Dai, Xiaoyan (Texas A&M UniversityTexas, A&M University, Dept. of Soil & CropCollege Station, TX, 77843; Phone: 979-845-1785; Email: xdai@ag.tamu.edu)

Soil Carbon Storage and Dynamics in Response to Fire Seasonality in a Temperate Mixed-grass Savanna

X. Dai *, T.W. Boutton, M. Hailemichael, R.J. Ansley, K.E. Jessup

Prescribed fires are often utilized to manage and manipulate the composition of plant communities. In the southern Great Plains, fire is utilized to control woody plant encroachment into grasslands. Although these fires have the potential to alter the quantity and quality of organic matter inputs to the soil, few studies have evaluated the impacts of this land management technique on soil organic carbon (SOC) storage. Therefore, the purpose of this study was to evaluate the impact of repeated vegetation fires and their season of occurrence on SOC storage and dynamics in temperate mixed-grass savanna in north-central Texas.

Four fire treatments (n = 3 replicates/treatment) were examined: summer only (SF), winter only (WF), alternate summer/winter fires (SWF), and unburned controls. Winter fires were conducted between January- March, and summer fires were between August- September. Fire treatments have been maintained for 13 yrs. In each replicate, soil cores were taken to 1 m under three vegetation types: C₃ grasses, C₄ grasses, and mesquite trees (n = 6/vegetation type). SOC and δ¹³C were determined using an elemental analyzer interfaced with a continuous flow isotope ratio mass spectrometer.

SOC storage in the 0-20 cm depth increment was significantly greater in SF (2700 g C m^-2) and SWF (2800 g C m^-2) treatments relative to the unburned controls (2400 g C m^-2). In contrast, WF (2600 g C m^-2) had no significant impact on SOC compared to controls. There were no treatment effects on SOC at depths > 20 cm. Vegetation cover type had no significant influence on SOC storage at any depth. δ¹³C values of SOC increased from approximately -21‰ at 0-10 to -15‰ at depths >20 cm. This indicates that the proportion of SOC derived from C₄ plants increased with depth, ranging from 55-60 % at 0-10 cm to 60%-90% below 10 cm. Thus, all treatments were once strongly dominated by C₄ grasses prior to woody plant invasion during the past century. δ¹³C of SOC was not affected by fire treatment implying that fire intensity and frequency are not sufficient to significantly change the vegetation pattern in this mixed-grass savanna.

In summary, repeated summer fires appeared to alter ecosystem processes in a manner that increased SOC storage in the upper 20 cm of the soil profile in this mesquite/mixed-grass savanna. Summer fire treatments (SF and SWF) increased SOC storage by 13-17% over the past 13 yrs. In contrast, winter fires had no effect on SOC storage. Studies of net primary productivity and soil microbial biomass and activity are currently underway in an effort to elucidate the mechanisms responsible for carbon sequestration in response to fire and its season of occurrence. These results will be of interest to scientists, policy makers, and land managers who are now evaluating the potential for land management practices to store atmospheric carbon and mitigate the potential for global climate change.