Concerns regarding the potential deleterious effects of chronic N deposition and N saturation on terrestrial and aquatic ecosystems have led to emissions controls that have resulted in declining N deposition in the NE US and Western Europe. However, the effects of this reduced N deposition on ecosystems that had achieved a state of N saturation are not certain, as lower, but still elevated rates of N deposition may be sufficient to maintain a state of N saturation, along with its consequences. In 2018, we addressed this possibility by measuring soil solution leaching losses of total dissolved N (TDN) and dissolved organic C (DOC) in a northern hardwood forest that had received experimental N deposition inputs of 3 g N m-2 y-1 above ambient from 1994 to 2017. We hypothesized that elevated leaching losses of TDN and DOC would continue, with leaching losses of N being similar ambient N deposition inputs, indicative of a continuing state of N saturation.
Results/Conclusions
Soil solution TDN in the former N deposition treatment declined significantly in 2018 relative to concentrations during the time of N additions. However, TDN concentrations were still elevated above control levels. The enhancement of 0.78 mg N L-1 relative to the control is equivalent to additional leaching losses of 0.4 g N m-2 y-1, given soil water recharge of 50 cm at the site during this very wet year. This is approximately the same as ambient wet plus dry N deposition at the study site, and thus suggests the site remains N saturated, with leaching losses of N similar to annual N deposition inputs. Soil solution DOC in 2018 remained as elevated as in years prior to the cessation of the N deposition treatment. This suggests that the microbial mechanisms responsible for elevated DOC production are continuing to occur, indicative of conditions of excess N availability. The findings suggest that after 24 years of elevated N deposition treatments, current ambient N deposition may be sufficient to sustain a state of N saturation and many of the measured responses.