Kinetics of crop residue decomposition: Variability among crops and years

A kinetic description of crop residue decomposition makes assessment of the global C cycle and nutrient cycling possible for a wide variety of crop production systems. It was the objective of this study to compare decomposition kinetics of soybean [Glycine max (L.) Merr.], rice (Oryza sativa L.) and grain sorghum [Sorghum bicolor (L.) Moench.] residues for 3 yr to determine if variability among years was similar to that among crops. Crop residues were incorporated into a Crowley silt loam (fine, montmorillonitic, thermic Typic Albaqualf) soil and incubated at 25 degrees C under optimum soil moisture for 54 to 66 d. Initial (0-2-wk) decomposition was related to crop residue organic N and C/N ratio, while subsequent decomposition was not related to these factors. Decomposition data were evaluated using first-order kinetics and sequential and simultaneous decomposition models. As a result of the variability among crops and years, which was not related to residue characteristics (organic N or C/N ratio), it was concluded that mean estimates of rate constants and the rapid fraction would provide a reasonable estimate of crop residue decomposition for a variety of crops using either decomposition model. For the sequential model, mean rapid- and slow-fraction rate constants were 0.025 and 0.0091 d(-1), respectively, while the rapid fraction was 31%. Parallel values for the simultaneous model were 0.21 and 0.0080 d(-1) and 20%.