Soil C and N Storage and Microbial Biomass Responses to Forest Management in Southern Pine Forests.

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 biogeochemical processes that can cause C and N losses from soils.  We determined the impact of forest harvest intensity, soil compaction, and their interaction on soil organic carbon (SOC), total nitrogen (TN), and soil microbial biomass C and N (MBC and MBN, respectively) in a Pinus taeda forest 15-years post-treatment at an LTSP site in Crockett National Forest in eastern Texas.  Although SOC did not differ between treatments, TN was significantly higher in the bole only treatment compared to the more severe harvest treatments. TN in all harvest treatments remained below those in unharvested control plots.  Higher soil δ¹⁵N in harvest treatments vs. unharvested controls were consistent with greater N-losses from the harvest treatments.  MBC and MBN were significantly impacted by harvest intensity, and both microbial indices decreased in the order: bole only > whole tree > whole tree+forest floor.  Soil compaction and the harvest intensity x soil compaction interaction had no effect on soil properties.  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 limits tree growth in forest ecosystems, and because soil microbial biomass plays a key role in N mineralization, data suggest that harvest practices that minimize litter and slash removal will favor soil N retention, maintain the size of the microbial biomass pool, and will help ensure the continuity of key ecosystem services that sustain the productivity of forestlands in this region.