Soil Enzyme Activity Varies Among Second-Growth Douglas-Fir Stands of the Pacific Northwest.

The soil C pool is the largest reservoir of C over which humans can have a measurable impact. Thus, the ability to understand and manipulate the processes that control the soil organic matter, namely the microbial community, is critical in learning to manage C inputs and outputs sustainably, and minimize anthropogenic forcing of climate change. This study seeks to elucidate how microbial enzymatic activity is correlated with C cycling and sequestration. Soil samples collected from nine forested second-growth Douglas-fir stands located throughout the Coast and Cascade ranges of Oregon and Washington serve as the basis for this study. Within each site, five composite samples (0-15 cm) and two samples at depth (15-30 and 30-100 cm) were collected, for a total of 63 samples. These samples were then analyzed using extracellular enzyme assays, specifically enzymes involved in decomposition and C turnover (peroxidase, phenol oxidase, β-glucosidase, cellobiohydrolase), as well as nutrient availability (N-acetyl glucosaminidase, phosphatase, leucine aminopeptidase). These activities were compared between sites as well as correlated with extractable C and microbial biomass C (via chloroform fumigation extraction). Unsurprisingly, enzyme activity decreased with depth. However, fewer notable trends existed between enzymes, or between sites. Enzyme activities were highly variable among locations, with cellobiohydrolase and phosphatase exhibiting the greatest variability (seven-fold differences among sites). β-glucosidase and N-acetyl glucosaminidase values were more stable across sites with less than three-fold differences among locations. A notable positive correlation existed between cellobiohydrolase and β-glucosidase activities, as might be expected for enzymes involved in cellulose degredation. The lack of many other immediately obvious correlations among the examined enzymes, given the similarity of sampling locations, could be an indication of the complexity of the mechanisms governing the interactions of soil C and enzyme production. However, these baseline data will be instrumental in understanding the short term effects of harvest on extracellular enzyme activity when the sites are sampled again post-harvest.