Soil nitrogen dynamics in a fire-maintained forest ecosystem: results over a 3-year burn interval

Longleaf pine ecosystems throughout the southeastern United States developed under recurring fire disturbance, which often leads to conditions of low fertility, particularly reduced N pools in the soil. While numerous studies have documented short-term effects of prescribed burning on nutrient dynamics, few have evaluated the consequences of repeated fire on N availability and ecosystem productivity over longer time frames, such as a 3-year fire return interval. We assessed the environmental controls on soil inorganic N flux over a typical burn interval (3 yr) and related N availability patterns to aboveground net primary productivity (ANPP) and N demand across a hydrologic gradient in a longleaf pine—wiregrass landscape. Net N mineralization was measured within 0–10 and 0–90 cm soil depths to determine the influence of belowground litter inputs (via fine root turnover) on N dynamics. After 42 mos, annual estimates of N mineralization were 11.8, 6.8 and 3.5 kg ha−1 yr−1 for the top 10 cm of soil and 15.5, 11.4, and 5.3 kg ha−1 yr−1 for the top 90 cm of soil for xeric, mesic and wet-mesic sites, respectively. Overall, N mineralization rates were negatively correlated with both ANPP and bulk soil moisture, and were insufficient to supply estimated ANPP-N demands. These results suggest that belowground litter inputs may indeed influence N cycling patterns in these frequently burned systems but may not entirely account for the discrepancy between N availability and ANPP-N demand.