Nitrogen deposition in drylands: How anthropogenic nitrogen inputs affect coupled biogeochemical cycles in the Four Corners Region, U.S.A.

Background/Question/Methods

Nitrogen (N) deposition in the western U.S. is on the rise and is significantly affecting terrestrial ecosystems; for example, N deposition has been shown to modify plant community composition, exotic plant invasion, and fire regimes. However, we know little about how increased anthropogenic N inputs will affect biological soil crusts and associated biogeochemical cycling, nor of how these changes will feedback to help determine ecosystem invasibility. Nitrogen deposition is increasing in the Four Corners area of the U.S., presenting a novel challenge to the region’s land managers and decision makers. Here we used a set of field fertilizations and laboratory incubations to assess how nitrogen deposition affects plant and soil communities for ecosystems in and near Mesa Verda and Arches National Parks. Our goals were to explore questions related to (1) how increased anthropogenic N inputs affect the success of the exotic grass Bromus tectorum; (2) how altering N and phosphorus alone and in concert affects natural inputs of N via N fixation; and (3) how N deposition affects the cycling of other elements including phosphorus (P), and how interactions among elements could result in ecosystem effects that would be impossible to explain when exploring N alone.

Results/Conclusions

Using an incubation experiment and a single species of N-fixing lichen, as expected, we found that additions of N and P altered rates of N fixation. However, the effects of fertilization varied between the two National Parks. In Mesa Verde NP, where N deposition values are much higher, we found that any increases in N resulted in declines in N fixation rates. However, in Arches NP, fertilization with small amounts of N (< 2 kg N/ha/yr) actually stimulated N fixation rates, while fertilization with larger amounts suppressed fixation. In both sites, fertilization with P alone stimulated rates, and when N and P were added together, the ratio of N:P helped determine how fertilization affected N fixation. Soil phosphatase activities also varied with fertilization treatment, and these changes help explain observed responses in N fixation. In the field, we selected sites that spanned textural variation and fire chronosequences in Arches and Mesa Verde, respectively and added a series of low-level N fertilization treatments (< 8 kg N/ha/yr). Taken together our data suggest that N deposition is affecting ecosystems of the Four Corners region, and that – through interactions with other biogeochemical cycles – anthropogenic N inputs significantly alter dryland plant and soil function.