|Cade-Menun, Barbara - STANFORD UNIVERSITY|
Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 1, 2002
Publication Date: N/A
Interpretive Summary: The organic phosphorus composition of semi-arid arable soils is largely unknown, but such information is important for understanding phosphorus dynamics in irrigated agriculture. This study reports the composition of organic phosphorus in a range of such soils from the western US obtained using solution phosphorus-31 nuclear magnetic resonance spectroscopy. The potential biological availability of the organic phosphorus to plants was assessed using phosphatase hydrolysis. Concentrations of organic phosphorus were relatively small, and most was in the form of relatively recalcitrant orthophosphate monoesters. However, of the organic phosphorus soluble in bicarbonate, between 37 and 87% was hydrolyzed by phosphatase enzymes, suggesting its bioavailability. Correlations between organic phosphorus fractions and soil properties indicated that equilibrium levels of organic phosphorus 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.
Technical 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 phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy and phosphatase hydrolysis to characterize organic P in semi-arid arable soils from the western US (organic C 2.0-30.7 g C per kg soil, clay 2-48%, pH 5.2-8.2, lime <1-480 g per kg soil). Total P concentrations ranged from 220 to 1210 mg P per kg soil, of which between 12 and 45% was extracted with NaOH-EDTA. Inorganic orthophosphate was the dominant P compound, but concentrations determined by phosphorus-31 NMR spectroscopy were consistently greater than those determined by molybdate colorimetry. Concentrations of organic P were relatively small, and were dominated by orthophosphate monoesters (11-132 mg P per kg soil), with smaller concentrations of orthophosphate diesters (0-7 mg P per kg soil). Pyrophosphate was present in almost all soils at concentrations up to 14 mg P per kg soil. Bicarbonate-extractable organic P ranged from 1.7 to 22.8 mg P per kg 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, lime 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.