Heterotrophic respiration in northern hardwood forest soils after 14 years of nitrogen addition

Background/Question/Methods

Increases in anthropogenic nitrogen (N) deposition and atmospheric carbon dioxide (CO₂) concentrations are two of the most important global environmental problems we face. Because C and N are linked through interacting cycles, understanding N effects on ecosystem C budgets is critical. Soils represent the largest terrestrial C reservoir, and the primary sink for added N in most forest ecosystems, but there is still considerable uncertainty about the response of soil C dynamics to changing N availability. Some studies have indicated that N addition can increase C storage in soils by decreasing microbial activity, whereas in other studies the effect is quite variable among plots or there is no significant effect. We examined the effect of 14-year N amendments to forest plots dominated by five different tree species (Acer saccharum Marsh, Tsuga canadiensis (L.) Carr, Fagus grandifolia Ehrh, Betula alleghaniensis Britton, Quercus rubra L.) in the Catskill Mountains, New York, by conducting a one-year laboratory incubation of forest floor and surface (0-10 cm) mineral soils.

Results/Conclusions

Across all species, N addition suppressed rates of heterotrophic respiration relative to unamended soils in the mineral soil (-22%), but not in the organic horizon, within the first week of incubation. The N effect disappeared as respiration slowed over the course of the year. The decrease in microbial respiration for N-amended soils in the first week of incubation is consistent with other studies and supports the hypothesis that N addition may increase C storage in forest soils by decreasing microbial activity. There were significant differences among the five tree species in the magnitude of the effect of N addition on the rate of heterotrophic respiration in mineral soils, with suppression values ranging from -39% in the case of B. alleghaniensis to -10% in the case of A. saccharum. Thus, our results also suggest that the impact of N addition on ecosystem C budgets is likely to depend, in part, on tree species composition.