Influence of fertilization and organic amendments on organic-carbon fractions in Heilu soil on the loess plateau of China

The influence of fertilization on organic-carbon fractions separated by density and particle size in Heilu soil (Calcic Kastanozems, FAO) was investigated in a 20-year (19791999) long-term experiment on the Loess Plateau of China. Compared to an unfertilized treatment, N application alone did not increase total organic carbon (TOC) and its fractions of density and particle size. However, the treatment of N + P fertilization significantly increased salty-solution-soluble organic carbon (SSOC), microbial biomass C (MB-C), and organic C associated with fine sift. When manure was applied alone and in combination with N and P fertilizer, the light fraction of organic C (LFOC), SSOC, and MB-C were increased significantly, and the TOC was as high as that of a native Heilu soil. Organic C associated with different particle-size fractions was also increased significantly, and the allocation of C among the fractions was altered: the proportions of C in sand (>50 mum), coarse-silt (20-50 mum), and fine-clay (<0.2 mum) fractions were increased whereas fine-silt (2-20 mum) and coarse-clay (0.2-2 mum) fractions were decreased. It is concluded that N fertilizer alone is not capable of restoring organic-matter content in the Heilu soils of the Loess Plateau and that C-containing material like manure and straw is necessary to produce significant increase in soil organic carbon in these soils.