Composition of the Soil Microbial Community under Sugarcane Production as Indicated by Phospholipid Fatty Acid Analysis

Phospholipid fatty acid (PLFA) analysis was used to characterise the microbial communities in the soil under long-term pre-harvest burning of sugarcane and several other land uses, and on a long-term experiment comparing sugarcane burnt before harvest and sugarcane harvested green with retention of a trash blanket. Trends in total PLFAs (total microbial biomass) followed the order: kikuyu pasture > native grassland > ryegrass pasture > maize > burnt sugarcane, and were higher under trashed than burnt sugarcane. Principal component analysis (PCA) of PLFA data revealed considerable differences with respect to PLFA composition in response to land use and trash management, suggesting substantial differences in microbial community structure. In both the comparison of land uses and the trash management trial, soil organic C content was significantly correlated with PC1, suggesting that changes in soil organic matter content can greatly affect soil microbial diversity. Among the land uses, burnt sugarcane had the lowest values for PLFA richness and Shannon’s diversity index (indicators of microbial diversity) and conversion to green cane harvesting resulted in an increase in these values. The ratio of 18:2Τ6 fungal fatty acid: bacterial fatty acids was highest under improved pastures. The ratio of ester-linked monosaturated fatty acids : ester-linked saturated fatty acids was increased in land uses with a high organic matter content, suggesting an increase in the ratio of Gram-negative : Gram-positive bacteria in response to greater substrate availability. It was concluded that sugarcane production under pre-harvest burning is particularly detrimental to the structural diversity of soil microbial communities and that the conversion to green cane harvesting with trash retention increases soil microbial diversity.