Dynamics of soil microbial biomass and activity in conventional and organic farming systems

Dynamics of microbial communities during two growing seasons were compared in soils under tomatoes managed by conventional (2- and 4-y rotations), low input, or organic practices. Fumigation extractable carbon (FEC) and nitrogen (FEN), potentially mineralizable N, arginine ammonification and substrate induced respiration (SIR) were significantly higher in organic and low input than conventional systems on most sample dates. Microbial variables were significantly negatively correlated with amounts of soil mineral N in the conventional 4 y system, whereas they were positively correlated with mineral N in the organic system. The C-to-N ratios of material released after fumigation extraction were significantly higher in the conventional than organic soils. In all farming systems, soil moisture was positively correlated with FEC or FEN, but negatively correlated with the C-to-N ratio of the microbial biomass and SIR. Soil temperature was negatively correlated with FEC and FEN, but positively correlated with the C-to-N ratio of microbial biomass.