The effects of increasing periods under sugarcane monoculture (managed by preharvest burning)
on soil organic matter content and related soil properties were investigated in the 0 to 10 cm layer
of a sandy coastal Ochric Cambisol (Glenrosa soils) and a red Rhodic Ferralsol (Hutton soil) from
the sugar belt of KwaZulu-Natal. The organic C content at both sites under undisturbed
vegetation is about 48 g C kg-I. This declined exponentially with increasing years under
sugarcane. For the Glenrosa site, organic C reached a new equilibrium level of about 20 g kg-I
while at the Hutton soil the equivalent value was 41 g kg-I. The higher organic matter content
maintained at the Hutton site was attributed mainly to clay protection of organic matter since the
clay content of the Hutton soil was 61 % compared to the 18 % for the Glenrosa soil. The loss
of soil organic matter under sugarcane resulted in a concomitant decline in soil microbial biomass
C, microbial quotient, basal respiration, aggregate stability, arylsulphatase and acid phosphatase
activity. The activities of arylsulphatase and acid phosphatase like those for concentrations of
microbial biomass and organic C, were higher for the Hutton than Glenrosa soils. At the Glenrosa
site, the natural OBC abundance in soils was used to calculate the loss offorest-derived, native soil
C and the concomitant input of sugarcane-derived C. Sugarcane-derived C increased over time
until it accounted for about 61 % of organic C in the surface 10 cm in soils that had been under
sugarcane for greater than 50 years.
The effects of agricultural land use (including burnt sugarcane) on organic matter content and
related soil properties were compared with those under undisturbed native grassland in KwaZulu-Natal.
Two separate farms situated on Oxisols were used and both contained fields with
continuous long-term (>20 years) cropping histories. At site 1, soil organic C content in the surface 10 cm followed the order permanent kikuyu pasture> annual ryegrass pasture> native
grassland> preharvest burnt sugarcane > maize under conventional tillage (CT). At site 2,
organic C in the surface 20 cm decreased in the general order kikuyu pasture> native grassland
> annual ryegrass pasture> maize under zero tillage (ZT) ~ maize (CT). Organic C, microbial
biomass C, microbial quotient, basal respiration and aggregate stability were substantially greater
in the surface 5 cm under maize ZT than maize CT. In the undisturbed sites (eg 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 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 was similar under maize CT and sugarcane, values for microbial biomass
C, microbial quotient, 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 soil organic matter, microbial activity and aggregate stability is
potentially problematic under maize CT and sugarcane and measures to improve organic matter
status should be considered. For sugarcane, this could include green cane harvesting and the use
of green manure crops in rotation. / Thesis (M.Sc.)- University of Natal, Pietermaritzburg, 2002.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/4913 |
| Date | January 2002 |
| Creators | Dominy, Carol Susan. |
| Contributors | Haynes, Richard J. |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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