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 denitrifrcation rates and litter N. (C) 1999 Elsevier Science Ltd. All rights reserved.