Soil organic carbon dynamics under long-term sugarcane monoculture

Comparisons of soil samples from virgin sites or sites recently planted to sugarcane (new) with sites that had been under cane production for many yea rs (old) were made to investigate the potential impact of cane production o n soil organic carbon (OC) levels and chemistry. The comparisons showed tha t very little change had occurred in total OC and in `light' fraction (<1.6 Mg/m(3)). Increasing pyrophosphate extractability throughout the profile a t some sites, as a result of cultivation, however, suggested that the organ ic matter generally became more `humified' with long-term cane production. Evidence is presented for a redistribution of OC within profiles under cane production. Old, well-established cane sites had soils with lower OC level s in the surface horizons and higher levels in the subsoils relative to new sites. The overall chemistry of the soil organic matter, as indicated by s olid state C-13 nuclear magnetic resonance spectroscopy, did not change sig nificantly at each site even though between site differences were large. So me soils contained substantial amounts of charcoal which was of pre-cane or igin. In some of the coarse-textured soils, smaller amounts of charcoal pro duced during the burning of cane appeared to accumulate below the A1 horizo ns in the profiles. It also appeared likely that the redistribution of carb on in the upper horizons of some soils resulted from the movement of charco al within the profile, probably as a result of tillage.