Methane and soil CO2 production from current-season photosynthates in a rice paddy exposed to elevated CO2 concentration and soil temperature

Quantification of rhizodeposition (root exudates and root turnover) represents a major challenge for understanding the links between above‐ground assimilation and below‐ground anoxic decomposition of organic carbon in rice paddy ecosystems. Free‐air CO2 enrichment (FACE) fumigating depleted 13CO2 in rice paddy resulted in a smaller 13C/12C ratio in plant‐assimilated carbon, providing a unique measure by which we partitioned the sources of decomposed gases (CO2 and CH4) into current‐season photosynthates (new C) and soil organic matter (old C). In addition, we imposed a soil‐warming treatment nested within the CO2 treatments to assess whether the carbon source was sensitive to warming. Compared with the ambient CO2 treatment, the FACE treatment decreased the 13C/12C ratio not only in the rice‐plant carbon but also in the soil CO2 and CH4. The estimated new C contribution to dissolved CO2 was minor (ca. 20%) at the tillering stage, increased with rice growth and was about 50% from the panicle‐formation stage onwards. For CH4, the contribution of new C was greater than for heterotrophic CO2 production; ca. 40–60% of season‐total CH4 production originated from new C with a tendency toward even larger new C contribution with soil warming, presumably because enhanced root decay provided substrates for greater CH4 production. The results suggest a fast and close coupling between photosynthesis and anoxic decomposition in soil, and further indicate a positive feedback of global warming by enhanced CH4 emission through greater rhizodeposition.