effects of field managements for soil organic matter stabilization on water-stable aggregate...

Journal of Geochemical Exploration 129 (2013) 45–51

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Journal of Geochemical Exploration

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Effects of field managements for soil organic matter stabilization on water-stableaggregate distribution and aggregate stability in three agricultural soils

Riccardo Spaccini ⁎, Alessandro PiccoloDipartimento di Scienze del Suolo della Pianta, dell'Ambiente e delle Produzioni Animali Università di Napoli Federico II, Via Università 100, 80055 Portici, ItalyCentro di Ricerca Interdipartimentale sulla Risonanza Magnetica Nucleare ( NMR) per l'Ambiente, l'Agro-Alimentare ed i Nuovi Materiali, via Università di Napoli Federico II,Via Università 100, 80055 Portici, Italy

⁎ Corresponding author at: Dipartimento di Scienze del Se delle Produzioni Animali Università di Napoli FedericPortici, Italy. Tel.: +39 0812539176.

E-mail address: [email protected] (R. Spacci

0375-6742/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.gexplo.2012.10.004

a b s t r a c t
a r t i c l e i n f o
Article history:Received 2 July 2012Accepted 6 October 2012Available online 13 October 2012

Keywords:Soil managementSoil organic matterWater-stable aggregatesMWDwCompostIron-porphyrin catalyst

The effect of different soil organic matter (SOM) managements on soil aggregate stability was evaluated inthree agricultural sites in Italy. The field plots were treated with traditional deep tillage, minimum tillage,green manuring, mature compost, and iron-porphyrin as a biomimetic catalyst. Although the original soil tex-ture exerted a strong influence on the overall soil physical quality, after 3 years of field experiments, the soiladditions with humified compost, under maize cropping, improved both, the yield of water stable macroag-gregates and the soil aggregate stability, with a pronounced effect on the aggregate distribution of the coarsetextured soil. Soil treatments with the biomimetic catalyst, under wheat cropping, promoted an initial stableincorporation of finer particle-sizes in water-stable macro-aggregates. This effect was progressively lost withincreasing experimental time due to either loss or inactivation of the catalyst. Innovative management prac-tices for SOM stabilization based on soil amendment of either humified mature compost or a water-solublecatalyst to promote in-situ photopolymerization of humic molecules, were found to improve distributionand stability of soil water stable aggregates, as compared to the conventional methods of SOM management.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Soil physical properties, such as textural composition, particle sizedistribution and dynamics of macro and microaggregates, representimportant factors for the maintenance of the overall soil quality. Thevariation on particle size distribution produced by aggregation and dis-aggregation processes, plays amarked influence on bothmacro andmi-croporosity of soil systems, thus determines amount and availability ofwater and nutrients and the rate of gas exchanges. The dynamics of soilorganic matter (SOM) is closely related to soil physical properties(Tisdall and Oades, 1982). SOM components, in fact, are involved inthe different levels of soil aggregate hierarchy, from the initial forma-tion of basic organo-mineral complexes up to the stabilization of largersize-aggregates (Oades andWaters, 1991; Six et al., 2004), thereby pro-moting the soil physical aggregation processes (Plante and McGill,2002a) and the overall soil stability (Kasper et al., 2009). In turn, stablemicro- and macro-aggregates exert a physical protection on organicmatter associated to soil particle sizes (Six et al., 2004).

Various soil management practices have been applied in order toincrease, both, soil aggregate stability and the soil organic carbon(SOC) content in croplands. These include reduced and zero tillage,

uolo della Pianta, dell'Ambienteo II, Via Università 100, 80055

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perennial crops, organic farming, improved rotations, and conversionof arable land to grassland or woodland (Bronick and Lal, 2005;Plaza-Bonilla et al., 2010). The accumulation of SOC has been concep-tually regarded as a consequence of the saturation of soil minerals byOC (Hassink and Whitmore., 1997; Six et al., 2002), and, hence,governed by the physical control on OM decomposition (Scott et al.,1996; von Lützow et al., 2006). Therefore, the most common SOMmanagements are focused on the physical protection and stabilizationof SOM pools (Six et al., 2004, 2006; Torn et al., 1997),by combiningminimum or zero tillage with crop rotation, and beneficial incorpora-tion of crop and plant residues (green manure, mulch treatment). Thesmall soil disturbance provided by reduced tillage methods inhibitsaerobic microbial decomposition of SOM and thus limits the degrada-tion of soil aggregates and the consequent losses of soil OC.

In recent years, the concepts of C saturation in soil were further de-veloped (Stewart et al., 2009; Zhao et al., 2006), with an increasing con-sideration on the relationship between OM biochemical recalcitranceand SOC stabilization processes (Lorenz et al., 2007; Mikutta et al.,2006). Nowadays it is currently ascertained that also the quality of or-ganic matter is involved in, both, the accumulation of OC and the stabi-lization of soil particle sizes and aggregates (Piccolo et al., 2004;Spaccini et al., 2002; Yamashita et al., 2006). The processes of SOC accu-mulation and decomposition closely depend on the molecular charac-teristics of the organic matter inputs (Derenne and Largeau, 2001;Lorenz et al., 2007; Spaccini et al., 2002). This affects not only theamount of OM incorporated in soil, but also the chemical reactivity

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Table 1Textural composition (%), bulk density (g cm−3) and TOC (g kg−1) content of soilsfrom field experiments.

Field sites Soil type Sand Silt Clay Bulk density TOC

Torino Typic ustifluvent 36.9 56.2 6.9 1.50 11.5Piacenza Udifluventic haplustept 17.9 47.1 35.0 1.30 17.2Napoli Vertic haploxeralf 47.0 20.1 32.9 1.40 10.5

46 R. Spaccini, A. Piccolo / Journal of Geochemical Exploration 129 (2013) 45–51

and the role of different SOC pools for themaintenance of aggregate sta-bility and of soil physical quality. While soil addition with bio-labile or-ganic inputs had a short term effects on the soil aggregation processes,the soil treatmentswith humified organicmaterialsmay lead to a stead-y improvement of the main soil physical parameters, such as aggregatestability, erosion, water retention and available water content (Piccoloand Mbagwu, 1999; Piccolo et al., 1996, 1997).

The soil addition with mature compost represents a valuable andeffective tool to increase both the content of humified and hydropho-bic organic components and the long term stability of soil aggregates(Celik et al., 2004; Ngo et al., 2011; Zebarth et al., 1999). A large liter-ature indicates that soil amendments with different compost mate-rials provide an effective improvement in aggregate stability andrelated physical properties (porosity, infiltration rate, surface erosion,etc.), regardless of soil type and crop rotation (Sodhi et al., 2009;Weber et al., 2003, 2007).

A novel understanding of humus chemistry has shown that the sta-ble humified pools in soil are build up by relatively small heterogeneousbiomolecules self-assembled by weak interactions in supramolecularassociations, rather than being composed by large molecular-weightmacropolymers as previously assumed (Piccolo, 2002). It has beenproved that humic molecules could be covalently linked to each otherby oxidative coupling reactions under appropriate catalysis, thereby en-hancing their molecular size and complexity (Smejkalova and Piccolo,2005). The results of soil application with iron-porphyrin solutionshowed that the in situ catalyst-assisted photo-polymerization of soilorganic carbon (SOC) increased the water stability of soil aggregatesafter 5 days incubation of three Mediterranean soils (Piccolo et al.,2011).

The aim of this work was hence to evaluate the effects on soil ag-gregate stability of SOM managements based on soil amendmentswith either mature compost or a water-soluble biomimetic catalyst,as compared with conventional soil treatments. In three different ag-ricultural sites in Italy, field plots under maize were treated with tra-ditional deep tillage, minimum tillage, green manuring and maturecompost, whereas field plots under wheat were added with theiron-porphyrin catalyst.

2. Materials and methods

2.1. Materials

The compost used for soil amendment was obtained by GeSeNuSrL, Perugia, Italy. The final products were obtained through an aero-bic composting process consisting of 30 days followed by a curingphases of 20 days of maturity.

A water-soluble iron-porphyrin (FeP) was synthesized in the lab-oratory as meso-tetra (2,6-dichloro-3-sulfonatophenyl)porphyrinateof iron(III) chloride, Fe-(TDCPPS) Cl, as previously reported (Piccoloet al., 2005).

2.2. Soils and experimental setup

Three soils, from the experimental sites of the Agricultural Univer-sities of Torino (TO), Piacenza (PC) and Napoli (NA) were chosen toset up the field treatments. Soil classification textural compositionand organic carbon content of the selected soils are shown inTable 1. The effect of SOMmanagement on soil aggregate distributionwas evaluated by comparing the following field managements duringa 3-year experiment, using a monoculture of mais (Zea mays L.) andwheat (Triticum durum) as annual crops:

2.2.1. Maize- Traditional (TRA): plowing at 35 cm depth, followed by surfaceharrowing and mineral fertilization with urea at the rate of130 kg ha−1of N

- Minimum tillage (MT): no plowing, with addition of mineralfertilizers as TRA treatment

- Green manure (GM): plowing at 30 cm depth, followed by surfaceharrowing. Leguminous crops were interlaced between two mainannual cycles and used as green manure to totally or partiallyreplace nitrogen fertilizer.

- Compost first rate (COM-1): same as the TRA plots but with theaddition of an amount of mature compost corresponding to2.7 ton ha of OC.

- Compost second rate (COM-2): same as the TRA plots but withaddition of an amount of mature compost corresponding to5.4 ton ha of OC.

Each treatment was established on a 4×4 m plot with four repli-cates, in a randomized block experiment.

2.2.2. Wheat- FeP. Plowing at 35 cm depth, followed by surface harrowingwith the addition of mineral fertilizer and 10 kg ha of a biomimeticcatalyst.

- No-FeP. Plowing at 35 cm depth followed by surface harrowingwith the addition of mineral fertilizers.

The experiment based on the addition of the biomimetic catalystwas performed on 1 m×1 m field plots (n=4).

2.3. Water-stable aggregates and soil stability index

The classical procedure described by Kemper and Rosenau (1986)was used to separate water-stable aggregates. Briefly, 20 g ofb4.75 mm air-dried soil samples was put on the topmost of a nestof three sieves with 1.00, 0.50, and 0.25 mm mesh size andpre-soaked in distilled water for 30 min. Then, the nest of sieveswas oscillated vertically in water 20 times, using a 4 cm amplitudeat the rate of one oscillation per second. Care was taken to ensurethat soil particles on the topmost sieve were always below thewater surface during each oscillation. After wet-sieving, thewater-stable soil materials left on each sieve and the unstable(b0.25 mm) aggregates were quantitatively transferred into beakers,dried in the oven at 50 °C for 48 h, weighed and stored for analysis.The percentage ratio of aggregates in each sieve represents thewater-stable aggregates for size classes: 4.75–1.00, 1.00–0.50,0.50–0.25, and b0.25 mm. Mean-weight diameter (MWDw) ofwater-stable aggregates was calculated by the following equation:

MWDw ¼ Si¼1XiWin

where Xi is the mean diameter of the ith sieve, and Wi is the amountof total aggregates in the ith fraction.

3. Results

3.1. Control soils

The distribution of water-stable aggregates obtained from initial soilsamples, before the start of field experiments, indicated amarked influ-ence of textural composition on soil structural properties in the three

Table 3Torino experimental site: percent distribution (%) of water-stable aggregate sizes(mm) under different treatments for 3 years of experimentation.

Soil treatments Aggregate size

4.75–1.00 1.00–0.50 0.50–0. 25 b0.25

Control soil 9.7 21.1 27.5 41.8

MaizeFirst year

TRA 15.7 27.1 26.9 30.4MT 13.2 30.9 27.6 28.3GM 10.8 26.5 31.1 31.6

47R. Spaccini, A. Piccolo / Journal of Geochemical Exploration 129 (2013) 45–51

field sites (Table 2). The lowest aggregate stability (MWDw=0.6) wasfound for the silty-loamy soil of Torino, that was characterized by boththe lowest clay content (6.9%, Table 1) and the largest yield (41.8%) ofunstable (b0.25 mm) micro-aggregates (Table 2). Conversely, the sig-nificantly larger stability index (Table 2) found, in the order, in thesandy-clay loam soil of Napoli (MWDw=1.43) and in the silty-clayloam soil of Piacenza (MWDw=1.76), may be associated with a largeyield of clay particles found in these soil samples (Table 1).

The effects induced by field treatments on soil aggregate distribu-tion and structural stability, for the three experimental sites, arereported in Tables 3–5 and in Fig. 1, respectively.

COM-1 9 27.9 32 31.1COM-2 14.7 29.3 27.2 28.9LSD 0.05 1.7 NS 3.2 1.2

Second yearTRA 8.8 25.5 39.8 25.9MT 9.6 23.4 38.7 28.2GM 10.3 24.1 39.7 25.9COM-1 9.6 24.1 38.5 27.8COM-2 9.7 26.6 39.9 23.8LSD 0.05 NS NS NS NS

Third yearTRA 11.4 25 36.2 27.5MT 9.8 21.2 38 30.9GM 9.4 19.4 38.3 33COM-1 15.3 25.2 34.5 25.1COM-2 20.3 27.7 29.8 22.2LSD 0.05 2.5 2.2 2.1 2.2

WheatFirst year

Fe-P 19.4 (4.2)a 25.0 (4.4) 25.8 (3.8)b 29.8 (8.3)No Fe-P 12.9 (2.2)b 28.5 (2.5) 32.3 (1.1)a 26.2 (0.9)

Second yearFe-P 9.5 (1.2)b 23.4 (1.2) 38.2 (3.8)a 28.9 (8.7)No Fe-P 11.3 (1.7)a 22.5 (1.3) 33.0 (1.7)b 33.1 (2.8)

Third yearFe-P 10.3 (1.5) 25.0 (1.9) 36.5 (1.4) 28.2 (2.7)No Fe-P 10.6 (1.4) 24.3 (5.2) 33.9 (3.3) 31.1 (5.4)

LSD least significant difference (n=4), NS not significant. Numbers in brackets forwheat plots represent standard deviation (n=4). Different small letters in columns in-dicate significant difference at 0.05 probability level.

3.2. Torino experimental site

An increase of aggregate stability was shown after the first year, bythe majority of field treatments on the maize plots of Torino with re-spect to the initial soil (Fig. 1a). Only the addition of green manure(GM) and the lower rate of compost (COM-1) produced smallerMWDw increases (Fig. 1a). All field treatments revealed a positiveeffect in the distribution of water-stable aggregates, with an overalldecrease in the yield of microaggregates (b0.25 mm), which weresteadily incorporated in upper size-classes (Table 3). The widespreadincrease of soil aggregationmay be explained with the physical actionof plant roots, fungal hyphae, root debris and microbial bio-products,which promote the association of small soil fractions into meso- andmacro-aggregates (Chan and Heenan, 1999; Six et al., 2004; Tisdalland Oades, 1982). For the GM and COM-1 treatments, the effect waslimited to the intermediate particle-size fraction (1–0.25 mm),while for traditional (TRA), minimum tillage (MT), and larger com-post rate (COM-2), there was a significant particle incorporationalso in the large macroaggregate class (Table 3), with a consequenthigher value of MWDw (Fig. 1a).

An improvement of soil stability (Fig. 1a) was observed for the soiladded with biomimetic catalyst (FeP), as compared to both initial(control) and untreated plots (No-FeP), indicating that this treatmentinduced a redistribution of macroaggregates into larger size-fractions.After the second year, an overall stabilization of structural propertiesof all soil treatments was revealed by the results of particle size frac-tionation, that was characterized by an even distribution ofwater-stable aggregates and a similarity of MWDw values (Table 3,Fig. 1a). The aggregate distribution (Table 3) showed a further de-crease of microaggregate sizes for all treatments, which were preva-lently incorporated into the next larger size-fraction (0.50–0.25 mm).

A decrease of stability index was found for TRA, MT, and COM-2,whose values approached those for GM and COM-1 (Fig. 1a), that, in-stead, maintained a steady effect on soil aggregates. The lower aggre-gate stability found for TRA, MT, and COM-2 was mainly related to thedecrease of large macroaggregates, whose values remained unvariedonly for GM and COM-1 treatments (Table 3). Also the soil samplestreated with the biomimetic catalyst (FeP) showed, in the secondyear, a soil redistribution from larger aggregates towards smallermacroaggregates (Table 3) thereby showing an MWDw value compa-rable to that found for the No-FeP treatment (Fig. 1a).

Table 2Percent (%) distribution of water-stable aggregates (mm) and mean-weight diameterindex (MWD, mm) of initial control soils (t0).

Field sites Aggregate size MWD

4.75–1.00 1.00–0.50 0.50–0.25 b0.25

Torino 9.7 21.1 27.5 41.8 0.59Piacenza 52.8 26.2 9.1 11.9 1.76Napoli 39.2 30.8 14.3 15.7 1.43LSD 0.05 8.5 NS 4.0 3.9 0.18

LSD least significant difference (n=4).

At final sampling time, no significant variations in structural sta-bility were found in soil samples under traditional management, min-imum tillage and green manure addition, except for a slight increasein the yield of microaggregates (Table 3). Conversely a positive effecton structural properties was revealed by the aggregate distribution infield plots treated with compost (Table 3). In fact, both COM-1 andCOM-2 revealed a significant increase of the stability index (Fig. 1a),mainly related to the incorporation of microaggregates and smallmacroaggregates (0.50–0.25 mm), into larger sized fractions. The im-provement in soil physical stability was consistent with the amountof added compost, with the higher compost addition (COM-2) pro-viding the larger response. In fact, the yield of large macroaggregatesfor COM-2 was twice as large as that for TRA, MT, and GM (Table 3).No difference in the distribution of water-stable aggregates andMWDw index was found between the FeP treatment and its controlfor the third year.

3.3. Piacenza experimental site

As noticed for the sandy soil of Torino, also the field treatmentsfrom the experimental site of Piacenza, showed a similar overall im-provement of soil aggregation after the first year of cultivation(Table 4). This finding was mainly associated to the greater yield oflarger aggregate size fractions, as compared to the initial distributionat the onset of experimentation (Table 2). The major effects wereachieved for the least disruptive MT management, and for COM-2,and FeP treatments. Both soil aggregation and stability index were

Table 4Piacenza experimental site: percent distribution (%) of water-stable aggregate sizes(mm) under different treatments for 3 years of experimentation.


4.75–1.00 1.00–0.50 0.50–0. 25 b0.25

Control soil 52.8 26.2 9.1 11.9

MaizeFirst year

TRA 55.6 21.5 11 11.9MT 64.3 17.7 9.1 9.0COM-2 63.6 18.3 9.3 8.7LSD 0.05 5.5 NS NS 1.7

Second yearTRA 36.1 33.4 15.6 14.9MT 45 27.1 14.7 13.2COM-2 45.4 29.5 14.6 10.6LSD 0.05 3.2 2.4 NS 1.6

Third yearTRA 53.7 25.9 10.8 9.6MT 50.4 28.1 11.7 9.9COM-2 58.8 24.1 9.3 7.8LSD 0.05 NS NS NS NS

WheatFirst year

Fe-P 55.1 (0.9)a 23.8 (1.1) 11.2 (0.3) 9.9 (0.3)bNo Fe-P 49.0 (1.9)b 25.7 (2.7) 10.5 (0.4) 12.5 (1.6)a

Second yearFe-P 38.4 (0.1)a 29.9 (0.4)b 16.2 (0.5)b 15.5 (0.8)No Fe-P 34.6 (1.5)b 30.8 (0.4)a 17.9 (0.8)a 16.7 (1.0)

Third yearFe-P 47.7 (2.9) 28.2 (1.3) 12.5 (0.6) 11.6 (1.4)No Fe-P 52.5 (5.5) 25.9 (3.0) 11.8 (1.4) 9.8 (1.1)


Table 5Napoli experimental site: percent distribution (%) of water-stable aggregate sizes(mm) under different treatments for 3 years of experimentation.


4.75–1.00 1.00–0.50 0.50–0. 25 b0.25

Control soil 39.2 30.8 14.3 15.7

MaizeFirst year

TRA 33.3 32.4 16.6 17.6MT 31.9 35.4 18.4 14.2GM 28.7 39.8 16.8 14.7COM-1 53.6 26.9 9.5 10COM-2 37.3 31.4 12.7 18.6LSD 0.05 3.3 1.1 3.7 4.7

Second yearTRA 26.4 40.1 20.1 13.4MT 33.1 41.6 15.8 9.5GM 32.4 38.4 17.3 11.9COM-1 37.6 37.2 14.9 10.4COM-2 32.6 36.5 18.1 12.8LSD 0.05 5.3 3.7 2.1 1.2

Third yearTRA 38.7 33.7 16.1 11.5MT 40.8 31.9 16.1 11.2GM 46.1 31.4 13.2 9.3COM-1 43.1 29.9 15.5 11.5COM-2 43.2 32.6 14.7 9.5LSD 0.05 2.2 NS NS 2.4

WheatFirst year

Fe-P 51.3 (2.4) 25.9 (1.9) 11.2 (2.3) 11.6 (1.7)No Fe-P 47.5 (7.3) 28.2 (4.3) 13.3 (2.0) 10.9 (2.7)

Second yearFe-P 31.5(2.7) 40.0(0.4) 17.1(1.7) 11.4(1.5)No Fe-P 36.7(6.0) 40.2(3.0) 13.2(1.9) 9.9(1.1)

Third yearFe-P 41.6(1.9) 32.7(0.3) 15.2(0.6) 10.4(0.9)No Fe-P 39.7(2.2) 35.0(1.4) 15.2(0.5) 10.1(0.4)



significantly larger than the respective TR and No-FeP control treat-ments (Table 4, Fig. 1b).

A sharp decrease of, both, stable macro-aggregates and stabilityindex, occurred in all soil treatments under maize after 2 years offield experiments (Table 4, Fig. 1b). However, although all soil sampleswere characterized by the redistribution of soil particles towardssmaller size aggregates, the loss of structural properties was differentfor various treatments (Table 4). The large preservation of water-stable macroaggregates (Table 4), and of the related MWDw indexwas shown by MT and COM-2 samples (Fig. 1b). Moreover, the parti-cle size fractionation of COM-2 provided the lowest yield of unstablemicroaggregates (10.6%), whose amount was still lower than that ofthe initial control soil (11.9%). Notwithstanding the overall loss ofstructural properties found, in the second year, for the wheat cultivat-ed plots, both size-aggregate distribution (Table 4) and MWDw index(Fig. 1b) showed that treatment with the biomimetic catalyst pre-served soil structural quality more than the control samples.

Compared to previous results only a moderate recovery of theoriginal structural stability was shown, at the end of the third exper-imental year, by the aggregate distribution of TRA, MT, FeP, andNo-FeP treatments (Table 4). Although the yields of size-aggregatesand the value of the stability index were slightly raised, they werestill lower with respect to those found at the onset of experiments,thereby suggesting a progressive lower effectiveness of both MT andFeP on soil structural quality. Conversely a positive effect on soil ag-gregate stability was confirmed by the maize field plots with compostaddition (Table 4, Fig. 1b). The COM-2 treatment showed a continu-ous decrease of both microaggregate (b0.25 mm) and intermediatemacroaggregate (0.25÷0.50 mm) fractions, which became firmly in-corporated into larger sized aggregates (+13.4%), thus improving theoverall structural stability index (1.92 Fig. 1b).

3.4. Napoli experimental site

Restricted variations of structural properties were found in soiltreatments from experimental site of Napoli throughout the experi-mental period. Different from other experimental sites, a slight dete-rioration of soil stability was shown by nearly all soil samples in thefirst year of cultivation (Table 5). With respect to the initial controlsoil, the data from soil fractionation revealed a general lower yieldof large size aggregates and a decrease of soil stability index. On thecontrary, the soil treated with the lower compost rate (COM-1),showed a significant improvement of MWDw (Fig. 1c), due to aneffective association of soil particles into larger aggregate fractions,which included more than 50% of total fraction mass (Table 5). Theaddition of the low compost rate may have promoted a microbiallyinduced priming effect with a temporary improvement of macroag-gregate formation (Chan and Heenan, 1999). In the case of wheatfields, both FeP and No-FeP treatments revealed an improvement ofsoil structural properties as compared to maize fields, thus suggestinga lower impact of this cropping systems on soil physical parameters(Table 5).

A limited modification of soil stability with slight different distri-bution of water-stable aggregates was observed in the maize plotsalso after two years of experimentation. For all treatments, soil frac-tionation indicated a decrease in microaggregate yield and a corre-sponding, though uneven, increase in different macroaggregateclasses (Table 5). A sharp decrease of MWDw was found in soil sam-ples added with the lower compost rate (Fig. 1c), thereby confirming

0.801.001.201.401.601.802.00

0.40

0.50

0.60

0.70

0.80

0.90

1.00

FePNo-FeP

FeP No-FePTR MT GM

GM

Com-1

Com-1

Com-2

Com-2

FePNo-FePCom-2

c

b

MW

D w

(m

m)

MW

D w

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m)

MW

D w

(m

m)

MWDw t0 MWDw 1st year MWDw 2nd year MWDw 3rd year

1.001.201.401.601.802.002.20

MTTR

TR MT

a

Fig. 1. Variation of aggregate stability index (MWDw) with soil treatments and exper-imental time: a Torino; b Piacenza; c Napoli.


the short-term feature of the previously observed behavior of COM-1.An overall maintenance of stability index was shown by the othertreatments, except for traditional management that showed a furthersignificant decrease of MWDw (Fig. 1c). The MWDw stability indexfor both FeP and No-FeP treatments was also reduced in the secondyear to the levels found for the maize fields.

Similar to other experimental sites, also the soil samples of Napolishowed a structural improvement in all treatments after three exper-imental years (Table 5). The soil fractionation of both maize andwheat plots produced a larger amount of macroaggregate fractionwith the consequent increase of the related MWDw index, whichvalues were larger than for the initial undisturbed control soil. Thebest structural quality was found for soil treatments with organicamendments, including green manure, and for FeP, although the lat-ter values were not significantly different from No-FeP (Table 5).

4. Discussion

The data of aggregate distribution and of soil stability index of ini-tial control soils, suggested a predominant effect of specific textureand clay content on the physical and structural properties of soils.The larger yield of macroaggregates and the higher stability indexwere found in the heavy textured soils of Piacenza and Napoli thatwere characterized by the higher content of clay fraction. The finestsoil mineral components, such as fillosilicates and Fe and Al hydrox-ides, exert a strong influence on soil structural properties. In fact, itis assumed that the large surface area of clay-sized particles allowsa close interaction between inorganic and organic colloidal constitu-ents, with consequent formation and stabilization of soil aggregates(Attou et al., 1998; Oades and Waters, 1991).

Nevertheless, the various soil managements induced, a comparablecyclic aggregation dynamics in the different experimental sites, regard-less of specific soil structural properties. The amount of macro- andmicro-aggregates released by soil fractionation revealed, respectively,

an initial slight improvement or degradation of structural properties,followed by amoderate decrease or recovery of soil stability. The resultsshowed a similar sequence of aggregate yields and stability indices,whose values varied annually around the average level of the respectiveundisturbed initial soils. In cultivated soils with low SOM content, thelow resilience of clay particles may result in a slow response to the ag-gregation induced by soil management (Spaccini et al., 2004), and a sig-nificant loss of structural stability for heavy textured soils. In fact, thesusceptibility of cultivated soils to lose structural stability is a functionof the initial aggregation, that is greater for stable clayey soils than forfragile sandy soils (Spaccini et al., 2001). Our findings on aggregate dis-tribution showed that a steady positive effect on soil stability, in respectto undisturbed control soils,was shown by various soil managements inthe more basically fragile sandy soil of Torino whereas lower relativevariation was found in the two heavy textured field sites of Piacenzaand Napoli.

Moreover, large soil aggregates, placed in the upper level of aggre-gate hierarchy, are usually characterized by large porosity, wideplanes of weakness, and low tensile strength (Oades and Waters,1991). Therefore, macroaggregates are mostly affected by land useand soil disturbance, and undergo rapid turnover cycles and fastaggregation/disaggregation dynamics, especially for agriculturalsoils with low OC content (Plante and McGill, 2002b). Since turnovertime of macroaggregates ranges from 10 to 90 days (De Gryze et al.,2006; Plante et al., 2002), with significant fluctuation during growingseasons, frequent samplings should better record the effect of man-agement practices on soil aggregation (Daraghmeh et al., 2009). Thesingle sampling date used in this experiment, set at the end of thegrowing season, only summarized the structural modifications oc-curred throughout the crop cycle and may have partly masked thedifferences among soil treatments. Nevertheless, despite the small ef-fect exerted by soil managements on the intrinsic soil physical prop-erties, significant differences on aggregate size distribution andMWDw were found among various treatments in the three sites.

A similar aggregation process was revealed by the conventionalTRA, MT, and GM soil treatments at any experimental site. This indi-cates that either a reduced soil disturbance by minimum tillage, orthe soil incorporation of residues from leguminous crops in greenma-nuring, did not significantly modify the aggregate dynamics, neitherin the sandy-loam soil of Torino, nor in the heavier textured soils ofPiacenza and Napoli. On the contrary, slight but significant improve-ments of soil aggregation and structural stability were found forboth compost treatments (COM-1 and COM-2), as compared toother soil managements.

Contrasting results have been reported on the relation betweensoil aggregate stability and green manuring, when in combinationwith either reduced tillage or conventional tillage (Biederbeck et al.,1998; Podwojewski and Germain, 2005). The main purpose of greenmanuring relies in to supply the soil with an available source oforganic nitrogen, that may reduce or even replace mineral fertiliza-tion. However, the organic residues added with green manuring areusually poor in lignified tissues and hydrophobicity and are easily de-composable (Carvalho et al., 2009). Therefore these biolabile mate-rials are recognized to provide at best only a transient effect on soilstability (Piccolo and Mbagwu, 1999). Current findings on soil man-agement methods based on no-tillage practices indicate an overallimprovement of soil physical properties and soil stability, as com-pared to conventional tillage (Six et al., 2000, 2004). On the otherhand, the main effects of reduced tillage methods on soil aggregationare limited to surface horizons, whereas smaller differences from con-ventional tillage are found below 10 cm of soil depth (Kasper et al.,2009; Liebig et al., 2004). This emphasizes the importance to investi-gate the whole soil profile when studying the suitability of differenttillage systems for aggregate stability (Plaza-Bonilla et al., 2010).

A large literature indicates that soil amendments with differentcomposts provide an effective improvement in aggregate stability and


related physical properties (porosity, infiltration rate, surface erosion,etc.), regardless of soil type and crop rotation (Sodhi et al., 2009;Tejada and Gonzalez, 2008;Weber et al., 2003, 2007). Although the im-provement of soil structure is closely associated with an increase of or-ganic carbon content, the molecular composition of added organicmatter plays a basic role in the long-term stabilization of soil aggregates(Piccolo and Mbagwu, 1999). Compost may promote two possible ef-fects on soil physical quality: (1) a transient or temporary soil aggrega-tion through the stimulation of microbial activity and consequentproduction of stabilizing exopolysaccharides, and, (2) a long-term sta-bilization due to addition of an effective amount of hydrophobic humi-fiedmaterials on soil (Albiach et al., 2001; Bipfubusa et al., 2008; Piccoloand Mbagwu, 1999; Romàn et al., 2003). In fact, depending on the typeand quality of compost, its amendment to soils may produce eithershort term, poor, or even negative effects on soil aggregate stability(de León-González et al., 2000; Kohler et al., 2008). As remindedabove, the transient effect observed on soil aggregates of the COM-1field plots of Napoli site in the first year (Table 5) may be related to anincreased amount of microbial bio-products due to a priming effect onSOC. However, the aggregate-size distribution and stability indexreported for COM-1 and COM-2 for all field sites after 3 years(Tables 3–5), indicate that the progressive incorporation of humifiedand hydrophobic organic matter from compost allowed a slow but per-sistent improvement of soil aggregation. The largest effect was foundfor Torino, where the intrinsically low initial soil structural stabilitywas more easily improved by stable compost, and favored a significantincorporation of fine aggregate sizes into larger stable aggregates.

An increase of soil aggregate stability was found, after the first yearof experimentation, in soil treatments with biomimetic catalyst, for allthe experimental sites. Fractionation of water-stable aggregates forthese soils revealed a general incorporation of smaller aggregatesinto larger stable aggregate sizes. However, the effect of the in situcatalysed photopolymerization of SOM on soil aggregation showed aprogressive decline with experimental time, thereby depriving thedistribution of water stable aggregates in the FeP-treated soils of anysignificant difference from control at the final sampling date. A shortterm effect of the biomimetic catalyst on water stable aggregateswas also found in a laboratory incubation experiment, that revealeda temporary improvement of soil aggregation followed by a decreaseof soil stability with increasing numbers of dry-wetting cycles(Piccolo et al., 2011).

The addition of biomimetic catalyst promotes the photo-polymerization of the aromatic components of soil humic substances(Smejkalova and Piccolo, 2005; Šmejkalova et al., 2006), thereby in-creasing the association among soil particles. In OC depleted soils,the effect of the in situ catalysis may be, hence, limited to the earlystage of soil treatments, as revealed by our findings from field exper-iments. In fact, after an initial impact of the photopolymerization onsoil physical quality, the low availability of reactive humic moleculesmay limit the effectiveness of additional treatments with the biomi-metic catalyst. Moreover, the reactivity of the photopolymerizationmay also be decreased by either an irreversible adsorption of the cat-alyst on mineral colloids or the loss by eluviation to lower soil depthsin coarse textured field plots.

5. Conclusion

Field application on three different agricultural soils of eithermature compost or a water-soluble biomimetic catalyst for SOM se-questration, showed positive effects on the distribution of water-stable aggregates, with a significant improvement of soil aggregatestability in respect to conventional SOM management practices.Although the specific textural compositions of the three experimentalsites strongly affected the distribution of water-stable aggregates,thereby partially hindering the effect of the different soil treatments,significant variation was observed among SOM managements. After

3 years of field experiments, both minimum tillage and green manur-ing treatments showed short-term or null effects on soil physicalquality, as compared to conventional tillage. Conversely, the annualaddition of humified mature compost effectively promoted the steadyincorporation of unstable microaggregates into larger macroaggre-gates, with an overall improvement of the soil stability index. Theaddition of the biomimetic catalyst revealed an initial positive effecton the distribution of water-stable aggregates in the first year ofexperimentation for all the experimental sites. The lower efficacy onwater-stable aggregates found in soil treatment with iron-porphyrinat increasing experimental time, may be related to low SOC contentsthat prevented the reactivity of biomimetic catalyst for the furtherstabilization of soil humic substances, thereby allowing the pro-gressive regression to the aggregate distributions regulated by thetextural composition. While SOM management with mature compostis a valuable current practice for the improvement of physical qualityin agricultural soils, the use of biomimetic catalyst for the in situ po-lymerization of humic molecules may represent a feasible innovativetechnology to also improve soil structural stability, provided that theefficacy of the catalyst is enhanced.

Acknowledgments

This publication is the result of MESCOSAGR project co-ordinated byProf. A. Piccolo, and funded by the Italian Ministry of Education, Univer-sity and Research (MIUR), under the Special Integrative Funding forResearch (FISR) initiative (Strategic Programme for “Sustainable Deve-lopment and Climatic Changes”).

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