Many temperate forests across North America and Europe are highly retentive of nitrogen (N), despite high rates of deposition and no net accumulation of N in forest biomass. Recent work shows that soil N may aggrade in soil pools over long timescales, but the mechanisms that permit this continued net accumulation in soils remain poorly understood. Here, we explore the role that legacy effects of acid deposition may play in soil retention of N through the lens of a watershed acid remediation experiment performed at Hubbard Brook. We asked: 1. How have soil N cycling rates and pool sizes have responded to the remediation treatment almost two decades after the treatment? 2. How have patterns of stream N export during stormflow changed as a result of the alteration of soil N processes? We used isotopic labelling approaches to measure gross and net rates of N transformations in soils, and we used high frequency sensor data to examine storm N export.
Results/Conclusions
We found that N cycling in surficial soils was accelerated in the remediated watershed. Gross N mineralization in leaf litter was 26% higher than in an adjacent reference watershed. Soil inorganic N pools were 4-55% higher in the remediated watershed as well. These increased N cycling rates and inorganic N availability occurred alongside enhanced stream N export, especially during storm events. We found that stream water N concentrations increased markedly during storm events in the treated watershed, but not the reference watershed, suggesting the mobilization of large standing pools of inorganic N during storms. These results suggest that, as forested ecosystems recover from acid deposition over the coming decades to centuries, forest N cycling may become faster and less retentive.