Abstract: A field experiment was conducted to study the effects of planting density and nitrogen application rate on the topsoil (0-15 cm) microbial activity under wheat/forage rape multiple cropping. The results showed that multiple-cropping forage rape with wheat could significantly increase soil microbial biomass C (C_mic), soil microbial biomass N (N_mic), soil bacteria number (SBN), soil fungi number (SFN) and soil actinomyces number (SAN), but decrease soil microbial biomass C/N (C_mic/N_mic). The C_mic/N_mic and SBN increased with increasing planting density of forage rape, while N_mic and SAN were in adverse. SFN increased significantly with increasing nitrogen application rate, but C_mic and N_mic decreased first, increased then, and decreased again, with the highest in treatment 1 000 kg·hm^-2 N. Also with increasing nitrogen application rate, the SFN and SAN during harvest stage of forage rape decreased first and increased then, while the SAN during seedling stage increased first and decreased then. Soil microbial activities at rape harvest stage were all higher than those at seedling stage, except for SAN in treatment 600 kg·hm^-2 N. SBN and SAN were positively correlated with C_mic and N_mic, but negatively correlated with C_mic/N_mic. No significant correlation was observed between SFN and C_mic, and SMBN and C_mic/N_mic.

Key words: Land use/cover, Dynamic monitoring, Loess hilly and gully area, RS & GIS