Fire Frequency Influences Soil Organic Carbon Stocks and Sequestration in a South African Mesic Grassland

Abstract

Abstract Background Grasslands are significant terrestrial carbon sinks, storing large amounts of carbon in aboveground vegetation and roots and soils. However, the potential of grasslands to store organic carbon (OC) depends on how they are managed, particularly with the application of anthropogenic fires, which present both positive and negative carbon feedbacks. South African grasslands are fire-prone ecosystems, and fire plays an important ecological role in shaping and maintaining grasslands. Fire acts as a driver for fluctuations in carbon sequestration, through removal of above-ground biomass, redistribution of nutrients, and direct contribution of inputs through thermal mineralisation. However, regular burning has been recorded to reduce carbon in the top layers of soil, through the reduction of organic matter content and microbial biomass.Results The study analysed the effects of different fire frequencies and seasons on soil organic carbon (SOC) stocks, total nitrogen (TN), and carbon to nitrogen (C:N) ratios in a mesic grassland ecosystem in South Africa. The results showed that there was no significant interaction between treatment and depth, but significant differences were observed in the main effects of 5 cm depth increments (to a depth of 30 cm) and burn treatments. SOC and TN exhibited the highest stocks in the top 5 and 10 cm of soil across all treatments, with decreasing amounts at deeper soil depths respectively. Grasslands burnt annually in winter and spring showed the highest SOC stocks and wider C:N ratios. Conversely, reduced SOC and TN stocks were observed in biennial spring and autumn burns, with wider C:N ratios. Triennial burns showed the lowest rates of sequestration in the top 5 cm of soil, and negative sequestration rates were observed in 10 cm soil depth increments. Winter annual burns showed the highest SOC sequestration, while triennial burn frequencies exhibited negative sequestration in 10 cm soil depth increments. exhibited negative sequestration in 10 cm soil depth increments.Conclusions The results indicate that prescribed burns in spring have a significant impact on SOC and N concentrations in the upper soil horizons. SOC sequestration increased over time and did not show signs of stabilization, with SOC loss observed in soils below 5 cm in areas burnt triennially. The findings have implications for grassland management decisions relating to productivity, land rehabilitation, or conversion to pastoral use.