Contribution of plant-derived carbon to soil microbial biomass dynamics in a paddy rice microcosm

An understanding of the microbial biomass dynamics in rice paddies is essential for managing their nutrient and C cycling. Our objectives were to determine whether the seasonal dynamics of microbial biomass C (MBC) was related to the release of organic substance from rice roots. MBC and the dissolved organic C (DOC) in soil solutions were measured over a growing period of rice plants in a pot experiment. The C-13 pulse labelling (through supplying rice plants with (CO2)-C-13 for 6 h) was performed at different growth stages of rice to estimate the contribution of photosynthesized C to MBC. DOC concentrations increased with plant growth, reflecting the release of soluble root exudates from rice roots. MBC declined in the early period, and then rapidly increased from the maximum tillering stage to the heading stage of rice. During this period of increase, MBC was positively correlated to the DOC concentration and root biomass. About 0.15-0.94% (mean 0.54%) of photosynthesized C-13 was incorporated into MBC immediately after pulse labelling, and 0.18-0.75% (mean 0.41%) still remained at the end of the season. The estimated total contribution of photosynthates to MBC amounted to 91 mg C plant(-1), corresponding to 28% of total MBC at the end of the season or a 100% increase in MBC over the growing season. The results suggest that MBC dynamics in rice soil are largely controlled by organic substances released from rice roots.