Gains in soil C storage under anthropogenic N deposition are rapidly lost following its cessation

In the northern Hemisphere, anthropogenic nitrogen (N) deposition
contributed to the enhancement of the global terrestrial carbon (C) sink,
partially offsetting CO2 emissions. Across several long-term field
experiments, this ecosystem level response was determined to be driven, in
part, by the suppression of microbial activity associated with the
breakdown of soil organic matter. However, since the implementation of
emission abatement policies in the 1970’s, atmospheric N deposition has
declined globally, and the consequences of this decline are unknown. Here,
we assessed the response of soil C storage, and associated microbial
activities, in a long-term field study that experimentally increased N
deposition for 24-years. We measured soil C and N, microbial activity, and
compared effect sizes of soil C in response to, and in recovery from, the
N deposition treatment across the history of our experiment (1994-2022).
Our results demonstrate that the accumulated C in the organic horizon has
been lost and exhibit additional deficits 5-years post-termination of the
N deposition treatment. These findings, in part, arise from mechanistic
changes in microbial activity. Soil C in the mineral soil was less
responsive thus far in recovery. If these organic horizon C dynamics are
similar in other temperate forests, the northern Hemisphere C sink will be
reduced and climate warming will be enhanced.