Land degradation impact on soil organic carbon and nitrogen stocks of sub-tropical humid grasslands in South Africa

• We evaluate the impact of degradation; decrease in grass cover on SOC stocks
• Sigmoidal decrease of SOC stock due to linear decrease in grass cover
• Degradation also decreased SOC in the silt + clay particle size fraction.
• SOC decrease accompanied by decreasing soil bulk density and aggregate stability

Land degradation is recognized as a main environmental problem that adversely depletes soil organic carbon (SOC) and nitrogen (SON) stocks, which in turn directly affects soils, their fertility, productivity and overall quality. While it is expanding worldwide at rapid pace, quantitative information on the impact of land degradation on the depletion of SOC and SON stocks remains largely unavailable, limiting the ability to predict the impacts of land management on the C losses to the atmosphere and associated global warming. The main objective of this study was to evaluate the consequences of a decrease in grass aerial cover on SOC and SON stocks. A degraded grassland showing an aerial cover gradient from 100% (Cov100, corresponding to a non-degraded grassland) to 50–75% (Cov75), 25–50% (Cov50) and 0–5% (Cov5, corresponding to a heavily degraded grassland), was selected in South Africa. Soil samples were collected in the 0.05 m soil layer at 48 locations along the aerial cover gradient and were subsequently separated into the clay + silt (2–20 μm) and sand (20–2000 μm) fractions, prior to total C and N analysis (n = 288). The decline in grass aerial cover from 100% to 0–5% had a significant (P < 0.05) impact on SOC and SON stocks, with losses by as much as 1.25 kg m− 2 for SOC and 0.074 kg m− 2 for SON, which corresponded to depletion rates of 89 and 76%, respectively. Furthermore, both the C:N ratio and the proportion of SOC and SON in the silt + clay fraction declined with grass aerial cover, which was indicative of a preferential loss of not easily decomposable organic matter. The staggering decline in SOC and SON stocks raises concerns about the ability of these acidic sandy loam soils to sustain their main ecosystem functions. The associated decrease in chemical elements (e.g., Ca by a maximum of 67%; Mn, 77%; Cu, 66%; and Zn, 82%) was finally used to discuss the mechanisms at stake in land degradation and the associated stock depletion of SOC and SON stocks, a prerequisite to land rehabilitation and stock replenishment.