Soil carbon and phosphorus fractions and microbial activity in sandy loam humic soil under contrasting sugarcane harvest systems.
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Date
2022
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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.
Description
Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.