Climate warming is predicted to increase the frequency of soil freeze-thaw cycles in northern ecosystems as a consequence of reduced snow accumulation.  The soil freezing dynamics of northern temperate systems may be particularly sensitive to climate warming over winter, as air temperatures remain near freezing over much of this season.  Rates of atmospheric nitrogen deposition are also expected to increase concurrently with climate warming in these systems.  We examined the interactive effects of warming and nitrogen deposition on soil microbial biomass, extracellular enzyme activity, and fungal:bacterial ratios across seasons in a temperate old field.  We collected soil samples at six time points from March of 2007 to April of 2008: spring melt, early summer, late summer, late fall, mid-winter and the following spring melt.  We measured microbial biomass using a modified chloroform fumigation method, the activities of the hydrolase enzymes xylosidase, cellobiosidase, phosphatase, N-acetyl-glucosaminidase, α-glucosidase, and β-glucosidase using fluorescent MUB-tagged substrates, the activities of the ligninase enzymes phenol oxidase and peroxidase using L-DOPA and hydrogen peroxide as substrates, and bacterial:fungal ratios through staining and direct microscopic counts. 

Results/Conclusions

At each sampling point, the activity of most enzymes increased slightly in response to nitrogen additions, but enzymes were generally unresponsive to warming treatments.  However, an exceptionally dry summer may have diminished treatment effects in this year. Total extracellular enzyme activity was much higher in both March and November than through the summer months.  Microbial biomass followed a similar trend, indicating that changes in specific enzyme activity are relatively insensitive to seasonal changes in this system.