wAbstract
@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.
Keywords: Acid buffering capacity; Aluminum; Base cations; Nitrogen
critical load; Soil acidificationx
1. Introduction
2. Materials and methods
@2.1. Study site and sample collection
@2.2. Soil pH and acid buffering capacity
@2.3. Extractable soil cations
@2.4. Data analysis
3. Results
@3.1. pH and cation responses
@3.2. Acid buffering capacity
4. Discussion
5. Conclusions
Acknowledgements
References