Evaluating the impact of prescribed burning and herbivore grazing on soil nutrients and carbon dynamics in a savanna grassland in Satara, Kruger National Park

Abstract

Savanna grasslands in South Africa are well adapted to fire and herbivory, yet these land management practices modify vegetative biomass and overtime affect forage productivity. Fire-herbivory interaction effects offer a more credible explanation for the occurrence of sporadic changes in savanna ecosystems and are crucial for maintaining tree-grass coexistence. Although, fire and herbivory have often been studied independently, they both act together in savanna ecosystems to influence soil quality leading to a depletion in soil nutrients and soil organic carbon (SOC). There is ongoing uncertainty regarding the underlying mechanisms controlling the decline in soil nutrients and little is known on the destabilization mechanisms involved in SOC depletion in grassland soils. The objectives of this study were (1) to quantify the effect of late burning and herbivore grazing on soil nutrients in the topsoil of a savanna grassland, (2) to determine the effect of late burning and herbivore grazing on soil aggregate stability and distribution of carbon (C) within micro-and-macro-aggregates in a savanna grassland. To achieve these goals, topsoil from Satara experimental sites in Kruger National Park (KNP), South Africa were examined where late burning plus grazing and fire-grazing exclusion (control) experiments had been conducted annually for 7 years. Within the 25-ha experimental plot, three 10 × 10 m sub-plots were laid out randomly and soil samples were collected in the 0-10 cm depth for laboratory analysis. Results revealed that late burning and herbivore grazing depleted SOC in loamy sand soils at the KNP site. The depletion of SOC associated with late burning and herbivore grazing were accompanied by a reduction in total C and nitrogen (N) by 26% and 18%, respectively. This led to a further decrease in soil aggregate stability and concomitant decrease in calcium (Ca) by 6%, effective cation exchange capacity (ECEC) by 3% and zinc (Zn) by 5%. The alteration of the soil structure and depletion of SOC and some nutrients was ascribed to the disruption in the physical and chemical nature of the soil associated with the disaggregating effects linked to fire and grazing disturbances. Collectively, the findings obtained in this study demonstrates that 7-years of frequent burning and herbivore grazing in savanna grasslands leads to in a decline in micro- and macronutrients, which overtime may reduce overall soil fertility.