Dominant mycorrhizal association mediates the impact of nitrogen deposition on ecosystem processes
Chronic nitrogen (N) deposition has been linked to changes in soil carbon (C) and N retention and loss in temperate forest ecosystems, but the magnitude and direction of such responses are highly variable. Identifying the drivers of this variation is critical for predicting ecosystem-scale feedbacks to climate change and mitigating the downstream effects of N deposition. Given that tree species composition often influences forest responses to N, we evaluated the hypothesis that variable responses may be simplified by considering mycorrhizal associations. Specifically, we evaluated soil enzyme, microbial biomass, mineralization, and nitrate leaching responses to N addition in two forest types: forests dominated by trees associated with either arbuscular mycorrhizal fungi (AM forests) or ectomycorrhizal fungi (ECM forests). As AM forest soils are richer in inorganic N than ECM forests soils, we hypothesized that microbes in ECM forests would be more N-limited and thus respond more strongly to increases in inorganic N than those in AM forests. Furthermore, we hypothesized that different soil C and N responses to N deposition would be driven by varied nutrient limitations and stoichiometric strategies of soil microbes in AM and ECM forests.
We found that ECM forest microbes increased the ratio of C-degrading enzymes to N-degrading enzymes (P<0.05) and decreased the ratio of C mineralization to N mineralization (P<0.05). In contrast, the stoichiometries of enzyme and mineralization activities of AM forest microbes were not altered by N addition (P>0.05). This suggests that ECM forest microbes are relatively more N-limited than AM forest microbes and will allocate more energy and resources to enhanced C acquisition and C use efficiency when N-limitation is alleviated. In contrast, nitrification rates increased in AM forests (P<0.001), and AM forests leached more nitrate under chronic N addition than ECM forests (P<0.05). Combined, these findings suggest AM forest microbes function similarly in response to increased inorganic N while ECM forest microbes adjust the stoichiometries of their enzyme production and C and N use efficiencies. Hence, N deposition may have greater down-stream consequences in forests rich in AM-associated trees while having greater effects on forest soils in ECM-rich forests. With respect to C cycling, N deposition may lead to increased C sequestration in ECM forests due to decreased soil organic matter decomposition rates while having no effect on soil C in AM forests.