Soil respiration, carbon and nitrogen leaching, and nitrogen availability in response to harvest intensity and competing vegetation control in Douglas-fir (Pseudotsuga menziesii) forests of the Pacific Northwest

Management practices following forest harvest can affect long-term soil productivity through alteration of soil carbon (C) and nitrogen (N) pools, but processes contributing to change are poorly understood. I assessed effects of three levels of logging-debris retention in combination with initial or annual applications of competing vegetation control (CVC) following forest harvesting on soil C flux, N leaching, foliar N of planted Douglas-fir, and changes in soil N and C pools for two years at two sites with contrasting soil properties. Soil C flux was lower when heavy amounts of logging debris were retained, due largely to lower bulk soil and microbial respiration as there was no difference in dissolved organic C (DOC) flux among logging-debris treatments. Increased soil C when heavy amounts of logging debris were retained at the site with lower initial soil C reflected the lower C flux, but soil C was increased at both sites when logging debris was removed, likely due to greater decomposition of belowground organic matter (OM). There was no difference in DOC leaching or soil C between CVC 3 treatments at either site, despite lower OM inputs to mineral soil with annual CVC. Higher bulk soil respiration in the initial CVC treatment indicated that OM inputs from competing vegetation were rapidly consumed, and contributed little to mineral soil C. The most pronounced effects on N leaching and foliar N were associated with annual CVC, which increased Douglas-fir foliar N at both sites, and total N leaching below the rooting zone at the high-N site. However, estimated mass of leached N was small relative to the site soil N pool, and it is unlikely that the loss will negatively affect soil productivity. Logging-debris retention had little influence on Douglas-fir foliar status or N leaching, but soil N was higher at the end of the experiment when heavy amounts of logging debris were retained at the low-N site. There appears to be small potential for logging-debris removal and annual CVC to reduce soil productivity at these sites after harvesting, but logging-debris retention may improve soil productivity, particularly at sites with low initial pools of C and N.