Woody overstorey effects on soil carbon and nitrogen pools in South African savanna

Woody plant encroachment in savannas may alter carbon (C) and nitrogen (N) pools over the long-term, which could have regional or global biogeochemical implications given the widespread encroachment observed in the vast savanna biome. Soil and litter %C and %N were surveyed across four soil types in two encroached, semi-arid savanna landscapes in northern South Africa. Litter at sampling points with a woody component had a higher %N and lower C: N ratio than litter at solely herbaceous points. Severely encroached areas had lower C: N ratios throughout the soil profile than less encroached areas. Soil %C and %N were highly influenced by soil texture but were also influenced by the presence of a woody overstorey, which increased surface soil %C on three soil types but decreased it on the most heavily encroached soil type. Soil C sequestration may initially increase with bush encroachment but then decline if bush densities become so high as to inhibit understorey grass growth. Key words