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

Different soil organic P (SOP) structures mineralize at different rates. Actual SOP composition reflects steady-state equilibrium of SOP build-up and loss. This study was conducted to investigate heterogeneity of SOP composition 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 horizons along a climosequence in the native steppe and forest steppe from Tula to Kalmykia Republik, 120 km south of Volgograd, Russia. The SOP composition was assessed by 31P nuclear magnetic resonance (NMR) spectroscopy on alkaline NaOH/NaF extracts obtained from bulk soils (<2 mm), clay (<2 μm), silt (2–20 μm), and sand (20–2000 μm) fractions. About 50–60% of extracted P were monoesters, the proportions being highest in the sand fractions. Diester-P structures, including teichoic and nucleic acids, amounted to 19–31% of extracted P. They were significantly enriched in the clay fractions. There was little alteration of SOP composition with increasing soil depth, i.e. we did not find indications that in the Mollisols labile P diesters 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 hand 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 31P NMR. Elucidating the ecological significance of P structures accessible to liquid-state NMR spectroscopy might thus warrant further attention.