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DIVISION S-8—NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Changes in Bicarbonate-extractable Inorganic and Organic Phosphorus by Drying Pasture Soils

Institute of Grassland and Environmental Research, North Wyke Research Station, Okehampton, Devon EX20 2SB, UK

* Corresponding author (bturner{at}nwisrl.ars.usda.gov)

	ABSTRACT

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Soils are commonly dried in the laboratory prior to the determination of P fractions, but this can profoundly influence the results. We investigated the impact of soil drying on bicarbonate-extractable inorganic and organic P in 29 permanent lowland pasture soils from England and Wales (total C 29–80 g C kg^-1 soil, clay 219–681 g kg^-1 soil, pH 4.4–6.8) by extracting soils at approximate field moisture capacity and after air-drying at 30°C for 7 d. Air-drying increased the mean bicarbonate-extractable inorganic P from 14.8 to 22.5 mg P kg^-1 soil, and the mean bicarbonate-extractable organic P from 17.4 to 25.7 mg P kg^-1 soil. Proportional increases for individual soils following drying were between 11 and 165% for inorganic P, and between -2 and 137% for organic P, being greatest in soils with low P concentrations. The results are unlikely to influence tests for plant-available P, because these are derived from analyses of air-dried samples, but have important implications for attempts to relate bicarbonate-extractable P fractions to processes operating under field conditions.

	INTRODUCTION

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BICARBONATE-EXTRACTION is amongst the most common soil-test P methods. Typically representing between 1 and 5% of the total soil P, this operationally defined extraction technique was originally developed for the estimation of plant-available soil P (Olsen et al., 1954), and is commonly used as an index of soil P status to determine fertilizer P requirements (Kamprath and Watson, 1980). It has proved a versatile extract being used to determine soil microbial P (Brookes et al., 1982), and recently to investigate the link between soil P and P transfer in runoff (Heckrath et al., 1995; Pote et al., 1996; Leinweber et al., 1999). Conventional procedures involve only the determination of inorganic orthophosphate in the bicarbonate extracts, but organic P compounds are also extracted and can be determined following a suitable digestion procedure. There is little information on the chemical composition of bicarbonate-extractable organic P. It may represent an active soil organic P fraction (Bowman and Cole, 1978; Dick and Tabatabai, 1978), although in some soils only small amounts are hydrolyzable by phosphatase enzymes (Otani and Ae, 1999; Hayes et al., 2000).

We recently reported that soil drying causes significant changes in water-extractable P, in particular organic P, and that these changes appeared to be related to direct P release from lysed microbial cells (Turner and Haygarth, 2001; Turner et al., 2002). Similar changes were reported for inorganic P in bicarbonate extracts of New Zealand pasture soils (Sparling et al., 1985), and resin extracts of a range of mainly low organic matter European soils (Olsen and Court, 1982). However, no information is available on changes in bicarbonate-extractable organic P induced by soil drying. The objectives of this work were to determine how soil drying affected the concentrations of bicarbonate-extractable inorganic and organic P in a wide range of soils under permanent pasture, and to investigate how these changes related to soils properties.

	MATERIALS AND METHODS

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Soil Sampling and Preparation
Twenty-nine lowland soils under permanent pasture were sampled during October 1998 from sites around England and Wales, selected from the National Soil Inventory database (National Soil Resources Institute, Cranfield University, UK) to give a wide range of organic matter, bicarbonate-extractable P, and textural characteristics. Soil blocks were taken to 10 cm from each site and transported to the laboratory in plastic trays. Soil moisture contents were standardized by saturating each turf with water and allowing drainage under cover for 48 h at ambient temperature. This moisture content, defined here as field moisture capacity, was determined by measuring the gravimetric water content of duplicate soil cores (dried at 105°C overnight). Each soil was sieved (<4 mm) to remove large roots, stones and macrofauna, then left to equilibrate for 1 wk at 10 to 15°C. These were designated as "moist" soils and were stored at 4°C for no more than 7 d prior to bicarbonate extraction. Subsamples were dried on shallow metal trays for 7 d at 30°C and were designated as "dry" soils.

Determination of Bicarbonate- Extractable Phosphorus
Bicarbonate-extractable P was determined in moist and dry soils by extracting triplicate samples (2.50 g on an oven-dry soil basis) for 30 min with 50 mL of 0.5 M NaHCO₃ (adjusted to pH 8.5 with NaOH) on an end-over-end shaker (Olsen et al., 1954). The exact volume of extractant was adjusted to account for soil moisture content to ensure equivalent solution/soil ratios for moist and dry soils. The extracts were filtered through Whatman No. 42 filter papers (Whatman Ltd, Clifton, NJ), and analyzed for inorganic P by acid-molybdate reaction (Murphy and Riley, 1962) following acidification with dilute H₂SO₄ to remove carbonates. Total P in the extracts was determined by the same procedure following acid-persulphate digestion (Rowland and Haygarth, 1997). Organic P was calculated as the difference between total P and inorganic P. Inorganic P in bicarbonate extracts determined by molybdate-reaction includes only inorganic orthophosphate (Coventry et al., 2001), although this does not preclude some hydrolysis of acid-labile condensed and organic P compounds during pretreatment. In both procedures, the absorbance was read at 880 nm against a calibration from standards prepared in 0.5 M NaHCO₃. Values were corrected for blanks (no soil) and are expressed on the basis of oven-dry weight of soil.

Determination of Soil Properties
Total soil C and N were measured simultaneously using a Carlo-Erba NA2000 analyzer (Carlo-Erba, Milan, Italy). Total soil P fractions were determined by the ignition method with extraction in 0.5 M H₂SO₄ (Saunders and Williams, 1955). Microbial P was determined by chloroform fumigation and bicarbonate extraction (Brookes et al., 1982). Soil texture was determined by wet sieving followed by analysis using a Micromeritics Sedigraph 5100 with a Micromeritics Mastertech 51 automatic sampler (Micromeritics, Norcross, GA). Soil pH was determined in a 1:2.5 soil/deionized water ratio. Oxalate-extractable Fe and Al were determined by extraction with ammonium oxalate-oxalic acid for 2 h in the dark, with detection using inductively coupled plasma atomic emission spectrometry (ICP–AES) (Schoumans, 2000).

Some physical and chemical properties of the soils are presented in Table 1. Total C contents ranged from 28.9 to 80.4 g C kg^-1 soil, total N from 2.85 to 8.70 g N kg^-1 soil, total P from 376 to 1981 mg P kg^-1 soil, clay from 219 to 681 g kg^-1 soil, and pH from 4.4 to 6.8. Microbial P concentrations ranged between 31 and 239 mg P kg^-1 soil (Turner et al., 2001).

	RESULTS

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Concentrations of bicarbonate-extractable inorganic and organic P were similar for individual soils, although on average there was slightly more organic P in extracts of both moist and dry soils, and more organic P was released following soil drying (Table 2). Bicarbonate-extractable inorganic P increased from between 5.8 and 42.8 mg P kg^-1 soil (mean 14.8 mg P kg^-1 soil) in moist soils, to between 9.1 and 47.6 mg P kg^-1 soil (mean 22.5 mg P kg^-1 soil) in dry soils. This represented increases after drying of between 2.5 and 21.4 mg P kg^-1 soil (mean 7.7 mg P kg^-1 soil), equivalent to proportional increases of between 11 and 165% (mean increase of 66%). Bicarbonate-extractable organic P increased from between 5.6 and 39.7 mg P kg^-1 soil (mean 17.4 mg P kg^-1 soil) in moist soils, to between 10.8 and 45.9 mg P kg^-1 soil (mean 25.7 mg P kg^-1 soil) in dry soils. This represented a decrease in bicarbonate-extractable organic P for one soil (0.4 mg P kg^-1 soil), but increases of up to 137% for the other soils (mean increase of 59%) (Table 2). The proportions of inorganic and organic P in bicarbonate extracts of individual soils changed little following soil drying. Thus, the proportion of inorganic P varied from between 18 and 74% (mean 45%) in moist soils, to between 29 and 71% (mean 46%) in dry soils, while the proportion of organic P varied from between 26 and 82% (mean 55%) in moist soils, to between 29 and 71% (mean 54%) in dry soils.

Relationships Between Phosphorus Fractions and Soil Properties
There were strong correlations between bicarbonate-extractable P concentrations in moist and dry soils, suggesting a consistent and proportional response to soil drying (Table 3). Bicarbonate-extractable inorganic P was strongly positively correlated with total soil inorganic P, and negatively correlated with clay content. Bicarbonate-extractable organic P was strongly negatively correlated with soil pH (Fig. 1) , and positively correlated with total soil organic P (P < 0.05), and microbial P (P < 0.01).

View larger version (21K): [in this window] [in a new window]   Fig. 1. The relationship between bicarbonate-extractable organic P (mg P kg-1 soil) and soil pH for 29 permanent lowland pasture soils from England and Wales extracted at approximate field-capacity moisture content and after air-drying at 30°C for 7 d. The curves describe the following models: [Bicarbonate organic P from moist soil] = 9023[soil pH]-3.82, R2 = 0.49, P < 0.0001; and [Bicarbonate organic P from dry soil] = 5034[soil pH]-3.22, R2 = 0.60, P < 0.0001.

View larger version (16K): [in this window] [in a new window]   Fig. 2. The relationship between oxalate-extractable Al (g Al kg-1 soil) and the increase in bicarbonate-extractable inorganic P (mg P kg-1 soil) following air-drying at 30°C for 7 d in 29 permanent lowland pasture soils from England and Wales.

View larger version (15K): [in this window] [in a new window]   Fig. 3. The relationships between the proportional increase (%) in (top) bicarbonate-extractable inorganic P and (bottom) bicarbonate-extractable organic P following air-drying at 30°C for 7 d and the initial concentrations (mg P kg-1 soil) in 29 permanent lowland pasture soils from England and Wales.

	DISCUSSION

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The changes in bicarbonate-extractable P following soil drying revealed a similar trend to those reported previously for extraction with water, resin, and bicarbonate (Olsen and Court, 1982; Sparling et al., 1985; Turner and Haygarth, 2001). The increases in bicarbonate-extractable total P reported here (5.2–26.9 mg P kg^-1 soil) are several times greater than those reported for water-extracts of the same permanent pasture soils from England and Wales (1.1–7.6 mg P kg^-1 soil) and five Australian pasture soils (0.42–1.76 mg P kg^-1 soil) (Turner and Haygarth, 2001; Turner et al., 2002). Bicarbonate-extractable inorganic P in a range of New Zealand pasture soils increased by 2.7 to 21.7 mg P kg^-1 soil following air-drying, although concentrations decreased by 7.0 and 10.7 mg P kg^-1 for two soils (Sparling et al., 1985). Seasonal changes in the inorganic fraction were negatively correlated to soil moisture in sandy soils under grassland and arable cropping in Denmark (Magid and Nielsen, 1992).

Phosphorus released to water following soil drying is mainly organic, and appears to be derived from lysed microbial cells (Salema et al., 1982; Turner and Haygarth, 2001). However, relatively similar amounts of inorganic and organic P were released to bicarbonate extracts, suggesting that for bicarbonate extractable P, changes in P solubility may be more important than direct release from the microbial biomass. A similar conclusion was reported by Magid and Nielsen (1992), who calculated that seasonal changes in bicarbonate-extractable inorganic and organic P in sandy Danish soils were far greater than those expected on the basis of biological processes alone. Microbial cell death during the drying and rewetting process is primarily induced by osmotic shock and cell rupture upon rewetting with a solution of low ionic strength (Salema et al., 1982; Kieft et al., 1987). Therefore, the high ionic strength of bicarbonate solution may reduce this effect compared with water extraction (Sparling et al., 1985). The hypothesis that non-biomass organic P dominates in bicarbonate extracts is supported by ³¹P nuclear magnetic resonance analysis of bicarbonate-extractable organic P in two Canadian (air-dried) soils under arable cropping (Zhang et al., 1999). This study revealed that the bicarbonate-extractable organic P was dominated by orthophosphate monoesters and was, therefore, similar to the whole-soil organic P extracted with NaOH. Orthophosphate diesters, the main organic P compounds in soil microorganisms, were present in only small concentrations.

The mechanisms by which soil drying could affect the solubility of nonbiomass inorganic and organic P are poorly understood, but probably include both physical and chemical changes. Rapid rehydration of dry soils commonly causes aggregate breakdown (Amézketa, 1999), which increases the surface area for desorption by exposing surfaces and associated P protected within aggregates (Nevo and Hagin, 1966). Such a process was linked to increases in resin-extractable inorganic P following soil drying (Olsen and Court, 1982). Aggregate breakdown would be exacerbated in sieved soils (Bartlett and James, 1980), but also occurs under field conditions (Kemper and Rosenau, 1984). A potentially more important process is the disruption of organic matter coatings on clay and mineral surfaces by the physical stresses induced during soil drying. This increases organic matter solubility, exposes formerly protected mineral surfaces, and was attributed to increases in oxalate-extractable Si of up to 200% following drying of Swedish spodic B horizons (Simonsson et al., 1999). Soil drying also increases the crystallinity of pure Fe and Al oxides, which reduces the specific surface area and P sorption capacity of these minerals (McLaughlin et al., 1981). However, this is inconsistent with reports of increased sorption capacity of dried soils for P and S (Haynes and Swift, 1985; Comfort et al., 1991), which supports the hypothesis that physical disruption of organic matter coatings is the primary mechanism contributing to increases in bicarbonate-extractable P following soil drying.

The greatest increases in bicarbonate-extractable inorganic P occurred in low P pasture soils, as found in other studies (Olsen and Court, 1982; Sparling et al., 1985). This suggests that plant-available P tests might substantially overestimate the P status of such soils. However, these soils also contained relatively large concentrations of organic P in the bicarbonate extracts. This may contribute to plant nutrition in such soils, but little information exists on its chemical nature or biological availability. Some studies suggest that bicarbonate-extractable organic P is relatively active (Bowman and Cole, 1978; Guggenberger et al., 1996), while others indicate its resistance to hydrolysis by phosphatase enzymes (Otani and Ae, 1999; Hayes et al., 2000). The differences may be partly linked to pH, because most organic P compounds undergo reduced solubility in acidic soils, and become stabilized by association with clays and humic compounds (Goring and Bartholomew, 1952; Anderson and Arlidge, 1962). This is reflected in the negative correlations between bicarbonate-extractable organic P and soil pH (Tiessen et al., 1984), and almost certainly influences the relative bioavailability of bicarbonate-extractable organic P amongst different soils. Clearly, more information on the chemical composition of bicarbonate-extractable organic P is needed to determine its potential as a biologically available pool of soil P.

Despite the possible errors in plant-available P tests induced by drying low P pasture soils, most soils are unlikely to be affected, because correlations between plant growth and soil P status are almost exclusively derived from analyses of air-dried soils. However, our results have important implications for attempts to understand patterns of P release and availability in the field, because seasonal changes in bicarbonate-extractable P tend to be small (e.g., Magid and Nielsen, 1992), and will almost certainly be masked by the relatively large changes induced by air-drying. Therefore, we suggest that such studies should use field moist samples, even though soil drying can greatly reduce the variability amongst replicate analyses.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Concentrations of bicarbonate-extractable inorganic and organic P in permanent lowland pasture soils from England and Wales were substantially increased by air-drying prior to extraction. Similar proportions of inorganic and organic P were released following drying, which were proportionally greatest in soils with low initial bicarbonate-extractable P concentrations. The mechanisms underlying these changes remain poorly understood, but a combination of microbial lysis, aggregate breakdown, and disruption of organic matter coatings appear to be important. Organic P constituted a large proportion of the total extractable P, and more information is needed on the chemical composition of this potentially bioavailable fraction. The results have important implications for analytical procedures that involve soil drying prior to extraction, and for attempts to relate such measurements to seasonal patterns of soil P availability.

	ACKNOWLEDGMENTS

 
Ben Turner was funded by the UK Natural Environment Research Council (NERC) and the Institute of Grassland and Environmental Research (IGER). IGER receives grant aid from the Biotechnology and Biological Sciences Research Council (BBSRC). The authors acknowledge the National Soil Resources Institute for access to the National Soil Inventory database, William Turner for assistance in the field and Dr. Dale Westermann for comments on the manuscript.

	NOTES

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B.L. Turner, current address: USDA–ARS, Northwest Irrigation and Soils Research Laboratory, 3793 N. 3600 E., Kimberly, ID 83341.

Received for publication February 22, 2002.

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TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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