Grass species recovery from combined effects of long-term drought and grazing, under nitrogen fertilization in semi-arid Savanna ecosystem, in Limpopo Province, South Africa

Abstract

Savanna ecosystems are naturally occurring ecosystems in arid or semi-arid regions that are well-suited for commercial and communal farming practices, particularly for grazing livestock. Climate change-induced drought is currently threatening this precious resource and increasing nitrogen (N) loss because of increased asynchrony between N mineralization and uptake by plants. A "DroughtAct" experiment was initiated to investigate the effects of nitrogen on ecosystem functions and services from grazed (G+) and ungrazed (G-) vegetation, under drought (D+) and non-drought (D-) conditions. The experiment consisted of four blocks that combined drought treatments with grazing treatments, which were replicated six times, resulting in 24 plots (10 x 10 m each) separated by 5 m wide corridors. For the purpose of this study, the rain-out shelter treatments were removed after six years to study vegetation recovery through evaluation of aboveground net primary production (ANPP). The data collected in the last year of drought was compared with the first two years of recovery (2021 and 2022). From the findings, no significant ANPP difference (P≤0.05) was found between the last year of drought and the first two years of recovery. However, a high ANPP was recorded under post-drought treatment at 13.93g/m2 compared to drought treatment at 2.24 g/m2 in 2022. Furthermore, a significant (P≤0.05) ANPP recovery was recorded under grazed treatment in the second year of recovery. Furthermore, the application of nitrogen fertilization exhibited negligible impact (P≥0.0.5) on the drought treatments’ aboveground net primary productivity (ANPP) range. The biomass production exhibited no significant variation across the different treatments, generally falling within the range of 31.73 to 32.12 (g/m2). The study showed that the combined effect of drought and grazing has a negative effect on ANPP recovery. The study highlights the resilience of savanna ecosystems in recovering from drought-induced stress on ANPP, while also emphasizing the importance of considering grazing management and water stress adaptation when studying ecosystem functions and services in the context of climate change