Soil solution chemistry weak response to long-term N addition points towards a strong resilience of northeastern American forests to past and future N deposition

Northern temperate and boreal forests are large biomes playing crucial ecological and environmental roles, such as carbon sequestration. Despite being generally remote, these forests were exposed to anthropogenic nitrogen (N) deposition over the last two centuries and may still experience elevated N deposition as human activities expand towards high latitudes. However, the impacts of long-term high N deposition on these N-limited forest ecosystems remain unclear. For 18 years, we simulated N deposition by chronically adding ammonium nitrate at rates of 3 (LN treatment) and 10 (HN treatment) times the ambient N deposition estimated at the beginning of the experiment at a temperate sugar maple and a boreal balsam fir forest site, both located in northeastern America. LN and HN treatments corresponded respectively to addition of 26 kgN.ha(-1).yr(-1) and 85 kgN.ha(-1).yr(-1) at the temperate site and 17 kgN.ha(-1).yr(-1) and 57 kgN.ha(-1).yr(-1) at the boreal site. Between 2002 and 2018, soil solution was collected weekly during summer and concentrations of NO3-, NH4+, Ca2+ and pH were measured, totalling similar to 12,700-13,500 observations per variable on the study period. N treatments caused soil solution NO3-, NH4+ and Ca2+ concentrations to increase while reducing its pH. However, ion responses manifested through punctual high concentration events (predominantly on the HN plots) that were very rare and leached N quantity was extremely low at both sites. Therefore, N addition corresponding to 54 years (LN treatment) and 180 years (HN treatment) of accelerated ambient N deposition had overall small impacts on soil solution chemistry. Our results indicate an important N retention of northeastern American forests and an unexpected strong resilience of their soil solution chemistry to long-term simulated N deposition, potentially explained by the widespread N-limitation in high latitude ecosystems. This finding can help predict the future productivity of N-limited forests and improve forest management strategies in northeastern America.