Mechanisms Regulating Composting of High Carbon to Nitrogen Ratio Grass Straw

Abstract

Utilization of high C:N ratio farm residues is difficult because of its low value and bulk. Composting is an ideal method for upgrading the residue, however, it was not thought possible without co-composting to lower the C to N ratio to 30 to 1 or lower. We have successfully composted grass straw with a C:N of 50 to 1 using existing farm machinery and normal precipitation (115 cm y−1 and occurs primarily between October and June). After turning and densification in the field, composting conditions were achieved with the rapid growth of thermophilic microorganisms. During laboratory incubations of the grass straw, bacteria ranged from 108-109 g−1 grass straw. Actinomycetes numbered 108 g−1 grass straw. Fungi ranged from 106-107 propagules g−1 grass straw. Microbial biomass C during composting represented four to six percent of total grass straw C. A significant amount of lignin C, 25 percent, was degraded during mesophilic decomposition while 39 percent was degraded during thermophilic decomposition. The acid insoluble lignin fraction became highly oxidized and accumulated N. The decomposed lignin C, N, H and O concentrations were similar to humic substances in soil. These data indicate that extensive lignin degradation occurred. A simulation of microbial production indicated that the microbial biomass was more efficient at mesophilic than at thermophilic temperatures.