Soil microbial community structure in a rice paddy field and its relationships to CH4 and N2O fluxes

Soil microbial community structure and gas exchange (CH4, N2O) were investigated in a rice paddy field, located in the Po plain, Italy. At eight sampling dates-representative for different soil conditions and crop stages-microbial community structure was characterized by phospholipid fatty acid (PLFA) analysis. Principal component analyses revealed differences in community composition as well as in soil fluxes during the monitored year. The vertical distribution of PLFAs was investigated by sampling at 0-15 and 15-30 cm; the uniformity in soil properties through the soil profile resulted in a vertical homogeneity of microbial biomass and community composition. Only temporary variations of soil conditions led to a distinction in microbial populations with depth. The ratio of Gram-positive to Gram-negative bacteria was higher in flooded than in non-flooded soils. The saturated and methyl-branched fatty acids typically present in Gram-positive bacteria appeared to be indicators of flooded and waterlogged conditions, which were favourable for CH4 emission as well as for N2O consumption. Gram-negative bacteria were instead most important for characterization of aerobic conditions and for the decomposition of fresh residues. In terms of microbial processes, the 16:1 omega 9 fatty acid appeared to be correlated with N2O fluxes.