Soil biological properties following additions of bmr mutant grain sorghum

Soil carbon (C) sequestration may be a viable technology to reduce increases in greenhouse gas emissions until cleaner fuel technology is available. Crop plants with increased lignin levels may lead to increased soil C sequestration. Grain sorghum (Sorghum bicolor) exhibiting lower lignin due to the naturally occurring brown midrib mutation (bmr) may allow an assessment of the potential of biotechnology to affect soil C sequestration by manipulating plant lignin levels. A 194-d laboratory microcosm experiment was conducted to investigate the mineralization of bmr and normal plant residue from four sorghum hybrids. Cross-polarization magic angle spinning ¹³C-nuclear magnetic resonance of the residue agreed with chemical analysis that the bmr residue contained altered lignin and less lignin per mass weight. Ground bmr or normal grain sorghum residue was added to soil, with or without an inorganic nitrogen (N) amendment. Initial C mineralization from microcosms receiving bmr residue was higher than from microcosms receiving normal residue, but the differences were not maintained through the 194-d experiment. Total residue C mineralization was not different between bmr or normal isolines, and accounted for only 26% of the originally added residue C. Greater variability was observed between sorghum lines than between bmr or normal isolines. The addition of N to soil resulted in increased soil C mineralization. With no added N, however, microcosm C mineralization was most strongly correlated with the lignin/N ratio. With added N, microcosm C mineralization was most strongly correlated with hemicellulose content. The soil microbial community, as assessed by phospholipid and neutral-lipid fatty acid analysis, was not affected by bmr or normal genotype, but the addition of N resulted in significant changes to the soil microbial community, most notably changes to the soil fungi. Results indicate that potential does exist to modify plant residue chemistry to increase soil C sequestration, but soil fertility and microbial community dynamics are important considerations and may further enhance C sequestration potential.