Change from paddy rice to vegetable growing changes nitrogen-cycling microbial communities and their variation with depth in the soil

Changes in land use are likely to affect the abundance and functioning of microorganisms in the soil. In China many paddy fields are being converted for vegetable growing. We wished to determine how these changes affected the microbial populations in one particular region in Hunan province, where the climate is subtropical monsoon, as an example. We sampled the soil down to 1m in several fields: three that were still growing paddy rice, three that had been converted for vegetable growing 2 years earlier and three that had been growing vegetables for 25 years. Quantitative polymerase chain reaction (qPCR) and terminal-restriction fragment length polymorphism (T-RFLP) were used to determine the abundance and community composition of nxrA-containing nitrifiers and narG-containing denitrifiers in the soil. The abundances of these organisms depended largely on the amount of organic matter in the soil and decreased with increasing depth, as did the potential nitrification rate (PNR) and nitrate reductase activity (NRA). Enzyme activity was significantly correlated with the abundance of nitrogen-cycling bacteria. The change from rice to vegetable growing resulted in more residual nitrate-N in the soil, which correlated more strongly with the abundance of nxrA-containing nitrifiers in the topsoil (0-20 cm) and narG-containing denitrifiers in the deeper soil (80-100 cm). In general, the numbers of nitrogen-cycling microorganisms decreased markedly with increasing depth, but were less affected by the change from rice to vegetable cultivation in the fields investigated. Our results suggest that the abundances of nitrogen-cycling microbial communities are affected more by depth in the soil than by change of land use in these circumstances.