Organic Phosphorus Composition and Potential Bioavailability in Semi-Arid Arable Soils of the Western United States

the need to understand the nature of organic P in these soils. The organic P composition of semi-arid arable soils is largely unSoil organic P determination has traditionally been known, but such information is fundamental to understanding P dynamics in irrigated agriculture. We used solution 31P nuclear magnetic hampered by difficulties with the extraction, separation resonance (NMR) spectroscopy and phosphatase hydrolysis to characand detection of these often recalcitrant compounds, terize organic P in semi-arid arable soils from the western USA (orbut analysis by solution 31P NMR spectroscopy obviates ganic C 2.0–30.7 g C kg 1 soil, clay 2–48%, pH 5.2–8.2, CaCO3 1–480 g many of these problems. When coupled with the singlekg 1 soil). Total P concentrations ranged from 220 to 1210 mg P kg 1 step NaOH–EDTA extraction procedure (Bowman and soil, of which between 12 and 45% was extracted with NaOH–EDTA. Moir, 1993) more organic P can be characterized than by Inorganic orthophosphate was the dominant P compound, but concenextraction with NaOH alone (Cade-Menun and Preston, trations determined by solution 31P NMR spectroscopy were consis1996). Indeed, more than 90% of the total P in high tently greater than those determined by molybdate colorimetry. Conorganic matter soils has been characterized by NaOH– centrations of organic P were relatively small, and were dominated EDTA extraction and solution 31P NMR spectroscopy by orthophosphate monoesters (11–130 mg P kg 1 soil), with smaller concentrations of orthophosphate diesters (0–7 mg P kg 1 soil). Pyro(Dai et al., 1996; Cade-Menun et al., 2000). Solution 31P phosphate was present in almost all soils at concentrations up to 14 mg NMR spectroscopy is less suitable for the analysis of P kg 1 soil. Bicarbonate-extractable organic P ranged from 1.7 to water or bicarbonate extracts, because the relatively 22.8 mg P kg 1 soil, of which between 37 and 87% was hydrolyzed high detection limits confound analysis of samples with by phosphatase enzymes, suggesting its bioavailability. Soil organic low P concentrations (Nanny and Minear, 1997). DeP concentrations were positively correlated with mean annual precipispite this, the organic P in these types of extracts can tation, organic C, clay, and oxalate-extractable metals (Al, Fe, Mn), be characterized by its susceptibility to hydrolysis by and negatively correlated with mean annual temperature and soil pH. phosphatase enzymes (Shand and Smith, 1997; Turner However, CaCO3 concentrations were not significantly correlated with et al., 2002a). This sensitive technique not only provides any soil property. These results indicate that equilibrium levels of structural information on the extracted organic P, but organic P in semi-arid arable soils are controlled by a balance between the physical protection offered by the soil matrix and the suitability also indicates its potential biological availability. of the environment for biological productivity. Our objectives were to determine the composition and potential bioavailability of organic P in a range of low organic matter, mainly calcareous, arable soils of the semi-arid western USA, and to investigate how these I on the chemical composition of soil orwere related to climate and soil properties. ganic P is fundamental to understanding plant nutrition and soil biogeochemical cycles. However, organic P remains poorly understood, despite constituting a large MATERIALS AND METHODS proportion of the total P in many soils, and providing Soil Sampling, Preparation, and Determination a source of P for plant uptake (Magid et al., 1996). of Basic Properties In particular, little information exists on the chemical composition of organic P in low organic matter calcareEighteen soils were sampled to 30-cm depth from locations around the western USA during 2000 (Table 1). Most soils ous soils, such as those common in the drier regions of were under irrigated arable cropping, although the Wahpeton the western USA. The semi-arid climate of the western soil (No. 18) (soil descriptions are given in Table 1) was not USA means that most agricultural soils are irrigated, irrigated and the native Portneuf (No. 14) was under nonand such soils are agronomically important because irriirrigated sagebrush (Artemisia tridentata Nutt.). The sampling gated agriculture produces nearly 40% of the total U.S. sites were semi-arid, with hot dry summers and cool moist crop value from only 15% of the total cropped land winters. Mean annual temperatures ranged from 5.0 C at (Bajwa et al., 1992). Therefore, information on the comFargo, ND, to 13.8 C at Amarillo, TX. Mean annual precipitaposition of organic P and its potential contribution to tion ranged from 209 mm at Othello, WA, to 547 mm at Pullplant nutrition in these soils must be better understood. man, WA. Furthermore, current interest in the transfer of soil orThe soils were air-dried (30 C), sieved ( 2 mm), and stored at ambient laboratory temperature before analysis. Soil texganic P compounds to watercourses and their impact tural information and the concentrations of organic C and on water quality (Haygarth and Jarvis, 1999) reinforces CaCO3 were determined by standard methods (Allison and Moodie, 1965; Day, 1965; Nelson and Sommers, 1982). Soil B.L. Turner and D.T. Westermann, USDA–ARS, Northwest IrrigapH was determined on a saturated paste using deionized water tion and Soils Research Lab., 3793 N. 3600 E., Kimberly, ID 83341; (1:1 ratio). Oxalate-extractable Al, Fe, and Mn were deterB.J. Cade-Menun, Dep. of Geological and Environmental Sciences, mined by extraction with ammonium oxalate/oxalic acid (pH Stanford Univ., Stanford, CA 94305. Received 1 July 2002. *Corre3.0) for 2 h, followed by detection using inductively coupled sponding author ([email protected]). plasma atomic-emission spectroscopy (ICP–AES) (Schoumans, 2000). Published in Soil Sci. Soc. Am. J. 67:1168–1179 (2003).