Investigating the Structure and Function of Soil Microbial Communities in Managed Douglas–Fir Forests.

Increasing concern regarding how alteration of natural ecosystems affects biogeochemical cycles, and in particular the global carbon (C) cycle, has motivated investigation of the response of soil microbial communities to land use practices such as timber harvest. Forests play a major role in global C storage, with forest soil C stocks responsible for more than 10% of this storage. However this function can be altered substantially in response to land use change. In this study, soil from nine managed Douglas-fir forest sites across the Cascade and Coastal regions of the Pacific Northwest was collected one year before and one year after conventional timber harvest. Quantification of extracellular enzymatic activity, measurement of dissolved organic and biomass carbon and nitrogen, isolation of soil microbial DNA, and sequencing of isolated DNA samples have been performed to build a profile of the soil community after biomass harvest. This was done in order to assess how microbial response and potential functional shifts may vary across the Pacific Northwest region. Microbial biomass C levels were consistently higher post-harvest, with average increases ranging from 15 to 105% across sites, whereas levels of dissolved organic C were on average 10 to 40% lower across sites. This is coincident with an average of 150% and 250% increased activity of cellobiohydrolase and beta-glucosidase, two enzymes associated with C cycling. Investigation of the soil community’s structural and functional change after timber harvest may provide insights regarding how changes in belowground ecosystems translate to changes in the cycling of C and nutrients.