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 years (old) were made to investigate the potential impact of cane production on soil organic carbon (OC) levels and chemistry. The comparisons showed that very little change had occurred in total OC and in ‘light’ fraction (<1·6 Mg/m3). Increasing pyrophosphate extractability throughout the profile at some sites, as a result of cultivation, however, suggested that the organic 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 levels 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 solid state 13C nuclear magnetic resonance spectroscopy, did not change significantly at each site even though between site differences were large. Some soils contained substantial amounts of charcoal which was of pre-cane origin. In some of the coarse-textured soils, smaller amounts of charcoal produced during the burning of cane appeared to accumulate below the A1 horizons in the profiles. It also appeared likely that the redistribution of carbon in the upper horizons of some soils resulted from the movement of charcoal within the profile, probably as a result of tillage.