Elevated N deposition often increases C storage in forest ecosystems, some of which is attributed to a decrease in soil CO2 respiration (Rs) and corresponding increase in soil organic matter. However, the wide variability of results from relatively short-term experiments hinders our ability to understand the response of Rs to elevated N deposition. Preliminary Rs data from a long-term, whole-watershed fertilization experiment at the Fernow Experimental Forest (FEF), WV, indicate no difference in Rs between the fertilized and reference watersheds, despite data suggesting that Rs should be reduced- decreases in fine root biomass, root exudation, mycorrhizal colonization and microbial enzyme activity in the fertilized watershed. As a result, we are conducting more intensive Rs measurements in the FEF to test whether an increase in sample size and seasonal range is needed to detect a difference or if a compensatory CO2 respiration mechanism, such as greater respiration per gram of root, accounts for the lack of difference between watersheds. We are measuring Rs, soil temperature at 5 and 10 cm depths, and soil % moisture in 10 plots per watershed, with 4 subplots per plot, for a total of 80 observations weekly in the summer, fall and spring, and monthly in the winter.
Although the mean respiration rate in the fertilized watershed is 11.23% lower than the unfertilized watershed- a value consistent with many studies- this apparent change is not statistically significant at α=0.05. However, a linear ANOVA model using the log-transformed Rs values and including the effects soil temperature (at 10 cm depth), soil % moisture, watershed, and plot nested within watershed found a trend towards lower Rs in the fertilized watershed (F=3.2655, p=0.0874). To explain the weak, if any, response of Rs to N additions, we hypothesize that other factors may influence CO2-respiring mechanisms. Specifically, the rate of autotrophic respiration per gram of fine root may be greater in a N-rich environment, the degree of organic C protection by soil minerals may be altered, and/or differences in species composition may diminish the overall response of Rs to fertilizer additions despite reductions in other C-releasing processes. At this point, our results suggest that long-term N additions to a temperate deciduous forest may not enhance C storage in soils or suppress the return of CO2 to the atmosphere as strongly as previous studies found.