The forms of phosphorus in humic and fulvic acids of a toposequence of alpine soils in the northern Caucasus

Abstract

Chemical fractionation and 31P nuclear magnetic resonance (NMR) spectroscopy of humic acids (HA) and fulvic acids (FA) were used to characterize the forms of phosphorus and their changes within a toposequence of alpine soils at the Mt. Malaya Khatipara (Teberda reserve, northwestern Caucasus). Sodium hydroxide extracted 66–82% of total phosphorus from A horizons and 28–51% from B horizons. Organic P amounted to 92–99% of NaOH-extractable P. HA represented the major part of extracted organic P (52–90%). The P species in HA and FA comprised phosphate monoesters (40–86%), phosphate diesters (up to 22%), phosphonates (up to 8%), sugar-diester phosphates (up to 14%), pyrophosphates (up to 11%), polyphosphates (up to 16%), and unknown compounds (up to 6%). Inorganic orthophosphate was found in appreciable proportions only in HA of organic horizons and in FA (up to 16%). The percentages of phosphonates and phosphate diesters were higher in HA, those of sugar diesters and pyrophosphates were higher in FA. Within the toposequence, the contribution of labile P species to total P in HA of surface soil layers increased with increasing thickness of snow cover during winter and correspondingly shorter vegetation periods. The P-species distributions in HA of subsoil horizons were rather similar throughout the toposequence. HA of the surface soils thus seemed to best characterize the P dynamics in these soils.