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DIVISION S-2-SOIL CHEMISTRY

Phosphorus Fractions and Fate of Phosphorus-33 in Soils under Plowing and No-Tillage

^a Crop and Soil Sci. Dep., 286 Plant and Soil Sci. Bldg., Michigan State Univ., East Lansing, MI 48824 USA

daroub{at}pilot.msu.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Minimum tillage may alter soil P fractions through the application of P fertilizers and the deposition of organic matter on the surface rather than being incorporated into the soil. This study was conducted to determine whether no-tillage (NT) systems affected soil organic and inorganic P fractions and the transformation of P from residues applied to soils. Surface soils (0–2 cm) under NT and conventional tillage (CT) were sampled from three long-term research sites. Inorganic and organic P was measured in the NaHCO₃, microbial, NaOH, NaOH after sonication, HCl, and residual fractions extracted sequentially. Soybean (Glycine max L.) residues labeled with ³³P were added to soils, incubated, and extracted periodically, and ³³P was counted in the different P fractions. Levels of ³¹P in NT were higher in some of the fractions compared with CT; however, there was no consistency in ³¹P fractionation across soil types due to tillage in any of the inorganic and organic fractions. At the start of incubation, 56 to 82% of the applied ³³P was extracted in the resin fraction in the three soils. Resin-³³P followed a three-parameter single exponential decay model with a corresponding increase in other pools depending on soil. The increase in these pools followed a quadratic model in the three soils. By the end of the incubation period, the NaOH fraction accounted for the majority of the ³³P released from the labile resin pool. An increase in the calcium phosphate pool occurred in the calcareous soil. Tillage had no effect on the fate of ³³P released from soybean residues during the incubation period.

Abbreviations: CT, conventional tillage • KBS, Kellogg Biological Station • MSU, Michigan State University • NT, no-tillage • Pi, inorganic phosphorus • Po, organic phosphorus

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
CONSERVATION TILLAGE SYSTEMS, particularly no-tillage (NT) systems, are characterized by minimal disturbance to the soil and the management of crop residues at or near the soil surface. The levels of organic matter, microbial activity, and available P are, in general, higher in the surface layers of NT soils compared with conventional tillage (CT) soils (Shaer and Moschler, 1969; Ellis and Howse, 1980; Follett and Peterson, 1988; Weil et al., 1988; Tracy et al., 1990; Eckert, 1991; Karlen et al., 1991; Pierce et al., 1994). Accumulation of available P in the surface layers of NT soils can be attributed to the non-incorporation of broadcast P fertilizer and to the maintenance of a high level of surface crop residues that decompose at the soil surface (Robbins and Voss, 1991; Eckert, 1991). Follett and Peterson (1988) found that total and organic P (Po) were higher on NT soils at the 0- to 5-cm depth. Weil et al. (1988) showed little or no increase in Po levels in NT soils compared with CT soils, despite the increased levels of organic C and higher microbial activity in the NT soils.

The effect of tillage on organic and microbial P is still unclear. O'Halloran (1993) evaluated the effect of CT, NT, and reduced-tillage practices implemented for 7 yr on the distribution of P between inorganic and organic pools in a clay soil and a sandy loam soil under a continuous corn-production system. Tillage did not affect soil organic C or Po in either soil. None of the measured Po fractions were significantly affected by tillage in the clay soil; however, CT had a lower level of Po extracted by NaOH in the 10- to 20-cm layer compared with NT in the sandy loam soil. Since NT practices typically accumulate crop residues and organic C in surface soil, we hypothesize that a larger proportion of P will be in organic and microbial forms, resulting in an increased pool size of these forms and slower cycling of P. This immobilization of P in organic forms may protect P for the short term from being fixed into largely insoluble inorganic compounds. The slower cycling of P is hypothesized to be more synchronized with crop P demands. It is critical to understand P dynamics in any cropping system that attempts to reduce reliance on external P inputs by increasing reliance on organic sources of P. Sequential fractionation schemes for soil P coupled with isotope-tracer studies may reveal the effect of different management systems on P fractions and dynamics.

The P fractionation procedure developed by Hedley et al. (1982) and Tiessen et al. (1984) has been the most commonly used procedure in the last 10 yr. It extracts both inorganic P (Pi) and Po, extracting labile P first and ending with the more resistant forms of P. The Pi fractions extracted include resin, NaHCO₃, microbial, NaOH, NaOH after sonication, HCl, and residual. The Po fractions include NaHCO₃, microbial, NaOH, NaOH after sonication, and residual.

Resin-extractable Pi is considered to be the most biologically available to the plant (Amer et al., 1955; Sibbesen, 1977). Sodium bicarbonate at pH 8.5 extracts labile organic compounds like ribonucleic acid and glycerophosphate (Bowman and Cole, 1978). The NaHCO₃–extractable Pi and Po fractions constitute labile pools and are readily available to plants. Phosphorus released after fumigation with chloroform and extracted with NaHCO₃ can constitute up to 40% of the microbial P present in the soil (Hedley and Stewart, 1982). The NaOH-extractable Pi and Po fractions are moderately labile P and are P chemisorbed on Fe and Al oxides (Ryden et al., 1977). Sonication with NaOH extraction allows the release of physically protected Pi and Po (Hedley et al., 1982). Calcium phosphates (Ca–P) are mainly extracted with HCl. Residue P may contain both Pi and Po that is very resistant to decomposition. Although this fractionation does not define the components of each group, it will relate to the pools of rapid- and slow-cycling organic matter and allow the detection of changes in P-cycling within an intermediate time frame.

This study assessed how long-term CT and NT management alters P fractions and affects the kinetics of P transformation in soils. Laboratory incubation experiments were performed in which soils from three long-term NT and CT experimental sites were incubated with ³³P-labeled soybeans. We followed the transformation of the ³³P from soybean residues into the different P fractions in the soil using a sequential-fractionation procedure with slight modifications. The original soil samples without any added residues were also fractionated into the different inorganic and organic P fractions.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Soils
Surface soil from the 0- to 2-cm depth of CT and NT treatments of three long-term tillage studies in Michigan were sampled for use in P fractionation and ³³P incubation studies. Previous studies in Michigan have shown that the CT soil was homogenous over the plow-layer depth, and the 0- to 2-cm depth of NT soil contained the greatest changes in soil properties (Pierce et al., 1994; F.J. Pierce, unpublished data, 1999). The surface depth is therefore most relevant to study the effect of tillage on residue decomposition. One tillage study at East Lansing, MI, was established on a Capac loam (fine-loamy, mixed, mesic Aeric Endoaqualf) in 1980 on the Michigan State University (MSU) Research Farm (Pierce et al., 1994; Pierce and Fortin, 1997). A second tillage study at Hickory Corners, MI, was established on the Kalamazoo soil (fine-loamy, mixed, mesic Typic Hapludalf) in 1989 at the MSU Kellogg Biological Station (KBS) as part of the long-term ecological research experiments (Robertson et al., 1997). A third tillage study at Saginaw, MI, was established on a Misteguay soil (fine, mixed, calcareous, mesic Aeric Endoaquept) in 1986 at the MSU Saginaw Bean and Beet Experiment Station (Martinson, 1993). The CT treatment on the Capac and Misteguay soils consisted of fall plowing with a moldboard plow to a depth of 0.2 m, followed by spring tillage consisting of two passes of a soil finishing tool. On the Kalamazoo soil, CT consisted of spring moldboard plowing to a depth of 0.2 m followed by a secondary tillage of disking and field cultivating. Crop rotations at the East Lansing location consisted of continuous corn (Zea mays L.) from 1980 to 1988 and a corn–soybean rotation thereafter. Crop rotation at the KBS location consisted of corn–soybean rotation. At Saginaw, a corn–dry bean (Phaesolus vulgaris L.) rotation was in effect from 1986 to 1991, followed by a corn–soybean rotation thereafter. No P fertilizers were applied to the Misteguay soil and the Capac soil since 1988 or to the Kalamazoo soil since 1989, since soil P tests were high.

Soil samples were obtained for the CT and NT plots in May, October, and September of 1993 for the Capac, Kalamazoo, and Misteguay soils, respectively. Samples were obtained from the 0- to 2-cm depth from each of four treatment replications at each location, sieved while moist through a 2-mm sieve, and stored at 4°C. Soils were kept at room temperature for 2 wk before starting each incubation experiment to ensure microbial activity was at normal levels. A subsample of each soil was air-dried then analyzed in duplicate for pH (1:1 water), particle size by the pipette method (Gee and Bauder, 1986), Bray–Kurtz P1 (Bray and Kurtz, 1945), organic C by dry combustion using a LECO C analyzer (Leco, St. Joseph, MI) (Nelson and Sommers, 1982), and cation exchange capacity (Chapman, 1965). The carbonates in the Misteguay soil were removed by acidification before C analysis (Nelson and Sommers, 1982).

The characterization of P fractions in each soil was performed for each tillage treatment on each of four replications using a sequential-fractionation procedure modified from Hedley et al. (1982). Inorganic P fractions extracted were resin, NaHCO₃, microbial, NaOH, NaOH after sonication, and HCl. Organic P fractions included NaHCO₃, microbial, NaOH, and NaOH after sonication. Residual P in the soil was finally determined with no distinction made between Pi and Po. Since these soils were sieved moist through a 2-mm sieve, we modified the original procedure by using a 5-g soil sample for the resin, NaHCO₃, and the microbial fraction to minimize the variability for the microbial P extracted. After extracting microbial P, 1 g of soil was subsampled from the original 5-g sample and the sequential fractionation continued to obtain the remaining fractions. Inorganic P was analyzed in all fractions by the method of Murphy and Riley (1962) using an automated flow injection analyzer after adjusting the pH of the extracted solutions. Total P was measured in the NaHCO₃, NaOH, and NaOH with sonication extracts after digesting the samples with sulfuric acid and ammonium persulfate on a hot plate (USEPA, 1978) and analyzed with the same method as above. Organic P was obtained as the difference between total and inorganic P in these three fractions. Composite soil samples consisting of equal amounts of the four replications were used for the main incubation experiments with ³³P-labeled soybean residues.

Preparation of the Phosphorus-33–Labeled Soybean
Soybean seeds were germinated in sand flats that had been rinsed with 0.1 M HCl solution and distilled water. Three seedlings were transplanted into each of five 2-L pots containing a modified Hoagland nutrient solution (Hoagland and Arnon, 1950) with Fe–EDDHA as the source of Fe and a reduction in P concentration to 0.25 mM to ensure adequate absorption of ³³P. The nutrient solution was changed weekly. The plants were grown in a growth chamber set at a day temperature of 27°C, a night temperature of 21°C, and a day length of 16 h. After 2 wk, ³³P was added to the nutrient solution as orthophosphoric acid for a final activity of 4625 KBq pot^-1. Iron fertilizers were not added to the nutrient solution at this time out of concern for possible formation of an iron phosphate precipitate in the solution. Plants were sprayed instead with an FeSO₄ solution. Plants were grown for an additional 8 d and whole plants were harvested individually. Leaves, stems, and roots were separated and stems discarded. The roots were washed with a solution of ³¹P then rinsed with distilled water to remove any ³³P that adsorbed to the root surface. After drying, leaves and roots were ground to pass a 4-mm sieve. Triplicate 0.2-g samples of the plant tissue were digested with nitric and perchloric acids (Olsen and Sommers, 1982) and analyzed for P with the ascorbic acid–molybdate method after neutralization (Murphy and Riley, 1962) using an automated flow injection analyzer. The ³³P in the plant tissue was counted in a liquid scintillation counter with an open channel (0–2000 KeV) by adding 1 mL of sample to 10 mL of cocktail mix. All counts were corrected for background and decay.

Incubation Study
Labeled soybean residues consisting of 0.15-g leaves and 0.05-g roots were added to 100 g of field-moist soil in 250-mL glass jar and thoroughly mixed. Selected properties of the soils are given in Table 1 . The P concentration and ³³P activity of the soybean tissue are summarized in Table 2 . The partitioning of ³³P in the plant tissue was as follows: resin, 70%; NaHCO₃, 8.6%; NaOH, 14.3%; HCl, 1.9%; and H₂SO₄, 5.2%. Water was added to adjust the soil water content to field capacity. Soil–residue mixtures were prepared in triplicate for each soil–tillage combination and extraction date. A new batch of the ³³P–labeled soybean residues was prepared for each soil separately. The soil-residue mixture was incubated at 25°C for 6, 12, 18, 26, and 34 d. A subsample of soil was removed from each jar initially and at each date and subjected to the P fractionation procedure. No correction factor was employed for the microbial P calculation (Hedley and Stewart, 1982; Brookes et al., 1982).

Statistical Analysis
Statistical analysis for the ³¹P data was performed with a two-way analysis of variance for the effect of tillage in each soil on P fractions in units of concentration and as a percentage of total P. To test differences among soils, a combined analysis of all three soils required that the variances were homogeneous. Therefore, the percentage values were subjected to the Fmax test for homogeneity of variances (Kuehl, 1994). The Fmax tests showed that the variances were not homogeneous precluding the analysis of variance for combined soils. The ³³P kinetics data were modeled using a three-parameter single exponential decay curve [ ] for the resin–P fraction data and a quadratic curve ( ) for the NaHCO₃, microbial, NaOH, NaOH after sonication, HCl, and residual fractions using the regression and curve-fitting routines in SigmaPlot. The effect of tillage and soil type on the nonlinear fitted models for the resin–P data was tested with the sum-of-squares reduction test using PROC NLIN in SAS (O. Schabenberger, personal communication, 1999). For the remaining fractions, the fitted models were subjected to an F test to compare the quadratic linear regression equations using PROC GLM in SAS (SAS, 1998).

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
P Pools
The extracted P fractions do not exactly correspond to soil P pools. However, because it is more meaningful to relate the P fractions to P pools in soils, we assigned extracted P fractions to P pools described in the literature (Table 3) . For the ³³P kinetic data, P fractions were assigned to P pools whenever possible, provided that no distinction can be made between inorganic and organic ³³P.

Total P in the Misteguay soil averaged 913 mg P kg^-1 soil and was 1.5 and 2.5 times higher than in the Capac and Kalamazoo soils, respectively (Table 3). To compare P fractions among soils across tillage systems, we calculated P fractions as a percentage of total P with the intention of using a combined analysis of variance across soils. However, a combined analysis requires that the variances among soils be homogeneous. The F_max test for homogeneity of variances showed that the variances among soils were not homogeneous precluding the analysis of variance for combined soils (Kuehl, 1994). In general, the chemistry of P in the high pH Misteguay soil was dominated by the Ca–P fraction, accounting for 42% of the total P compared with 20 and 15% for the Capac and Kalamazoo, respectively. The size of the Ca–P pool in the Misteguay soil is similar to what is reported in the literature for calcareous soils (Yang and Jacobsen, 1990). Correspondingly, the Fe–P and Al–P pools of the Misteguay were lower than the Capac and Kalamazoo soils, again related to the high pH. The dominance of the Ca–P fraction in the Misteguay soil reduced the Po fractions compared with the other soils, even though organic C contents (Table 1) were intermediate to the Capac and Kalamazoo soils. In general, the fractionation of P was more similar in the Capac and Kalamazoo soils than the Misteguay.

Phosphorus Transformations from Phosphorus-33–Labeled Soybean Plants
The incubation curves for the ³³P-labeled soybean residues are plotted for both tillage systems for each soil in Fig. 1 . Statistical tests of the fitted models for tillage effects showed that no differences existed between tillage systems for any of the P fractions in any soil. We conclude, therefore, that tillage had no effect on the fate of ³³P released from soybean residues during the incubation period. The fitted models did vary by soil, depending on the P pool (Tables 4 and 5) . Therefore, our discussion of the fate of ³³P-labeled soybean residue will focus only on soil effects.

View larger version (27K): [in this window] [in a new window]   Fig. 1 Percent recovery of 33P with time in the resin (a–c), NaHCO3 (d–f), microbial (g–i), NaOH (j–l), NaOH after sonication (m–o), HCl (p–r), and residual (s–u) fractions after incubating with 33P–labeled soybean plants in the Capac, Kalamazoo, and Misteguay soils under no-tillage (NT) and conventional tillage (CT). Solid lines represent regression equations described in Table 4

The decline in resin–³³P corresponded with an increase in other P pools, depending on the soil (Fig. 1, Tables 4 and 5). The NaHCO₃–³³P pool followed a quadratic model, increasing to a maximum by about 18 d but declining thereafter. The model differed for all soils, with the Capac soil showing the largest pool of NaHCO₃–³³P and the Misteguay showing little activity in this pool. A quadratic model also fit the microbial–³³P pools with each soil having a different fit to the model but similar recoveries of ³³P. Like the NaHCO₃–³³P pool, the microbial–³³P pool maximum occurred about Day 18 and then declined.

The NaOH–³³P fraction corresponds best to the Fe–P and Al–P and moderately labile Po pools and was an important fraction for all three soils. This fraction continued to increase during the incubation period for the Capac soil but appeared to have reached a maximum in the Kalamazoo and Misteguay soils. By the end of the incubation, this fraction accounted for the majority of ³³P released from the labile resin–³³P pool. The transformation of ³³P into the NaOH fraction of the Kalamazoo was rapid with most of the ³³P going into that fraction by Day 6. This rapid increase is due to either adsorption of the ³³P onto Fe and Al oxides–hydroxides and organic matter or to precipitation of P compounds. Phosphorus is precipitated as Al–P and Fe–P in this pH range and the reaction may be rapid in a matter of days to weeks. The difference between the Capac and the Kalamazoo soils could be due to the differences in the organic C content, which is higher in the Capac soil. The microbial population seems to exert a greater influence on P transformations in the Capac soil, keeping more of the P in the labile pools (resin and NaHCO₃) at least for the first 18 d of incubation compared with the Kalamazoo soil. We hypothesize that part of the ³³P in the NaOH fraction is organic, in particular in the Capac soil, since the decrease of ³³P in the NaHCO₃ and microbial fractions after Day 18 corresponded to an increase in the NaOH fraction.

The physically protected pool was minimal for the three soils; the quadratic model fit only the Kalamazoo soil well; and the model fit did not differ by soil. Therefore, this was not an important pool for ³³P during this incubation. The Ca–³³P pool was important for the Misteguay soil and appeared to reach a maximum by the end of the incubation. This pool would be expected to be important to the Misteguay, given this soil's high pH and large ³¹P pool size (Table 3). This pool was minimally important to the Capac and Kalamazoo soils although the model fit was different for these two soils (Table 5). The higher magnitude of transformation of ³³P into the Ca–P in the Misteguay soil, in contrast to the other two soils, is attributed to the competition between the CaCO₃ and Fe and Al oxides and hydroxides for the P released from decomposition of plant residues. More of the ³³P ends up as Ca–P in the Misteguay soil, compared with the other two soils where less Ca is available to fix the released P. The percentage of ³³P going into the NaOH fraction in the Misteguay soil was less, however, than that found in both the Capac and Kalamazoo soils. The residual–³³P pool was small for the Capac soil but appeared to increase as the incubation proceeded. This pool reached a maximum in the Kalamazoo and Misteguay soils and was similar in size to the NaHCO₃ and microbial–³³P pools in these soils.

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
There were few indications from this study that tillage is greatly affecting soil P fractions. Where the few differences occurred, they were inconsistent among soils. Furthermore, tillage had no effect on the fate of ³³P added to soil in soybean residues. While it is commonly reported that conservation tillage systems, particularly NT, stratify P, this study indicates that tillage does not affect the fractionation or fate of added P in any major way. Therefore, there are no special soil sampling or sample preparation requirements when comparing P in tillage systems.

Overall, resin–³³P was released early in the incubation, indicating a rapid release of a significant portion of P in soybean residues soon after they reach the soil. Any P remaining in soybean residues after this will release slowly. By comparing Fig. 1 with Table 3, it appears that the pools that were important in the ³¹P fractionation were also important to the fate of ³³P. For example, P extracted with NaOH was an important pool for all three soils for both ³¹P and ³³P. Furthermore, the NaHCO₃ fraction was more important for the Capac while the Ca–P was most important for the Misteguay soil. Of the P released from the soybean residues, some moved into the more available pools but these pools generally declined over time. By the end of the incubation, most of the ³³P was found in the NaOH pools described here as consisting of Fe–P, Al–P, and moderately labile Po, with only a small portion moving into the residual pool.

	ACKNOWLEDGMENTS

 
We greatly acknowledge the statistical help provided by Dr. Oliver Schabenberger. Support for this project was provided in part by the National Science Foundation through the KBS LTER project (NSF award 92-1177) and by the Michigan Agricultural Experiment Station.

Received for publication December 1, 1998.

	REFERENCES

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