Carbon mineralization and microbial biomass dynamics in soil amended with plant residues and residue fractions

To study the dynamics of C mineralization and microbial biomass in soil amended with residues and residue fractions, a laboratory experiment was conducted using sorghum straw and Gliricidia prunings and their fractions. Analysis of residue fractions indicated the preponderance of decomposition-resistant fibre fraction (78.6%) in sorghum straw and easily decomposable soluble fraction (53.3%) in Gliricidia prunings. Most of the N in the residues was associated with the soluble fraction in both the residues. The pattern of C mineralization from control soil as well as soil amended with residues and residue fractions was exponential, being fast in the initial stages and slowing down with the progress of time. Peak rate of C mineralization was observed immediately following incorporation in case of sorghum straw, Gliricidia prunings and fibre fraction of prunings, whereas in case of fibre fraction of sorghum straw, the mineralization rate reached peak level after 10 days. The rates of C mineralization declined rapidly and stabilized by 40–50 days after incubation in all the treatments. In response to amendment and incubation, soil microbial biomass C increased from the initial value of 123 mg kg−1 and reached a peak level by 5 days of incubation in all cases except fibre fraction of sorghum straw, where peak level was reached only after 15 days. After attaining peak values, the biomass C in all treatments declined, and at the end of 90 days of incubation, the biomass C in control soil and soil amended with Gliricidia prunings and fibre fraction of prunings fell back to initial level, while it remained higher in soil amended with sorghum straw and fibre fraction of straw. These results indicate the dependence of decomposition and C mineralization of plant residues on the proportion of soluble and fibre fractions in the residues, and their N concentration and C/N ratio.