rhizosphere phosphorus depletion induced by heavy nitrogen fertilization in forest nursery soils

7
DIVISION S-7-FOREST & RANGE SOILS Rhizosphere Phosphorus Depletion Induced by Heavy Nitrogen Fertilization in Forest Nursery Soils Y. Teng and V. R. Timmer* ABSTRACT The cause of N-induced P deficiency in white spruce [Picea glauca (Moench) Voss] seedlings was investigated in a greenhouse pot trial testing factorial additions of ammonium nitrate (AN) and phosphoric acid (PA) to root zone and root-free compartmentalized soils. Plant growth was significantly improved by combined N and P topdressings during the growing season. However, N-only fertilization induced P deficiency symptoms and reduced biomass and P status in shoots, demonstrating an apparent N antagonism on P. Phosphorus availability in this treatment also was 35% lower in the rhizosphere soil than in the root-free bulk soil, reflecting rhizosphere P depletion. Soil acidity and Al activity were increased most by AN-only applications, probably contributing to reduced P availability. A diagnosis of induced Al toxicity by N fertilization based on plant analysis data was supported by symptoms of root injury. Phosphorus capture by ion-exchange resin bags at the base of the pots was lower than that of N with combined N-P applications, suggesting rapid fixation and low mobility of P in the soil compared with N. Nitrogen-induced rhizosphere P depletion in this nursery soil was attributed to restricted root develop- ment of seedlings due to Al toxicity, reduced P availability by Al- phosphate precipitation, and low P replenishment because of slow diffusion from the bulk soil. Topdressing with both PA and AN in- creased P availability in the rhizosphere (320 mg kg-'), reduced soil extractable Al by 40% compared with AN-alone treatments, and increased plant uptake of N and P by 270%, resulting in positive N x P interactions on plant growth. XTiTROGEN FERTILIZATION in agricultural crops is usu- J.^1 ally associated with promotion of P absorption by plants (Grunes, 1959). Experience with some forestry crops, however, has shown that N additions may depress P uptake in tree seedlings (Dumbroff and Michel, 1967; Fowells and Krauss, 1959; Maftoun and Pritchett, 1970; Pritchett, 1972; Teng and Timmer, 1994). These nega- tive responses, often occurring on low-P soils, have been attributed to reduced P availability resulting from Faculty of Forestry, Univ. of Toronto, 33 Willcocks Street, Toronto, ON, Canada M5S 3B3. Received 30 Nov. 1993. ""Corresponding author (timmer @larva. forestry. utoronto. ca). Published in Soil Sci. Soc. Am. J. 59:227-233 (1995). increased Fe and Al activity as soil pH declines (Merifield and Foil, 1967), reduced root growth of N-treated seed- lings (Teng and Timmer, 1994), and reduced root-soil contact due to inhibited mycorrhizal activity (Pritchett, 1972). With Al-sensitive species and acid soils, such conditions may be indicative of excess Al availability because root injury and mycorrhizal inhibition are com- mon symptoms of Al toxicity (Foy, 1974; Thompson and Medve, 1984). This might explain why tests of several fertilization predictors for loblolly pine (Pinus tadea L.), an Al-sensitive species (Paganelli et al., 1987), revealed that soil indices of extractable Al, rather than N or P availability, correlated more significantly with growth response to N-P fertilization (Hart et al., 1986). White spruce, which also is considered an Al-sensitive species (Hutchinson et al., 1986; Nosko and Kershaw, 1988), has been shown to respond to high Al activity in growing media by reducing root growth and P absorption. Similar growth and nutrient response were noted by Teng and Timmer (1994) when white spruce seedlings were topdressed with AN on relatively high-P soils. They hypothesized that the reduced P absorption was due to rhizospheric P depletion, because the apparent P defi- ciency was corrected by adding P either as MAP or PA. The AN additions acidified the soil (Teng and Timmer, 1994), promoting soil Al activity (Foy et al., 1978) which inhibited root development. Thus, an antagonism of Al on P was suspected to be involved in the underlying mechanism of P depletion. In our study, we examined this hypothesis in more detail. The objectives were to assess possible interactions among N, P, and Al in the soil and plant system, with particular emphasis on the possible mechanisms of rhizospheric P depletion. The mechanisms may explain the improved growth response associated with MAP additions compared with AN-only fertilization in nursery conifer seedling production (van den Driessche, 1988; Teng and Timmer, 1994). Abbreviations: AN, ammonium nitrate; MAP, monoammonium phos- phate; PA, phosphoric acid; IER, ion-exchange resin.

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Page 1: Rhizosphere Phosphorus Depletion Induced by Heavy Nitrogen Fertilization in Forest Nursery Soils

DIVISION S-7-FOREST & RANGE SOILS

Rhizosphere Phosphorus Depletion Induced by Heavy Nitrogen Fertilizationin Forest Nursery SoilsY. Teng and V. R. Timmer*

ABSTRACTThe cause of N-induced P deficiency in white spruce [Picea glauca

(Moench) Voss] seedlings was investigated in a greenhouse pot trialtesting factorial additions of ammonium nitrate (AN) and phosphoricacid (PA) to root zone and root-free compartmentalized soils. Plantgrowth was significantly improved by combined N and P topdressingsduring the growing season. However, N-only fertilization induced Pdeficiency symptoms and reduced biomass and P status in shoots,demonstrating an apparent N antagonism on P. Phosphorus availabilityin this treatment also was 35% lower in the rhizosphere soil than inthe root-free bulk soil, reflecting rhizosphere P depletion. Soil acidityand Al activity were increased most by A N-only applications, probablycontributing to reduced P availability. A diagnosis of induced Altoxicity by N fertilization based on plant analysis data was supportedby symptoms of root injury. Phosphorus capture by ion-exchangeresin bags at the base of the pots was lower than that of N withcombined N-P applications, suggesting rapid fixation and low mobilityof P in the soil compared with N. Nitrogen-induced rhizosphere Pdepletion in this nursery soil was attributed to restricted root develop-ment of seedlings due to Al toxicity, reduced P availability by Al-phosphate precipitation, and low P replenishment because of slowdiffusion from the bulk soil. Topdressing with both PA and AN in-creased P availability in the rhizosphere (320 mg kg-'), reducedsoil extractable Al by 40% compared with AN-alone treatments, andincreased plant uptake of N and P by 270%, resulting in positiveN x P interactions on plant growth.

XTiTROGEN FERTILIZATION in agricultural crops is usu-J.̂ 1 ally associated with promotion of P absorption byplants (Grunes, 1959). Experience with some forestrycrops, however, has shown that N additions may depressP uptake in tree seedlings (Dumbroff and Michel, 1967;Fowells and Krauss, 1959; Maftoun and Pritchett, 1970;Pritchett, 1972; Teng and Timmer, 1994). These nega-tive responses, often occurring on low-P soils, havebeen attributed to reduced P availability resulting from

Faculty of Forestry, Univ. of Toronto, 33 Willcocks Street, Toronto,ON, Canada M5S 3B3. Received 30 Nov. 1993. ""Corresponding author(timmer @ larva. forestry. utoronto. ca).

Published in Soil Sci. Soc. Am. J. 59:227-233 (1995).

increased Fe and Al activity as soil pH declines (Merifieldand Foil, 1967), reduced root growth of N-treated seed-lings (Teng and Timmer, 1994), and reduced root-soilcontact due to inhibited mycorrhizal activity (Pritchett,1972). With Al-sensitive species and acid soils, suchconditions may be indicative of excess Al availabilitybecause root injury and mycorrhizal inhibition are com-mon symptoms of Al toxicity (Foy, 1974; Thompsonand Medve, 1984). This might explain why tests ofseveral fertilization predictors for loblolly pine (Pinustadea L.), an Al-sensitive species (Paganelli et al., 1987),revealed that soil indices of extractable Al, rather thanN or P availability, correlated more significantly withgrowth response to N-P fertilization (Hart et al., 1986).

White spruce, which also is considered an Al-sensitivespecies (Hutchinson et al., 1986; Nosko and Kershaw,1988), has been shown to respond to high Al activity ingrowing media by reducing root growth and P absorption.Similar growth and nutrient response were noted by Tengand Timmer (1994) when white spruce seedlings weretopdressed with AN on relatively high-P soils. Theyhypothesized that the reduced P absorption was due torhizospheric P depletion, because the apparent P defi-ciency was corrected by adding P either as MAP or PA.The AN additions acidified the soil (Teng and Timmer,1994), promoting soil Al activity (Foy et al., 1978)which inhibited root development. Thus, an antagonismof Al on P was suspected to be involved in the underlyingmechanism of P depletion. In our study, we examinedthis hypothesis in more detail. The objectives were toassess possible interactions among N, P, and Al in thesoil and plant system, with particular emphasis on thepossible mechanisms of rhizospheric P depletion. Themechanisms may explain the improved growth responseassociated with MAP additions compared with AN-onlyfertilization in nursery conifer seedling production (vanden Driessche, 1988; Teng and Timmer, 1994).

Abbreviations: AN, ammonium nitrate; MAP, monoammonium phos-phate; PA, phosphoric acid; IER, ion-exchange resin.

Page 2: Rhizosphere Phosphorus Depletion Induced by Heavy Nitrogen Fertilization in Forest Nursery Soils

228 SOIL SCI. SOC. AM. J., VOL. 59, JANUARY-FEBRUARY 1995

MATERIALS AND METHODSPlant Material and Culture

White spruce seedlings, started from stratified seeds, weregrown in pots in a controlled greenhouse at 25-15 °C (day-night) temperature and 16-h photoperiod for 6 mo. The growingmedium was a loamy sand collected from the plow layer ofseedbeds at the Midhurst Provincial Forest Nursery, Ontario,Canada, where N-induced P deficiency was previously diag-nosed (Teng and Timmer, 1994). The soil was consideredhigh in available P (310 mg kg"1 by Bray 0.03 M NH,F +0.025 M HC1 extraction), with pH 6.0, cation-exchange capac-ity 7.8 cmolc kg"1, and organic matter content 35 mg kg"1.Each of 16 plastic pots (20-cm diam. and 20 cm deep) wasfilled with 3.5 kg of the air-dried, sieved soil. The surfacewas covered with a 2-cm layer of peat that served temporarilyas a germination medium to prevent damping-off disease. Thepeat layer was carefully replaced by 0.5 kg of soil 3 wk aftergermination, when the roots had penetrated the mineral soilbelow. The soil in the pots was partitioned into two concentriczones: a central vertical core (defined as the rooting soil)confined by a 5-cm cylindrical nylon mesh stocking, and theouter core, defined as the root-free bulk soil. The seeds weresowed in the inner zone to restrict root development to thecentral core (Fig. 1).

Prior to seeding, a single IER bag (Binkley and Matson,1983), serving as ion sink (Sibbesen, 1978), was buried inthe middle of the central core, 5 cm from the base of the pot.Each bag contained 4.6 g dry weight of resin beads (FisherScientific, Fair Lawn, NJ, 1-300 mixed ion-exchange resins)providing 9.4 mmolc cation and 9.6 mmolc anion exchangecapacity. Exchange sites were saturated with OH" and H+.

Experiment Design and TreatmentsThe pots (considered as experimental units) were arranged

in a randomized block design with four replications testing

Mesh nylon stockingBulk soil

Rooting soilRhizosphere soilIER bag

Plastic pot

Fig. 1. Schematic cross section of the pot showing the partitioning ofthe soil into root-free bulk soil and rooting soil and the positionof the ion-exchange resin (IER) bag. Strongly root-adhering soilparticles were collected as rhizosphere soil.

factorial combinations of two levels of N (0 and 0.8 g N pot"',as AN) and two levels of P (0 and 1.7 g P pot"1 as PA). Thehigh dose levels were adopted from Teng and Timmer (1994)and were equivalent to 360 kg N and 730 kg P ha"1 in thefield. Three weeks after germination, the surface peat layerwas replaced by soil and the seedlings were thinned to 20plants per pot. Biweekly topdressings of one-eighth of thefertilizer dosage commenced at this time and continued to 16wk, ceasing 4 wk before final harvest. The fertilizers wereapplied as 90-mL aqueous solution. The pot surface was dividedinto nine equal sections and each received 10 mL per applicationto ensure even delivery. The pots were irrigated daily withtap water to container capacity (White and Mastalerz, 1966).No leaching from the pots was observed.

Sampling and Chemical AnalysisAt harvest, seedlings were removed from the soil and cut

at the root collar. Weakly adhering soil was shaken off theroots and returned to the central core. The more stronglyadhering soil, considered as rhizosphere soil (Riley and Barber,1971), was carefully brushed and washed off the roots andcollected. Because both root length and root branching aresensitive indicators of Al toxicity (Schaedle et al., 1989), totalroot length and number of root tips were determined for eachtree (Paganelli et al., 1987); means of these measurementswere calculated for each pot. Specific root length, which repre-sents the fineness of the roots, was calculated as the ratio ofroot length to root biomass (Tan and Keltjens, 1990). Theshoots and roots of all 20 seedlings were washed and ovendried separately for 48 h at 70°C for dry mass (grams perpot) determination and then composited for chemical analysis.Analysis of N, P, K, Ca, Mg, Zn, and Cu followed proceduresdescribed by Teng and Timmer (1994). Aluminum in planttissue was determined by neutron activation analysis using theSLOWPOKE nuclear reactor at the University of Toronto.

Fresh soil samples from the bulk zone, rooting zone, andrhizosphere were used to determine pH and KC1 extractableNHJ-N and NOf-N. Available P was measured in air-driedsoil samples (Bray and Kurtz, 1945). The KC1 extracts alsowere analyzed for extractable Al by atomic absorption (Hartet al., 1986). Assessment of KC1 extractable Al, P, NH/-N,and NO3"-N for IER bags followed Binkley and Matson (1983).Because other forms of inorganic N are negligible in soilsunder normal conditions (Keeney and Nelson, 1982), the sumof NH|-N and NOf-N was defined as total inorganic N.

Statistical Analysis and Data InterpretationTreatment responses in plant dry mass, chemical composi-

tion, root morphology, and soil chemistry were evaluatedstatistically by analysis of variance for a randomized complete-block design and followed by Tukey's Honestly SignificantDifference test (SAS Institute, 1985). Pearson correlations ofavailable P levels in the three soil compartments (bulk soil,rooting soil, and rhizosphere) were conducted using SAS.Shoot dry mass and chemical composition data were evaluatedby vector analysis (Timmer, 1991) to identify elements limitingor toxic to plant growth and to examine possible nutrientinteractions involved (Teng and Timmer, 1994).

RESULTS AND DISCUSSIONBiomass Production and Root MorphologyGrowth and color of the seedlings varied markedly

among treatments during the 6-mo growing period. Withtime, needle color of the control seedlings became more

Page 3: Rhizosphere Phosphorus Depletion Induced by Heavy Nitrogen Fertilization in Forest Nursery Soils

TENG & TIMMER: NITROGEN FERTILIZATION INDUCING PHOSPHORUS DEPLETION 229

Table 1. Morphological response in 6-mo-old white spruce seedlings to topdressings of ammonium nitrate (AN) and phosphoric acid (PA)in greenhouse culture.

Treatment

ControlANPAAN + PASignificance (P >Source

Replicate (3)tAN(1)PA(1)AN x PA (1)

Shoot

2.89at2.76a3.44b5.80c

F from ANOVA)

NSNS**

***

Dry mass

Root

1.47ab1.31a1.41ab1.89b

NSNSNS#*

SIR

1.95a2.42b2.33b3.07c

NS**

NS

Length

cm plant"1

182bc63a

21 Ic174b

NS***NS**

Root characteristics

Specific length

cm mg~"2.45ab0.96a2.80c1.84b

NS******

Tips

no. plant"1

371dlOla277c248b

NS****

**

*, **, *** Significant at P < 0.05, 0.01, and 0.001, respectively, for the main effects (AN and PA) and interactions (AN x PA); NS = nonsignificant.t Column values followed by different letters are significantly different (P < 0.05) determined by Tukey's HSD test.$ Degrees of freedom in parentheses.

yellow and exhibited die-back at the tips, probably re-flecting a multinutrient deficiency. Topdressings withAN-alone had no significant effect on seedling growthcompared with the control (Table 1) but resulted indistinct needle purpling, often symptomatic of P defi-ciency (Armson and Sadreika, 1979). The purpling, how-ever, may also be indicative of Al toxicity, as noted forthe same species under Al stress in solution culture(Hutchinson et al., 1986), where Al may have interferedwith uptake, transport, and use of P (Foy et al., 1978). Incontrast to the AN-only treatment, additions of PA-onlystimulated shoot dry mass production by 20% (Table 1)but induced pronounced needle yellowing, a symptomof N deficiency. Plants receiving combined topdressingsof AN and PA were free of nutrient deficiency symptomsand increased shoot and root growth (by 101 and 28%,

respectively) compared with the control. The N X Pgrowth interactions were highly significant and positive(Table 1), as was previously observed in the field (Tengand Timmer, 1994).

Fertilization with AN-only also reduced root length(65%), specific root length (61%), and root tip number(73%; Table 1). Because root biomass was unchanged,these roots were much shorter and thicker than the con-trol. Restricted root elongation and branching are typicalresponses of Al toxicity in plants (Schaedle et al., 1989;Kelly et al., 1990) and are presumably due to inhibitedcell division (Foy et al., 1978). Similar root responsesin sorghum [Sorghum bicolor (L.) Moench] were attrib-uted to Al toxicity rather than simple P deficiency (Kelt-jens and van Loenen, 1990), hence a possible antagonismof Al on P also may be involved in the growth disorders

Table 2. Effects of application of ammonium nitrate (AN) and phosphoric acid (PA) on chemical composition of 6-mo-old white spruceseedlings.

Treatment

ControlANPAAN + PA

N

24.6b15. la25.7b

P

1.71b0.69a3.56c3.12c

K

kc"1

8.58a8.18a8.22a7.83a

Ca

Shoot9.50a8.94a

10. lalO.Sa

Mg

2.61b1.91a2.86bc3.08c

Zn

81.3b37.3a45.3a28.6a

Cu

22.2b17.2ab16. lab13.4a

Al

mgkg"1 —————

44a144c75ab80b

P/Alt

33.7b4.2a

41. 3b33.9b

Significance (P > F from ANOVA)Source

Replicate (3)§AN(1)PA(1)AN x PA (1)

NS**NSNS

NS********

NSNSNSNS

NSNSNSNS

Root

NSNS

NS NS

NS

NS NS**#**

ControlANPAAN + PASignificance (P >Source

Replicate (3)AN(1)PA(1)AN x PA (1)

13.9a25.6b14.4a25.3b

F from ANOVA)

NS***NSNS

1.84a1.23a4.70b4.64b

NS****

6.75a5.15a5.95aS.SOa

NSNSNSNS

7.89b5.36a8.28b7.58b

NS*

NSNS

1.67a1.52a1.97al.SOa

NSNSNSNS

87.1b55.0a51. la46.0a

NS***

17.8b9.8a7.8a7. la

NS***

1008a1305b1605c1545c

NS****

1.59ab0.73a2.93b2.61b

NS******

*, *«, *** Significant at P< 0.05, 0.01, and 0.001, respectively, for main effects (AN and PA) and interactions (AN x PA); NS = nonsignificant.t Molar ratio of P to Al.t Column values followed by different letters in shoot or root are significantly different (P < 0.05) determined by Tukey's HSD test.§ Degrees of freedom in parentheses.

Page 4: Rhizosphere Phosphorus Depletion Induced by Heavy Nitrogen Fertilization in Forest Nursery Soils

230 SOIL SCI. SOC. AM. J., VOL. 59, JANUARY-FEBRUARY 1995

observed in our study. Combined N and P fertilizationseemed to counter the growth inhibition as reflected bysignificant and positive N x P interactions on dry massof both shoots and roots from AN + PA treatment (Table1). This was achieved without depressing root elongationand branching, although root tip number was reducedby 33%.

Tissue Chemistry and Nutrient DiagnosisDiffering growth and morphological responses in the

seedlings were associated with distinct changes in tissuechemical compositions. Topdressings of N or P or bothgenerally increased concentrations of respective elementsin plant components (Table 2). Clearly, both N and Padditions were necessary for optimum seedling produc-tion, because separate addition of either element induceddeficiency symptoms of the other. On the other hand,combined AN + PA treatment doubled biomass (Table1) and raised tissue N and P concentration (Table 2),increasing uptake of these nutrients nearly threefold (Fig.2). The largest right pointing vectors (signifying thedeficiency zone) in Fig. 2 were associated with the AN +PA treatment, indicating diagnostically that both ele-ments (N and P) were equally limiting (Shift C, seeTimmer, 1991).

Vector patterns reflected strong interrelationships be-tween tissue N, P, and Al. Although the fertilizer sourceslacked Al, all treatments increased tissue Al concentra-tions. On a relative basis, the largest shifts were inducedby the AN-only treatment where Al concentration wasincreased by 220% and P concentration was reduced by60% (lowering shoot P/A1 ratio eightfold; Table 2),illustrating an apparent N-P antagonism (Fig. 2). Com-parisons of vector size and orientation suggested thatexcess accumulation of Al antagonistically depressed Pstatus in this treatment. The Al vector was not onlylarger than the N vector but also was larger and oppositein direction to the P vector (Shifts E and F in Timmer,1991). The diagnosis was supported by symptoms ofboth Al (root injury) and P (shoot purpling) stresses inthe seedlings. High Al status in plant tissues can inactivateP by precipitating Al-phosphates on the cell wall, low-ering higher bond energy P, and increasing the redoxpotential of the tissues (Foy et al., 1978), thus exacerbat-ing P deficiency. In contrast to AN-only treatments, PAaddition not only increased shoot P concentration butalso countered Al buildup in shoots by reducing Alabsorption (Fig. 2). Phosphorus fertilization, with orwithout AN, appears to stimulate P allocation to shootswhile depressing Al transportation from roots to shoots(Fig. 3). Elevated P status in these seedlings alleviatedP limitation, maintaining P/A1 balance in the shoots(Table 2), which probably prevented Al toxicity andstimulated seedling growth (Table 1).

Soil ChemistryComparisons of NH^-N and NO3~-N levels in the

N-treated soils indicate that nitrification was at a highrate, because the 1:1 NH4

+/NO3~-N ratio of the ANfertilizer dropped to 1:10 in soil extracts (Fig. 4). The

400

300

200

100

co5"c0)ocoo

300

.1 1 2°°•t ^3C

V

IS0)rr

100

300

200

100

Relative biomass(Control = 100)y £.

100

PA

AN+PA

100

120

100

202

100 200 300Relative nutrient content

(Control = 100)

400

Fig. 2. Relative response of nutrient concentration, nutrient content,and shoot mass of 6-mo-old white spruce seedlings to (a) ammoniumnitrate (AN) (0.80 g N pot"1), 0>) phosphoric acid (PA) (1.70 g Ppot""1), and (c) AN + PA topdressing under greenhouse conditions.Control treatments were normalized to 100. To minimize clutter,only major vectors (N, P, and Al) are shown.

ratio decrease was apparently not the result of preferentialabsorption for NH^-N by plants because NOa~-N levelswere highest in the bulk soil (Fig. 4a), which was freeof roots. Nitrification also led to soil acidification (Fig.5a) and high Al activity (Fig. 5b). The lowest pH values(4.4-4.6) were found in the AN-only treated rhizosphere,which was 1.4 to 1.6 units less than that of the control.Extractable Al concentrations (Fig. 5b) in the AN-onlytreatment increased 5 times (to 8 mg kg"1) in IER bagsand nearly 50 times (to 125 mg kg"1 or 4.6 \iM) in therhizosphere. These levels were well above critical levels(>1 nM) associated with root inhibition of Al-sensitivespecies shown in solution culture (Parker et al., 1988) andprobably contributed to the restricted root development(Table 1).

Page 5: Rhizosphere Phosphorus Depletion Induced by Heavy Nitrogen Fertilization in Forest Nursery Soils

TENG & TIMMER: NITROGEN FERTILIZATION INDUCING PHOSPHORUS DEPLETION 231

100

OO

CO

«Q.

"CO

§

"oCD

20>O0>

Q_

OO

EC

80 •

100

Control AN AN+PA

TreatmentFig. 3. Effects of topdressing with ammonium nitrate and phosphoric

acid on relative P and Al allocations between shoots and roots of6-mo-old white spruce seedlings grown in greenhouse. All values aremeans of four replications with vertical bars representing standard

Chemical analysis of the bulk soil, rooting soil, andrhizosphere soil revealed that nutrient gradients betweenthe root-free soil and the roots were much steeper foravailable P (Fig. 5c) than those for inorganic N (Fig.4c). Without N fertilization, total inorganic N levels inthe soil were low (<5 mg kg~') and showed no consistentgradient. As expected, AN addition elevated NO^-N,NH^-N, and total inorganic N levels (Fig. 4, especiallyin AN-alone treated soils in which NOf-N was 202 mgkg~' in root-free bulk soil and 180 mg kg-1 («10%lower) in the rooting zone and rhizosphere soil. Thesegradients were similar in the AN + PA treatment, al-though inorganic N concentrations in both bulk soil andrhizosphere were =30% lower than those in AN-alonetreated soils, presumably due to increased N uptake bylarger plants.

In contrast to inorganic N, available P levels in bulksoil, rooting soil, and the rhizosphere demonstrated clearP depletion in the rhizosphere (Fig. 5c), confirming ourprevious speculation of slow P replenishment to seedlingroots in nursery culture (Teng and Timmer, 1994). With-out P fertilization, rhizosphere P status fell below bulksoil levels (by 30 % in the control and 42 % in the AN-only

200

COE

O 100

O>

200

100

o>

E>oc

200

100

Bulk zoneRooting zoneRhizospherelERbag 40 ^

20

40

20

.aen

OrrLU

CDsCD

ccLU

40

ICD

20 CCLU

Control AN+PATreatment

Fig. 4. Effects of topdressings with ammonium nitrate and phosphoricacid on KCl-extractable (a) NH,-N, (b) NO4-N, and (c) total inor-ganic N in the soil and buried ion-exchange resin bags. All values aremeans of four replications with vertical bars representing standard

treatment) and well short of the desirable target level(300 mg kg-1, see Armson and Sadreika, 1979). Unlikeinorganic N, rhizosphere P replenishment from sur-rounding soil was low because bulk soil P levels werelittle changed (Fig. 5c), even though steep P concentra-tion gradients developed. These differing gradients mayreflect the lower mobility of phosphate than that of nitratein the soil (Jungk, 1991). Phosphorus capture by IERbags (3.5 mg bag"1) in AN + PA treatments was 4.6times lower than that of N (16 mg bag""1), despite similarIER exchange capacities for both cations and anions,and a high P/N ratio (2:1) of applied fertilizers. Thelow recovery seems to indicate the rapid fixation andslow diffusion of P in the soil during the season. HighAl activity owing to rapid nitrification of AN fertilizer

Page 6: Rhizosphere Phosphorus Depletion Induced by Heavy Nitrogen Fertilization in Forest Nursery Soils

232 SOIL SCI. SOC. AM. J., VOL. 59, JANUARY-FEBRUARY 1995

t

3o>,§<JB

«T3

6.0

4.0

2.0

0.0

150

100

5 50

g

400O)̂

D)

0> 200JD_«

1<

Bulk zoneRooting zoneRhizospherelERbag

O>

2D)

cc111

1.5

1.0

0.5

0.0

4.0 ^

§>no

2.0 Q.EC

0.0Control PA

Treatment

AN+PA

Fig. 5. Effects of topdressings with ammonium nitrate and phosphoricacid on (a) soil pH, (b) KCI-extractable Al, and (c) available P inthe soil and buried ion-exchange resin bags. All values are meansof four replications with vertical bars representing standard errors.

may have further depressed available P by precipitatingAl phosphates (Foy, 1974).

Our study results suggest three factors contributingto the rhizospheric P depletion from N fertilization:restricted root extension into the surrounding soil dueto Al toxicity, slow P replenishment because of low Pdiffusion in the nursery soil, and increased Al-phosphateformation resulting from high Al activity associated withnitrification. Phosphorus topdressings applied with Nappears to contribute to the positive N X P fertilizerinteraction on seedling growth in two ways. First, al-though PA-only applications led to similar rhizosphereacidification as AN treatments did (Fig. 5), extractableAl levels were depressed by likely formation of Al-phosphates (Alva, 1986) due to increased P availability,hence alleviating Al inhibition of roots and increasing

plant uptake of P, K, Ca, and Mg (Fig. 2). Secondly,PA addition raised available P levels in the rhizosphereabove 300 mg kg"1, a level considered adequate forconifer seedling growth (Armson and Sadreika, 1979),thus countering rhizospheric P depletion. Maximumgrowth coincided well with this critical level in therhizosphere (Table 1; Fig. 5), confirming its validity forprescribing P fertilization in bareroot seedling produc-tion. However, because the correlation of rhizosphereP with rooting zone P (r = 0.80) was higher than thatwith root free zone P (r = 0.65), care should be takenthat soil samples originate from the intense rooting zonerather than plow layer as a whole.

ACKNOWLEDGMENTSThis research was funded by the Natural Science and Engi-

neering Research Council of Canada (NSERC) and the OntarioMinistry of Natural Resources. Aluminum analysis providedby the SLOWPOKE nuclear reactor facility at the Universityof Toronto is gratefully acknowledged. The senior author waspartly supported by an Ontario Graduate Scholarship and aUniversity of Toronto Open Fellowship.

Page 7: Rhizosphere Phosphorus Depletion Induced by Heavy Nitrogen Fertilization in Forest Nursery Soils

MACDONALD ET AL.: TEMPERATURE EFFECTS ON RESPIRATION AND MINERALIZATION 233