Biol Fertil Soils (1999) 28 : 244–252 Q Springer-Verlag 1999 ORIGINAL PAPER P. F. Grierson 7 N. B. Comerford 7 E. J. Jokela Phosphorus mineralization and microbial biomass in a Florida Spodosol: effects of water potential, temperature and fertilizer application Received: 30 October 1997 Abstract Phosphorus mineralization and microbial biomass were measured in the surface 5 cm of a Spodosol (sandy, siliceous hyperthermic Ultic Alaquod) from north-central Florida. Soils from fertilized and unfertilized plantations of loblolly pine (Pinus taeda L.) were incubated at a range of water potentials (F0, P3, P8, P10 and P1500 kPa) and temperatures (15 7C, 25 7C and 38 7C) for 14 days and 42 days. Increasing water potential and temperature increased specific P mineralization (mineralization expressed as a percentage of total P) regardless of fertilizer treatment. An increase in water potential from P10 kPa to P0.1 kPa resulted in an increase of between 38% and 239% in the concentration of KCl-extractable inorganic P, depending on incubation temperature and time. An increase in incubation temperature from 15 7C to 38 7C resulted in an increase of between 13% and 53% in KCl-extractable inorganic P. Changes in specific P mineralization with change in water potential or temperature were not affected by fertilizer application. This suggests that, although specific P mineralization was greater in the fertilized soils, environmental control of P mineralization was the same for both treatments. Specific P mineralization was most sensitive when soils were at higher water potentials, and decreased logarithmically to water potentials of between P3 kPa and P8 Contribution of the Florida Agricultural Experiment Station Journal No. R-05487 P. F. Grierson (Y) 1 7 E. J. Jokela School of Forest Resources and Conservation, 118 Newings-Ziegler Hall, University of Florida, Gainesville, FL 32611-0303, USA N. B. Comerford Department of Soil and Water Science, 2169 McCarty Hall, University of Florida, Gainesville, FL 32611-0303, USA Present address: 1 Department of Botany, University of Western Australia, Nedlands, WA 6907, Australia e-mail: pfgblue@cyllene.uwa.edu.au; Fax: c618-9380-7925 kPa. Specific P mineralization was relatively insensitive to changes in water potential when water potential was lower than P8 kPa. Microbial biomass C showed no consistent responses to changes of temperature or water potential and was not significantly correlated with specific P mineralization. Our results suggest that field estimates of P mineralization in these Spodosols may be improved by accounting for changes in soil water potential and temperature. Key words Phosphorus mineralization 7 Microbial biomass C 7 Water potential 7 Temperature 7 Spodosol Introduction Nutrient availability, particularly of P, limits productivity of loblolly pine (Pinus taeda L.) and slash pine (Pinus elliottii Engelm. var. elliottii) plantations growing on Spodosols in north-central Florida (Neary et al. 1990). The growth of these plantations increases in response to fertilizer application (Pritchett and Llewellyn 1966; Colbert et al. 1990) and to weed control (Neary et al. 1990), and it is likely that continued cycling of P within the forest floor will increase productivity of the site for some time, possibly for many years after the initial benefits derived from direct fertilizer uptake (Polglase et al. 1992a). In the surface horizons of Spodosols in north-central Florida, mineralization and immobilization are the dominant processes determining the concentration of P in the soil solution (Fox et al. 1990), primarily because of a lack of sorptive surfaces. Rates of P mineralization calculated from field measurements for these sites may be considerably less than estimated rates of P uptake. For example, Polglase et al. (1992a) found a large disparity between field estimates of P mineralization measured by an in situ core method (Adams et al. 1989), and “potentially mineralizable P” estimated from a laboratory incubation at constant moisture and 245 temperature. Potentially mineralizable P also varied with season, probably due to factors such as temporal variation in litterfall, or to changes in soil moisture (wetting and drying cycles) and temperature. While it is well known that the mineralization of organic P increases with increasing temperature (e.g. Thompson and Black 1947), the effect of increasing soil moisture is less clear (Dalal 1977). Models which describe both the influence of temperature and moisture on nutrient dynamics are essential to reconcile rates of mineralization observed under optimum conditions in the laboratory (generally conducted under high temperatures and at constant moisture) with those observed in the field, particularly where soil moisture contents fluctuate over short periods of time. To apply models across a range of soils, it is also necessary to assess how the influence of soil moisture and temperature on P mineralization differs with substrate quality and quantity. Soil microbial biomass plays an intrinsic role in P dynamics through biochemical transformations of organic matter and as both source and sink for mineral nutrients (Jenkinson and Ladd 1981). There is, generally, a negative relationship between soil moisture content and microbial biomass (Wardle and Parkinson 1990). Microbial biomass also varies seasonally in response to changes in soil moisture and temperature (Patra et al. 1990; Srivastava 1992), but there are few estimates of mineralization rates as a function of change in the microbial biomass or its activity at different soil water potentials. The aim of this study was to determine how changes in water potential and temperature influence P release from soil organic matter and the microbial biomass in the surface horizon of a Florida Spodosol. Direct measurement of P mineralization in most soils is often made problematic by sorption reactions with mineral surfaces. However, the surface soils of Spodosols in Florida have little or no ability to sorb mineral P (Ballard and Fiskell 1974; Fox et al. 1990) and are therefore suitable for describing some of the mechanisms which influence P mineralization. To assess whether the influence of these factors on P mineralization rates varied with substrate quality, we compared soils from unfertilized and fertilized plantations of loblolly pine in laboratory incubations. nantly quartz sand with low organic matter content, low CEC (~5 cmolckg P1) and few primary or secondary minerals. Sorption of added inorganic P is undetectable (Ballard and Fiskell 1974; Fox et al. 1990). The study site was part of a long-term field experiment initiated in 1983 by the Intensive Management Practices Assessment Center (IMPAC) of the USDA Forest Service, to ascertain the growth potential of loblolly and slash pine. The IMPAC experiment consisted of three replicates of a factorial of species (loblolly and slash pines), weed control (with or without sustained mechanical and/or herbicide elimination of all understorey competition), and fertilizer application (with or without annual additions of complete fertilizer), arranged in a randomized, split-plot (species) design. A more detailed description of the site is given in Neary et al. (1990). Because earlier studies by Polglase et al. (1992a) indicated significant impacts of fertilizer application on rates of P mineralization, the study described here focused on comparing rates of mineralization of P and microbial biomass 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 ha P1 year P1): 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. Laboratory incubations Soil was sampled from unfertilized and fertilized treatments of loblolly pine in March 1993. A metal corer (2.5 cm in diameter) was used to collect 40 samples of the surface horizon (0–5 cm) of each of 3 block replicates. Samples were collected from the interbeds, at least 2 m away from the fertilizer bands, and separated by furrows. Samples were bulked by treatment, sieved (~2 mm) and mixed thoroughly. For each bulked soil, mineralization potential and microbial biomass were determined by aerobic incubation. A subsample of air-dried soil was adjusted gravimetrically to a range of soil moisture contents, each corresponding to a known water potential (P0.1 kPa, P3 kPa, P8 kPa, P10 kPa, P1500 kPa) (Fig. 1). Thirty grams (equivalent dry weight) of adjusted soil were incubated in capped 120 ml polyethylene bottles at three temperatures (15 7C, 25 7C and 38 7C), for a period of 14 days. The procedure was repeated for each water potential/temperature combination for an incubation period of 42 days. Incubation times were selected to allow for initial responses to rewetting (Amato and Ladd 1988) and for sufficient development of differences between treatments. Bottles were removed every 3 days for aeration, at which time moisture contents were adjusted gravime- Materials and methods Site description The study site is about 10 km north of Gainesville, Fla., USA (29780bN, 82720bW). 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 21 7C (U.S. National Oceanic and Atmospheric Administration 1989). The soils are poorly drained Pomona fine sands (sandy, siliceous, hyperthermic Ultic Alaquods) (Soil Survey Staff 1998), generally with a spodic horizon beginning at 20–50 cm, and an argillic horizon beginning at 90–120 cm. The A horizon is predomi- Fig. 1 Moisture/potential curve for surface horizon (0–5 cm) of a Florida Spodosol (Pomona fine sand, Ultic Alaquod) 246 trically as required. There were three replicates of each moisture by temperature by incubation period combination. After 14 days or 42 days, subsamples of the soil were analyzed for microbial biomass and inorganic P. Total P was measured prior to incubation. To account for differences in total substrate quality between treatments, we calculated mineralization on the basis of the total concentration of P in the soil. This specific mineralization (Frazer et al. 1990) has been suggested to provide a functional index of substrate quality in studies on N mineralization (Vitousek et al. 1983), and has been used by Polglase et al. (1992a) to describe mineralization of both N and P at this study site. Chemical analyses Inorganic P was extracted in 0.1 M KCl (Polglase et al. 1992a). Previous work by Polglase et al. (1992a) and the present authors have confirmed 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 effective 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 340 7C 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 biomass C Soil microbial biomass was estimated by the fumigation-extraction technique recommended by Wardle and Parkinson (1990). The net gain of ninhydrin-reactive N in extracts of fumigated soils was the basis for estimation of biomass C (Amato and Ladd 1988). Duplicate sub-samples of soil (;5 g equivalent dry weight) were weighed into 50 ml polypropylene centrifuge tubes and incubated for 6 days at 25 7C. After the 6-day pre-incubation period, samples were extracted and analyzed by the following procedure: 0.5 ml of alcohol-free chloroform was added to half the tubes, then all were incubated at 25 7C for a further 6 days. After fumigation, chloroform was evaporated and samples were extracted with 20 ml of 2 M KCl by shaking for 1 h. Samples were centriTable 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 kg P1) Phosphorus (mg kg P1) Total C/P ratio Organic P Cold TCA b Hot TCA 0.5 M NaHCO3 Inorganic P Mehlich 1 Water Cold TCA 0.1 M KCl 0.5 M NaHCO3 a b 3.9 16.72 (1.51) a Fertilized 3.8 18.31 (0.72) 36.74 (5.21) 455 51.12 (6.79) 358 2.05 (0.53) 3.39 (0.21) 3.29 (0.80) 3.32 (0.24) 3.73 (0.19) 5.63 (1.69) 3.25 2.49 2.63 2.39 0.63 4.27 3.19 3.89 3.75 1.89 (0.18) (0.13) (0.27) (0.13) (0.07) Standard deviations (np3) in parentheses Trichloroacetic acid (Chapin and Kedrowski 1983) (0.53) (0.11) (0.42) (0.11) (0.02) fuged and the supernatant analyzed for ninhydrin-reactive N (Amato and Ladd 1988). Biomass C, when based on ninhydrinreactive N released from soils fumigated for 6 days, was derived from the equation: biomass Cp21! the release of ninhydrinreactive N (Amato and Ladd 1988). Statistical analyses Analysis of variance (ANOVA) for a factorial experiment was used to examine treatment main effects (fertilizer, water potential and temperature) on KCl-extractable inorganic P, specific P mineralization and microbial biomass, and to test for differences in KCl-extractable inorganic P, specific P mineralization, and microbial biomass among incubation treatments. Relationships between variables were examined using linear regression analyses. Analysis of covariance was used to compare slopes and intercepts of regressions of specific P mineralization with water potential among treatments. The SAS software (SAS Institute 1989) was used for all statistical analyses (ANOVA, REG and GLM procedures). The significance level was P^0.05 unless otherwise stated. Results KCl-extractable inorganic P and P mineralization There was significantly more KCl-extractable inorganic P in the fertilizer treatment than in the unfertilized after 14 days and after 42 days (Table 2). Concentrations of P mineralized were affected by water potential, temperature and interactions between them, for both treatments and incubation times (Table 2). There was no significant interaction of fertilizer treatment with water potential and temperature. With decreasing water potential, there was a decrease in the mean amount of KCl-extractable inorganic P across all treatments and incubations. There was also an overall increase in KClextractable P with temperature, although less P was extracted at 25 7C than at 15 7C. Although both effects were significant in their influence on P mineralization (P~0.005), water potential explained between 55% and 80% of the variance, while temperature explained between 9% and 35%, depending on treatment and incubation time. The variance explained by interactions between water potential and temperature was always less than 10%. The significant interaction described a trend of increasing KCl-extractable inorganic P with increasing water potential, where the degree of change between water potentials appears to be greater at 38 7C compared to 15 7C (Table 2). After 14 days, an increase in temperature from 15 7C to 38 7C resulted in a 14% increase in the amount of P mineralized in the unfertilized treatment across all water potentials (range P5% to 24%) and a 33% increase in the fertilized treatment (range 19–50%) (Table 2). After 42 days there 53% more P mineralized at 38 7C than at 15 7C in the unfertilized treatment (range 42– 61%), but only a 13% increase in the fertilized treatment (range P26% to 39%) due to net immobilization in the period 14 days to 42 days. Assuming a constant rate of mineralization per day, the ratios of kinetic con- 247 Table 2 Effect of soil water potential and temperature on KClextractable inorganic P and P mineralization in unfertilized and fertilized soils incubated for 14 days and 42 days. *, **, *** Main Temperature Unfertilized a Water potential 14 days 7C kPa 15 P 0.1 P 3 P 8 P 10 P1500 6.40 4.51 4.02 4.62 3.17 P 0.1 P 3 P 8 P 10 P1500 P 0.1 P 3 P 8 P 10 P1500 42 days 14 days Inorganic P 6.52 3.97 2.91 3.26 2.40 (1.09) (0.61) (0.14) (0.15) (0.20) 3.81 (0.57) (0.85) (0.07) (0.15) (0.12) 4.03 7.76 5.57 4.96 4.38 3.29 Mean (0.62) (0.29) (0.14) (0.51) (0.12) 4.54 6.97 4.12 3.43 2.97 2.68 Mean 38 Ferilized 42 days mg kg P1 Mean 25 effects of water potential, temperature and interactions are significant at P~0.05, 0.01, and 0.001, respectively. Standard deviation (np3) in parentheses 4.72 3.04 2.30 1.87 1.19 5.19 10.07 6.39 4.51 4.79 3.41 (0.67) (0.62) (0.39) (0.22) (0.28) 4.97 (1.13) (0.03) (0.14) (0.04) (0.29) 2.62 (0.12) (0.25) (0.66) (0.54) (0.39) 7.15 5.28 4.95 4.07 3.39 6.75 5.01 4.25 3.98 3.56 5.84 Sources of variation 10.73 6.36 6.08 5.75 4.04 (1.52) (1.08) (0.25) (1.09) (1.36) 5.28 (0.10) (0.66) (0.32) (0.35) (0.21) 4.71 (0.55) (0.41) (0.53) (0.72) (0.60) 7.36 5.70 4.28 5.35 3.68 6.62 4.65 4.21 3.86 2.97 (1.28) (0.48) (0.77) (0.51) (1.00) 4.46 (1.18) (0.57) (1.05) (0.25) (0.61) 6.59 9.76 7.90 5.77 3.99 3.10 (0.80) (1.01) (1.00) (0.29) (0.68) 5.99 Mean squares (type III) Water potential Temperature Water potential!temperature 20.22*** 5.05*** 0.63** 30.68*** 39.59*** 1.43** 26.25*** 15.90*** 1.54** 28.67*** 10.12*** 2.75* P mineralization rate mg kg P1 day P1 15 P 0.1 P 3 P 8 P 10 P1500 0.457 0.322 0.287 0.329 0.226 Mean 25 P 0.1 P 3 P 8 P 10 P1500 38 P 0.1 P 3 P 8 P 10 P1500 0.155 0.095 0.069 0.078 0.057 (0.026) (0.015) (0.003) (0.004) (0.005) 0.091 (0.041) (0.061) (0.005) (0.011) (0.009) 0.288 0.554 0.398 0.355 0.313 0.234 Mean a 0.325 0.498 0.294 0.245 0.212 0.191 Mean (0.045) (0.021) (0.010) (0.037) (0.009) 0.112 0.072 0.055 0.045 0.028 0.371 0.239 0.152 0.108 0.114 0.081 (0.048) (0.045) (0.028) (0.016) (0.020) 0.355 (0.027) (0.001) (0.003) (0.001) (0.007) 0.062 (0.009) (0.018) (0.045) (0.038) (0.028) 0.511 0.377 0.354 0.291 0.242 0.483 0.358 0.304 0.284 0.254 0.139 0.766 0.454 0.434 0.411 0.289 0.471 (0.036) (0.026) (0.006) (0.026) (0.032) 0.126 (0.007) (0.047) (0.023) (0.025) (0.015) 0.337 (0.013) (0.010) (0.013) (0.017) (0.015) 0.176 0.134 0.102 0.127 0.088 0.158 0.111 0.100 0.092 0.071 (0.030) (0.012) (0.018) (0.012) (0.024) 0.106 (0.084) (0.041) (0.075) (0.018) (0.044) 0.232 0.188 0.137 0.095 0.074 (0.019) (0.024) (0.024) (0.007) (0.016) 0.145 Unfertilized treatment is significantly different from fertilized treatment (P~0.001) stants at 38 7C to those at 15 7C would then range between 0.73 (fertilized treatment after 42 days at P10 kPa) and 1.61 (unfertilized treatment after 42 days at P3 kPa). However, the mean mineralization rate (mg kg P1 day P1) was ;3 to 4 times greater in the first 14 days compared to the rate over 42 days (Table 2). This indicates initial mineralization is rapid, followed by a marked slow-down in the mineralization rate in the lat- ter part of the incubation period. At 38 7C there was no significant difference in the net mineralization rate between treatments in the 42-day incubation (Table 2), with the result that specific P mineralization was greater in the unfertilized treatment. Increasing water potential increased specific P mineralization, regardless of treatment or incubation time (Figs. 2, 3). Figures 2 and 3 demonstrate that P mineral- 248 Fig. 2 Relationship of specific P mineralization to soil water potential after a 14-day incubation. Specific mineralization is the amount of nutrient mineralized as a percentage of the total present originally. Regressions are indicated by dashed lines. l fertilized; } unfertilized ization was related logarithmically to soil water potential. Coefficients of determination (r 2) greater than 0.78 at 14 days and greater than r 2p0.52 after 42 days, were estimated for all relationships between water potential (up to P10 kPa) and specific P mineralization (Figs. 2, 3). There were no significant differences (P~0.01) in the slopes of the regressions between treatments. This suggests that the response of the rate of specific P mineralization to change in water potential was independent of soil treatment. However, the intercepts of the regressions for the unfertilized and fertilized soils were significantly different after 14 days at 38 7C, and after 42 days at 15 7C and 25 7C. Consequently, for these incubations the fertilizer treatment, for any given water potential, had a greater amount of inorganic P in solution per unit of labile P, than the unfertilized treatment. Specific P mineralization was most sensitive when soils were at higher water potentials. Sensitivity decreased logarithmically as water potentials decreased to about P3 kPa to P8 kPa. At water potentials less than P8 kPa, P mineralization was relatively insensitive to changes in soil water potential. Fig. 3 Relationship of specific P mineralization to soil water potential after a 42-day incubation. Specific mineralization is the amount of nutrient mineralized as a percentage of the total present originally. Regressions are indicated by dashed lines. l fertilized; } unfertilized Microbial biomass C Microbial biomass C was greatest in the fertilized soil (72–988 mg kg P1), with significant effects (P 1 0.001) due to fertilizer treatment and incubation period (Table 3). However, microbial C showed no consistent trends with water potential and temperature (Table 3). There was a significant temperature effect and water potential!temperature interaction, after a 14-day incubation in the unfertilized treatment. Temperature explained 34% of the variance, while the interaction between water potential and temperature explained 31% of the variance. In the fertilized treatment, both water potential and temperature had a significant effect on microbial C after incubation for 42 days. Water potential explained 11% of the variance and temperature 20% of the variance. Specific P mineralization was not significantly correlated with microbial biomass C after incubation for 14 days or 42 days (Fig. 4), although both microbial biomass C and P mineralization were higher in the fertilizer treatment. 249 Table 3 Effect of soil water potential and temperature on microbial C in unfertilized and fertilized soils incubated for 14 days and 42 days. *, **, *** Main effects of water potential, temperature Temperature and interactions are significant at P~0.05, 0.01, and 0.001, respectively. Standard deviation (np3) in parentheses Unfertilized a Water potential 14 days 7C kPa 15 P 0.1 P 3 P 8 P 10 P1500 108 140 141 105 105 P 0.1 P 3 P 8 P 10 P1500 P 0.1 P 3 P 8 P 10 P1500 110 120 181 149 77 14 days Inorganic P 42 days (77) (73) (10) (59) (178) (208) (99) (71) (40) (125) ND 123 (31) 185 (18) 235 (93) 191 (42) 845 870 631 685 700 184 746 181 38 77 80 147 619 615 639 478 724 575 (77) (40) (120) (99) (142) 606 c (181) (63) (53) (104) (208) 104 Sources of variation Water potential Temperature Water potential!temperature (56) (15) (50) (20) (19) 128 267 659 681 727 704 323 Mean (16) (25) (26) (42) (73) 120 162 165 133 114 762 Mean 38 42 days mg kg P1 Mean 25 Ferilized 440 605 307 426 988 72 137 162 241 287 (47) (106) (68) (107) (172) 180 (82) (42) (306) (353) (176) 179 200 253 212 205 (35) (73) (48) (47) (400) 210 (85) (400) (63) (95) (347) 553 272 150 301 376 468 (72) (79) (24) (91) (209) 313 Mean squares (type III) 7951 921 521*** 82 908*** a Unfertilized treatment is significantly different from fertilized treatment (P~0.001) 7402 6803 16 228 90 237 113 809 62 056 37 785** 69 207** 12 240 b 14 day incubation is significantly different from 42 days (P~0.001) c ND – not determined Discussion Fig. 4 Relationship of specific P mineralization to microbial biomass C when soils incubated for 14 days and 42 days. l fertilized; } unfertilized The mechanisms by which changes in water potential may influence P mineralization is not clear. Several field studies on soils from the temperate climates have shown large seasonal variation of labile inorganic P, and control of mineralization by soil moisture and temperature is often inferred. For example, in a study of seasonal variation of P fractions in pasture soils, Magid and Nielsen (1992) reported an inverse relationship between inorganic P and soil moisture. There was no significant effect of temperature but there were significant interactions between temperature and moisture. They attributed the relationship between inorganic P fractions and soil moisture to changes in soil structure, and to the effects of drying on the soil microbial biomass. Changes in organic P fractions were considered to be a reflection of changes in solubility of organic matter, rather than biological turnover of organic matter. In our study, P mineralization increased with increasing soil water potential and increasing temperature. Changes in inorganic P concentration may be attributable to a number of factors including (1) an increase in solubility of organic matter, (2) an increase in the rate of diffusion of organic P to the microbial biomass and, 250 (3) an increase in the activity of the microbial biomass. However, from our data it is not possible to ascertain which of these processes is most important. Quality of soil organic matter has always been difficult to define. However, we used the ratio of mineralized P : total P (specific P mineralization) to characterize quality. While pools of P were greater in fertilized soils, the change in the rates of mineralization of organic P of the two different treatments responded in a similar manner to changes in water potential and temperature. Mineralization of P was related to soil water potential as previously described for mineralization of N (Stanford and Epstein 1974). The change in the absolute amount of P mineralized between P0.1 kPa and P8 kPa (the range where mineralization was most sensitive) ranged from 61% to 114%. While these water potentials represent soils that would be at field capacity or wetter, it should be remembered that these soils often have a high water table (Neary et al. 1990). Consequently, such water potentials may persist for significant periods in the annual hydrologic cycle. Most laboratory studies of N, S and P mineralization are conducted at constant temperature and water potential (e.g. Frazer et al. 1990; Ellert and Bettany 1992; Polglase et al. 1992a) and calculated rates of “potential mineralization” may bear little relevance to field situations. We measured specific P mineralization of ;12– 13% in both fertilized and unfertilized plantations of loblolly pine after a 42-day incubation (at 38 7C and water potential ;10 kPa). However, under similar conditions and a 42-day incubation, Polglase et al. (1992a) measured specific P mineralization between 0.3% and 4.5% in unfertilized soils, and between 3.6% and 9% in fertilized soils, depending on the time of year when soils were sampled. The differences between estimates of potential P mineralization may be attributable to a number of factors. These include a difference in sampling scheme (we used soils from interbeds and not from furrows), the time of year when samples were collected for laboratory incubation, the difference in age of the plantations, and time since fertilizer was last applied. We found no clear relationship between the amount of microbial C and specific P mineralization (Fig. 4), probably due to the large variation in estimates of microbial C (Table 3), even though both microbial biomass C and concentrations of P mineralized were higher in the fertilized soils. Raubuch and Beese (1995) also reported poor correlations of P content in soils with microbial biomass C and concluded that microbial biomass is not closely related to soil chemical parameters. This is in contrast to other studies (e.g. Harrison 1985) which have demonstrated a close relationship between mineralizable P and the soil microbial biomass. There has been mainly qualitative examination of the relationships between edaphic factors and seasonal estimates of microbial populations in soils, and how substrate quality might affect these. To better understand the relationship between P mineralization and the microbial biomass, more information is required on the P fraction held in soil microorganisms, and on the activity of the microbial biomass (through indices such as soil respiration, and microbially-derived phosphatase enzymes, for example) rather than simple estimates of the microbial biomass itself. Likewise, there have been few attempts to correlate P mineralization with turnover of the microbial pool even though this pool is an important component of the labile P in the soil (Brookes et al. 1985; McLaughlin et al. 1988; Sperling et al. 1994). Fertilizer application caused a significant increase in microbial C after 14 days, but after 42 days microbial C was less, possibly due to substrate limitation (Table 3). Hossain et al. (1995) also described a stimulatory effect of fertilizer application on microbial biomass C, but the magnitude of the response appeared dependent on the type of fertilizer added. We collected soil samples from the interbeds of loblolly pine plantations where there were no direct effects of fertilizer type on the microbial biomass. Rather, the effects of fertilizer on both P mineralization and the microbial biomass (Table 3; Fig. 4) are due to differences in organic matter inputs from litterfall, i.e. higher substrate quality and increased organic matter in the fertilized plots (Polglase et al. 1992b). Microbial biomass C in the soils of our treatments were within the range described for coniferous forests growing on sandy soils (e.g. Díaz-Raviña et al. 1995) and reflect the warm climate. In forest soils, microbial biomass C is generally between 1% and 3% of total soil C (Fritze et al. 1993; Sparling et al. 1994). In these sandy Spodosols, microbial C was 0.5–4.6% of the total C in unfertilized soils, and 0.4–5.4% in fertilized soils. Temperature is a major factor influencing N and S mineralization, but mineralization shows a diminishing response to temperature in soils of warmer climates (Ellert and Bettany 1992). Nadelhoffer et al. (1991), in a study of six Arctic soils, observed both an increase and decrease in the rate of P release in response to increasing temperature depending on site, and concluded that the quality of soil organic matter had a greater effect than temperature on microbial activity under field conditions. Soil moisture also affects microbial respiration, and this effect probably increases with temperature (Nadelhoffer et al. 1991). Increased P mineralization with increasing temperature also occurred in our study, particularly at temperatures above 25 7C (Table 2; Figs. 2, 3). However, the low ratios of P mineralized at 38 7C compared to 15 7C, and the negligible change in P mineralization between 15 7C and 25 7C, suggests a lesser response of P mineralization to temperature in these warm soils compared to those of colder areas, than to changes in moisture. Likewise, although substrate quality influences mineralization rates, its impact is less obvious than that of water potential or temperature. Some of the differences in amounts of P mineralized for each incubation period (Table 2) and the response of the microbial biomass to temperature and 251 water (Table 3) may be due to relative differences in forms of mineralizable P between the fertilized and unfertilized treatments. For example, if there are fractions of “easily mineralizable” and more recalcitrant forms of P present in different ratios in each treatment, then the average mineralization rate for each incubation period is the net result of differing rates of mineralization from the two pools (Grierson et al. 1998). However, the rate constants for each of these pools may respond to temperature in different ways, as has been described for N mineralization (Watanabe et al. 1996). This may in turn be a reflection of the variety of phosphatase enzymes present in the soil which, although catalysing the same reaction, can differ dramatically in Km and Vmax values (Nannipieri et al. 1990). While there is evidence to suggest that fertilized treatments may have a greater proportion of “easily mineralizable” P than unfertilized treatments (Polglase et al. 1992b), detailed studies of the kinetics of P mineralization would be necessary to ascertain the validity of this explanation. The Rothamsted model (Jenkinson 1990) and the CENTURY model (Parton et al. 1988) include both temperature and moisture as rate-modifying factors on the decomposition and turnover of C, N, S and P in grasslands and pasture. Less complex models, such as that used by Gonçalves and Carlyle (1994) to predict net N mineralization in forested sandy soils, combine soil moisture and soil temperature functions with an index of potentially mineralizable N. However, few previous studies have considered the impact of changes in soil moisture and temperature on mineralization in combination with fertilizer treatment. Experiments that include factorial combinations of soil moisture, temperature and organic matter quality are needed in order to develop general predictive models of mineralization (Powers 1990). Our data suggests that estimates of P mineralization in situ would be improved by accounting for soil moisture and soil temperature changes. These data would be useful to describe the response of P mineralization to moisture and temperature across a range of sandy soils, as they describe P mineralization in terms of water potential (which accounts for differences in physical properties such as soil texture and porosity), and of substrate quality (by expressing as specific mineralization). 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