Riparian soil response to surface nitrogen input: the indicator potential of free-living soil nematode populations

Nitrogen saturation of riparian ecosystems may lead to leaching and loss of surface water quality. In the search for early warning signs of N-saturation, we conducted an N-addition experiment to evaluate the indicator potential of free-living nematodes. During 6 months, we measured changes in feeding and life strategy groups, following single high and repeated low inorganic N-additions to a near-stream zone (‘zone 1’) and an upslope area (‘zone 2’) within a riparian forest in the southeast Coastal Plain of Georgia. In both zones, N-addition significantly increased the number of bacterivores but not fungivores. Only bacterivores with r-selected life strategies increased. In this group, Rhabditinae multiplied after N-addition in both zones, but Neodiplogasteridae and Myolaimidae responded to N only in zone 1, and Cephalobidae only in zone 2. Microbivore correlations with microbial data, collected concomitantly (Ettema et al., 1999. Riparian soil response to surface nitrogen input: temporal changes in denitrification, labile and microbial C and N pools, and bacterial and fungal respiration. Soil Biology & Biochemistry 31, 1609–1624), were considerably stronger in N-amended treatments than in controls, suggesting that N-addition synchronized microbial-microbivorous dynamics. At the end of the experiment, bacterivore abundance returned to control levels, probably partly due to predation, as predator populations had considerably increased several months after the first N-addition. The increase in predator abundance was greater in the single high than repeated low N-addition treatments, as was the case for bacterivore populations. These results suggest that nematodes can be indicators of N-saturation. However, for practical application, it appears that these indicators could only be meaningful when monitored together with other system characteristics, such denitrification rates and litter N.