Abstract

Our SERDP/SEMP-funded project is comparing vegetation and nitrogen cycling among upland mixed pine-hardwood forests at Fort Benning that are managed primarily through prescribed burning and differ in soil texture (from sandy to clayey) and intensity of military training (lighter dismounted infantry vs. heavier mechanized training). Forest management and land use influence soil nitrogen cycling, which can affect forest sustainability. We analyzed soil nitrogen parameters before and after prescribed fire to determine if fire effects on nitrogen cycling differ with soil texture or military use. Prescribed burns were conducted prior to our research in 2000, and again in 2002. Pooled soil organic layers were collected from all sites in 2001, 2002 and 2003, and laboratory incubations were used to measure soil mineralization and nitrification during the growing seasons. In 2002 and 2003, field soil incubations were conducted in both the dormant and growing seasons, and tension lysimeters were used to measure soil nitrogen leaching in eight sites. Initial pre-burn laboratory results indicate greater initial extractable mineral soil nitrogen in clayey sites with lighter land use intensity. After 84 days, heavier use areas with sandy soils had the greatest nitrate production and overall mineral nitrogen pool. Initial organic layer dry mass (1166 g/m2) and nitrogen pool (8.7 g N/m2) were greatest in clayey sites with lighter military training. Fire minimally consumed the soil organic layer, which ranged among sites from 2% to 36% of the pre-burn mass. Immediate fire effects on mineral soil extractable total nitrogen were variable, ranging from a 54% decline to a 327% increase. In combination with other data from these sites, the initial post-burn results suggest management toward more frequent burning can sustain desired longleaf pine vegetation, with a more pronounced result in sites with sandy soil. Further analyses are being conducted to assess longer-term responses to fire and identify thresholds of sustainable burn frequency over variation in soil texture and military training. This information will help guide adaptive management of upland forests at Fort Benning.