Harvesting Douglas-fir Stands Shifts Soil Microbial Activity and Biogeochemical Cycling

Clear-cutting is a common timber extraction method used across the Pacific Northwest, but the variability in soil microbial community functional response across the range of soil types and stand ages in the region is largely unknown. Our objective was to determine if shifts in soil microbial carbon (C) and nitrogen (N) pools and activities shortly following harvest are similar among similarly treated, but environmentally diverse sites. At nine second-growth Douglas-fir forests, paired soil samples were collected one year before and after clear-cutting. Samples were analyzed for total, extractable, and microbial biomass C and N, and seven extracellular enzyme activities. Total C and N did not change following harvest, but extractable organic C decreased and total extractable N increased. Microbial biomass C generally increased, as did microbial C/N. Activities of beta-glucosidase and cellobiohydrolase increased significantly, whereas activities of phenol oxidase and peroxidase decreased. N-acetylglucosaminidase, leucine aminopeptidase, and phosphatase showed site-specific responses. Cumulative respiration during a 3-mo incubation was lower in post-harvest soils. Although the total N leached during the incubation was similar in soils sampled before and after harvest, it was dominated by dissolved organic N pre-harvest and NO3- post-harvest. Collectively, the changes in soils following harvest were related to shifts in available C and N. Pre-harvest measurements suggested that soil microbial communities may be co-limited by C and N, whereas C limitation was suggested post-harvest. Analyzing short-term microbial responses following harvest may bolster understanding of post-harvest successional processes, which is essential for determining the long-term sustainability of these practices.