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 31 P 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, CaCO 3 <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 31 P 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, CaCO 3 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.