Backing the burn: carbon sequestration in South african mesic grasslands through sustainable fire management.

Abstract

Emphasising the ancient origins of the grassland biome in South Africa, much research has supported the role of grasslands in biodiversity, ecosystem services, and economic benefits for local populations. Despite this, the grassland biome faces challenges. It is highly transformed, poorly conserved, and urgently in need of preservation and restoration. Fire-adapted mesic grasslands are distinct ecologically from arid-adapted climatic climax grassland communities in that they are adapted to frequent defoliation, necessitating burning to maintain productivity and biodiversity. To fully understand the role of grasslands in the global climate change arena, it is necessary to quantify the impacts of grassland management, including fire and livestock grazing, on carbon exchange (source versus sink). Research in this thesis looks particularly at management with prescribed fire as a form of defoliation in mesic grasslands. To do this, I quantified soil total carbon stocks, fractionated stable pyrogenic carbon stocks, and soil carbon sequestration rates in various fire regime treatments at the Ukulinga Grassland Fire Experiment (UGFE), Pietermaritzburg. Additionally, I examined the impact of fire frequency on grazed grassland at Wakefield research farm, quantifying carbon stocks and release while exploring potential mechanisms behind observed patterns in this grazed system. Lastly, I monitored Eddy Covariate carbon flux data over a four year period at research catchment six, Cathedral Peak, KwaZulu-Natal, to understand seasonal and interannual flux within mesic grassland and observe patterns in source versus sink dynamics in these ecosystems. Research has emphasised the necessity of frequent fires to maintain grassy biomes and sustain the role of biotic and abiotic factors in this biome, through biochemical soil alteration in the form of ash deposition, and the alteration of above and below ground biomass. The complexity of managing this grassland is emphasized by the need to balance the impact of herbivory, prescribed fires, and the nature of biomass accumulation in these biomes, all of which influence carbon cycling. Differences were observed between different prescribed burning regimes. Substantial differences in soil organic carbon (SOC) and total nitrogen (TotN) stocks at different soil depths were observed, with the highest stocks observed in the top 5 and 10 cm of soil across all treatments. Annual winter and spring burns exhibit the highest SOC stocks and wider C:N ratios. Triennial burns display the lowest sequestration rates in the top 0 to 5 cm of soil, with negative rates within the 5 to 10 cm horizon. Over a period of 20 years, SOC sequestration increased in a 70-year-old experiment with no signs of stabilization within the 0 to 5 cm soil horizon, but SOC loss is noted below 5 cm in areas burnt triennially. Increased fire frequency in grassland also caused a reduction in the stable fraction of black carbon (BC), and contrastingly - increased levels of BC quantified in grassland burnt infrequently or excluded from fire. This pattern may be due to reduced alkaline ash deposition and subsequently greater soil acid saturation, suggested to result in increased pyrogenic carbon particulate size and reduced breakdown of this carbon in the soil. When considering the inclusion of livestock and grazing into grassland managed with prescribed fire, findings showed no significant differences in SOC and TotN levels between annually burned grasslands and those excluded from fire. Grassland managed with annual burning showed greater soil respiration rates compared to unburned sites, indicating greater soil microbial activity and root turnover. Annual burning and heavy grazing were both associated with reduced aboveground biomass accumulation compared to the adjoining unburned grassland. Additionally, annually burnt grasslands exhibit reduced aboveground biomass lignin and fibre percentage relative to adjacent unburned areas. Findings highlighted that increased fire frequency in grazed grassland influences livestock grazing behaviours through improvement of forage palatability and available biomass, contributing to greater belowground carbon turnover. Considering the mechanisms governing carbon dynamics in fire-dependent grassland, four years of flux data showed that following rainfall events, increased soil water content is linked to a rapid rise in soil respiration, aligning with heightened biological and photosynthetic activities during warmer growing seasons. These processes determine the rate and variability of grassland uptake and release of CO2. The findings support evidence that mesic grasslands managed with regular long term prescribed fires consistently act as carbon sinks, absorbing more carbon than they emit over periods exceeding 70 years. Findings from this research advocate for management practices utilizing a frequent burn regime, suggesting that such practices maintain persistent carbon sinks in South African mesic grasslands. This approach enhances the resilience and capacity of mesic grasslands to act as effective and consistent carbon sinks, even in the face of potential future climate change impacts. The evidence from these studies shows that prescribed fire during late winter / spring in South African mesic grasslands should enhance carbon sequestration and the role of these grasslands as a carbon sink.