Phosphorus mineralization kinetics and response of microbial phosphorus to drying and rewetting in a Florida Spodosol

PII:

Soil Biol. Biochem. Vol. 30, No. 10/11, pp. 1323±1331, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0038-0717(98)00002-9 0038-0717/98 $19.00 + 0.00

PHOSPHORUS MINERALIZATION KINETICS AND RESPONSE OF MICROBIAL PHOSPHORUS TO DRYING AND REWETTING IN A FLORIDA SPODOSOL P. F. GRIERSON,1* N. B. COMERFORD2 and E. J. JOKELA1 School of Forest Resources and Conservation, 118 Newins-Ziegler Hall, University of Florida, Gainesville, FL 32611-0303, U.S.A. and 2Department of Soil and Water Science, 2169 McCarty Hall, University of Florida, Gainesville, FL 32611-0303, U.S.A. 1

(Accepted 14 November 1997) SummaryÐSurface soils of a north central Florida Spodosol (sandy, siliceous hyperthermic Alaquod) from fertilized and unfertilized plantations of loblolly pine (Pinus taeda L.) were conditioned, dried, rewet and incubated at 388C for up to 26 d with periodic sampling for inorganic P and microbial P. Undried samples were also incubated and sampled periodically. Several kinetic models were evaluated to describe patterns of net P mineralization. Cumulative net mineralization of P in undried samples of both fertilized and unfertilized treatments was best described by zero-order kinetics. In contrast, a segmented model with two pools was most appropriate for describing cumulative net mineralization of P in dried and rewet samples, with one pool following zero-order kinetics, and the other following ®rstorder kinetics. Net mineralization in rewet soils progressed in three stages: (i) An initial ¯ush where inorganic P was brought into solution, the source most likely being turnover of the microbial biomass from the previous drying period and mineralization of organic substrates; (ii) a lag of a few days where there was no net release of P; and (iii) a period that followed similar kinetics to the undried soil, where the microbial biomass had recovered suciently to mineralize P from soil organic matter. Although more P was mineralized in fertilized soils, the kinetics of the reactions were similar to those in unfertilized soils. Generalized models were used to predict net P mineralization expressed as a percent of total P (speci®c P mineralization) for all data (both fertilized and unfertilized treatments). A zero-order model best described mineralization of P in undried soils (R2=0.884), and a segmented two-pool model was the best ®t for soils that had been dried and rewet (R2=0.923). There was a negative relationship between inorganic P (as a % of total P) and microbial P in undried soils (R2=0.715). When soils were dried and rewet, microbial P increased over the entire incubation period but this relationship ¯uctuated with time and was not signi®cantly correlated with P mineralization. The kinetic models proposed here should be useful in improving predictions of P mineralization in other sandy soils of low adsorption capacity. # 1998 Elsevier Science Ltd. All rights reserved

INTRODUCTION

Laboratory incubation experiments have been used to model kinetics of N mineralization, and less frequently S mineralization, in soils and organic residues (Stanford and Smith, 1972; Bonde and Lindberg, 1988; Ellert and Bettany, 1988). These studies have improved our understanding of nutrient cycling, particularly of N and C (Bonde and Rosswall, 1987; Fyles and McGill, 1987). Fitting kinetic equations to patterns of nutrient release can provide useful indices of mineralization and may help to describe the mechanisms involved. However, such procedures have not been used to estimate P mineralization in the past, largely because of the confounding e€ects of adsorption reactions with mineral surfaces. Similarly, most models have been developed using data derived from long-term incu*Author for correspondence. Present address: Department of Botany, University of Western Australia, Nedlands, WA 6907, Australia.

bations at constant temperature and moisture, thereby failing to incorporate the largely short-term e€ects of wetting and drying events, and of ¯uctuations in temperature (Cabrera, 1993). Sudden changes in soil moisture play an important role in the mineralization of nutrients. Wetting and drying cycles are known to a€ect both C and N mineralization (Birch, 1960; Sorensen, 1974; Cabrera, 1993; Bauhus and Khanna, 1994), particularly through their e€ects on microbial biomass and activity (Kieft et al., 1987). Drying and rewetting also increases the release of P from air-dried soils and sediments (Sparling et al., 1985; Qiu and McComb, 1995). Increased mineralization may result from an increase in decomposable organic substrates derived from the death of microorganisms and from non-living soil organic matter which becomes available for decomposition by the physical disruption of the soil structure, by substrate desorption from aggregate surfaces, and from increased mobility of soluble organic compounds

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(Adu and Oades, 1978). The temperature at which soil dries (Jager and Bruins, 1975), the duration of the dry period, and the frequency of the wetting and drying cycles also in¯uence mineralization rates (Seneviratne and Wild, 1985; Cabrera, 1993) and may alter the balance between bacteria and fungi in soil. While fungal hyphae can extend to moist microsites if the soil does not dry uniformly, bacteria are dependent on the moisture content of their location. Thus, drying may favor fungi over bacteria and consequently result in di€erent net mineralization rates (Bauhus and Khanna, 1994). Several comprehensive studies describe models for N mineralization (e.g., Stanford and Smith, 1972, Campbell et al., 1988; Cabrera, 1993; Gonc° alves and Carlyle, 1994). However, only a few studies have attempted to measure ¯ushes of P release due to drying and rewetting and to separate mineralization reactions from adsorption±desorption reactions (Sparling et al., 1985; Qiu and McComb, 1995). Drying and rewetting cycles are common in the coarse-textured soils of north-central Florida, where rainfall is distinctly seasonal. Such factors are likely to be important in controlling rates of litter decomposition and net P mineralization, as well as contributing to turnover of the microbial biomass (Sparling et al., 1985). The lack of adsorptive surfaces in the sandy A horizon of these Spodosols, where the Langmuir adsorption maximum is e€ectively zero (Ballard and Fiskell, 1974) and the predominance of both inorganic and organic P in water-soluble forms (Fox et al., 1990), allows P mineralization in these soils to be estimated directly as the change in labile inorganic P (Polglase et al., 1992b). Our objectives were to investigate the ¯ush of P release in the surface soil of a Spodosol after rewetting, and to develop a model that describes this process. We tested several kinetic models to describe net P mineralization observed in laboratory incubations of forest soil. The approach used was initially based on the procedure described for N mineralization by Cabrera (1993), in order to contrast the kinetics of P mineralization to those of N, and to compare net P mineralization in the surface horizon of unfertilized and fertilized soils. Finally, the transformation of microbial P was examined in relation to mineralization kinetics. MATERIALS AND METHODS

Site description The study site is about 10 km north of Gainesville, FL (29880'N, 82820'W). The climate is warm temperate-subtropical, characterized by wet summers and dry autumns and springs. The mean annual rainfall is 1350 mm and mean annual temperature is 218C (U.S. National Oceanic and Atmospheric Admin., 1989). The soils are poorly

drained Pomona ®ne sands (sandy, siliceous, hyperthermic Ultic Alaquods) (Soil Survey Sta€, 1994), generally with a spodic horizon beginning at 20±50 cm, and an argillic horizon beginning at 90± 120 cm. The A horizon is predominantly quartz sand with low organic matter content, low CEC (<5 cmolc kgÿ1) and few primary or secondary minerals. Sorption of added inorganic P is undetectable (Ballard and Fiskell, 1974; Fox et al., 1990). Soil samples were collected from a long-term ®eld experiment, initiated in 1983 by the Intensive Management Practices Assessment Center (IMPAC) of the USDA Forest Service to assess potential biological productivity of pine and the processes controlling it (Colbert et al., 1990; Dalla-Tea and Jokela, 1991, 1994; Polglase et al., 1992a,b,c,d). The experiment consisted of three replicates of a 2  2  2 factorial of species (loblolly and slash pines), weed control (with or without sustained elimination of all understory competition), and fertilizer application (with or without annual additions of complete fertilizer), arranged in a randomized, split-plot (species) design. A complete description of the site is given in Swindel et al. (1988); Neary et al. (1990). Because earlier studies by Polglase et al. (1992b) indicated signi®cant e€ects of fertilizer application on rates of P mineralization, the study described here has focused on comparing rates of mineralization of P and microbial biomass P in soil from fertilizer treatments to unfertilized (reference) treatments. The fertilizer was a mix of ammonium nitrate, diammonium phosphate, muriate of potash and additional micronutrients, and supplied the following nutrients (kg haÿ1 yÿ1): N (60), P (24), K (50), Ca (20), Mg (10), S (13), B (0.06), Cu (0.06), Fe (0.05), Mn (0.05), and Zn (0.05). Fertilizer was applied in narrow bands (30 cm semi-circle) around the base of each tree, which enabled soil samples to be collected away from these areas and analyzed for nutrient content without interference from residual fertilizer. The most recent fertilizer application was more than 12 months prior to sample collection. Treatment and experimental design Twenty soil cores (7.5 cm i.d.) were collected from the surface 0±5 cm of mineral soil from the interbed of each plot for all three replicates of both fertilized and unfertilized treatments of loblolly pine. Samples for each treatment were bulked and sieved (<2 mm) prior to wetting to ®eld capacity (110 kPa) and then conditioned at 388C for 14 d to obtain a relatively steady rate of P mineralization (Polglase et al., 1992b). After conditioning, soils were analyzed for initial amounts of KCl-extractable inorganic P, and microbial P. The conditioned samples were divided in two, and one half placed in a self-sealing polyethylene bag at 388C (``undried'' treatment). The undried soils were subsampled daily for chemical

Microbial phosphorus mineralization

analysis for the ®rst 7 d, and then every 7 d for 4 weeks. The other half was dried at 308C for 48 h in a constant temperature room and then rewet to ®eld capacity prior to incubation in a polyethylene bag at 388C (``dried and rewet'' treatment). The soils were subsampled every 8 h for the ®rst 2 d, then daily for the following 5 d, and then every 2 d until d 26 (624 h). All soil samples were mixed and aerated daily. While disturbance of the soil may enhance rates of P release, these soils have poor aggregate formation and it was considered more important to prevent uneven drying of the samples and anaerobic conditions. This sampling procedure was previously used by Cabrera (1993) to investigate the e€ect of drying and rewetting on the kinetics of N mineralization. Soil water content was monitored throughout the incubation and was at least 85% of the original at the conclusion of the experiment for both treatments. Each treatment was replicated three times. To account for di€erences in total substrate quality (C to P ratio, Table 1) between treatments, we calculated speci®c P mineralization (ratio of net mineralized P to total concentration of soil P) (Polglase et al., 1992b) as well as net P mineralized. Net P mineralized was measured as the increase in KCl-extractable inorganic P from the beginning of the incubation (t0). Chemical analyses Inorganic P was extracted in 0.1 M KCl (Polglase et al., 1992b). Previous work by Polglase et al. (1992b) and ourselves have con®rmed the validity of 0.1 M KCl as an extractant for inorganic P on these soils (Table 1), primarily because these soils lack adsorptive surfaces due to their extremely weathered, sandy nature. The Langmuir adsorption maximum has been shown to be zero (Ballard and Fiskell, 1974). Consequently, 0.1 M KCl is e€ective in removing all inorganic P in solution and any that might be held by outer sphere sorption (anion exchange). Total P in soils was measured after digestion at 3408C in concentrated H2SO4±H2O2. Inorganic P in the acid digests and KCl extracts was measured colorimetrically by the procedure of Murphy and Riley (1962). The pH was determined in a 1:2 (w/v) soil±water slurry, and soil organic matter was determined by the Walkley±Black method (Nelson and Sommers, 1982) (Table 1). Microbial P was measured by fumigation-extraction using a modi®cation of the method of McLaughlin et al. (1986). Duplicate sub-samples of soil (15 g equivalent dry weight) were weighed into 50 ml polypropylene centrifuge tubes and 0.5 ml of alcohol-free chloroform was added to half the tubes prior to incubation at 258C for 36 h. After fumigation, the chloroform was evaporated and samples extracted by shaking for 1 h in 0.5 M NaHCO3. Samples were then centrifuged and the supernatant

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Table 1. Selected chemical properties and phosphorus fractions of air-dried bulked soil collected from interbed areas of a loblolly pine plantation (0±5 cm) Unfertilized pH Total C (g kgÿ1) Total C/P ratio Cold TCA$ Hot TCA 0.5 M NaHCO3 Mehlich 1 Water Cold TCA 10 mM KCl 0.5 M NaHCO3

3.9 16.72 (1.51)* Phosphorus (mg kgÿ1) 36.74 (5.21) 455 Organic P 2.05 (0.53) 3.39 (0.21) 3.29 (0.80) Inorganic P 3.25 (0.18) 2.49 (0.13) 2.63 (0.27) 2.39 (0.13) 0.63 (0.07)

Fertilized 3.8 18.31 (0.72) 51.12 (6.79) 358 3.32 (0.24) 3.73 (0.19) 5.63 (1.69) 4.27 3.19 3.89 3.75 1.89

(0.53) (0.11) (0.42) (0.11) (0.02)

* Standard deviations (n = 3) in parentheses. $ Trichloroacetic acid (Chapin and Kedrowski, 1983).

analyzed for inorganic P. Microbial P was estimated from the di€erence between the fumigated and unfumigated samples. Because the extraction eciency of microbial P varies with the composition of the microbial population and soil sorption of lysed P (McLaughlin et al., 1986), no kp values were used for the estimates presented here (where kp is the proportion of the microbial P that is extracted from the soil, and correcting for adsorption). Statistical analysis The amounts of P mineralized during each incubation were described as functions of time (t, h) from the start of the incubation by subtracting the inorganic P at the start of the incubation from that measured at each sampling. Non-linear regression (NONLIN procedure, SAS Institute, 1989) was used to ®t mineralization models to the mean values of the three replicates of each treatment. The model o€ering the best description of the data was based on R2 values and the residual sum of squares (RSS) left unexplained by the regression. For generalized models of speci®c P mineralization, the models that gave the best description of the kinetics of net P mineralization were applied to treatment means that had been corrected for di€erences in substrate due to fertilizer treatment, i.e. mineralization was expressed as a percent of the total P originally present, and simple linear regressions were used to describe the relationship between predicted and observed values. Simple correlations between microbial P and P mineralization were examined using the CORR procedure, and treatment e€ects on net P mineralization and change in microbial P were examined by analysis of variance and Tukey's HSD test. RESULTS

Net P mineralization and microbial P The amount of P mineralized in the ®rst 168 h (7 d) in the undried, fertilized soil was 54% higher

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Table 2. Changes with incubation time in KCl-extractable P (mg kgÿ1) and microbial P (mg kgÿ1) for fertilized and unfertilized soils Unfertilized Treatment

0h

Fertilized

168 h

624 h

0h

168 h

624 h

KCl-extractable P*

Undried Dry-rewet

2.20 (0.02) 3.49 (0.21)

2.88 (0.11) 5.70 (0.04)

3.77 (0.12) 7.60 (0.28)

7.12 (0.14) 7.16 (0.24)

8.17 (0.32) 10.48 (0.07)

9.08 (0.04) 12.59 (0.21)

Microbial P$

Undried Dry-rewet

0.99 (0.03) 0

0.91 (0.02) 0.10 (0.04)

0.67 (0.04) 0.19 (0.08)

0.75 (0.04) 0

0.57 (0.06) 0.10 (0.12)

0.37 (0.09) 0.27 (0.08)

Values are means of three replicates. SEM are given in parentheses. * KCl-extractable P in fertilized soils was signi®cantly greater than unfertilized soils when undried, and when dried and rewet (p < 0.05). $ Microbial P in fertilized soils was signi®cantly di€erent to unfertilized soils when samples were undried (p < 0.05).

than in the unfertilized soil (Table 2). After 624 h (26 d), the fertilized soil was 25% higher. When soils were dried and rewet, the amount of P mineralized after 168 h was 3.2 times greater than the undried sample, and 2.6 to 2.8 times greater after 624 h. At the conclusion of the incubation, more P had been released in samples that had been dried and rewet; 5.43 mg P kgÿ1 was mineralized in the fertilized sample and 4.11 mg P kgÿ1 in the unfertilized soil. In the samples that were undried, 1.96 mg P kgÿ1 was mineralized in the fertilized sample and 1.57 mg P kgÿ1 in the unfertilized soil. Within the undried treatment there was signi®cantly more microbial P (p < 0.05) extracted from the unfertilized soils than from the fertilized soils after 168 h, with a pattern of slow but consistent decrease in microbial P with time (Fig. 1; Table 2). By the conclusion of the incubation, microbial P had decreased by a similar amount in the unfertilized and fertilized soils. When soils were dried and rewet, microbial P ¯uctuated throughout the study after a rapid initial ¯ush, with a net increase of 0.19±0.27 mg microbial

Fig. 1. Change in microbial P in fertilized and unfertilized soils with incubation time. Data are the means of three replicates per treatment. Bars indicate2SEM where these exceed the size of the symbol.

P kgÿ1 (Fig. 1; Table 2). There was no signi®cant di€erence between the unfertilized and fertilized soils. Net mineralization kinetics The kinetics of net P mineralization after drying and rewetting were similar to those described for N and S (e.g., Bonde and Lindberg, 1988; Ellert and Bettany, 1988; Cabrera, 1993). The amount of P mineralized in undried samples increased linearly with time for both unfertilized and fertilized soils, suggesting that mineralization followed zero-order kinetics (Fig. 2; Table 3, Model A). However, zeroorder mineralization cannot be sustained inde®nitely because of substrate limitations and ®rstorder mineralization in vitro is more likely in the longer term. In the ®eld, where substrate is continually replaced by above- and below-ground inputs, zero-order kinetics should adequately explain mineralization rates at constant soil moisture and temperature. A ®rst-order model was ®tted to the data for the fertilized soil, but the standard error increased and the R2 decreased (Table 3, Model I), o€ering no improvement over the zero-order model. The ®rst-order model failed to produce a ®t meeting of the convergence criterion required for non-linear regression for the unfertilized soil and was therefore discarded.

Fig. 2. Relationship between net P mineralized and incubation time in undried soils. Data are the means of three replicates per treatment. Bars indicate2SEM where these exceed the size of the symbol. Mineralization model is Model A: Pmin=k0t. Pmin=cumulative net P mineralized; k0=rate constant of mineralization; t = time.

Microbial phosphorus mineralization

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Table 3. Phosphorus mineralization models with parameter estimates and statistics for unfertilized and fertilized soils maintained at constant moisture Soil Unfertilized Fertilized

Model A. Zero-order* A. Zero-order I. First-order$

R2 0.947 0.849 0.622

Parameter estimates k0=0.003 k0=0.004 P1=10.716 k1=0.000

Asymptotic SE 0.000 0.001 67.788 0.003

* Zero-order model (Model A): Pmin=k0t; Pmin=cumulative net P mineralized; k0=rate constant of mineralization; and t = time. $ One-pool ®rst-order model (Model I): Pmin=P1(1ÿeÿk1 t ); Pmin=cumulative net P mineralized; P1=pool of mineralizable P made available; k1=rate constant of mineralization of P1; and t = time.

In contrast, the release of P from samples that were dried and rewet showed an initial P ¯ush, which followed ®rst-order kinetics in both fertilized and unfertilized treatments (Fig. 3). Amounts of P released by these samples were ®tted to a number of models including: Model I, a one-pool model in which net P mineralization was assumed to proceed according to ®rst-order kinetics and; Model II, a two-pool model, in which one pool was assumed to mineralize according to ®rst-order kinetics, and the second pool according to zero-order kinetics (Table 4). The two-pool model was based on the premise that the ¯ush of P-mineralization was superim-

Fig. 3. Net P mineralized in dried and rewet soils. Data are the means of three replicates per treatment. Bars indicate2 SEM where these exceed the size of the symbol. Mineralization models are: (I) Pmin=P1(1ÿeÿk1 t ); (II) Pmin=P1(1ÿeÿk1 t ) + k0t; (III) Pmin=P1(1ÿeÿk1 t ) t < 192, Pmin=k0t + b t r192; (IV) Pmin=P1(1ÿeÿk1 t )i. Pmin= cumulative net P mineralized; P1=pool of mineralizable P made available after drying and rewetting; k1=rate constant of mineralization of P1; k0=rate constant of mineralization; t = time; b = intercept corresponding to the net P mineralization after the lag phase; i = constant determining the position of the point of in¯ection of a sigmoid curve.

posed on the background mineralization observed in undried samples (zero-order kinetics) (Cabrera, 1993). A modi®cation of the two-pool model described by Cabrera (1993) better described the biological processes resulting in the pattern of P mineralization observed for these soils (Fig. 3). From these data, it appeared that the mineralization of P after rewetting dried soil proceeded in three phases: (i) An initial ¯ush following ®rst-order kinetics, where inorganic P was brought into solution, a major component of which would be turnover of the microbial biomass from the previous drying period; (ii) a lag lasting of a few days where there was immobilization or no net mineralization; and (iii) a period that followed similar kinetics to the undried soil, where the microbial biomass had recovered and net mineralization of the soil organic matter proceeded at a relatively constant rate. Thus, the data were best described by a segmented model comprised of a ®rst-order relationship describing the initial P ¯ush and ensuing lag phase, and a linear relationship describing net mineralization of the soil organic matter (Fig. 3; Table 4, Model III: Two-pool segmented model). The time at which the rate of net P mineralization was a minimum was used to de®ne the in¯ection point of the model (t = 192 h). A logistic model (Model IV) was also ®tted to the data. Such models, when i>1, produce a sigmoidal curve, which can account for a lag phase in the data (Ellert and Bettany, 1988). This model, however, was not a good ®t for describing the release of P in either soil treatment and was subsequently discarded (Table 4, Fig. 3). When dry soils were rewet, 2.27 mg P kgÿ1 was released according to ®rst-order kinetics in unfertilized soil, and 3.08 mg kgÿ1 in fertilized soil, suggesting that a considerable amount of P, relative to the mineralizable P in the soil, may be released when wetting and drying cycles are common (Table 4). These amounts represent ca. 55±57% of the total amount of P released during the incubation, and correspond to about 6% of the total P in both the unfertilized and fertilized soils. The rate constant for the ®rst-order pool was 0.19 hÿ1 for the unfertilized treatment, and 0.21 hÿ1 for the fertilized treatment, indicating that ca. 99% of the P

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P. F. Grierson et al.

Table 4. Phosphorus mineralization models with parameter estimates and statistics for unfertilized and fertilized soils that were dried and rewet Soil Unfertilized

Model I. One-pool* II. Two-pool

Fertilized

R2 0.480

$

0.860

III. Segmented two-pool%

0.850

IV. Logistic}

0.554

I. One-pool

0.556

II. Two-pool

0.892

II. Segmented two-pool

0.890

IV. Logistic

0.554

Parameter estimates

Asymptotic SE

P1 (mg P kgÿ1 soil)

P1=2.638 k1=0.111 P1=1.932 k1=0.295 k0=0.003 P1=2.273 k1=0.188 k0=0.004 b = 1.608 P1=2.927 k1=0.260 i = 0.020

0.177 0.058 0.134 0.260 0.001 0.074 0.051 0.001 0.237 0.365 0.537 0.035

2.638

P1=3.633 k1=0.095 P1=2.710 k1=0.989 k0=0.004 P1=3.076 k1=0.211 k0=0.005 b = 1.985 P1=3.72 k1=0.006 i = 1.070

0.211 0.039 0.144 15.684 0.001 0.113 0.076 0.001 0.386 0.467 0.013 0.000

3.633

1.932 2.273

2.927

2.710 3.076

3.782

* One-pool model: Pmin=P1(1ÿeÿk1 t ); Pmin=cumulative net P mineralized; P1=pool of mineralizable P made available after a wetting and drying event; k1=rate constant of mineralization of P1; and t = time. $ Two-pool model: Pmin=P1(1ÿeÿk1 t ) + k0t; k0=rate constant of mineralization of the more stable P pool. % Segmented two-pool model: Pmin=P1(1ÿeÿk1 t ) t < 192; Pmin=k0t + b t = 192; b rintercept corresponding to net P mineralization after a lag phase. } Logistic model: Pmin=P1(1ÿeÿk1 t )i; when i>1, the model is sigmoidal.

released by ®rst-order kinetics occurred during the ®rst 24 h after rewetting. These results emphasize the need for intense sampling immediately after soil rewetting in order to obtain useful data for model-

Fig. 4. Actual and predicted speci®c P mineralization in fertilized and unfertilized soils. Pmin=cumulative net P mineralized; P1=pool of mineralizable P made available after drying and rewetting; k1=rate constant of mineralization of P1; k0=rate constant of mineralization; t = time; b = intercept corresponding to the net P mineralization after the lag phase; i = constant determining the position of the point of in¯ection of a sigmoid curve.

ing the kinetics of net mineralization in systems where wetting and drying cycles are common. To test the validity of the selected models, we developed a generalized equation by standardizing the data for both treatments. To account for di€erences in total substrate quantity, we calculated speci®c P mineralization (P mineralization per unit of total P) (Fig. 4). A zero-order model (Model A) adequately described net P mineralization in undried soils (R2=0.884), and the segmented, twopool model (Model III) best described net P mineralization in soils that had been dried and rewet (R2=0.923) (Table 5). In the undried soils, there was a negative relationship (R2=0.701) between the amount of inorganic P in solution and microbial P, regardless of treatment (Fig. 5). Drying and rewetting showed a small net increase in microbial P by the end of the incubation, but which only accounted for between 19±36% of the original amount before drying (Table 2). However, the general pattern of microbial P over the incubation period was similar to that of P release, i.e. an initial ¯ush, followed by a relatively steady state (Fig. 1). DISCUSSION

The net mineralization ¯ush Several factors may contribute to the P ¯ush produced when dried soils were rewet (Figs 3 and 4). A

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Table 5. Generalized models of speci®c P mineralization with parameter estimates and statistics for sandy soils that were undried, or dried and rewet Soil treatment Undried

Dried and rewet

Model

R2

Parameter estimates

Asymptotic SE

Prob>F

A. Zero-order* I. First-order$

0.884 0.891

k0=0.009 P1=19.432 k1=0.001

0.000 30.642 0.000

0.0001

III. Segmented two pool%

0.923

P1=5.952 k1=4.018 k0=0.011 b=4.134

0.142 0.000 0.001 0.519

0.0001

0.0001

* Zero-order model: Pmin=k0t; Pmin=cumulative net P mineralized; k0=rate constant of mineralization; and t = time. $ First-order model: Pmin=P1(1ÿeÿk1 t ); Pmin=cumulative net P mineralized; P1=pool of mineralizable P made available; k1=rate constant of mineralization of P1; and t = time. % Segmented two-pool model: Pmin=P1(1ÿeÿk1 t ) t < 192; Pmin=k0t + b tr 192.

signi®cant proportion of the soil microorganisms may die during soil drying (van Gestel et al., 1991) or from rewetting the soil (Kieft et al., 1987), resulting in a large pool of microbial cells for decomposition and mineralization. The P ¯ush may also be partly explained in some soils by an increase in the availability of organic substrates through desorption from soil surfaces (Seneviratne and Wild, 1985) and through an increase in the exposed organic surfaces (Birch, 1959). The relative contribution of each of these factors is not well understood, but the results of our study show that the initial ¯ush of inorganic P after rewetting appeared to follow ®rst-order kinetics in a similar way to N mineralization (Cabrera, 1993). In addition, the P ¯ush was extremely rapid, being almost completely contained within the ®rst 24 h. This would suggest that a signi®cant proportion of the P release is probably due to the e€ects of drying and rewetting on the microbial biomass. This is consistent with previous studies on soils (Sparling et al., 1985) and sediments (Qiu and McComb, 1995), where the entire increase in soluble P on air-drying was attributed to killed microbial cells. The microbial P available for the ¯ush observed in this study would be that amount contained in the microbial biomass prior to drying. Assuming this to be equivalent to about 1 mg kgÿ1 for unfertilized

Fig. 5. Relationship between microbial P and inorganic P (expressed as a percentage of the total P) for undried soils (R2=0.72; p < 0.01).

soils, and about 0.8 mg kgÿ1 for fertilized soils (Table 2), then between ca. 30% and 45% of the P released in the mineralization ¯ush would be accounted for by microbial death, with the contribution greater in the unfertilized than fertilized soil. The remaining ca. 55±70% could be attributed to rapid depletion of an easily mineralizable fraction of organic P, a situation that has been described for both N and S mineralization (Ellert and Bettany, 1988; Cabrera, 1993). Air-drying may also increase the solubility of organic matter (Magid and Nielsen, 1992), possibly also increasing that fraction of organic P which is easily mineralized. The lower amounts of microbial P in the fertilized treatment are consistent with the data presented by Clarholm (1993), where repeated additions of fertilizer resulted in a decline in microbial biomass P, possibly re¯ecting a decrease in mycorrhizal hyphae or a general reduction in the storage of P in hyphae. The net mineralization lag phase Mineralization patterns may re¯ect changes in substrate availability, microbial activity, or the balance between immobilization and mineralization. Kinetic models provide mathematical descriptions of patterns of net mineralization, and specify how mineralization rates change over time. Models of mineralization also attempt to describe biological processes in soils, but such models are certain to oversimplify the processes involved (Ellert and Bettany, 1988). Other investigators have observed an initial lag and subsequent increase in mineralization rates of N and S, but the lags are generally unaccounted for in kinetic models (Stanford and Smith, 1972; Fyles and McGill, 1987). The lag phase that we observed occurred for up to about 1 week after rewetting, and thus may be comparable to initial lag phases observed in mineralization studies of N and S, where observations have been made weekly, rather than hourly (Ellert and Bettany, 1988). There are several hypotheses which may describe the processes responsible for the lag phase observed in this study. First, we

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might assume that the mineralization rate is proportional to the cumulative nutrient release, as this re¯ects the activity of microbial or enzymatic processes responsible for mineralization. The proportionality factor, however, decreases over time to re¯ect declining e€ectiveness of the mineralization processes and exhaustion of the mineralizable substrate (Ellert and Bettany, 1988). Second, kinetic models should recognize that mineralization depends on both substrate concentration and on the catalytic activity of the biomass, or capacity of the microbial community to mineralize P. Once the pool of P released from microbial turnover and the easily mineralizable pool is depleted i.e., the mineralization ¯ush phase ®nishes, observed rates of net mineralization shall slow (Figs 3 and 4) as the more refractory components concentrate in the mineralizable pool (Cabrera, 1993). The microbial biomass also has a role as a sink or source of P. In our case, the lag phase might be best accounted for as a period of acclimation of the microbial biomass, where there is no net mineralization, or where the amount immobilized is similar to that mineralized from organic matter. The linear rates of mineralization observed after the lag phase may be attributed to mineralization of a more recalcitrant pool of P, where mineralization and microbial turnover exceeds immobilization by the microbial biomass (Fig. 5, this paper; Ellert and Bettany, 1988; Cabrera, 1993). However, as emphasized by Ellert and Bettany (1988), while statistical tests can indicate which model provides the best ®t, measurements of microbial growth, community structure, enzyme activities and C ¯ows are necessary to explicitly determine the cause of the lag. Our data suggest that turnover of microbial P has the potential to make a signi®cant contribution to the pool of labile P in soils maintained at a constant moisture, as well as in soils that are dried and rewet (Fig. 5). Generally, only poor correlations have been found between rates of nutrient mineralization and changes in the microbial nutrient pool. Changes in microbial P during the incubation were not signi®cantly correlated to net P mineralization, but did closely correlate with inorganic P pools (when expressed as a proportion of the total P). Thus, by accounting for di€erences in substrate availability (in our case, by expressing the inorganic P in solution as a proportion of the total P), the relationship between the microbial P fraction and P mineralization was more apparent (Fig. 5). When soils were dried and rewet, a signi®cant proportion of the P ¯ush was also due to turnover of microbial P. However, our measurements of the change in microbial P pool did not account for the concurrent uptake and loss of P from the microbial biomass, i.e., the turnover rate of the pool. This component of turnover has the potential to contribute substantially to N mineralization (Holmes and Zak, 1994),

and it is likely that a similar mechanism exists for P mineralization. A more accurate determination of microbial P dynamics (e.g. by using isotopic tracers to identify P transformations (McLaughlin et al., 1988), and by monitoring respiration rates) would improve understanding of the mineralization mechanisms involved. Further, Ellert and Bettany (1988) point out the diculty of identifying a kinetically homogeneous ``potentially mineralizable pool'' because of complex mineralization schemes composed of several substrates, biochemical pathways and microbial communities. The possible involvement of intermediate components (perhaps the microbial biomass) or ¯uctuating mineralization rate (possibly caused by changing catalytic activity, element ratios or energy supply) merit further investigation. Few studies have attempted to estimate P mineralization because of confounding e€ects of adsorption reactions with mineral surfaces. Our results show that drying and rewetting events may have signi®cant e€ects on P dynamics in both fertilized and unfertilized plantations of loblolly pine in north central Florida, with the initial ¯ush of P release representing a signi®cant pool of potentially available P in such nutrient-impoverished ecosystems. This is particularly true of soils which have not been fertilized. We found that a simple model which incorporates an initial ¯ush followed by a lag phase, and which is standardized for di€erences in substrate availability, is equally applicable to soils that have been subjected to di€erent treatments. Thus, the kinetic models proposed here should be useful in predicting P mineralization in other sandy soils of low adsorption capacity of the southeastern U.S.A. AcknowledgementsÐFinancial support was provided primarily by the industrial members of the Cooperative Research in Forest Fertilization (CRIFF) program at the University of Florida. Many thanks to Dr. Ken Clark and Arlete Freitas for laboratory assistance. REFERENCES

Adu J. K. and Oades J. M. (1978) Physical factors in¯uencing decomposition of organic material in soil aggregates. Soil Biology & Biochemistry 10, 109±115. Ballard R. and Fiskell J. G. A. (1974) Phosphorus retention in coastal plain forest soils: I. Relationship to soil properties. Soil Science Society of America Proceedings 38, 250±255. Bauhus J. and Khanna P. K. (1994) Carbon and nitrogen turnover in two acid forest soils of southeast Australia as a€ected by phosphorus addition and drying and rewetting cycles. Biology and Fertility of Soils 17, 212± 218. Birch H. F. (1959) Further observations on humus decomposition and nitrogen. Plant and Soil 11, 262±286. Birch H. F. (1960) Nitri®cation in soils after di€erent periods of dryness. Plant and Soil 12, 81±96. Bonde T. A. and Lindberg T. (1988) Nitrogen mineralization kinetics in soil during long-term aerobic incu-

Microbial phosphorus mineralization bations: A case study. Journal of Environmental Quality 17, 414±417. Bonde T. A. and Rosswall T. (1987) Seasonal variation of potentially mineralizable nitrogen in four cropping systems. Soil Science Society of America Journal 51, 1508± 1514. Cabrera M. L. (1993) Modeling the ¯ush of nitrogen mineralization caused by drying and rewetting soils. Soil Science Society of America Journal 57, 63±66. Campbell C. A., Jame Y. D. and De Jong R. (1988) Predicting net nitrogen mineralization over a growing season: Model veri®cation. Canadian Journal of Soil Science 68, 537±552. Chapin F. S. and Kedrowski R. A. (1983) Seasonal changes in nitrogen and phosphorus fractions and autumn retranslocation in evergreen and deciduous taiga trees. Ecology 64, 376±391. Clarholm M. (1993) Microbial biomass P, labile P, and acid phosphatase activity in the humus layer of a spruce forest, after repeated additions of fertilizers. Biology and Fertility of Soils 16, 287±292. Colbert S. S., Jokela E. J. and Neary D. G. (1990) E€ects of annual fertilization and sustained weed control on dry matter partitioning, leaf area, and growth eciency of juvenile loblolly and slash pine. Forest Science 36, 905±1014. Dalla-Tea F. and Jokela E. J. (1991) Needlefall, canopy light interception, and productivity of young, intensively managed slash and loblolly pine stands. Forest Science 37, 1298±1313. Dalla-Tea F. and Jokela E. J. (1994) Needlefall and resorption rates of nutrients in young, intensively managed slash and loblolly pine stands. Forest Science 40, 650±662. Ellert B. H. and Bettany J. R. (1988) Comparison of kinetic models for describing net sulfur and nitrogen mineralization. Soil Science Society of America Journal 52, 1692±1702. Fox T. R., Comerford N. B. and McFee W. W. (1990) Kinetics of phosphorus release from Spodosols: e€ects of oxalate and formate. Soil Science Society of America Journal 54, 1441±1447. Fyles J. W. and McGill W. B. (1987) Nitrogen mineralization in forest pro®les from central Alberta. Canadian Journal of Forest Research 17, 242±249. Gonc° alves J. L. M. and Carlyle J. C. (1994) Modelling the in¯uence of moisture and temperature on net nitrogen mineralization in a forested sandy soil. Soil Biology & Biochemistry 26, 1557±1564. Holmes W. E. and Zak D. R. (1994) Soil microbial biomass dynamics and net nitrogen mineralization in northern hardwood ecosystems. Soil Science Society of America Journal 58, 238±243. Jager G. and Bruins E. H. (1975) E€ect of repeated drying at di€erent temperatures on soil organic matter decomposition and characteristics, and on the soil micro¯ora. Soil Biology & Biochemistry 17, 153±159. Kieft T. L., Soroker E. and Firestone M. K. (1987) Microbial biomass response to a rapid increase in water potential when dry soil is wetted. Soil Biology & Biochemistry 19, 119±126. Magid J. and Nielsen N. E. (1992) Seasonal variation in organic and inorganic phosphorus fractions of temperate-climate sandy soils. Plant and Soil 144, 155±165. McLaughlin M. J., Alston A. M. and Martin J. K. (1986) Measurement of phosphorus in the soil microbial biomass: a modi®ed procedure for ®eld soils. Soil Biology & Biochemistry 18, 437±443. McLaughlin M. J., Alston A. M. and Martin J. K. (1988) Phosphorus cycling in wheat-pasture rotations. III.

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Organic phosphorus turnover and phosphorus cycling. Australian Journal of Soil Research 26, 343±353. Murphy J. and Riley J. P. (1962) A modi®ed single solution method for the determination of phosphate in natural waters. Analytical Chimica Acta 27, 31±36. Neary D. G., Jokela E. J., Comerford N. B., Colbert S. R. and Cooksey T. E. (1990) Understanding competition for soil nutrients Ð The key to site productivity on southeastern Coastal Plain Spodosols. In Sustained Productivity of Forest Soils. Proceedings of the American Forest Soils Conference 7th, Vancouver, B.C. 24±28 July 1988, eds S. P. Gessel et al., pp. 432±450. University of British Columbia, Faculty of Forestry Publication, Vancouver. Nelson D. W. and Sommers L. E. (1982) Total carbon, organic carbon, and organic matter. In: Methods of Soil Analysis. Part 2. Chemical and Microbiological Properties, eds Page A. L., Miller R. H. and Keeney D. R. pp. 539±579. Soil Science Society of America, Madison. Polglase P. J., Comerford N. B. and Jokela E. J. (1992a) Leaching of inorganic phosphorus from litter in southern pine plantation. Soil Science Society of America Journal 56, 573±577. Polglase P. J., Comerford N. B. and Jokela E. J. (1992b) Mineralization of nitrogen and phosphorus from soil organic matter in southern pine plantations. Soil Science Society of America Journal 56, 921±927. Polglase P. J., Jokela E. J. and Comerford N. B. (1992c) Nitrogen and phosphorus release from decomposing needles in southern pine plantations. Soil Science Society of America Journal 56, 914±920. Polglase P. J., Jokela E. J. and Comerford N. B. (1992d) Phosphorus, nitrogen and carbon fractions in litter and soil of northern pine plantations. Soil Science Society of America Journal 56, 566±573. Qiu S. and McComb A. J. (1995) Planktonic and microbial contributions to phosphorus release from fresh and air-dried sediments. Marine and Freshwater Research 46, 1039±1045. SAS Institute (1989) SAS User's Guide. Statistics. Version 6.03. SAS Inst. Inc., Cary. Seneviratne R. and Wild A. (1985) E€ect of mild drying on the mineralization of soil nitrogen. Plant and Soil 84, 175±179. Soil Survey Sta€ (1994) Keys to Soil Taxonomy. U.S. Government Printing Oce, Washington. Sorensen L. H. (1974) Rate of decomposition of organic matter in soil as in¯uenced by repeated air drying-rewetting and repeated additions of organic material. Soil Biology & Biochemistry 6, 287±292. Sparling G. P., White K. N. and Ramsay A. J. (1985) Quantifying the contribution from the soil microbial biomass to the extractable P levels of fresh and air-dried soils. Australian Journal of Soil Research 23, 613±621. Stanford G. and Smith S. J. (1972) Nitrogen mineralization potential of soils. Soil Science Society of America Proceedings 36, 465±472. Swindel B. F., Neary D. G., Comerford N. B., Rockwood D. L. and Blakeslee G. M. (1988) Fertilization and competition control accelerate early southern pine growth on ¯atwoods. Southern Journal of Applied Forestry 12, 116±121. United States National Oceanic and Atmospheric Administration. (1989) Climatological data, Florida. National Climate Center, Environmental Data Service, Asheville, North Carolina. van Gestel M., Ladd J. N. and Amato M. (1991) Carbon and nitrogen mineralization from two soils of contrasting texture and microaggregate stability: In¯uence of sequential fumigation, drying and storage. Soil Biology & Biochemistry 23, 313±322.