Chemical stabilisation of carbon stocks by polyvalent cations in plinthic soil of a shrub-encroached savanna grassland, South Africa

Abstract

Shrub encroachment into open grasslands is an extensive land cover change that influences the accrual and depletion of soil carbon (C) stocks, yet the mechanisms underpinning the direction and magnitude of soil C gain or loss due to encroachment are poorly understood. In this study, we quantified and compared surface (0–5 cm) C stocks in shrub-encroached and open grassland savanna on similar plinthic soil and topographic position, and determined controlling edaphic factors. Soil C content and C stocks were on average 148% and 117% greater in shrub-encroached grassland soil, with recorded values of 14.19 g C kg−1 and 0.91 kg C m−2 respectively, compared to 5.73 g C kg−1 and 0.42 kg C m−2 determined in open grassland soil, respectively. The shrub-encroached grassland exhibited a lower soil C:N ratio compared to the open grassland, indicating greater N mineralisation. The soil C content and stocks were positively correlated with effective cation exchange capacity (r = 0.73, P < 0.05) and exchangeable calcium (r = 0.71), manganese (r = 0.64) and zinc (r = 0.61), but negatively correlated with soil bulk density (r = −0.69). These findings suggest that total C content and stocks in the surface plinthic soil of the studied shrub-encroached grassland are chemically stabilized via complexation interactions with polyvalent cations. As a result, it has been concluded that stabilisation offered by the polyvalent cations is critical in controlling the accrual of soil C stocks in the shrub-encroached grassland.