Nitrogen deposition decreases acid buffering capacity of alpine soils in the southern Rocky Mountains

Deposition of anthropogenic N can potentially alter biogeochemistry of ecosystems, acidifying soils and surface waters, lowering availability of some nutrient cations, and increasing concentrations of toxic metals. Remote regions in western North America are exhibiting symptoms of ecological change due to N deposition. We used an existing N addition experiment to empirically estimate the thresholds for the loss of acid buffering capacity and nutrient base cations, decreases in pH, as well as increases in toxic metals in response to N deposition in alpine soils of Niwot Ridge in the southern Rocky Mountains. Soil acid buffering capacity was evaluated using laboratory acid titration, and concentrations of extractable cations and soil pH were evaluated in field-collected soils from plots at ambient deposition (8 kg N ha− 1 yr− 1), and plots with N added at 20, 40, and 60 kg per hectare per year above the ambient level. Soil acid buffering capacity decreased with increasing N inputs (40% decrease at highest input), and was associated with a decrease in pH, loss of extractable magnesium and increases in aluminum and manganese. The threshold at which acidification occurred was around 28 kg N ha− 1 yr− 1. Significant variation in soil acid buffering capacity, base cation concentrations, and pH was noted among experimental blocks, independent of treatment effects, possibly reflecting variation in the amount of winter snow cover and its influence on inputs of anthropogenic N, and differences in local dust deposition. The results of this study indicate granitic-derived alpine soils of the southern Rocky Mountains are susceptible to acidification at relatively modest levels of N deposition.