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Soil carbon dynamic in Afromontane grasslands (SAEON Seminar, Sept 2024)

Presenter: Dr Lindokuhle Dlamini (SAEON Grasslands-Wetlands-Forests Node) Title: Soil carbon dynamic in Afromontane grasslands: reaping the benefits of carefully designed 1940s Cathedral Peak Research Catchments Abstract: Despite the importance of South Africa's Afromontane grasslands for ecosystem services (water supply and biodiversity), soil organic carbon (SOC) research remains limited. Afromontane grasslands are ancient grasslands and fire has been an integral evolutionary force, driving ecological functions and maintaining plant diversity. While most soils in South Africa have low SOC stocks, Afromontane grasslands in temperate high-rainfall regions are huge reservoirs of SOC. It is crucial to understand terrestrial ecosystems that act as C sinks and their sensitivity to land-use change and climate change, especially with the urgent threat posed by climate change and the need to develop nature-based solutions to C mitigation. It is also important to quantify soil respiration (Rs), a crucial biogeochemical process that is a major pathway for CO2 emission from ecosystems. Due to its temperature dependence, Rs is expected to increase with global warming, particularly in high-altitude mountainous regions. This study sought to investigate the impact of fire-exclusion-driven afforestation, aspect, post-afforestation grassland degradation, and climate variability on SOC dynamics at a catchment scale in Afromontane grasslands. To accomplish this, the Cathedral Peak Research Catchments, initiated in the 1940s, were utilised. The study focused on three catchments: the degraded north-facing Catchment III which was planted with Pinus patula in 1958 but burned down by wildfires in 1981, the natural north-facing Catchment VI maintained through prescribed biennial burns, and the south-facing Catchment IX with a fire exclusion treatment. Accidental fires still occurred every 2 to 5 years, maintaining periodically burnt grassland and an afforested patch in Catchment IX. Soil samples were collected at set increments down to 1 m depth across a slope gradient (top, middle, bottom) to determine soil bulk density, SOC stocks, δ13C signature, active C, total microbial activity, and water-stable aggregates. Measurements of Rs were performed using an 8-chamber LI-8100A automated system (C-VI) and a monthly static chamber-based manual technique (all sites, approximately 3 years dataset).These carefully designed catchments provided a unique opportunity to study the impact of fire-exclusion-driven afforestation, aspect, and degradation on SOC dynamics, as well as providing insight into the impact of afforestation programs, which are being promoted as nature-based solutions for climate change. This study addressed three questions 1) Does fire-exclusion-driven afforestation result in greater SOC storage and reduced soil CO2 emissions, 2) How does slope aspect influence SOC dynamics in these grasslands? and 3) What are the consequences of planting trees in Afromontane grassland on SOC storage and soil CO2 emissions? This presentation will answer these questions and highlight future research needs.