PS 40-98 - Examining soil biogeochemical recovery from long-term simulated nitrogen deposition in a northeastern temperate forest

Abstract: While atmospheric nitrogen (N) deposition rates have historically increased because of human activity in industrialized and agricultural areas, some regions, including the Northeastern United States, are now experiencing a decrease in N deposition with the enforcement of government regulations. To make informed predictions about ecosystem nutrient cycling and forest health, ecological and biogeochemical studies must refocus on the recovery of terrestrial communities in the wake of decreasing N availability. The goal of this study was to characterize soil microbial community and biogeochemical responses to recovery from high rates of N enrichment. Since the soil community in one experiment at the Harvard Forest Long Term Ecological Research (LTER) site has been exposed to excess N for more than three decades before treatment discontinuation, it is likely that the long-term N excess will be slow to leave the system, especially considering the susceptibility of northeastern forests to N saturation. We hypothesized a lag in recovery, predicting that microbial and ecosystem parameters would more closely resemble plots receiving a lower dose of N amendment before approaching the control. To test this hypothesis, we collected soil samples from the Harvard Forest Chronic Nitrogen Amendment Study (Est. 1988, Petersham, MA, USA). These plots in a hardwood dominated stand contain three treatments: control (no added N), N50 (+50 kg N/ha/yr), and discontinued N150 (+150 kg/ha/yr until 2019). We measured microbial biomass using phospholipid fatty acid (PLFA) analysis and biogeochemical responses (soil respiration, total soil C and N, potential extracellular enzyme activity). Soil respiration and microbial enzyme activity in N50 plots continued to follow previously reported patterns of suppression. Microbial biomass remained lower in plots where N amendment treatments continued. Suppression in biomass was paired with N-induced shifts in bacterial and fungal community composition in previously reported studies, so the effects of N150 on community composition is of interest for future work even after discontinuation of N fertilization. Accumulated N in these soil communities may remain and continue to suppress organic matter decomposition, explaining increased soil C levels previously observed in the N-amended plots. Since soil microbes are vital players in the cycling of nutrients in forest ecosystems, understanding their response to these changes is necessary in monitoring the health of forests.