Soil carbon persists under prescribed fire and experimental drought in a semi-arid savanna

Abstract:

Soils are the largest terrestrial sink of carbon, and are therefore, an important component of climate change mitigation. Drought, however, can alter the quality and quantity of carbon stored in soils, which is especially relevant in drought-prone savannas of the Southern Great Plains. Texas savannas are also experiencing displacement of native vegetation due to woody plant encroachment, a problem often solved with the use of prescribed fire. While prescribed fires have proven useful in curbing vegetation displacement, the combined effects of drought and fire on the quality and quantity of soil organic carbon (SOC) are not well understood. We tested the combined effects of prescribed fire and drought on SOC in a semi-arid savanna using a fully factorial experimental design with 8 replicate plots of the following four treatments: experimental drought with rainout shelters, prescribed fire, experimental drought and prescribed fire, and a control (i.e., absence of drought or fire). Prescribed fire and drought plots (N = 32) were established at the Texas A&M AgriLife Sonora Center in March 2018 and plots were burned again in August, 2019. Samples were collected in all plots immediately following the second prescribed burn and processed for chemical and physical properties. We hypothesized that fire and drought would lead to compositional differences of SOC. We also hypothesized that the prescribed fire would reduce SOC stocks due to volatilization, whereas carbon may accumulate in plots subjected to drought from decreased decomposition. Our results show that there were no effects of fire or drought on percent SOC and SOC stocks. However, soil moisture, pH, and bulk density were correlated with SOC, highlighting the importance of soil physical and chemical properties on SOC storage. The composition of soluble SOC, determined via optical measurements (absorbance and fluorescence), differed between burned soils and control soils. Unexpectedly, optical indices representing aromatic or highly decomposed SOC were lower in burned plots relative to the control plots. Overall, our results suggest that managing these semi-arid savanna ecosystems with prescribed fire will not adversely alter soil stocks even following short-term drought.