Nitrogen cycling across the globe has been fundamentally altered due to regional elevated N deposition, and there is a cascade of ecosystem consequences including shifts in species composition, eutrophication, and soil acidification. The alpine is an ecosystem type that exhibits these changes under relatively low levels of N depositions due to short growing seasons and shallow soils limiting N storage. While recent work provided estimates for the magnitude of N associated with ecosystem changes, less is known about the within-site factors that may interact to stabilize or amplify the differential response of N pools under future conditions of resource deposition. To examine numerous potential within-site and regional factors (both biotic and abiotic) affecting ecosystem N pools we examined the relationship between those factors and a suite of ecosystem pools of N followed by model selection procedures. Measurements were conducted at Niwot Ridge LTER and in Rocky Mountain National Park in three distinct alpine meadow communities (dry, moist, and wet meadows). These meadows span a moisture gradient as well as plant community composition, thereby providing high variability of potential biotic and abiotic drivers across small spatial scales in the alpine.
Results/Conclusions
In general, regional scale abiotic factors such as site levels of annual average N deposition or precipitation were poor predictors of soil resin extractable N, although predictions were improved by including blocking factors such as the plant community type. Within-site univariate factors also were poor predictors of soil resin extractable N, with soil pH the best abiotic factor, and plant species richness the best biotic factor, suggesting that other unexamined within-site factors or multivariate factors such as plant community composition may play an important role in determining growing season soil N. Pore water concentrations of nitrate following snow melt were equally uncorrelated with N deposition and precipitation, but were significantly correlated negatively with elevation, positively with the percent of clay in soils, and negatively with plant species diversity. As increases in pore water nitrate is associated with declining soil pH and buffering capacity over time these findings stress the importance of maintaining species diversity in the alpine. Future work will include potential interactions among with-site factors affecting N pools in the alpine, as well as statistically examine the bidirectional relationship between plant communities and on soil pools of N.