Anthropogenic nitrogen deposition is a growing influence on ecosystems the world over. The effect on aboveground biomass seems to be largely positive, but the impact of elevated nitrogen on soil microbial systems is established to be negative. Twenty-four years of experimental nitrogen deposition at a rate of 3 g N/m2/y broadly suppressed soil microorganisms over the course of the active nitrogen loading across four northern Michigan hardwood sites. Methods similar to those used to establish this suppression were applied to samples collected in 2020, three years after the cessation of active nitrogen treatments to assess soil microorganism populations. Soil and sugar maple root samples from the same four study sites were collected and assessed for phospholipid fatty acid (PLFA) and neutral lipid fatty acid (NLFA) contents. These acids are functional group specific and their levels in samples indicative of various microbial groups. We hypothesized that population levels would still be suppressed as the soils were nitrogen saturated when nitrogen deposition ended, and the intervening three years may not be long enough for this situation to ameliorate.
Results/Conclusions
Results of this only partially supported this hypothesis. PLFA and NLFA indicative of fungi in soil were suppressed in areas that had previously received nitrogen deposition, as were PLFA levels associated with several bacterial groups and with arbuscular mycorrhizal fungi (AMF). Other bacterial indicators, actinomycete indicators, and several measures of total PLFA and NLFA levels showed no significant difference between treated and control plots. No functional group found in root samples showed suppression. These results suggest that populations and activities of soil microorganisms may be in the early stages of recovery from their suppressed state since the end of active nitrogen loading. During active nitrogen loading, suppressed bacterial and fungal communities were associated with lowered rates of litter decomposition a corresponding accrual of surface soil carbon. Therefore, these recovering communities may return decomposition rates to their pre-treatment rates over time. Increased AMF abundance could also have long-term impacts on carbon allocation within trees, potentially decreasing the amount of carbon allocated to aboveground growth, which was increased during active nitrogen loading.
