on an end-over-end shaker.
One ml of a P solution containing (2 10 Bq) was added to the 99 ml of suspension at
time zero and shaken. At time t = 1, 10, 40, 100, 1440, 4320 and 10080 min, 2 ml of the
suspension were removed with a plastic syringe and the solution immediately separated
from the solid phase by filtration at 0.2 \\m. Phosphate ions concentration in solution (Cp)
and pH were determined after 100 min and 10080 min. The introduced amount of
radioactivity at time 0 (R, Bq) and the amount of radioactivity remaining in the solution
after t minutes of isotopic exchange (r(t), Bq) were measured. The quantity of
isotopically exchanged P at time t, E(t) {\ig P m"2 ferrihydrite) was calculated assuming
31 33that (i) P and P have the same fate in the system and (ii) whatever the time t, the
specific activity of the phosphate in the soil solution is identical to that of the isotopically
exchangeable phosphate in the whole system (Equation [1]).
r(t)/(ACP) = R/E(t) [1]
The factor A represents the soil / solution ratio of 1 g of ferrihydrite in 100 ml of water so
that (ACp) is equivalent to the water-soluble P content of the ferrihydrite expressed in \ag
P m"2 ferrihydrite.
Therefore,
E(t) = ACPR/r(t) [2]
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 20
The amount of P isotopically exchangeable within 1 min, E i mjn, is an estimate of the
pool of the free phosphate ions. This is the sum of the phosphate ions in soil solution plus
the phosphate ions in the solid soil phase that instantaneously exchange with phosphate
ions in the solution (Salcedo et al. 1991). In this work the amount of P isotopically
exchangeable within a week (E i week) was calculated as follows.
E i week = ACPR/r( 10080) [3]
The amount of P non isotopically exchangeable within a week (E > i week) was calculated
as the difference between the total amount of P sorbed onto the ferrihydrite and E i week.
Statistical analysis
Standard errors calculation was performed using Excel software (Microsoft office 97).
Analysis of variance was carried out using Statgraphics statistical software (one-way
ANOVA, Multiple Range Test).
Results and Discussion
Plant growth, P and Fe uptake
Plant growth
Effect of P supply. The total amount of P brought to the system strongly affected plant
growth (Table 3). The biomass increased with increasing P supply, while the roots/aerial
parts ratio decreased. Excepted for the highest P supply (P4= 66.010"3 fig P m"2
ferrihydrite), P deficiency symptoms were observed on leaves and were characterized by
anthocyanidin formation: red-purple interveinal chlorosis on length of leaves and by deep
purple tints on the limb of the leaves. At the lowest P supply treatment (treatment Pi)
plant growth stopped after 4-5 weeks.
Effect of the ferrihydrite. Ferrihydrite properties affected roots and aerial parts biomass
production, and roots/aerial parts ratio (Table 3). These three parameters were higher for
ferrihydrite I than ferrihydrite II treatments except for the lowest P supply.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 21
Table 3. Influence of ferrihydrite properties and P nutrition on roots and aerial parts dry weight
after 12 weeks plant growth. P1= 6.6 (ig P m"2 ; P2= 24.8 |ig P m"2 ; P3= 33.0 |ig P m"2 ; P4= 66.0
(lg P m"2; Aerial parts: leaves + shoot + spikes.
Cultivar P Dry weight (g) Roots / Aerial parts
Ferrihydrite I Ferrihydrite II Ferrihydrite
Roots Aerial parts Roots Aerial parts I II
Corso Pi 0.62ab
1.83aa 0.51ab 1.37ba 0.34 a 0.37 a
P2 4.16ab 14.25aa
1.10bb
5.51ba
0.29a
0.20b
P3 9.92ab
34.16aa 3.57bb
17.25ba
0.29a 0.21b
P4 17.1ab
76.15aa 8.75bb 58.01 ba 0.23 a 0.15b
Sikuani Pi 1.13aa 1.64aa 0.94aa 1.28ba 0.69a 0.73a
P2 7.98aa ll.llab 2.52ba 5.73ba 0.72 a 0.44 b
P3 16.56aa 29.78ab
6.03ba 13.33bb 0.56a 0.45 b
P4 28.34aa 71.04ab 15.13ba 52.32bb
0.40a 0.29b
Dry weight: within the same row and the same parameter (roots, aerial parts, roots/aerial parts
ratio) mean values followed by the same first letter are not statistically different at a
probability level a = 0.05. Within the same column and the same P treatment mean values
followed by the same second letter are not statistically different at a probability level a = 0.05.
n = 6.
Ratio roots/aerial parts: within the same row mean values followed by the same letter are not
statistically different at a probability level a = 0.05. n = 6.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 22
Effect ofthe maize cultivar. Differences between the two cultivars were observed in terms
of total biomass production and root-aerial part ratio. Aerial parts production was higher
for the cultivar Corso while total biomass and roots production, and the ratio root/aerial
part were higher for the cultivar Sikuani. The larger root system for Sikuani than for
Corso might contribute to the better adaptation of Sikuani to low P availability, through a
larger occupation of soil. The vegetative development of the cultivar Sikuani was much
slower than that of the cultivar Corso (Figure 1). After 12 weeks of growth no spikes
were present for Sikuani, treatment P4, while for Corso these vegetative organs already
represented 44% of the dry weight of the aerial parts in the ferrihydrite II treatment. This
phenomenon has to be seen as a specific variety property.
P acquisition and P concentration ofplants.
Effect of P supply. P content and P concentration of plants increased with increasing P
supply (Table 4; Table 5 A-D). For the four distinct levels of P supply, P concentration in
the plant tissues was below the critical value (3 mg P g dry weight) reported for maize
(Reuter et Robinson, 1997).
Effect of the ferrihydrite. P concentration and uptake in plant were higher for ferrihydrite
I than for ferrihydrite II treatments although the absolute quantities of P added per pot
were lower in the treatment ferrihydrite I than in the treatment ferrihdrite II (Table 4;
Table 5 A-D). These results suggest that ferrihydrite properties, porosity and specific
surface area, significantly affect P acquisition by both maize cultivars.
Effect of the maize cultivar. Differences between both cultivars were noticed. After 12
weeks of plant growth P concentration in the whole plant and P acquisition was higher
for Sikuani than for Corso (Table 4; Table 5 A-D). However, during the first 9 weeks of
plant growth, P acquisition from ferrihydrite was higher for cultivar Corso than for
cultivar Sikuani, in spite of the higher root development observed for Sikuani (Table 3).
The faster vegetative development of Corso might also explain the higher P acquisition
during the first weeks of plant growth. On the other hand the lower P requirement of
Sikuani in the initial growth period, related to slower plant development, might partly
explain its adaptation to low-P soils.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 23
Plant growth (weeks)
Figure 1. Biomass production for two maize cultivars (Corso and Sikuani) grown in the
presence of a phosphated ferrihydrite as the sole source of P and Fe. Treatment ferrihydrite JJ,
P input 66 tag m"2. Mean value; n = 4 ; • : roots; A : stalk (shoot + leaves); : ears (spikes);
: male flowering time; open symbols: Corso; full symbols: Sikuani.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 24
Table 4. Influence of ferrihydrite properties and P nutrition on P
-2concentration in plant after 12 weeks plant growth. P]= 6.6 flgPm" ; P2=
24.8 (ig P m"2 ; P3= 33.0 jag P m"2 ; P4= 66.0 |ig P m"2 ; mean value ± SE;
n = 4.
Cultivar P P concentration in total plant (mg P g" DW)
Ferrihydrite I Ferrihydrite II
Corso Pi 0.32 ± 0.006 0.39 ± 0.006
P2 0.30 ± 0.006 0.29 ± 0.008
P3 0.54 ± 0.020 0.39 ±0.012
P4 0.73 ± 0.025 0.68 ± 0.028
Sikuani Pi 0.30 ± 0.006 0.35 ± 0.006
P2 0.35 ±0.010 0.37 ±0.017
P3 0.57 ± 0.027 0.48 ± 0.025
P4 0.84 ± 0.029 0.78 ± 0.023
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 25
Table 5 A. Ferrihydrite I: total and isotopically exchangeable P, P in solution and P mobilized by Zea mays L.
-2. -2Corso. Pi= 6.6 u.g P m"z ; P2= 24.8 u,g P m"z ; P3= 33.0 tig P m ; P4= 66.0 ug P m . n.d.: not detected.
P Plant P P P P P P
growth remaining on in mobilized exchangeable non-exchangeable exchangeable
ferrihydrite solution by plant in 1 week (E(t)) in 1 week (E(t)) in 1 week (E(t))
(weeks) (ig P m"2) (% P total)
p2 3 24.2aA
0.007aA
0.6c
9.5aA
14.7aA
39.4aA
6 23.2b
0.001aA
1.3b
7.9b
15.2a
34.3b
9 21.7c
n.d. 1.7a
7.2c
14.5a
33.2b
12 21.5cB
n.d. 1.8a
6.7cB
17.8aA
31.3cB
Control 3 24.7A
0.015A
- 9.9A
14.8A
40.1A
Control 12 24.4A
0.008A
- 8.9A
15.5A
36.5A
p3 3 30.9aA
0.037aB
1.3c
15.8aA
15.1aA
51.1aA
6 27.9b
0.021b
3.9b
12.0b
15.9a
42.9b
9 24.4c
0.008"
7.4a
9.0c
15.5a
36.6c
12 23.6cB
0.001cB
7.8a
7.8cB
15.8aA
33.1cB
Control 3 32.5A
0.076A
- 16.9A
15.6A
52.0A
Control 12
3
31.8A
0.045A
- 16.6A
15.2A
52.2A
P4 56.9aA
0.215aB
5.4c
35.9aA 21.0aA 63.1 aA
6 48.3b
0.023b 14.8" 28.4 b 19.8 a 58.9 b
9 40.9c
0.007c
22.5a
21.3c
19.6a
52.0c
12 40.3cB
0.005cB
22.6a
19.6cB
20.7aA
48.7cB
Control 3 57.8A
7.524A
- 36.5A
21.0A
63.1A
Control 12 55.5A
5.973A
- 36.7A
18.8A
66.1A
Within the same P treatment and the same column, mean values followed by the same small letter are not
statistically different at a probability level a = 0.05 ; Within the same P treatment, the same column and the
same sampling time, mean values followed by the same capital letter are not statistically different at a
probability level a = 0.05
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 26
Table 5 B. Ferrihydrite I: total and isotopically exchangeable P, P in solution and P mobilized by Zea mays L.
-2Sikuani. Pi= 6.6 (Xg P m"z ; P2= 24.8 (ig P m'z ; P3= 33.0 Lig P m"z ; P4= 66.0 Lig P m . n.d.: not detected.
Plant P P P P P P
growth remaining on in mobilized exchangeable non-exchangeable exchangeable
ferrihydrite solution by plant in 1 week (E(t)) in 1 week (E(t)) in 1 week (E(t))
(weeks) (LigPrn"2) (% P total)
p2 3 24.5aA
0.009aA
0.5b
9.8aA
14.7aA
40.2aA
6 23.6a
0.002a
0.8b
8.3b
15.3a
35.2b
9 21.8" n.d. 1.6 a 7.6 b 14.2 a 35.2 b
12 21.1bB
n.d. 2.2a
6.2cB
14.9aA
29.3cB
Control 3 24.7A
0.015A
- 9.9A
14.8A
40.1A
Control 12 24.4B
0.008A
- 8.9A
15.5A
14.6aA
36.5A
p3 3 31.4aA
0.045aB
1.0d
16.8aA
53.4aA
6 29.4b
0.030b
2.9c
12.9b
16.5a
44.0b
9 24.8c
0.011c
6.6b
9.5c
15.3a
38.5c
12 22.6dB 0.001cB 8.8 a 6.5 dB 16.1aA 28.6 dB
Control 3 32.5A
0.076A
- 16.9A
15.6A
52.0A
Control 12 31.8A
0.045A
- 16.6A
15.2A
52.2A
p4 3 57.0aA
0.306aB
4.1d 35.9aA 21.1aA 63.0 aA
6 53.6" 0.107 b 9.9 c 32.2 b 21.4 a 60.1 b
9 40.5c
0.006c
23.2b
20.6c
19.9a
50.9c
12 36.7dB
n.d. 27.4a
14.9dB
21.8aA
40.6dB
Control 3 57.8A
7.524A
- 36.5A 21.0A 63.1 A
Control 12 55.5A
5.973A
- 36.7A
18.8A
66.1A
Explanation for the small and capital letters following the mean values see Table 5 A.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 27
Table 5 C. Ferrihydrite II: total and isotopically exchangeable P, P in solution and P mobilized by Zea mays L.
-2 -2Corso. Pi= 6.6 fig P m ; P2= 24.8 ixg P m"z ; P3= 33.0 tig P m"z ; P4= 66.0 (ig P m . n.d.: not detected
Plant P P P P P P
growth remaining on in mobilized exchangeable non-exchangeable exchangeable
f h A tsolution by plant in 1 week (E(t)) in 1 week (E(t)) in 1 week (E(t))
(weeks) (jig P m"2) {% P total)
p2 3 24.1aA
0.005aA
0.2a
7.5aA
16.7bA
30.9aA
6 24.1a
n.d. 0.3a
1.3b
22.8a
5.4b
9 23.8a
n.d. 0.3a
1.1b
22.7a
4.6b
12 23.7aA
n.d. 0.3a
0.9bA
22.8aA
3.9bA
Control 3 24.5A
0.012A
- 7.6A
16.9A 31.0A
Control 12
3
24.3A
0.003A
- 1.7A
22.6A
6.2A
p3 31.7aA
0.029aB
0.4c
14.3aA
17.4bA
45.1aA
6 31.0a 0.008b 0.9" 12.5 a 18.5" 40.3 a
9 30.4a
n.d. 1.4a
7.9b
22.5a
25.9b
12 30.3aA
n.d. 1.5a
6.9bA
23.3aA
22.9bA
Control 3 32.0A
0.091A
- 14.2A
17.8A
44.4A
Control 12 31.5A
0.062A
- 8.6A
22.9A
27.3A
p4 3 63.7aA
0.128aB
1.6d
36.2aA
27.5cA
56.8aA
6 60.0b
0.012b
5.2c
27.0b
33.0b
45.0b
9 57.0c
n.d. 7.4b
16.7c
40.3a
29.3c
12 56.1cB
n.d. 8.1a
13.7dB
42.4aA
24.5dB
Control 3 64.4A
3.866A
- 36.7A
27.7A
23.6B
Control 12 63.0A
0.340A
- 20.3A
42.7A
32.2A
Explanation for the small and capital letters following mean values see Table 5 A.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 28
Table 5 D. Ferrihydrite II: total and isotopically exchangeable P, P in solution and P mobilized by Zea mays L.
Sikuani. Pj= 6.6 jig P m~z ; P2= 24.8 jig P rnz
; P3= 33.0 \Lg P m"z ; P4= 66.0 ug P m". n.d.: not detected.
P Plant P P P P P P
growth remaining on in mobilized exchangeable non-exchangeable exchangeable
ferrihydrite solution by plant in 1 week (E(t)) in 1 week (E(t)) in 1 week (E(t))
(weeks) (ug P ml) {% P total)
P2 3 24.1aA 0.007aA 0.2b 8.2aA 15.9bA 33.9aA
6 24.0a n.d. 0.3b 1.6" 22.5a 6.6"
9 23.8a n.d. 0.3b 1.3b 22.5a 5.4b
12 23.6aA n.d. 0.6a 0.9 bB 22.7aA 3.6bA
Control 3 24.5A 0.012A - 7.6A 16.9A 31.0A
Control 12 24.3A
31.8aA
0.003A
- 1.7A 22.6 A 6.2 A
P3 3 0.034aB 0.3c 14.4aA 17.4bA 45.3 aA
6 31.2a 0.010b 0.8 b 12.8a 18.4b 40.9"
9 30.5a n.d. 1.4a 8.1" 22.5a 26.5c
12 30.1aA n.d. 1.7a 6.3 bA 23.8 aA 21.0dA
Control 3 32.0A 0.091 A - 14.2 A 17.8 A 44.4 A
Control 12 31.5A 0.062 A - 8.6 A 22.9 A 27.3 A
P4 3 64.1aA 0.301aB 0.8 d 34.9 aA 29.1 dA 54.5aA
6 60.4b
0.048bB
3.9C 24.9b
35.5c 41.2b
9 56.4c n.d. 7.4b 17.5c 38.9b 31.Ie
12 54.6cB n.d. 9.5a 11.6dB 43.0aA 22.5 dB
Control 3 64.4A 3.866A - 36.7 A 27.7 A 23.6B
Control 12 63.0A
0.340A
- 20.3A
42.7A
32.2A
Explanation for the small and capital letters following mean values see Table 5 A.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 29
Fe acquisition and concentration by plants.
Effect of P supply. Symptoms of Fe deficiency were visible within each P treatments.
Except for the highest P supply (P4= 66.0T0"3 (j,g P m"2 ferrihydrite), P deficiency
symptoms strongly dominated Fe deficiency symptoms on the leaves. Fe deficiency
symptoms were observed as bright yellow interveinal chlorosis (striping) on entire length
of new leaves. Fe concentration in the aerial parts increased with increasing P supply up
to a maximum and then decreased (Figure 2). Since the total amount of Fe in the aerial
parts continuously increased with increasing P supply (Figure 3), the lower Fe
concentration observed at the highest rate of P application was related to higher increase
in dry matter.
Effect of the ferrihydrite. The Fe content of maize was higher with the ferrihydrite I than
with the ferrihydrite II (Figure 3). The larger root system within the ferrihydrite I than the
ferrihydrite II treatments might account for this difference. However, the ratio of Fe
content in the aerial parts to the root biomass was lower for ferrihydrite I than ferrihydrite
II treatments (Figure 4). This suggests that maize roots were more efficient in acquiring
Fe from the ferrihydrite II than from ferrihydrite I. This can be explained by the highest
porosity and specific surface of the ferrihydrite II.
Effect of the maize cultivar. Fe concentration and content in the aerial parts was higher in
Sikuani than in Corso (Figure 2 and Figure 3). As reported by Brown (1967) Fe uptake in
maize is genetically controlled. The higher Fe content in the aerial parts of Sikuani than
of Corso might be explained by the larger root system for Sikuani than for Corso. Since
the ratio Fe content in the aerial parts/root biomass was lower for Sikuani than for Corso
(Figure 4), the higher content in the aerial parts in Sikuani than in Corso did not appear to
be related to a more efficient Fe acquisition mechanism in Sikuani. The difference
between both cultivars could also be due to a higher efficiency in Fe transport from roots
to shoot in Sikuani.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 30
0.28
H
'- ^—sC3ft £73 A•F-N u +^
QJ b
C« a
aA^N
cC«
o i-•FN 0J
«Jcd
^^r^ diu
o>
o U.S 0£
0.24 -
0.20
0.16 -
0.12
0.08
Ferrihydrite I Ferrihydrite II
0 20 40 60 0 20 40 60
-2
P treatment (jig P m )
Figure 2. Relation between the P inputs and the Fe concentration in the aerial parts for two
maize cultivars (Corso and Sikuani) grown for 12 weeks in the presence of a phosphated
ferrihydrite as the sole source of P and Fe. Pi= 6.6 fig P m"2 ; P2= 24.8 fig P m"2 ; P3= 33.0 fig P
m"2 ; P4= 66.0 fig P m"2 ; mean value ± SE ; n = 4 ; O : Corso; À : Sikuani.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 31
14
12
10 -
ft
a ft
es s
ft
2 -
0
Ferrihydrite I 4 Ferrihydrite II
n i i i
0 20 40 60 0 20 40 60
-2
P treatment (|Lig P m )
Figure 3. Relation between the P inputs and the Fe uptake in the aerial parts for two maize
cultivars (Corso and Sikuani) grown for 12 weeks in the presence of a phosphated ferrihydrite
as the sole source of P and Fe. P1= 6.6 fig P m"2 ; P2= 24.8 fig P m"2 ; P3= 33.0 fig P m"2 ; P4=
66.0 fig P m~2 ; mean value ± SE ; n = 4 ; O : Corso; A : Sikuani.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 32
1.0
0.8 -
ucd /^
ft £CO Q;-
o0.6
«s o
e^'•pN 1
0>OX)
0.4^-»
ft DiD
=5 S
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 33
Changes in some physico chemical properties of the mineral substrate
pH value in solution
Plant growth resulted in an increase in pH values (Figure 5). This was related to the
uptake of nitrate as the sole source of nitrogen during plant growth which led to the
release of HCO3" or OH" into the solution to maintain pH equilibrium in plant tissue
(Marschner, 1995b; Nye, 1986). Increase of pH values were higher for Sikuani than for
Corso. These increases were positively correlated for both cultivars to their root biomass
production. In the treatments incubated without plant the higher pH values for
ferrihydrite II than for ferrihydrite I, observed in particular for P4, reflected the higher
quantity of sorbed P on ferrihydrite II and was related to a higher release of OH group
from the surface of the oxide.
Evolution ofP in the substrate during plant growth.
Inorganic P in the solution. The concentration of inorganic P (Pi) in the solution
decreased very rapidly with plant growth, due to plant P uptake (Table 5 A-D). As shown
by the control treatment, Pi sorption onto the ferrihydrite during the experiment was
relatively low for ferrihydrite I, but high for ferrihydrite II treatments. The high specific
surface and porosity and therefore the higher number of sorption sites on ferrihydrite II
explain this observation. This in turn is coherent with the lower P acquisition and total
biomass production of both maize cultivars with the ferrihydrite II than with the
ferrihydrite I (Table 3). No significant differences were measured between total P and Pi
in the solution (data not shown).
P isotopically exchangeable on the ferrihydrite. The amount of P isotopically
exchangeable within one week (E 1 week) remained constant in the presence of ferrihydrite
I without plants during the 12 weeks of incubation while it decreased in the presence of
both maize cultivars (Table 5A and B, Figure 6). In the presence of the ferrihydrite I, the
amount of P taken up by maize during the entire growth period is totally accounted for by
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 34
the changes in P remaining in the solution and in E i week (Figure 7, Table 5 A-B). The
pool of P non-exchangeable within one week (E >j week) did not change during the 12
weeks of the plant growth. This suggests that in the presence of ferrihydrite I the main
sources of P for both maize cultivars were the P in the solution and the P isotopically
exchangeable within a week.
Because of its higher specific surface and porosity, E i wee)j decreased steadily in the
presence of ferrihydrite II without plant during the 12 week of incubation while E >i week
increased (Table 5 C-D). In the presence of maize, E i week decreased more rapidly than
in the samples incubated with the ferrihydrite II without plant (Tab 5 C-D). As for the
experiment with the ferrihydrite I, a tight relation was observed between the quantity of P
remaining in the solution plus E i week and the quantity of P taken up by the plant of the
one side in the cultivated treatments, and the quantity of P remaining in the solution plus
E i week in the incubated sample on the other side. This suggests that also in the presence
of ferrihydrite II the main sources of P for both maize cultivars were the P in the solution
and the P isotopically exchangeable within a week.
The transfer of P ions into the pool of P non exchangeable in one week in the presence of
the ferrihydrite II explained also the lower P acquisition and total biomass production by
both maize cultivars grown in the presence of this mineral than when grown in the
presence of the ferrihydrite I. Differences between both cultivars in term of P mobilized
by plants and P exchangeable within one week (Table 5 A-D) were well related with the
biomass production of both cultivars (Table 3).
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 35
8.2
4 S 8.0ft ©
© CO
« 'S0>
u
äCO
aft
o7.8
In
o
57.6
©
7.4
7.2
Ferrihydrite I Ferrihydrite II 4
20 40 60 20 40 60
-2
P treatments (jug P m )
Figure 5. Effect of plant growth on the pH value in the solution for two maize cultivars (Corso
and Sikuani) grown for 12 weeks in the presence of a phosphated ferrihydrite as the sole source of
P and Fe P1= 6.6 fig P m"2 ; P2= 24.8 fig P m"2 ; P3= 33.0 fig P m"2 ; P4= 66.0 fig P m2
; mean
value ± SE ; n = 4 ; O : Corso 12 weeks; A : Sikuani 12 weeks; D : Control t = 0; : control 12
weeks.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 36
Isotopic exchange time (days)
Figure 6. Changes in the isotopically exchangeable P content of the ferrihydrite I and II
after 3, 6, 9 and 12 weeks of plant growth (maize cultivar Corso) as compared to the
isotopically exchangeable P content of the same substrate incubated without plant for 0
and 12 weeks. Treatment P4 = 66.0 fig P m"2 ; mean value ± SE ; n = 4 ; O: 3 weeks
growth; A: 6 weeks growth; V: 9 weeks growth; O: 12 weeks growth; D: control t =
0; : control 12 weeks.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 37
a
+
C
to
s
vi
Ph r*
m
40
30
+ S? &H 20I WD
10
0
Ferrihydrite I
0
r =0.999
y = 0.92x + 1.28
Ferrihydrite II
P, ir r = 0.978
y = 0.91x + 1.22
20 40
Control: P
0 20 40
+ Pi.e. (t = 1 week) solution
(|Lig P m"2)
Figure 7. Relation between the amount of P isotopically exchangeable within a week (Eiweek) and the P
remaining in the solution in the substrate incubated without plant (axis X), and the amount of P taken
up by the plant plus the amount of P remaining in the solution and E i week after plant growth (axis Y).
Pi= 6.6 fig P m"2 ; P2= 24.8 fig P m"2 ; P3= 33.0 fig P nf2 ; P4= 66.0 fig P m"2 ; mean value ± SE ; n =
4.#: Corso; A: Sikuani; open symbols: 3 weeks ; full symbols: 12 weeks.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 38
Evolution ofP sorbed on ferrihydrite fractions adhering and non-adhering to the roots.
The total P content of the ferrihydrite of the substrate adhering to the roots was always
lower than in the fraction of ferrihydrite non-adhering to the roots (Table 6 A-B). The
concentration of P ions in solution during the isotopic exchange kinetic (Cp), the ratio
rl/R and the amount of P exchangeable within 1 min (E i mjn) were also affected by the
adherence of the ferrihydrite to the roots. In the beginning, the plant took up P adsorbed
on oxides particles in the vicinity of the roots. After 3 weeks of plant growth, within both
ferrihydrite treatments the Cp, E \ ^n and rl/R were lower in the fraction of ferrihydrite
adhering to the roots than in the one non-adhering to the roots. A lower ratio rl/R is
related to a higher P sorption capacity (Fardeau, 1993). At the end of the experiment, the
values of the Cp and E i mjn were still lower, but the ratio rl/R higher, in the fraction of
ferrihydrite adhering to the roots than in the non-adhering fraction. According to Fardeau
(1993) the stronger decrease of water soluble P in the fraction of ferrihydrite adhering to
the roots should have resulted in an decrease of the ratio rl/R, i.e. to an increase of the P
sorption capacity, rather than in an increase in rl/R which denotes a decrease in the P
sorption capacity. This points out to significant modifications in the P chemistry in the
root vicinity after 12 weeks of plant growth. These could be due to a change in the oxide
sorption properties related either to changes in the ferrihydrite porosity following root
colonisation, and/or to the adsorption of organic compounds at its surface following root
exudation. Indeed phosphate and organic compounds can compete for the same
adsorption sites on the iron oxides (Staunton and Leprince, 1996).
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 39
Table 6 A. Adhering- non-adhering ferrihydrite I to the roots by Zea mays L Corso and Sikuani. ; P2= 24.8 fig P
m"2 ; P3= 33.0 fig P m"2 ; P4= 66.0 fig P nf2; Cp: P concentration in solution within the isotopic exchange
kinetic; R: the introduced amount of radioactivity at time 0; rl: the amount of radioactivity remaining in the
solution after 1 minute of isotopic exchange; E \ ^^ amount of P adsorbed on the oxide exchangeable within 1
min; Adh.: ferrihydrite adhering to the roots; Non-adh.: ferrihydrite non-adhering to the roots.
P Cultivar Sampling P total Cp in solution rl/R E i mm
(weeks) OigPrn2) (mgPl"1) (figPrn"2)
Adh. Non-adh. Adh. Non-adh. Adh. Non-adh. Adh. Non-adh.
P2 Corso 3 23.988aa 24.247aa 0.023ba 0.031aa 0.117bb 0.142aa 0.114aa 0.126aa
12 20.321bb 21.809ab 0.002ab 0.002ab 0.236aa 0.110bb 0.005bb 0.011ab
Sikuani 3 24.213aa
24.602aa 0.028ba 0.039aa 0.138bb 0.161aa 0.117ba 0.140aa
12 19.922bb 21.746ab 0.003ab 0.002ab 0.209aa 0.092bb 0.008ab 0.013 ab
Control 12 - 24.418 - 0.042 - 0.233 - 0.104
p3 Corso 3 30.207aa 30.968aa 0.050ba 0.064aa 0.212bb 0.256aa 0.136aa 0.145aa
12 23.033bb 25.791 ab 0.006bb 0.012ab 0.350aa 0.213bb 0.010bb 0.033ab
Sikuani 3 30.970aa 31.632aa 0.066aa 0.071aa 0.227bb 0.282aa 0.168aa 0.146aa
12 22.418bb 24.012ab 0.003ab 0.006ab 0.329aa 0.190bb 0.005bb 0.018ab
Control 12 - 31.241 - 0.098 - 0.471 - 0.120
p4 Corso 3 56.110ba 57.268aa 0.306ba 0.421aa 0.435bb 0.480aa 0.389ba 0.507aa
12 39.788bb 44.180ab 0.020bb 0.030ab 0.479aa 0.269bb 0.024bb 0.065ab
Sikuani 3 58.247ba 59.732aa 0.455ba 0.619aa 0.467ba 0.512aa 0.564ba 0.699aa
12 35.328bb 40.690ab 0.010bb 0.019ab 0.483 aa 0.240bb 0.012bb 0.046ab
Control 12 - 55.526 - 0.663 - 0.609 - 0.630
Explanation for the letters following the mean values see Table 6 B.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 40
Table 6 B. Adhering- non-adhering ferrihydrite II to the roots by Zea mays L Corso and Sikuani.
Explanation on determined parameters (Cp; rl/R; E i,^; Adh.; Non-adh.) see Table 5 A.
P Cultivar Sampling P total Cp in solution rl/R E i min
(weeks) (fig P m"2) (mgPF1) (fig P rn 2)
Adh. Non-adh. Adh. Non-adh. Adh. Non-adh. Adh. Non-adh.
P2 Corso 3 24.069aa 24.141aa 0.002aa 0.005aa 0.021bb 0.034aa 0.030ba 0.046aa
12 22.637bb 23.827aa 0.003aa 0.003aa 0.064aa 0.028ba 0.015bb 0.034ab
Sikuani 3 24.000aa 24.068aa 0.003aa
0.009aa 0.019bb 0.034aa 0.050ba 0.083aa
12 22.314bb 23.763aa 0.002aa 0.002aa 0.063 aa 0.020bb 0.010bb 0.031 ab
Control 12 - 24.307 - 0.010 - 0.029 - 0.108
P3 Corso 3 31.420aa 31.762aa 0.012ba 0.027aa 0.040bb 0.065aa 0.094ba 0.131aa
12 29.072bb 30.560ab 0.011 aa 0.013ab 0.179aa 0.050bb 0.019bb 0.082ab
Sikuani 3 31.456aa 31.852aa 0.014ba 0.038aa 0.045bb 0.073 aa 0.098ba 0.164aa
12 28.803bb 30.504ab 0.008aa 0.011 ab 0.155aa 0.036bb 0.016bb 0.096ab
Control 12 - 31.565 - 0.040 - 0.079 - 0.159
P4 Corso 3 61.367ba 63.848aa 0.162ba 0.295aa 0.220bb 0.251aa 0.232ba 0.370aa
12 54.633bb 58.550ab 0.064bb 0.080ab 0.413aa 0.181 bb 0.049bb 0.139ab
Sikuani 3 61.780ba 64.153aa 0.201 ba 0.317aa 0.235bb 0.288aa 0.269ba 0.346aa
12 53.213bb 56.837ab 0.027bb 0.046ab 0.382aa 0.150bb 0.022bb 0.096ab
Control 12 - 63.042 - 0.616 - 0.424 - 0.457
Within the same P treatment, the same cultivar, the same sampling time and the same parameter (P total, Cp in
solution, rl/R and E \ m,n) mean values followed by the same first letter are not statistically different at a
probability level a = 0.05; Within the same P treatment, the same cultivar, the same parameter and the sameferrihydrite fraction mean values followed by the same second letter are not statistically different at a probabilitylevel a = 0.05; n = 4
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 41
Modification of the oxalate-extractable Fe (Fe0) content offerrihydrite. The amount of
oxalate-extractable Fe (Fe0) decreased in the ferrihydrite I and II during plant growth,
while Fe0 remained constant in the incubation experiments without plants (Figure 8).
After 12 weeks of plant growth this decrease was higher for ferrihydrite II than for
ferrihydrite I, and higher by Sikuani than by Corso. Fe0 decreased much more in the
fraction adhering to the roots than in non-adhering fraction. Fe0 decrease was stronger at
high P supply and related high plant and root development. Dithionite-citrate-
bicarbonate-extractable Fe (Fed) (Mehra and Jackson, 1960) remained constant for both
ferrihydrites during the entire experiment. The Feo/Fed value decreased during plant
growth.
The decrease of oxalate-extractable Fe (Fe0) might result from the crystallisation of the
ferrihydrite during plant growth, and/or from a mobilisation of Fe0 by the plant. A
positive correlation existed between the uptake of Fe in the aerial parts and the decrease
of oxalate-extractable Fe (Fe0) (Figure 9). However, and even without any determination
of the Fe content in roots, the difference between the decrease of Fe0 and the uptake of P
by the plant is too high to be only explained by a plant mobilization.
Modification of the specific surface area (SSA) and of the porosity of the ferrihydrites.
With the presence of plants the SSA of both ferrihydrites decreased during the entire
experiment, while for the control treatments the SSA remained constant (Figure 10). The
decrease of the SSA of the fraction of ferrihydrite adhering to the roots was much higher
than that of the fraction of ferrihydrite non-adhering to the roots. Furthermore, increasing
concentration of P in the system and related higher plant and root development also
enhanced this decrease.
The decrease of the specific surface area of both ferrihydrites during plant growth was
closely related to the decrease of the porosity of the ferrihydrite aggregates (Figure 11).
The decrease of the porosity was higher for Sikuani than for Corso, and was higher for
the fraction of ferrihydrite adhering to the roots.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 42
The decrease in SSA and porosity could be related to solubilization/precipitation cycles
due to varying redox conditions. Although maize is classified as a strategy II plant in Fe
acquisition (Römheld and Marschner, 1986), which means that maize roots have a low
reducing capacity, saturation with water, the presence of organic exudates and iron
reducing micro-organisms can result in the apparition of reducing conditions in the
vicinity of the roots (de Willigen and van Noordjwijk, 1984; Uren, 1984; Uren and
Reisenauer 1988; Munch and Ottow, 1982; Fischer, 1988). Adsorption of high molecular
weight compounds root exudates at the surface or/and in the aggregates of ferrihydrite
might also reduce the porosity and the specific surface area.
Iron oxide XRD determination. At the end of the experiment for both 2-lines ferrihydrites
adhering and non-adhering to the roots, no Fe-(hydr)oxide other than ferrihydrite was
detected (data not shown). However it is important to notice that the sensitivity of the
XRD method is relatively low. As shown by Schwertmann and Murad (1983) storage of
ferrihydrite in aqueous suspension and pH changes can result in the formation of goethite
and hematite. Nevertheless the presence of phosphate retards the transformation of
ferrihydrite into crystalline products (Paige et al., 1997). Observed reduction in porosity,
SSA and Fe0 could be explained by the apparition of a more crystallized form of iron
oxide such as hematite or goethite.
Modification of the organic matter content of ferrihydrite during plant growth.
The content of organic matter in ferrihydrite increased during plant growth when
compared to the control treatment without plant (Table 7). The sorption of organic
compounds was higher on ferrihydrite II than on ferrihydrite I. The concentration of
organic C was higher in the fraction of ferrihydrite adhering to the roots than in the non-
adhering fraction. The concentration of organic C in the fraction of ferrihydrite both
adhering and non-adhering to the roots was higher for Sikuani than for Corso. These
results support the suggested influence of adsorbed organic compounds on the observed
(i) decrease of the ferrihydrite porosity on the fraction adhering to the roots, and (ii)
decreased P sorption capacity on the fraction of ferrihydrite adhering to the roots.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 43
S
to
q? 2.0
2.0 -Pl P2P3 p4
Pl P2P, p41.5 -
Sikuani
Ferrihydrite I
1.0 -
0.5 -
on
Sikuani TÄ
Ferrihydrite II
1.5
1.0 -
0.5 -
0.0
pi P2P,
Corso
Ferrihydrite I
P, P2 F
Corso
Ferrihydrite II
0 20 40 60 0 20 40 60
P treatment ([ig P m )
Figure 8. Influence of the presence of plant on the oxalate-extractable Fe (Fe0). Pi= 6.6
fig P m"2 ; P2= 24.8 fig P m"2 ; P3= 33.0 fig P m"2 ; P4= 66.0 fig P m"2 ; mean value ± SE ; n
= A.%: ferrihydrite adhering to the roots; : ferrihydrite non-adhering to the roots; open
symbols: 3 weeks; full symbols: 12 weeks; : Control 12 weeks.
-
Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 44
4.0 -
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Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 45
DX
c*
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172
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in
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1 "O'«
=3t/2
S 169
ftCZ3
168
Ferrihydrite I Ferrihydrite II- 320
310
- 300
- 290
280
270
WD
fS
es
u
*-
20 40 60 20 40 60
P treatment (j^g P m )
Figure 10. Modification of the specific surface area (SSA) after 12 weeks plant growth. Pi= 6.6
fig P m"2 ; P2= 24.8 fig P m"2 ; P3= 33.0 fig P m"2 ; P4= 66.0 fig P m"2. mean value ± SE ; n = 3.
• : Corso; A: Sikuani; B : Control; full symbols: ferrihydrite fraction non-adhering to roots;
open symbols: ferrihydrite fraction adhering to roots.
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Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 46
45.2
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43.6
Ferrihydrite -. FerrihydriteII
- 94.0
97.0
91.0 £©U
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88.0
i n ' n n i ' i ' i '
167 169 171 173 270 290 310 330
Specific surface area (SSA) (m g" )
85.0
Figure 11. Change of the ferrihydrite porosity related to the specific surface area
(SSA) after 12 weeks plant growth. P1= 6.6 fig P m"2 ; P2= 24.8 fig P m"2 ; P3= 33.0
fig P m"2 ; P4= 66.0 fig P m"2; mean value ± SE ; n = 3. On fig 10 : ?u P2, P3, P4 for
A; •: Corso; A: Sikuani; : Control; full symbols: ferrihydrite fraction non-
adhering to roots; open symbols: ferrihydrite fraction adhering to roots.
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Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 47
Table 7. Influence of 12 weeks plant growth and P supply on organic matter in ferrihydrite for
Zea mays L. Corso and Sikuani. P2= 24.8 fig P nf2 ; P3= 33.0 fig P m"2 ; P4= 66.0 |ig P m"2; Adh
oxide adhering to the roots; Non-adh.: oxide non-adhering to the roots, mean value ± SE; n = 4.
Cultivar Organic matter content on ferrihydrite (fig C m"2)
Ferrihydrite I Ferrihydrite II
Adh. Non-adh. Adh. Non-adh.
p2 Corso 6.43 ±0.342 1.39 ±0.220 7.52 ± 0.577 1.10 ±0.211
Sikuani 12.26 ±0.708 1.68 ±0.244 9.59 ±0.612 1.45 ±0.