Elevated levels of inorganic nitrogen (N) deposition and earthworm invasion have the potential to alter N dynamics in eastern North American temperate forests. A regional comparison was conducted across 21 sugar maple (Acer saccharum Marsh) stands in southern Ontario, where forest floor C:N ratios ranged from 17 to 38 showed that, similar to many other studies, rates of potential net mineralization and nitrification increased below a forest floor C:N ratio threshold of approximately 25 and that nitrification rates are positively correlated with foliar N concentration. However, detailed measurements at four representative stands, receiving between 9.8 and 19 kg N ha−1 y−1 in throughfall, showed that foliar N levels were highest at the site with the lowest N deposition. The primary difference amongst these sites was the presence of invasive earthworms. Specifically, sites without earthworms had significantly higher forest floor N with a lower C:N ratio than the sites with earthworms. There was no significant difference in the rate of sugar maple litter decomposition or chemistry amongst the sites assessed after 540 days using fine (2-mm mesh) litter bags, suggesting that differences in forest floor N levels were most likely due to consumption of litter by large earthworm species and that the lower C:N ratio of the forest floor in sites without earthworms is brought about primarily by a much longer residence time. This work supports the conclusions that forest floor N concentration (or C:N ratio) has a very strong control on N dynamics in forests, but shows that the presence of earthworms can have an impact on forest floor C:N ratio and hence N dynamics that is greater than current levels of atmospheric inorganic N deposition in temperate forests of Ontario.
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The authors would like to thank Shanel Raney for assisting in the presentation of the manuscript and Bradley Jennings for the collection of earthworm data. This work was funded by an NSERC grant awarded to SAW, with additional support from Environment Canada and the Ontario Ministry of Environment.
Watmough conceived the idea for the paper, obtained the funding, and wrote the manuscript. Meadows performed all the field and lab work and commented on drafts of the manuscript.
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Watmough, S.A., Meadows, M.J. Do Earthworms Have a Greater Influence on Nitrogen Dynamics Than Atmospheric Nitrogen Deposition?. Ecosystems 17, 1257–1270 (2014). https://doi.org/10.1007/s10021-014-9794-0