Differences in Englemann spruce forest biogeochemistry east and west of the Continental Divide in Colorado, USA

We compared Englemann spruce biogeochemical processes in forest stands east and west of the Continental Divide in the Colorado Front Range. The divide forms a natural barrier for air pollutants such that nitrogen (N) emissions from the agricultural and urban areas of the South Platte River Basin are transported via upslope winds to high elevations on the east side but rarely cross over to the west side. Because there are far fewer emissions sources to the west, atmospheric N deposition is 1–2 kg N ha⁻¹ y⁻¹ on the west side, as compared with 3–5 kg N ha⁻¹ y⁻¹ on the east side. Species composition, elevation, aspect, parent material, site history, and climate were matched as closely as possible across six east and six west side old-growth forest stands. Higher N deposition sites had significantly lower organic horizon C:N and lignin:N ratios, lower foliar C:N ratios, as well as greater %N, higher N:Ca, N:Mg, and N:P ratios, and higher potential net mineralization rates. When C:N ratios dropped below 29, as they did in east-side organic horizon soils, mineralization rates increased linearly. Our results are comparable to those from studies of the northeastern United States and Europe that have found changes in forest biogeochemistry in response to N deposition inputs between 3 and 60 kg ha⁻¹ y⁻¹. Though they are low by comparison with more densely populated and agricultural regions, current levels of N deposition, have caused measurable changes in Englemann spruce forest biogeochemistry east of the Continental Divide in Colorado.