Streamside interactions among salamanders, earthworms, and soil nitrogen cycle processes

Background/Question/Methods

In North American forests, exotic earthworms are expanding their ranges into forests that were previously earthworm free. Earthworms can disrupt native forests by altering understory plant composition, nutrient dynamics, and trophic interactions. The main objective of this study was to determine if quantity and diversity of earthworms and salamanders along stream banks are associated with differences in nitrogen cycling in these soils: focusing, in particular, on assessing the rates of nitrification, mineralization and denitrification along the stream bank. Soil samples were collected from stream banks in the Adirondack, Catskill and Helderberg regions of New York State where salamander and earthworm abundances, level of co-occurrence, and diversity were measured. Nitrification, mineralization and denitrification rates were compared across 16 streams, with more detailed studies in three of the streams.  Potential denitrification rates were determined using a denitrification enzyme assay, while potential nitrification and mineralization rates were determined using laboratory incubations.    

Results/Conclusions

There was a positive, though not significant, relationship between denitrification potential and the number of earthworms encountered (all species pooled). In streams sampled at finer scales, denitrification was highest in areas with both earthworms and salamanders present. Preliminary comparisons of nitrification and mineralization rates indicate no differences among sites on the basis of total abundances. As some earthworm species are thought to exert more of an effect on nitrogen cycle processes than others, further analysis will be done on a species by species basis (from 16 sites, a total of 1138 salamanders were identified to species in situ, and 1237 earthworms were collected for identification). Analysis is underway to determine if earthworms and/or salamander activity leads to increases in denitrification, or if environmental conditions favor high levels of earthworms, salamanders and denitrification. Future work will focus on designing controlled experiments to further examine whether earthworms affect nitrogen cycling along the stream edge, as they do in the upland forest floor, and whether there is potential for salamanders to modify those interactions.