Soil microbial communities in tilled organic and no-till conventional crop production systems

The world acreage of organic crop production systems is increasing, but the soil microbial dynamics in these systems are not fully understood. We studied the composition and functioning of soil microbial communities in 4-year organic or conventional rotations. The organic systems were tilled to control weeds, and N-2-fixing legumes or compost supplied nutrients. The conventional systems were managed under no-till; herbicides controlled weeds, and compost or fertilizers supplied nutrients. Soil microbial biomass C (MBC), the diversity and composition of bacterial communities, and activities of enzymes that mediate C, N, P, and S cycling were determined. The bacterial classes Gemmatimonadetes, C0119 (phylum Chloroflexi), and Thermomicrobia (phylum Chloroflexi) were more abundant in organic than conventional cropping systems, so were some genera from the class Actinobacteria. The bacterial beta-diversity showed similar cropping system differences. However, acid phosphomonoesterase activity was greater in conventional than organic cropping systems, presumably because the soil P from the large amounts of compost applied in the organic system suppressed this enzyme. MBC, bacterial alpha-diversity, the relative abundances of the bacterial classes delta-Proteobacteria, gamma-Proteobacteria, and Bacilli (phylum Firmicutes) were all greater in compost than no-compost treatments. The relative abundances of three genera from Actinobacteria class were negatively correlated with acid phosphomonoesterase activity due to the high relative abundances of these genera, but low acid phosphomonoesterase activities, where compost was applied in the organic system. Therefore, there were soil bacterial compositional differences between organic and conventional cropping systems, but only differences in the activities of a P cycling enzyme were detected.