Long-term Effects of Green Cane Harvesting versus Burning on the Size and Diversity of the Soil Microbial Community

The long-term effects of green cane harvesting with trash retention, compared with burning, on soil microbiological properties were evaluated using data from the long-term trash management trial at Mount Edgecombe (BT1). Total organic C content was greater under green cane harvesting than burning in the surface 0-10 cm layer of soil. Microbial biomass C was greater under green cane harvesting than burning to a depth of 30 cm. Both K 2 SO 4 -extractable C and light fraction C were also increased markedly by trash retention to a depth of 30 cm reflecting the downward leaching of soluble C and/or deposition of particulate C at depth below the trash blanket. This demonstrates the value of these labile C fractions as indicators of changes in soil C. Soil community structure was examined based on extraction and identification of phospholipid fatty acids (PLFA). Data from PLFA composition of soils were subjected to principal component analysis which accounted for over 78% of the total variance. Trash retention markedly changed the composition of the microbial community compared with burning. Fertiliser applications had very little effect on the community composition in the burnt treatments but changed the community greatly where trash was returned. The total quantity of PLFAs extracted was greater under trashed than burnt treatments reflecting the larger microbial biomass under trash retention. Principal component analysis was also used to separate PLFA profiles from soils under long-term burnt sugarcane, maize , annual ryegrass, kikuyu grass and undisturbed veld. Results clearly showed that the community composition under conventional sugarcane production where burning is practiced prior to harvest was very different from that under the other land management practices.