Forms of organic phosphorus in zonal steppe soils of Russia assessed by P-31 NMR

Different soil organic P (SOP) structures mineralize at different rates. Ac tual SOP composition reflects steady-state equilibrium of SOP build-up and loss. This study was conducted to investigate heterogeneity of SOP composit ion among particle-size fractions in zonal steppe soils. Composite samples were taken from four surface (0-10 cm) and subsurface (ca. 50-60 cm) soil h orizons along a climosequence in the native steppe and forest steppe from T ula to Kalmykia Republik, 120 km south of Volgograd, Russia. The SOP compos ition was assessed by P-31 nuclear magnetic resonance (NMR) spectroscopy on alkaline NaOH/NaF extracts obtained from bulk soils (< 2 mm), clay (< 2 mu m), silt (2-20 mum), and sand (20-2000 mum) fractions. About 50-60% of extr acted P were monoesters, the proportions being highest in the sand fraction s. Diester-P structures, including teichoic and nucleic acids, amounted to 19-31% of extracted P. They were significantly enriched in the clay fractio ns. There was little alteration of SOP composition with increasing soil dep th, i.e. we did not find indications that in the Mollisols labile P diester s were degraded in preference to the more stable monoester-P structures as in other soils. The enrichment of diester P in the clay fractions increased as the ratio of mean annual precipitation (MAP) to potential evaporation ( pET) decreased; in turn, the proportion of monoester P decreased. However., also P extractability declined with increasing MAP/pET ratio. On the one h and this may indicate different P availability in the zonal soils, however, on the other hand this complicates the assessment of SOP dynamics with the help of P-31 NMR. Elucidating the ecological significance of P structures accessible to liquid-state NMR spectroscopy might thus warrant further atte ntion. (C) 2001 Elsevier Science B.V. All rights reserved.