Belowground carbon allocation and dynamics under rice cultivation depends on soil organic matter content.

Background and aims: The cycling of photosynthate carbon (C) released in the rhizosphere has significant implications for C sequestration, microbial activities, and nutrient availability in the soil. It is known that the soil organic matter (SOM) content affects the nutrient status, root growth, rhizodeposition, and microbial composition and activity; however, the effects of SOM and consequently of soil fertility on the belowground allocation and dynamics of photosynthetic C remain unknown. Methods: To examine the effects of SOM on the allocation and dynamics of photosynthetically fixed C, rice plants grown on soils with low (0.5 %), moderate (1.4 %), or high (3.4 %) C content were labeled with CO and harvested six times in one month. Results: The highest C amount was released from the roots into the soil with high SOC content, whereas the opposite pattern was observed for CO losses. Microbial C increased with C in SOM, when soil C content was low or moderate, but decreased when C content was high. At 30 d after labeling, rice plants allocated 2560 kg C ha, 3030, kg C ha, and 4580 kg C ha in the soil with low, moderate, and high SOC content, respectively, accounting for a rhizodeposition of approximately 13 %, 15 %, and 30 %, respectively. Most of the root-derived C in low SOM soil was mineralized quickly. In contrast, high and moderate SOM content led to higher incorporation of rhizodeposits into SOM and higher belowground C protection against microbial decomposition. Conclusions: We concluded that SOM content and consequently, soil fertility play a crucial role in the amount of photosynthates allocated by the plant into the soil and C stabilization. A high SOM level is maintained by the high C input and has longer stability. [ABSTRACT FROM AUTHOR]