FAME Profiling and Activity of Microbial Communities During Jatropha curcas L. Residue Decomposition in Semiarid Soils

Abstract

There is widespread interest in the renewable biofuel potential of Jatropha curcas L., a shrub that exists under indigenous conditions in India. Following oil extraction from seeds, the remaining shell and cake residue can be used as a soil amendment. Microbial decomposition of organic residues can improve the quality of soils, but little is known about Jatropha as a soil amendment. Therefore, the objective of this study was to determine the influence of Jatropha residue on soil microbial community structure during decomposition. Soil microcosms amended with Jatropha leaves, cake, or fruit shell or an unamended control were incubated in the laboratory and destructively sampled at 3, 10, 20, 40, and 60 days. At each sampling, soil microbial communities were analyzed using fatty acid methyl ester (FAME) profiles, and enzyme assays performed for β-glucosidase, cellulase, and urease. Microbial biomass (FAMEtot) and bacterial and fungal FAME concentrations were significantly higher in cake-amended soil compared with other treatments. Averaged across all treatments, FAMEtot and fungal and bacterial FAMEs were highest on Day 3 and subsequently decreased significantly. Higher fungal-to-bacterial FAME ratios were found in residue-amended soils over control, with the highest ratio occurring in fruit shell-amended soil (except Day 3). A significantly lower ratio of the stress indicator (saturated to monounsaturated FAMEs) was found in cake-amended soil compared with other amendments. Fungal, bacterial, and actinomycetes biomarkers were significantly correlated with β-glucosidase and cellulase activities. The study shows that decomposition of Jatropha affected the soil microbial community, differently depending on residue type, but overall had positive effects in promoting the microbial diversity and activity, thus making Jatropha residue a favorable amendment for soils.