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

Abstract

The organic P composition of semi-arid arable soils is largely unknown, but such information is fundamental to understanding P dynamics in irrigated agriculture. We used solution 31P nuclear magnetic resonance (NMR) spectroscopy and phosphatase hydrolysis to characterize organic P in semi-arid arable soils from the western USA (organic C 2.0–30.7 g C kg−1 soil, clay 2–48%, pH 5.2–8.2, CaCO3 <1–480 g kg−1 soil). Total P concentrations ranged from 220 to 1210 mg P kg−1 soil, of which between 12 and 45% was extracted with NaOH–EDTA. Inorganic orthophosphate was the dominant P compound, but concentrations determined by solution 31P NMR spectroscopy were consistently greater than those determined by molybdate colorimetry. Concentrations of organic P were relatively small, and were dominated by orthophosphate monoesters (11–130 mg P kg−1 soil), with smaller concentrations of orthophosphate diesters (0–7 mg P kg−1 soil). Pyrophosphate was present in almost all soils at concentrations up to 14 mg P kg−1 soil. Bicarbonate-extractable organic P ranged from 1.7 to 22.8 mg P kg−1 soil, of which between 37 and 87% was hydrolyzed by phosphatase enzymes, suggesting its bioavailability. Soil organic P concentrations were positively correlated with mean annual precipitation, organic C, clay, and oxalate-extractable metals (Al, Fe, Mn), and negatively correlated with mean annual temperature and soil pH. However, CaCO3 concentrations were not significantly correlated with any soil property. These results indicate that equilibrium levels of organic P in semi-arid arable soils are controlled by a balance between the physical protection offered by the soil matrix and the suitability of the environment for biological productivity.