RELATIONSHIPS BETWEEN CLIMATIC FACTORS AND C, N POOLS IN PARTICLE-SIZE FRACTIONS OF STEPPE SOILS, RUSSIA

Many soils of the Russian steppe are characterized by high soil organi c matter contents and similar parent material. Thus, they are suitable for investigations of a climatic impact on C and N pools. We sampled 10 topsoils of the zonal Russian steppe at 0-10 and about 50-60 cm dep th intervals. After particle-size fractionation into clay (<2 mu m), s ilt (2-20 mu m), fine sand (20-250 mu m) organic C and N concentration s were determined in bulk soils and fractions. The results suggest tha t especially the older organic matter of the subsoil (in the silt frac tion) is correlated with climatic factors. Topsoils show less evidence for climatic influences on C and N pools. As the ratio of mean annual precipitation to potential evaporation (=N/V) subsoil. Obviously the degree of soil organic matter alteration was more pronounced in the or der Greyzem (N/V = 1.0) > Chernozem, Phaeozem (N/V = 0.89) > Haplic Ka stanozem (N/V = 0.6) > Calcic (N/V = 0.34), and Gypsic Kastanozem (N/V = 0.32). The organic carbon contents of the bulk subsoil are highest in the subsoil of the Chernozem and Phaeozem, and decrease with increa sing N/V ratio (i.e., increasing heat input and dryness) to the Calcic Kastanozem. This is accompanied by an increasing enrichment of organi c carbon in the silt fractions (r = -0.99** for the correlation of th e C enrichment in silt with N/V).