Published online 11 April 2005
Published in Soil Sci Soc Am J 69:630-633 (2005)
DOI: 10.2136/sssaj2004.0295
© 2005 Soil Science Society of America
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Soil Chemistry
Storage-Induced Changes in Phosphorus Solubility of Air-Dried Soils
Benjamin L. Turner*
Smithsonian Tropical Research Institute, Box 2072, Balboa, Ancon, Republic of Panama
* Corresponding author (turnerbl{at}si.edu)
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ABSTRACT
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Soil is commonly stored in an air-dried state for extended periods before chemical analysis. The effect of storage on P solubility was assessed by determining NaHCO3–extractable P concentrations in a range of pasture soils from England and Wales (total C = 28.9–80.4 g kg–1, clay = 219–681 g kg–1, pH = 4.4–6.8) immediately following air drying and after 3 yr of storage at ambient laboratory temperature. Following storage, NaHCO3–extractable inorganic P concentrations decreased by between 2 and 60% (mean decrease = 21%), while NaHCO3–extractable organic P concentrations increased by between 48 and 156% (mean increase = 95%). The greatest changes occurred in soils of pH < 5.3. The changes appear to result from the disruption of organic matter coatings on mineral surfaces, continuous solid-phase diffusion of phosphate into soil particles, and decomposition of microbial cells. The results have important implications for the determination of NaHCO3–extractable P in stored soils and highlight the importance of working with fresh samples to derive information with relevance to field conditions.
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INTRODUCTION
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PHOSPHORUS EXTRACTION in NaHCO3 is used as an agronomic test of plant-available P (Olsen et al., 1954) and an environmental test to predict the potential risk of P loss in runoff (Heckrath et al., 1995; Turner et al., 2004). It is also used to estimate labile pools of inorganic and organic P in sequential extraction schemes (Bowman and Cole, 1978; Hedley et al., 1982). Soil is commonly air-dried before NaHCO3 extraction, but this can increase both inorganic and organic P concentrations compared with the equivalent fresh soil (Sparling et al., 1985; Turner and Haygarth, 2003). The mechanisms involved are unclear, but include the disruption of organic matter structures and the lysis of microbial cells.
Dried soil is assumed to be chemically stable and may be stored for long periods before extraction and analysis. However, substantial changes can occur during storage (Bartlett and James, 1980). In an Australian loamy sand stored at temperatures between 5 and 61°C, solid-phase diffusive penetration of phosphate into soil particles continued for >100 d, although the reaction was virtually halted by freezing (Bramley et al., 1992). There is no comparable information on changes in organic P during storage, although organic matter solubility increased during the storage of dry tropical soils (Birch, 1959).
Given the widespread use of NaHCO3 extraction in agronomic and environmental assessment of soil P, artifacts induced by storage are of potential importance. This was assessed by determining NaHCO3–extractable inorganic and organic P in a range of temperate pasture soils extracted immediately after air drying and following air-dry storage for 3 yr at ambient laboratory temperature. The extracted organic P was characterized further by phosphatase hydrolysis to provide information on the sources of organic P contributing to the observed changes.
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MATERIALS AND METHODS
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Soils under lowland permanent pasture were sampled to 10-cm depth during October 1998 from sites around England and Wales, selected to give a wide range of physical and chemical properties. Some properties of the soils are presented in Table 1. Total C (28.9–80.4 g C kg–1 soil) and N (2.85–8.70 g N kg–1 soil) were determined simultaneously using a Carlo-Erba NA2000 analyzer (Carlo-Erba, Milan, Italy). Total P (0.57–1.98 g P kg–1 soil) was determined by ignition and H2SO4 extraction (Saunders and Williams, 1955). Clay content (219–681 g kg–1 soil) was determined by wet sieving followed by analysis using a Micromeritics Sedigraph 5100 with a Micromeritics Mastertech 51 automatic sampler. Soil pH (4.4–6.8) was determined in a 1:2.5 soil-to-deionized water ratio. Microbial P (31–239 mg P kg–1 soil) was determined on fresh soil by CHCl3 fumigation and NaHCO3 extraction (Brookes et al., 1982).
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Table 1. Physical and chemical properties of 15 permanent pasture soils from England and Wales. The soils are ranked in order of their total C concentrations.
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Each soil was sieved (4 mm) and left to equilibrate for 1 wk at 10–15°C. Subsamples were then dried on shallow metal trays for 7 d at 30°C and extracted within 1 wk of drying. The soils were then stored air-dried in sealed plastic bags at approximately 22°C for 3 yr before reextraction. In both cases, the soils were extracted in 0.5 M NaHCO3 using a standard procedure and analyzed for phosphate by molybdate colorimetry (Olsen et al., 1954). Total P in the extracts was determined by colorimetry following acid-persulfate digestion with modifications for NaHCO3 extracts (Turner and Haygarth, 2003). Organic P was estimated as the difference between total P and phosphate. Each soil was extracted three times. Phosphate is termed inorganic P for simplicity, although some inorganic pyrophosphate is present in these soils (Turner et al., 2003b) and may have been included in the organic P fraction.
NaHCO3–extractable organic P was characterized by phosphatase hydrolysis following a method modified for NaHCO3 extracts (Turner et al., 2003a). The hydrolyzable organic P was separated into labile monoester P (hydrolyzed by alkaline phosphatase), phospholipids (the difference between organic P hydrolyzed by phospholipase C + alkaline phosphatase and alkaline phosphatase alone), and nucleic acids (the difference between organic P hydrolyzed by phosphodiesterase + alkaline phosphatase and alkaline phosphatase alone). Phytase was not included in the current study due to its inhibition in some extracts.
Concentrations are expressed on the basis of oven-dried soil (105°C). A correlation matrix (r values) was calculated to investigate relationships between soil properties and P fractions. Means differences were determined by ANOVA. All analyses were performed using standard procedures in Microsoft Excel.
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RESULTS
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NaHCO3–extractable P concentrations changed markedly following storage, although the changes were quantitatively and proportionally greater for organic P than for inorganic P (Table 2). Inorganic P concentrations decreased by between 0.2 and 11.4 mg P kg–1 soil (mean 4.7 mg P kg–1 soil), equivalent to proportional decreases of between 2 and 60% (mean 21%) (Table 2). In contrast, organic P concentrations increased by between 9.9 and 49.9 mg P kg–1 soil (mean 26.9 mg P kg–1 soil), equivalent to proportional increases of between 48 and 156% (mean 95%) (Table 2).
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Table 2. Concentrations of NaHCO3–extractable P fractions in a range of permanent pasture soils extracted immediately after air drying at 30°C for 7 d and after air-dry storage for 3 yr at ambient laboratory temperature. Values are means of three replicate extracts with SE < 5%.
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The decreases in inorganic P were greatest, and only statistically significant, in soils with pH < 5.3 (Fig. 1). Increases in organic P were greatest in soils with pH < 5.3 (Fig. 1), even though the differences were statistically significant for all soils. The more acidic soils tended to contain high initial concentrations of NaHCO3–extractable organic P (Table 2). Correlation coefficients (r values) between soil pH and the increase in NaHCO3–extractable P following storage were 0.73 (P < 0.01) for inorganic P and –0.67 (P < 0.01) for organic P. The increases in organic P were also positively correlated with microbial P (r = 0.67; P < 0.01), while all other correlations with physical and chemical soil properties were not significant (P > 0.05).
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Fig. 1. Scatter plots of the relationships between the changes in NaHCO3–extractable inorganic and organic P fractions after air-dry storage for 3 yr at ambient laboratory temperature and the soil pH (determined in a soil/water ratio of 1:2.5). Note the different scales on the Y axes.
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Concentrations of organic P hydrolyzed by phosphatase enzymes ranged between 6.7 and 27.1 mg P kg–1 soil, equivalent to between 22 and 39% of the extracted organic P (Table 3). Of this, between 4.4 and 20.7 mg P kg–1 soil was labile monoesters (17–30% extracted organic P) and between 1.3 and 7.2 mg P kg–1 soil was nucleic acids (3–10% extracted organic P). Phospholipids were detected in only small quantities (<2.6 mg P kg–1 soil and <5% extracted organic P). Concentrations of total hydrolyzed P and labile monoesters were negatively correlated with soil pH (P < 0.01) and positively correlated with microbial P (P < 0.01).
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Table 3. Concentrations of phosphatase hydrolyzable P fractions in NaHCO3 extracts of 15 permanent pasture soils from England and Wales extracted after air-dry storage for 3 yr at ambient laboratory temperature. Values are mean ± SE of three replicate extracts. Values in parentheses are the proportion (%) of the NaHCO3 organic P.
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DISCUSSION
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The marked changes in soil P solubility following 3 yr of air-dry storage occurred in addition to the changes induced by the initial drying of fresh samples (Turner and Haygarth, 2003). Analysis of fresh soils is therefore critical to obtain information with relevance to field conditions. This has important implications for the determination of NaHCO3–extractable P in archived soils or samples stored from long-term field trials because it cannot be assumed that air drying will prevent further changes in P solubility. In this respect, a previous recommendation that stored soils should be frozen to prevent changes in inorganic P solubility (Bramley et al., 1992) seems appropriate. Freezing can increase the solubility of organic P in fresh soils (Ron Vaz et al., 1994), but is probably acceptable for air-dried samples.
Changes in chemical properties of stored soils are well known (Bartlett and James, 1980), although the mechanisms involved remain unclear. Soil drying disrupts organic matter coatings on mineral surfaces, which is exacerbated during storage. The effect is analogous to an elastic band (Bartlett and James, 1980) which remains sound when relaxed (analogous to organic matter in moist soil), but eventually fails if held continuously or too long under tension (analogous to organic matter in dry soil). This process almost certainly explains observed increases in organic matter solubility during storage of dried tropical soils (Birch, 1959) and the changes in organic P solubility in the pasture soils analyzed here. It is likely that decomposition of microbial cells also contributed to the observed changes because fresh soils contained large microbial biomass concentrations. Many cells survive initial air drying (Sparling et al., 1985), but would be susceptible to prolonged storage.
The decreases in inorganic P are probably due largely to continuous solid-phase penetration of sorbed phosphate into soil particles (Bramley et al., 1992), or diffusion into the interior of small aggregates. However, the transformation of phosphate minerals to more crystallized (lower energy) states may also have contributed (Haynes and Swift, 1985). Such processes appear to depend in part on ambient humidity, as demonstrated by changes in phosphate solubility during storage of air-dried calcareous soils (Castro and Torrent, 1993). This effect could not be assessed for the samples analyzed here because humidity was not determined during storage.
Disruption of organic matter coatings would be expected to further increase P sorption by exposing fresh mineral surfaces (Haynes and Swift, 1985). Both inorganic and organic P would be affected, although sorption of organic P would depend in part on its composition. For example, extracts of stored soils in the current study contained large proportions of phosphate diesters and simple phosphate monoesters that sorb weakly in soil (Frossard et al., 1989). In contrast, the presence of phytic acid (not determined here) would have indicated the potential for strong sorption to freshly exposed surfaces (Celi and Barbaris, 2004).
The relatively large proportions of labile monoesters and phosphate diesters measured here in NaHCO3 extracts are similar to the values (6–49% extracted organic P) reported for extracts of semiarid arable soils from the western USA (Turner et al., 2003a), but are in direct contrast to the small proportions detected in air-dried arable soils from Australia and Japan (Otani and Ae, 1999; Hayes et al., 2000). The western U.S. soils were stored dry for 2 yr before analysis, so it seems possible that storage increases the susceptibility of NaHCO3–extractable organic P to enzymatic hydrolysis, perhaps through degradation of high molecular weight organic structures. Storage time may therefore be an important consideration when assessing the potential bioavailability of soluble organic P using phosphatase hydrolysis.
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CONCLUSIONS
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Storage of air-dried pasture soils for 3 yr at ambient laboratory temperature caused marked changes in NaHCO3–extractable P concentrations. These changes were in addition to the increases in extractable P that followed initial drying of fresh soils. The results have profound implications for the determination of NaHCO3–extractable P in dried, stored soils. Air-dried soils should be frozen for prolonged storage to minimize changes in P solubility, whereas fresh soils should be analyzed to obtain results with relevance to field conditions.
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ACKNOWLEDGMENTS
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I thank Dr. Dale Westermann (USDA–ARS, Kimberly) for comments on the manuscript.
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NOTES
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Research was conducted in the Soil and Water Science Dep., Univ. of Florida. Florida Exp. Stn. journal series number R-10445.
Received for publication September 1, 2004.
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REFERENCES
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- Celi, L., and E. Barbaris. 2004. Abiotic stabilization of organic phosphorus in the environment. p. 113–132. In B.L. Turner et al. (ed.) Organic phosphorus in the environment. CAB International, Wallingford, UK.
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ABSTRACT
INTRODUCTION
