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

Distributions of organic C (Corg), total P (Pt), and organic P (Porg) among particle-size fractions were investigated in Humic Cambisols and Umbric Leptosols of the northwestern Caucasus. In all investigated soils, Pt and Porg increased with decreasing particle size. In fractions <50 μm of A horizons, Porg accounted for 58–95% of Pt. In sandy fractions, the proportions of Porg varied significantly from 29% to 80%. The greatest proportions of soil Corg, Pt, and Porg in A horizons were concentrated in the 5–50 μm fraction. The C/Porg 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 Porg). Diester-P (DNA+phospholipid–teichoic acids) accounted for 10.9–33.4% of extractable Porg 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.