Storage-Induced Changes in Phosphorus Solubility of Air-Dried Soils

This was assessed by determining NaHCO3–extractable inorganic and organic P in a range of temperate pasture Soil is commonly stored in an air-dried state for extended periods soils extracted immediately after air drying and followbefore chemical analysis. The effect of storage on P solubility was assessed ing air-dry storage for 3 yr at ambient laboratory temperaby determining NaHCO3–extractable P concentrations in a range of pasture soils from England and Wales (total C 28.9–80.4 g kg 1, ture. The extracted organic P was characterized further clay 219–681 g kg 1, pH 4.4–6.8) immediately following air drying by phosphatase hydrolysis to provide information on the and after 3 yr of storage at ambient laboratory temperature. Following sources of organic P contributing to the observed changes. storage, NaHCO3–extractable inorganic P concentrations decreased by between 2 and 60% (mean decrease 21%), while NaHCO3– MATERIALS AND METHODS extractable organic P concentrations increased by between 48 and Soils under lowland permanent pasture were sampled to 156% (mean increase 95%). The greatest changes occurred in soils 10-cm depth during October 1998 from sites around England of pH 5.3. The changes appear to result from the disruption of and Wales, selected to give a wide range of physical and organic matter coatings on mineral surfaces, continuous solid-phase chemical properties. Some properties of the soils are presented diffusion of phosphate into soil particles, and decomposition of microin Table 1. Total C (28.9–80.4 g C kg 1 soil) and N (2.85–8.70 g bial cells. The results have important implications for the determinaN kg 1 soil) were determined simultaneously using a Carlotion of NaHCO3–extractable P in stored soils and highlight the imErba NA2000 analyzer (Carlo-Erba, Milan, Italy). Total P portance of working with fresh samples to derive information with (0.57–1.98 g P kg 1 soil) was determined by ignition and H2SO4 relevance to field conditions. 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 MicroP extraction in NaHCO3 is used as an meritics Mastertech 51 automatic sampler. Soil pH (4.4–6.8) was determined in a 1:2.5 soil-to-deionized water ratio. Microagronomic test of plant-available P (Olsen et al., bial P (31–239 mg P kg 1 soil) was determined on fresh soil 1954) and an environmental test to predict the potential by CHCl3 fumigation and NaHCO3 extraction (Brookes et risk of P loss in runoff (Heckrath et al., 1995; Turner al., 1982). et al., 2004). It is also used to estimate labile pools of Each soil was sieved (4 mm) and left to equilibrate for 1 wk inorganic and organic P in sequential extraction schemes at 10–15 C. Subsamples were then dried on shallow metal (Bowman and Cole, 1978; Hedley et al., 1982). Soil is trays for 7 d at 30 C and extracted within 1 wk of drying. commonly air-dried before NaHCO3 extraction, but this The soils were then stored air-dried in sealed plastic bags at can increase both inorganic and organic P concentraapproximately 22 C for 3 yr before reextraction. In both cases, the soils were extracted in 0.5 M NaHCO3 using a standard tions compared with the equivalent fresh soil (Sparling procedure and analyzed for phosphate by molybdate colorimeet al., 1985; Turner and Haygarth, 2003). The mechatry (Olsen et al., 1954). Total P in the extracts was determined nisms involved are unclear, but include the disruption of by colorimetry following acid-persulfate digestion with modifiorganic matter structures and the lysis of microbial cells. cations for NaHCO3 extracts (Turner and Haygarth, 2003). Dried soil is assumed to be chemically stable and may Organic P was estimated as the difference between total P be stored for long periods before extraction and analysis. and phosphate. Each soil was extracted three times. Phosphate However, substantial changes can occur during storage is termed inorganic P for simplicity, although some inorganic (Bartlett and James, 1980). In an Australian loamy sand pyrophosphate is present in these soils (Turner et al., 2003b) stored at temperatures between 5 and 61 C, solid-phase and may have been included in the organic P fraction. NaHCO3–extractable organic P was characterized by phosdiffusive penetration of phosphate into soil particles phatase hydrolysis following a method modified for NaHCO3 continued for 100 d, although the reaction was virtuextracts (Turner et al., 2003a). The hydrolyzable organic P ally halted by freezing (Bramley et al., 1992). There is no was separated into labile monoester P (hydrolyzed by alkaline comparable information on changes in organic P during phosphatase), phospholipids (the difference between organic storage, although organic matter solubility increased durP hydrolyzed by phospholipase C alkaline phosphatase and ing the storage of dry tropical soils (Birch, 1959). alkaline phosphatase alone), and nucleic acids (the difference Given the widespread use of NaHCO3 extraction in between organic P hydrolyzed by phosphodiesterase alkaline agronomic and environmental assessment of soil P, artiphosphatase and alkaline phosphatase alone). Phytase was not included in the current study due to its inhibition in some extracts. facts induced by storage are of potential importance. Concentrations are expressed on the basis of oven-dried soil (105 C). A correlation matrix (r values) was calculated to Smithsonian Tropical Research Institute, Box 2072, Balboa, Ancon, investigate relationships between soil properties and P fracRepublic of Panama. Research was conducted in the Soil and Water tions. Means differences were determined by ANOVA. All Science Dep., Univ. of Florida. Florida Exp. Stn. journal series number analyses were performed using standard procedures in MicroR-10445. Received 1 Sept. 2004. Soil Chemistry. *Corresponding ausoft Excel. thor ([email protected]). Published in Soil Sci. Soc. Am. J. 69:630–633 (2005). RESULTS doi:10.2136/sssaj2004.0295 NaHCO3–extractable P concentrations changed mark© Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA edly following storage, although the changes were quan630 Published online April 11, 2005