| Riparian ecosystems serve as buffers between aquatic bodies and upland land use, protecting water quality from pollutants such nitrogen (N). The buffer mechanism results from complex interactions between hydrologic flows, plant uptake, decomposition and denitrification in the riparian zone. However, this function may be impaired when the riparian ecosystem becomes N-saturated. The objective of my dissertation research was to evaluate whether soil nematodes could be useful 'early warning signs' or 'indicators' of riparian soil N-saturation. Nematodes, microscopic soil animals, are abundant and diverse, have responded to N-enrichment in other ecosystems, and are intimately involved in soil processes.; My approach included reviewing which factors influence nematode assemblage structure and function, to be able to answer practical questions like "how predictable are nematode species composition and species responses to N-saturation?". Secondly, I surveyed the spatial distributions of soil N and bacterial-feeding nematode taxa in an N-loaded riparian wetland downslope of a dairy waste application site. Thirdly, I have analyzed nematode and microbial dynamics following experimental N-additions to an undisturbed riparian forest.; I conclude from the literature review that, at the species level, nematode assemblage structure may be rather unpredictable, as it appears to be influenced by stochastic factors such as predation, disturbance, and dispersal. In the riparian wetland, patch dynamics of species belonging to the bacterivore genus Chronogaster illustrated the importance of such stochastic factors. However, the wetland data also showed that populations of some other taxa were consistently favoring locations where soil nitrate-N was highest. The same taxa (Rhabditinae and Cephalobidae, r-strategists sensu lato) significantly increased in response to experimental N-addition in the riparian forest, and provided a clearer signal than underlying microbial changes. The N-response of r-selected bacterivores corroborates results of other N-enrichment studies, and suggests that sufficiently sensitive (and more predictable) nematode indicators may be found at a taxonomic resolution lower than the species-level. However, because r-selected taxa increase in response to any disturbance that temporarily raises microbial production, nematode indicators will be only meaningful in an N-saturation monitoring program when measured in tandem with other system characteristics, such as litter N and denitrification rates. |
