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Pietermaritzburg, South Africa

Mchunu C.,Soil Fertility and Analytical Services | Chaplot V.,University of KwaZulu - Natal
Geoderma | Year: 2012

Worldwide concerns with global change and its effects on our future environment require an improved understanding of the impact of land cover changes on the global C cycle. Overgrazing causes a reduction in plant cover with accepted consequences on soil infiltration and soil erosion, yet the impact on the loss of soil organic carbon (SOC) and its associated processes remain unaccounted for. In this study performed in South Africa, our main objective was to evaluate the impact of plant cover reduction on (i) SOC erosion by water in both particulate (POC) and dissolved (DOC) forms, and (ii) soil CO 2 emissions to the atmosphere. The study performed under sandy-loam Acrisols investigated three proportions of soil surface coverage by plants (Cov), from 100% (Cov100) for the "non-degraded" treatment to 25-50% (Cov50) and 0-5% (Cov5). POC and DOC losses were evaluated using an artificial rainfall of 30mmh -1 applied for a period of 30min on bounded 1×1m 2 microplots (n=3 per treatment). CO 2 emissions from undisturbed soil samples (n=9) were evaluated continuously at the laboratory over a 6-month period. At the "non-degraded" treatment of Cov100, plant-C inputs to the soil profile were 1950±180gCm -2y -1 and SOC stocks in the 0-0.02m layer were 300.6±16.2gCm -2. While soil-C inputs by plants significantly (P<0.05 level) decreased by 38.5±3.5% at Cov50 and by 75.4±6.9% at Cov5, SOC, the losses by water erosion of 0.75gCm -2 at Cov100 increased from 66% at Cov50 (i.e. 3.76±1.8gCm -2) to a staggering 213% at Cov5 (i.e. 7.08±2.9gCm -2). These losses were for the most part in particulate form (from 88.0% for Cov100 to 98.7% for Cov5). Plant cover reduction significantly decreased both the cumulative C-CO 2 emissions (by 68% at Cov50 and 69% at Cov5) and the mineralization rate of the soil organic matter (from 0.039 gC-CO 2gC -1 at Cov100 to 0.031gC-CO 2gC -1 at Cov5). These results are expected to increase our understanding of the impact of land degradation on the global C cycle. Further in-situ research studies, however, need to investigate whether or not grassland degradation induces net C-emissions to the atmosphere. © 2012 Elsevier B.V.. Source

Chaplot V.,University Pierre and Marie Curie | Chaplot V.,University of KwaZulu - Natal | Abdalla K.,University of KwaZulu - Natal | Alexis M.,University Pierre and Marie Curie | And 11 more authors.
Agriculture, Ecosystems and Environment | Year: 2015

The impact of agricultural practices on CO2 emissions from soils needs to be understood and quantified to enhance ecosystem functions, especially the ability of soils to sequester atmospheric carbon (C), while enhancing food and biomass production. The objective of this study was to assess CO2 emissions in the soil surface following tillage abandonment and to investigate some of the underlying soil physical, chemical and biological controls. Maize (Zea mays) was planted under conventional tillage (T) and no-tillage (NT), both without crop residues under smallholder farming conditions in Potshini, South Africa. Intact top-soil (0-0.05m) core samples (N=54) from three 5×15m2 plots per treatment were collected two years after conversion of T to NT to evaluate the short-term CO2 emissions. Depending on the treatment, cores were left intact, compacted by 5 and 10%, or had surface crusts removed. They were incubated for 20 days with measurements of CO2 fluxes twice a day during the first three days and once a day thereafter. Soil organic C (SOC) content, soil bulk density (ρb), aggregate stability, soil organic matter quality, and microbial biomass and its activity were evaluated at the onset of the incubation. CO2 emissions were 22% lower under NT compared with T with CO2 emissions of 0.9±0.10 vs 1.1±0.10mg C-CO2gC-1 day-1 under NT and T, respectively, suggesting greater SOC protection under NT. However, there were greater total CO2 emissions per unit of surface by 9% under NT compared to T (1.15±0.03 vs 1.05±0.04g C-CO2m-2 day-1). SOC protection significantly increased with the increase in soil bulk density (r=0.89) and aggregate stability (from 1.7±0.25mm to 2.3±0.31, r=0.50), and to the decrease in microbial biomass and its activity (r=-0.59 and -0.57, respectively). In contrast, the greater NT CO2 emissions per m2 were explained by top-soil enrichment in SOC by 48% (from 12.4±0.2 to 19.1±0.4gkg-1, r=0.59). These results on the soil controls of tillage impact on CO2 emissions are expected to inform on the required shifts in agricultural practices for enhancing C sequestration in soils. In the context of the study, any mechanism favoring aggregate stability and promoting SOC allocation deep in the soil profile rather than in the top-soil would greatly diminish soil CO2 outputs and thus stimulate C sequestration. © 2015. Source

Chaplot V.,University of KwaZulu - Natal | McHunu C.N.,University of KwaZulu - Natal | Manson A.,Soil Fertility and Analytical Services | Lorentz S.,University of KwaZulu - Natal | Jewitt G.,University of KwaZulu - Natal
Agriculture, Ecosystems and Environment | Year: 2012

The acceleration of soil erosion by water in most regions of the world in response to the anthropogenic modification of landscapes is a serious threat to natural ecosystem functionalities because of the loss of invaluable constituents such as soil particles and organic carbon (OC). While soil OC erosion is likely to be a major component of the global C cycle, water erosion-induced CO 2 emissions remain uncertain. In this study, our main objective was to compare the release of CO 2 from eroded topsoils and from the sediments exported by diffuse erosion during an entire rainy season. Conventional tillage (CT) and no-tillage (NT) maize treatments were considered in an attempt to set up best management practices to mitigate gaseous OC losses from agricultural soils. The study was conducted in the KwaZulu-Natal province in South Africa, whereas in many other areas of the developing world, erosion is severe and crop residue scarcity is the main challenge. CO 2 emissions from undisturbed 0-0.02m soil samples collected within 2.25m×10m runoff plots and from exported sediments by water erosion, were evaluated continuously at the laboratory over a 140-day period and compared to soil OC stocks. NT significantly reduced CO 2 emissions from both soils and sediments. Overall NT, which exhibited a greater carbon density than CT (17.70 vs 13.19kgCm -3), reduced soil gaseous emissions by 4.4% (10.40 vs 10.88gCO 2-Cm -2, P<0.05) but had a much greater impact on the release of CO 2 from eroded sediments (0.185 vs 0.778gCO 2-Cm -2), which corresponded to a 76.3% decrease. For CT, cumulative 141-day emissions were, 19% greater in sediments (0.048gCO 2-CgC -1) compared to soils (0.040gCO 2-CgC -1), while for NT, emissions were 33% lower in sediments (0.024gCO 2-CgC -1) compared to soils (0.032gCO 2-CgC -1), these differences being significant at P<0.05. The lower erosion-induced CO 2 emissions under NT could be explained by a high soil aggregate stability (mean weight diameter of 2.29±0.05mm for NT vs 1.59±0.07mm for CT, P<0.05) and the associated enhanced protection of SOC from the decomposers. These results on a land management control of water erosion-induced CO 2 emissions, might allow improving the impact of terrestrial ecosystems on greenhouse gases concentration in the atmosphere and associated climate change. © 2012 Elsevier B.V. Source

McHunu C.N.,University of KwaZulu - Natal | Lorentz S.,University of KwaZulu - Natal | Jewitt G.,University of KwaZulu - Natal | Manson A.,Soil Fertility and Analytical Services | Chaplot V.,University of South Africa
Soil Science Society of America Journal | Year: 2011

Although no-till (NT) is now practiced in many countries of the world, for most smallholders, the crop residues are of such a value that they cannot be left on the soil surfaces to promote soil protection, thus potentially limiting NT benefits and adoption. In this study our main objective was to evaluate runoff, soil, and soil organic carbon (SOC) losses from traditional small-scale maize (Zea mays) field under conventional tillage (T) and NT, with crop residues cover of less than 10% during the rainy season, in South Africa. Six runoff plots of 22.5 m 2 (2.25. 10 m) under NT and T since 2002 were considered. At each plot, soil bulk density ( ρb) and SOC content of the 0-0.02 m layer were estimated at nine pits. Top-soil SOC stocks were 26% higher under NT than under T (P = 0.001). The NT reduced soil losses by 68% (96.8 vs. 301.5 g m -2 yr -1, P = 0.001) and SOC losses by 52% (7.7 vs. 16.2 g C m -2 yr -1, P = 0.001), and diff erences in runoff were not significant. Dissolved organic carbon accounted for about 10% of total SOC losses and showed significantly higher concentrations under T than NT (1.49 versus 0.86 mg C m -2 yr -1). The less erosion in NT compared to T was explained by a greater occurrence under NT of indurated crusts, less prone to soil losses. These results showed the potential of NT even with low crop residue cover (<10%) to significantly reduce soil and SOC losses by water under small-scale agriculture. © Soil Science Society of America. Source

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