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

摘要

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.