COS 39-4 - The effect of fire regimes on the storage of recalcitrant organic carbon fractions in mesic grassland soils

Abstract: Fire plays a fundamental part in shaping and maintaining the ecological integrity of grassy biomes. The frequency of applied high-intensity fire regimes in grasslands influences the composition of available biomass and the rate at which these grassland species sequester Carbon. While C sequestration is primarily noted in the upper layers of grassland soils, it has been proposed that carbon resulting from the application of pyrogenic events, may occur in more recalcitrant forms in deeper soil horizons. Research into the organic geochemical process of Organic Carbon (OC) conversion into pyrogenic forms of carbon (PyC), has been coming to light, with much literature attempting to comprehend the role of PyC in the carbon cycle. These PyC fractions are suggested to have a significantly longer period of existence in soil due to its inert ability to decompose in terrestrial environments, potentially contributing toward slow-cycling carbon pools in the soil. Refractory organic matter (refractory OM) and Black organic carbon (BOC) fractions have been detected in soils globally and considered to be a highly stable fraction of the PyC component occurring in soils. We investigated the effect of prescribed fire regimes on the accumulation of refractory OM and BOC fractions within mesic grassland soils, to a depth of 15 cm. Soils were collected from a long-term (50-year) burning experiment at Ukulinga research farm, Pietermaritzburg, South Africa and analysed for soil carbon fractions using a Thermogravimetric analyser (TGA).

We hypothesized that stable PyC would exhibit variations within treatments and soil depth in response to burn frequency (no burn, annual, biennial, triennial) and season (winter, spring, autumn). The results indicated that an increased frequency of fire resulted in a reduction of BOC within the soil. Prescribed triennial burns in the spring, autumn, and winter were found to be significant contributors to the variation in BOC as well as the no-burn control and mowed treatments exhibited greatest quantities of BOC. Additionally, it was observed that there were no significant differences in refractory OM within the burn treatments, but there were significant decreases in refractory OM with increasing depth increments below 5 cm. The observed increase in BOC in prescribed burns with a greater burn interval is suggested to be related primarily related to available organic matter as well as temperature and moisture sequences associated with triennial burns and the absence of fire specifically. It is suggested that BOC may consist of both thermally labile and stable components.