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Soil Carbon and Nitrogen Storage in Response to Fire in a Temperate Mixed-Grass Savanna

^a Department of Rangeland Ecology and Management, Texas A&M University, College Station, TX 77843-2126
^b Texas Agricultural Experiment Station, P.O. Box 1658, Vernon, TX 76384, X. Dai, present address: Department of Soil and Crop Sciences, Texas A&M University, College Station, TX 77843-2474

^* Corresponding author (xdai{at}ag.tamu.edu)

Received for publication June 29, 2005. Vegetation fires may alter the quantity and quality of organic matter inputs to soil, rates of organic matter decay, and environmental factors that influence those processes. However, few studies have evaluated the impacts of this land management technique on soil organic carbon (SOC) and total N in grasslands and savannas. We evaluated the impact of repeated fires and their season of occurrence on SOC and total N storage in a temperate mixed-grass–mesquite savanna where fire is used to control woody plant encroachment. Four fire treatments varying in season of occurrence were examined: summer only (SF), winter only (WF), alternate summer and winter fires (SWF), and unburned controls. In each treatment, soils were sampled to 1 m under three vegetation types: C₃ grasses, C₄ grasses, and mesquite trees. The SOC storage at 0 to 20 cm was significantly greater in SF (2693 g C m^–2) and SWF (2708 g C m^–2) compared to WF (2446 g C m^–2) and controls (2445 g C m^–2). The SWF treatment also increased soil total N (271 g N m^–2) relative to all other treatments (228–244 g N m^–2) at 0 to 20 cm. Fire had no effect on SOC or total N at depths of >20 cm. Vegetation type had no significant influence on SOC or total N stocks. The ¹³C value of SOC was not affected by fire, but increased from –21 at 0 to 10 cm to –15 at depths of >20 cm indicating that all treatments were once dominated by C₄ grasses before woody plant encroachment during the past century. These results have implications for scientists, land managers, and policymakers who are now evaluating the potential for land uses to alter ecosystem C storage and influence atmospheric CO₂ concentrations and global climate.

Abbreviations: SF, summer fire • SOC, soil organic carbon • SOM, soil organic matter • SWF, alternate summer and winter fire • WF, winter fire