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

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.