Effects of increased nitrogen deposition on soil nematodes in alpine tundra soils

Because of anthropogenic activities, atmospheric deposition of nitrogen has increased on some high elevation ecosystems in North America. On Elk Mountain in SE Wyoming, USA, we found that ice, formed by the impaction of supercooled cloud droplets, contains nitrogen levels (NH4+ = 58 +/- 47 mu mole/l, NO3- = 52 +/- 40 mu mole/l) significantly higher than snow (NH4+ = 7 +/- 5 mu mole/l, NO3- = 14 +/- 7 mu mole/l). At this site the impaction process (riming) occurs on krummholz and is an important mechanism of water and nutrient deposition. We sampled nematodes in alpine soil for two seasons under this rime ice deposition and in adjacent meadow and krummholz soil with only snow deposition. No significant difference was found in nematode density and trophic composition between snow and rime ice deposition zones in krummholz; and nematode densities were significantly higher in meadow soil than in the krummholz rhizosphere. Densities of active nematodes were highest immediately after snow melt and a positive correlation was found between nematode density and percent soil moisture. With subsequent soil drying, nematodes gradually entered anhydrobiosis and this process began earlier in the exposed meadow than under the krummholz canopy. In a subsequent microcosm experiment we exposed nematodes in two alpine soils to four different nitrogen treatments [ammonium nitrate (NH4NO3) at 0, 20, 40 and 80 kg N/ha/yr] and two temperatures (5 and 25 degrees C). None of the three variables had a significant effect on nematode density. But at the first sampling (1 month) the interaction between soil type and nitrogen level was significant, and at the second and final sampling (6 months) the interaction between soil type and temperature was significant. Nematode response to increasing nitrogen deposition rates varied in a complex way with soil type and temperature. Under the microcosm conditions, nematode community composition shifted to opportunistic rhabditid species. Our work indicates that higher levels of nitrogen deposition (>80 kg N/ha/yr) were necessary to produce detectable effects on nematode density in this alpine region.