Soil carbon and phosphorus fractions and microbial activity in sandy loam humic soil under contrasting sugarcane harvest systems.

Abstract

Soils have the potential to store large quantities of organic carbon, with benefits of mitigating climate change and improved crop/pasture productivity. Humic soils, which are only known to occur in South Africa, are highly weathered soils with high acidity, low base status, > 1.8% soil organic carbon (SOC) and good internal drainage and the main land uses on these soils include forestry, grassland, maize and sugarcane production. Where sugarcane is produced, pre-harvest burning is practiced with the aim of removing excess trash, for easy harvesting and improving sucrose recovery, while policy shifts appear to be developing globally towards green cane production, in view of climate change. There is a paucity of research findings on the effects of green cane relative to pre-harvest burning on SOC, phosphorus and microbial activity in these acidic and carbon-rich humic soils. Therefore, the objective of this study was to determine the effects of green cane relative to pre-harvest burning on concentrations of stocks and fractions of SOC, P fractions, soil microbial biomass and activities of enzymes associated with cycling of carbon and P in sandy loam humic soils. The soils were analysed for SOC and its fractions, soil aggregates stability, microbial biomass carbon (MBC), β-glucosidase enzyme, charcoal C, P fractions, MBP, acid and alkaline phospho-monoesterase and organo-mineral complexes. Soil C stocks and soil microbial quotient (SMQ) were calculated from SOC concentrations and MBC. Green cane retention resulted in higher SOC content and stocks, MBC, β-glucosidase activity and SMQ when compared to pre-harvest burning in these humic soils. The carbon content in the macro-aggregates fraction constituted > 60% of total SOC making it the primary C storage fraction for both green cane and burnt cane. The aggregate stability (only in the top 10 cm) and SOC in macro- and micro-aggregate were higher, while mineral associated C in μSilt+μClay was lower under green cane than under burnt cane. Additional analysis of effects of sugarcane production relative to forest showed that soils under sugarcane had lower charcoal-C than forest only at Wartburg but not Eshowe and Eston, while the SOC, Fe and Fe+Al in Al/Fe-OM complexes were significantly higher under sugarcane than forest only at Eshowe but not Eston and Wartburg. Soil charcoal-C was significantly higher under pre-harvest burning than green cane, with no differences in SOC, Al and Fe in Al/Fe-OM complexes, between the production systems. Green cane reduced pH and increased available P, P fixation through precipitation with Al and Fe and as CDB-P when compared to burnt cane. The MBP in the top 20 cm, and activity of acid phospho-monoesterase was significantly higher, while that of alkaline phospho-monoesterase was lower under green cane when compared to burnt cane. The findings of this study imply that green cane production on sandy loam humic soils increase SOC storage, especially in macro-and micro-aggregates, microbial activity, and P availability, when compared to pre-harvest and that charcoal C and organo-mineral complexes, contribute to the high OC in these soils. The findings suggest that green cane production has a potential for sustainable sugarcane production when compared to burnt cane. Green cane production could contribute to lowering of greenhouse gas emissions when compared with the burnt cane.