Abstract
Atmospheric deposition of biologically active nitrogen (N) has increased dramatically over the past 60 years, with far‐reaching impacts on the structure and function of many ecosystems. Much research has examined the initial impacts of N enrichment; however, few studies have been multidecadal, and even fewer long‐term studies have examined the longevity of N‐induced impacts on N cycling after inputs cease. Here, we address this gap by reporting the state of key N pools and fluxes in a Minnesota grassland for plots that received N addition for 10 years and then none for 12 years, in comparison with plots that received annual N treatment for the entire 22 years. We found weak evidence for long‐term N retention in plots that ceased receiving treatment; and in plots that continued to receive N over the 22‐year period, retention that was high after 12 years (50–100% of inputs) was greatly reduced after 22 years (to 15%). In spite of this, net N mineralization rates remained elevated in plots that ceased receiving treatment 12 years prior, likely because N‐rich litter maintained higher N‐cycling rates. These results suggest (1) some systems do not retain much deposited N, with potentially large impacts on downstream habitats; (2) the previously reported high retention efficiencies for this and many other terrestrial ecosystems may be relatively short‐lived as N sinks become saturated over time; and (3) the effects of even small amounts of retained N in N‐limited environments may be particularly long‐lasting. In total, these findings highlight the importance of long‐term studies in evaluating the impacts of chronic N deposition to ecosystems, and urge additional research examining dynamics following N cessation to evaluate the reversibility of these impacts.