In view of the significance of agricultural soils in affecting global C balance, the impact of manipulation of the quality of exogenous inputs on soil CO 2-C flux was studied in rice-barley annual rotation tropical dryland agroecosystem. Chemical fertilizer, Sesbania shoot (high quality resources), wheat straw (low quality resource) and Sesbania+wheat straw (high+low quality), all carrying equivalent recommended dose of N, were added to soil. A distinct seasonal variation in CO 2-C flux was recorded in all treatments, flux being higher during rice period, and much reduced during barley and summer fallow periods. During rice period the mean CO 2-C flux was greater in wheat straw (161% increase over control) and Sesbania+wheat straw (+129%) treatments; however, during barley and summer fallow periods differences among treatments were small. CO 2-C flux was more influenced by seasonal variations in water-filled pore space compared to soil temperature. In contrast, the role of microbial biomass and live crop roots in regulating soil CO 2-C flux was highly limited. Wheat straw input showed smaller microbial biomass with a tendency of rapid turnover rate resulting in highest cumulative CO 2-C flux. The Sesbania input exhibited larger microbial biomass with slower turnover rate, leading to lower cumulative CO 2-C flux. Addition of Sesbania to wheat straw showed higher cumulative CO 2-C flux yet supported highest microbial biomass with lowest turnover rate indicating stabilization of microbial biomass. Although single application of wheat straw or Sesbania showed comparable net change in soil C (18% and 15% relative to control, respectively) and crop productivity (32% and 38%), yet they differed significantly in soil C balance (374 and -3 g C m -2 y -1 respectively), a response influenced by the recalcitrant and labile nature of the inputs. Combining the two inputs resulted in significant increment in net change in soil C (33% over control) and crop yield (49%) in addition to high C balance (152 g C m -2 y -1). It is suggested that appropriate mixing of high and low quality inputs may contribute to improved crop productivity and soil fertility in terms of soil C sequestration.
