Short communicationActivity of an introduced earthworm (Lumbricus terrestris) increases under future rates of atmospheric nitrogen deposition in northern temperate forests

•Experimental atmospheric nitrogen (N) deposition suppresses soil decay processes.•Forest floor mass increases with an increase in litter N concentration.•Forest conditions positively affect non-native earthworm introduction.•Activity of Lumbricus terrestris increases under experimental N deposition.•Earthworm activity may negate soil carbon storage under experimental N deposition.

When temperate forests of North America are exposed to long-term experimental levels of atmospheric nitrogen (N) deposition that mimic predicted future rates, elevated concentrations of inorganic N suppress microbial decay processes and increase the mass of the forest floor, storing more carbon (C) as soil organic matter. However, when non-native earthworms were serendipitously introduced to forests subjected to long-term experimental atmospheric N deposition, the greater forest floor mass with a higher N concentration had a positive effect on earthworm activity by increasing earthworm abundance and consumption of the forest floor. Here, we present evidence from a long-term experimental N deposition study showing how the abundance of the introduced earthworm, Lumbricus terrestris, increased significantly under elevated rates of atmospheric N deposition (+363%) in one of four study sites and contributed to a decline in the forest floor (−50%). In addition, mineral soil C (+97%) and N (+117%) concentrations increased as earthworms redistributed a greater proportion of organic matter belowground under experimental N deposition. We conclude that earthworm-induced changes to the forest floor can supersede a decline in microbial decay under experimental N deposition that has previously increased both forest floor mass and turnover time, thereby potentially negating increases in soil C storage.