PS 22-6 - Persistence of N saturation effects four years after the cessation of experimental N deposition to northern hardwood forests

Background/Question/Methods

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. We have assessed this possibility by measuring soil solution leaching losses of total dissolved N (TDN) and dissolved organic C (DOC), soil CO₂ efflux and aboveground tree growth in four northern hardwood forest for four years following the cessation of 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 to ambient N deposition inputs, indicative of a continuing state of N saturation, and that tree growth would remain elevated and soil respiration would remain suppressed due to suppression of the microbial community.

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 0.8 mg N L^-1 relative to the control was equivalent to additional leaching losses of 0.4 g N m^-2 y^-1. This was approximately equal to ambient wet plus dry N deposition, and thus suggests the sites remained N saturated, with leaching losses of N similar to annual N deposition inputs. From 2018 to 2021, soil solution TDN was only slightly less than in 2018, suggesting the sites remained largely N saturated. Soil solution DOC also remained elevated following cessation of the N deposition treatment. Soil respiration continued to be suppressed and tree growth enhance, but by a lesser degree than during the period of N addition. This suggests that the microbial mechanisms responsible for elevated DOC production, decreased soil respiration and enhanced tree growth are continuing, indicative 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 near N saturation and many of the measured responses.