Distribution of 13C-Labelled Rhizodeposition of Sweet Sorghum Into Soil Microbial Biomass Under Treatments Applied With Organic Matter.

Carbon is fixed by plant in photosynthesis, and then released into the soil by rhizodeposition and subsequently utilized by soil microbial biomass. The objective of this study was to evaluate how utilization of sweet sorghum rhizodeposition carbon by soil microbial biomass is affected by organic matter application. Sweet sorghum bagasse (SSB: TC 45.1%, TN 0.347%, C/N 133) was applied to soil as an organic matter. Sweet sorghum plants were grown in potted soil with incorporated SSB (INC), surface-applied SSB (SUR), and without SSB as control (CON). All pots (0.05 m2) were fertilized with inorganic fertilizer (N:P2O5:K2O = 24.0:10.7:12.0 g m-2). In INC and SUR, SSB was applied at rates of 1000 g dry weight m-2. The cultivation was conducted in greenhouse with four replications. On June 17, 2011, after application of inorganic fertilizer and SSB, nine seeds of sweet sorghum were seeded in each pot and thinned to three seedlings at 15 days after sowing. At 46 days after sowing (DAS), sweet sorghum plants were exposed to 13CO2. Plants and soil were taken at 24 h after 13CO2 exposure, and 13C content in the samples (plant shoots, roots, and soil microbial biomass) was measured. Root dry weight / aboveground ratio was lower in INC and SUR than that in CON. While, distribution rate of 13C into sweet sorghum root was higher in INC and SUR. Moreover, soil microbial biomass C and 13C (g / pot) were also higher in INC and SUR than those in CON. These results show a close relationship between plant and microbial biomass in soil applied with organic matter. Utilization of sweet sorghum rhizodeposition carbon by soil microbial biomass was significantly increased by organic matter application.