Changes in Soil Nitrogen Storage and Dynamics in Response to Forest Management in Southeastern Pine Forest

Forest harvesting increases radiation reaching the soil surface, decreases transpiration and rainfall interception, and increases the amount of precipitation reaching and infiltrating into the soil. The magnitude of these impacts varies with intensity of tree harvest method, but generally results in warmer, moister soils that favor microbial activity and accelerate N cycling processes that can result in leaching and gaseous N losses from the ecosystem. More intense forest harvest (i.e., removal of increasing amounts of aboveground biomass) results in greater N losses, potentially limiting the productivity and sustainability of future rotations. This study determined the impact of forest harvest intensity, soil compaction, and their interaction on N-cycling in a loblolly pine (Pinus taeda L.) forest 15-years following treatment. We quantified N and δ¹⁵N in litter, root, and soil compartments at the Long Term Soil Productivity Experiment in Crockett National Forest in eastern Texas. Treatment plots of 0.4 ha were established in 1997 and consisted of three harvest methods (bole only, whole tree, and whole tree + forest floor removal) in factorial combination with three levels of soil compaction (none, intermediate, and severe); all treatment combinations were replicated (n=3). Sampling was conducted quarterly from March 2011 through March 2012. Litter N pool sizes were reduced significantly by increasing harvest intensity and varied significantly through time. Litter N was highest in the bole only treatment (10-13 g N m^-2) and lowest in the whole tree+forest floor removal treatment (9-10 g N m^-2). Soil total N storage in the 0-10-cm increment was also reduced significantly by increasing harvest intensity and was highest in the bole only treatment (74-99 g N m^-2) and lowest in the whole tree+forest floor removal treatment (61-83 g N m^-2). Tree harvest methods had no effect on the total root N pool (fine + coarse roots) in the upper 10 cm of the soil profile; however, root N varied significantly through time. Soil δ¹⁵N was significantly impacted by harvest intensity and time. Soil δ¹⁵N values were always lowest (1.21-1.96‰) in the bole only treatment, while those in the more intensely harvested treatments were higher (1.41-2.72‰). Higher soil δ¹⁵N values suggest that N-losses following tree harvest were greater in the more intense harvest treatments, consistent with the smaller soil N pool sizes observed in those treatments. Neither soil compaction nor the harvest method x soil compaction interaction had an effect on any response variables. Results indicate that more severe forest harvest practices that maximized biomass and litter removal resulted in significant ecosystem N-losses that have not yet been replenished even 15 yrs following treatment. Since N is a limiting nutrient for tree growth in the sandy soils of the Gulf Coastal Plain, tree harvest practices that favor N-retention will help ensure the continuity of key ecosystem services and sustain the productivity of forestlands in this region.