Influence of agricultural land management on organic matter content, microbial activity and aggregate stability in the profiles of two Oxisols

The effects of agricultural land use on organic matter content and related soil microbial and physical properties were compared with those under undisturbed native grassland in KwaZulu-Natal, South Africa. Two separate farms situated on Oxisols were used and both contained fields with continuous long-term (>20 y) cropping histories. At site 1, soil organic C content in the surface 30 cm followed the order permanent kikuyu pasture > annual ryegrass pasture > native grassland > sugarcane > maize under conventional tillage (CT). At site 2, organic C in the surface 30 cm decreased in the order kikuyu pasture > native grassland > annual ryegrass pasture > maize under zero tillage (ZT) greater than or equal to maize CT. Organic C, microbial biomass C, percentage organic C present as organic C, basal respiration and aggregate stability were substantially greater in the surface 5 cm under maize ZT than maize CT but this trend tended to be reversed in the 10- to 30-cm layer. In the undisturbed sites (e.g. native grassland and kikuyu pasture) the metabolic quotient increased with depth. By contrast, under maize CT and sugarcane there was no significant stratification of organic C, yet there was a sharp decrease in the metabolic quotient with depth. Aggregate stability was high under both native grassland and kikuyu pasture and it remained high to 40 cm depth under the deep-rooted kikuyu pasture. Although soil organic C content was similar under maize CT and sugarcane, values for microbial biomass C, percentage of organic present as microbial biomass, basal respiration and aggregate stability were lower, and those for metabolic quotient and bulk density were higher, under sugarcane. This was attributed to the fallow nature of the soil in the interrows of sugarcane fields. It was concluded that the loss of organic matter, microbial activity and aggregate stability is potentially problematic under maize CT, sugarcane and annual pasture and measures that improve organic matter status should be considered.