Soil nutrients and biogeochemical cycling in the forest-alpine tundra ecotone

The patterns of soil nutrients and biogeochemical cycling were investigated on several spatial scales in the forest-alpine tundra ecotone (FATE) in the Front Range of Colorado. The FATE is predicted to be a hotspot for atmospheric deposition, but the interaction of trees and the wind may create unique source and sink locations for nutrients at multiple spatial scales.;This dissertation had three goals: (1) evaluate if aspect and side of the Continental Divide affected patterns of soil nutrients at abrupt vegetation transitions, (2) determine the relationship between snow depth, dust inputs, and soil properties at three spatial scales in the FATE on Niwot Ridge, (3) investigate the relationships between total and available pools of nitrogen (N) and phosphorus (P) on Niwot Ridge to examine nutrient limitation.;Results from the abrupt transitions indicate that aspect and side of the Continental Divide affected the patterns of elemental concentrations. Calcium and nitrogen in forest mineral soils were higher on east aspects consistent with the prediction that deposition of nutrients should be higher when the forest was located downwind of the alpine tundra. The low organic horizon C:N ratio measured in these subalpine forests at treeline suggests that N deposition may already be altering N cycling.;On Niwot Ridge, snow depth was related to soil temperatures, plant community, and surface litter decomposition, but had little relationship with most other soil properties. The patterns for the concentrations of soil carbon, nitrogen, and acid cations was generally predictable at all three sampled spatial scales. The high variability across the ecotone obscured most spatial patterns in base cations. However, the concentrations of base cations were higher downwind than upwind of trees perhaps related to dust deposition.;Finally, there were clear relationships in N and P across the ecotone. Total pools of soil N and P were highly correlated and were highest near tree limit. Available N and P were generally inversely related across the ecotone. In general, both the available and total N:P ratios increased across the ecotone from the forest to the tundra. This shift may indicate a change from N limitation to P limitation.