Grassland degradation and rehabilitation of soil organic carbon and nitrogen stocks.
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Date
2014
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Abstract
Land degradation is widely considered to adversely affect soil fertility, soil quality,
constrain productivity, subsequently leading to a decline in soil organic carbon (SOC) and
nutrients in soils, yet little is known about the stocks, environmental controls,
destabilization mechanisms and carbon sequestration potential of degraded grassland soils.
The aim of this dissertation was to evaluate (1) the impact of land degradation on SOC and
nitrogen (N) stocks, distribution and SOC quality, to elucidate the environmental controls,
in a communal rangeland with varying intensities of degradation, (2) to examine the
rehabilitation potential of the same rangeland (3) to assess the spatial variability and
replenishment potential of SOC and N stocks in a typically degraded grassland catchment.
A meta-analysis was conducted to provide a quantitative review of the impact of land
degradation on SOC stocks in grassland soils, worldwide. Subsequently, the impact of
degradation on SOC and N stocks and organic matter quality was investigated in a
communal rangeland in the KwaZulu-Natal province, South Africa with varying intensities
of degradation. Thereafter, different rehabilitation techniques were applied in the same
communal rangeland to replenish SOC and N stocks. Advantage was also taken of 23 ha
degraded grassland catchment to assess the spatial variability, carbon replenishment
potential of SOC and N and to elucidate the main environmental controls.
Degradation resulted in a significant depletion of SOC stocks in grassland soils, both in the
meta-analysis and field experiment. The meta-analysis indicated that the depletion of SOC
stocks as a result of degradation was more pronounced in sandy acidic soils under dry
climate than clayey soils under wet climate. The field experiment showed that degradation
significantly depleted SOC stocks by 89% and N stocks by 76% in sandy acidic soils at the
study site. The reduction of the stocks due to degradation was accompanied by an increase
in soil bulk density, a decrease in soil aggregate stability and concomitant decrease of
macro and micronutrients (e.g, Ca by 67%; Mn, 77%; Cu, 66% and Zn, 82%). SOC and N
stocks decreased sigmodially with a linear decrease in grass aerial cover. After two years,
the “Savory and fertilization techniques increased SOC stocks by 6.5% and 3.9%,
respectively. At catchment level, degradation led to high spatial variability of SOC and N
stocks controlled primarily by soil surface characteristics, including grass cover, soil
surface crusting and secondarily by topography. The carbon replenishment potential of
degraded grassland catchment was estimated to be 4.6 t C ha-1, with clay-rich Acrisols
having a greater capacity to replenish SOC stocks than sandy Luvisols and Gleysols.
In conclusion, the results of this dissertation indicate that degradation results in high
depletion of SOC and N stocks. However, rehabilitation has the potential for carbon
sequestration and can lead to more sustainable grassland ecosystems.
Description
Ph. D. University of KwaZulu-Natal, Pietermaritzburg 2014.
Keywords
Soils -- Carbon content., Carbon sequestration., Land degradation., Soil exhaustion., Grassland ecology., Theses -- Soil science.