Incorporation of 13C-labelled rice rhizodeposition carbon into soil microbial communities under different water status

The overall processes by which carbon is fixed by plants in photosynthesis then released into the soil by rhizodeposition and subsequently utilized by soil micro-organisms, links the atmospheric and soil carbon pools. The objective of this study was to determine the plant derived C-13 incorporated into the phospholipid fatty acid (PLFA) pattern in paddy soil, to test whether utilization of rice rhizodeposition carbon by soil micro-organisms is affected by soil water status. This is essential to understand the importance of flooded conditions in regulating soil microbial community structure and activity in wetland rice systems. Rice plants were grown in soil derived from a paddy system under controlled irrigation (CI), or with continuous waterlogging (CW). Most of the C-13-labelled rice rhizodeposition carbon was distributed into the PLFAs 16:0, 18:1 omega 7 and 18:1 omega 9 in both the CW and CI treatments. The bacterial PLFAs i15:0 and a15:0, both indicative of gram positive bacteria, were relatively more abundant in the treatments without rice plants. When rice plants were present rates of C-13-incorporation into i15:0 and a15:0 was slow: the microbes containing these PLFAs may derive most of their carbon from more recalcitrant C (soil organic matter). PLFAs, 18:1 omega 7 and 16:1 omega 7c, indicative of gram negative bacteria showed a greater amount incorporation of labelled plant derived carbon in the CW treatment. In contrast, 18:2 omega 6,9 indicative of fungi and 18:1 omega 9 indicative of aerobes but also potentially fungi and plant roots had greater incorporation in the Cl treatment. The greater root mass concomitant with lower incorporation of C-13 into the total PLFA pool in the CW treatment suggests that the microbial communities in wetland rice soil are limited by factors other than substrate availability in flooded conditions. In this study differing soil microbial communities were established through manipulating the water status of paddy soils. Steady state C-13 labelling enabled us to determine that the microbial community utilizing plant derived carbon was also affected by water status. (C) 2012 Elsevier Ltd. All rights reserved.