Organic phosphorus compounds in particle-size fractions of mountain soils in the northwestern Caucasus

Abstract

Distributions of organic C (C org), total P (P t), and organic P (P org) among particle-size fractions were investigated in Humic Cambisols and Umbric Leptosols of the northwestern Caucasus. In all investigated soils, P t and P org increased with decreasing particle size. In fractions <50 μm of A horizons, P org accounted for 58–95% of P t. In sandy fractions, the proportions of P org varied significantly from 29% to 80%. The greatest proportions of soil C org, P t, and P org in A horizons were concentrated in the 5–50 μm fraction. The C/P org ratios decreased with decreasing particle size, indicating a relative enrichment of P in the organic matter associated with finer particle-size fractions. Phosphorus-31 nuclear magnetic resonance spectroscopy of dialyzed NaOH extracts was used to characterize the forms of phosphorus in particle-size fractions. As expected, monoesters were the major P species in all investigated particle-size separates (64–88% of extractable P org). Diester-P (DNA+phospholipid–teichoic acids) accounted for 10.9–33.4% of extractable P org in particle-size fractions of A horizons and for only 9.0–22.9% in fractions of B horizons. Diester-P occurred in uniform proportions in fractions of A horizons while its relative enrichment in clay fractions was evident for the B horizons. Up to 5.2% of organic P was present as phosphonates, the maximum being in the 1–5 μm fraction.