Soil Respiration and Carbon Responses to Logging Debris and Competing Vegetation

Management practices following forest harvesting that modify organic matter (OM) inputs and influence changes in the soil environment have the potential to alter soil C pools, but there is still much uncertainty regarding how these practices influence soil C flux. We examined the influence of varying amounts of logging-debris retention (0, 40, and 80% coverage) and vegetation control (initial or annual applications) on in situ bulk soil respiration, microbial respiration, and total soil C at two Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco] sites. Annual vegetation control decreased bulk respiration, which was attributed to reduced root respiration and OM inputs when competing vegetation was absent. There was no difference in potential microbial respiration or total soil C pools between vegetation-control treatments, indicating that OM inputs from competing vegetation were rapidly consumed in situ. Logging-debris retention reduced bulk respiration, microbial respiration, and soil temperature, but the significance and magnitude of the difference were variable. A soil temperature function explained between 44 and 76% of the variation in microbial and bulk respiration, but there was no effect of reduced temperature on microbial respiration with 40% coverage. Total soil C at the end of the experiment was higher with 80% coverage at the site with relatively low initial total soil C, but there was also a significant increase in total soil C at both sites when the logging debris was removed. The results suggest that root decomposition following harvesting causes an increase in total soil C, which is dependent on the magnitude of logging-debris retention, its influence on the soil environment, and ultimately the microbial response.