Afforestation and wildfire-induced grassland degradation: Impact on soil carbon dynamics in temperate grasslands, Drakensberg, South Africa

Mountains in high-rainfall temperate regions store significant amounts of soil organic carbon (SOC). Afromontane grasslands in South Africa, coevolved with fire and provide various ecosystem services (biodiversity, water delivery, carbon storage). While afforestation is proposed as a nature-based solution to climate change, knowledge on its postfire impact on SOC dynamics in fire-prone grasslands is limited. This study, conducted in the Cathedral Peak Research Catchments (initiated in the 1940 s), investigates the impact of afforestation and the subsequent wildfire on SOC dynamics in Afromontane grasslands. This study compared a degraded catchment (CP-III), afforested with Pinus patula in 1958 but burned in a 1981 wildfire, to a natural grassland catchment (CPVI) maintained through prescribed biennial burns. The wildfire completely removed P. patula, resulting in extensive soil erosion and bracken fern (Pteridium aquilinum) invasion. Soil samples were collected across slope positions to a 1-meter depth to quantify SOC stocks, delta 13C signature, active carbon (C), and total microbial activity. Results showed SOC is transported downslope in the degraded catchment due to accelerated soil erosion. This catchment had two-fold SOC stocks, active C, and microbial activity in the bottom slope compared to the natural grassland catchment. Despite the dominance of bracken fern, SOC in this catchment was primarily derived from C4 grasses and indicated little legacy of P. patula plantation. These findings are highly relevant to current policies that advocate tree planting, even in regions with substantial pre-existing SOC stocks. This study cautions against large-scale tree planting in fire-adapted systems because of the unintended consequences of afforestation, where wildfire-induced degradation leads to SOC loss and redistribution. Afromontane grasslands show better long-term SOC storage than afforestation in fire-prone regions.