Greenhouse assessment of microbial biomass carbon and nitrogen as influenced by compaction, Bradyrhizobium inoculation and nitrogen fertilizer application in maize-soybean cropping systems

Soil microbial biomass (SMB) is the main driving force in nutrient cycling and good indicator of soil productivity. A greenhouse experiment was designed to assess the effect of soil compaction, cropping system [sole maize, rotation 1 (inoculated soybean-maize), rotation 2 (un-inoculated soybean-maize) and intercrop 1(inoculated soybean-maize)and intercrop 2(un-inoculated soybean-maize)] and nitrogen fertilizeron soil microbial biomass C (SMB-C) and N (SMB-N) and their proportion to soil organic C and total N. SMB-C and SMB-N were higher in un-compacted than compacted soils with percent differences of 2.63 and 6.04% respectively. However, they were 19.32 and 36.36% lower in sole maize compared to rotation 1, 7.83 and 15.36% for rotation 2, 22.19 and 20.06% for intercrop 1 and 14.62 and 12.54% for intercrop 2. The results also showed that the application of 120 kg N ha-1 produced the highest soil microbial biomass as a percent of soil organic carbon, followed by 80 kg N ha-1, while the least value was obtained under zero application of nitrogen. Microbial biomass carbon and nitrogen as a percent of soil total nitrogen was significantly higher up to 80 kg N ha-1 before it decline at 120 kg N ha-1 suggesting better soil productivity improvement at 80 kg N ha-1 under the cropping systems with inoculated soybean. The findings indicate the need for inoculation in soybean-maize cropping systems to improve soil microbial biomass especially under less soil disturbances.