Microbial response to rhizodeposition depending on water regimes in paddy soils

Rhizodeposit-carbon (rhizo-C) serves as a primary energy and C source for microorganisms in the rhizosphere. Despite important progress in understanding the fate of rhizo-C in upland soils, little is known about microbial community dynamics associated with rhizo-C in flooded soils, especially depending on water regimes in rice systems. In this study, rice grown under non-flooded, continuously flooded and alternating water regimes was pulse labeled with (CO2)-C-13 and the incorporation of rhizo-C into specific microbial groups was determined by C-13 in phospholipid fatty acids (PLFAs) at day 2 and 14 after the labeling. A decreased C released from roots under continuously flooded condition was accompanied with lower total C-13 incorporation into microorganisms compared to the non-flooded and alternating water regimes treatments. Continuous flooding caused a relative increase of C-13 incorporation in Gram positive bacteria (i14:0, i15:0, a15:0, i16:0, i17:0, a17:0). In contrast, Gram negative bacteria (16:1 omega 7c, 18:1 omega 7c, cy17:0, cy 19:0) and fungi (18:2 omega 6, 9c, 18:1 omega 9c) showed greater rhizo-C incorporation coupled with a higher turnover under non-flooded and alternating water regimes treatments. These observations suggest that microbial groups processing rhizo-C differed among rice systems with varying water regimes. In contrast to non-flooded and alternating water regimes, there was little to no temporal C-13 change in most microbial groups under continuous flooding condition between day 2 and 14 after the labeling, which may demonstrate slower microbial processing turnover. In summary, our findings indicate that belowground C input by rhizodeposition and its biological cycling was significantly influenced by water regimes in rice systems. (C) 2013 Elsevier Ltd. All rights reserved.