Assessing the Fate of Nitrogen Deposition on the Colorado Plateau

Abstract:

Anthropogenic nitrogen (N) deposition is a growing concern in the western U.S., and is known to modify ecosystem composition and function. Arches National Park is near an identified ‘hotspot’ of N pollution in the Southwest U.S. There is real concern regarding how the area’s drylands will respond to elevated deposition. Yet, our understanding of potential responses remains poor. Uncertainties persist regarding how these systems will hold and cycle additional N, and how N inputs will affect other aspects of biogeochemistry. We used a field fertilization experiment in Arches NP to explore three questions: 1) does added N stay within soil and for how long; 2) are N additions primarily utilized by native grass and the soil microbial community; and 3) is N leaving the system as gas (NO_x, N₂O) or in leached forms? Results indicate added N remained in the plots even 90 days after fertilization: soil NO₃^- concentrations were consistently elevated and were related to the amount of fertilization. Significant amounts of N left the system through leaching and as gas (N₂O and NO_x); in particular, NO_x losses were greater in the plots receiving 8 kg N/ha/yr relative to the control plots, a pattern that matched soil NO₃^- concentrations. While treatment effects were significant, soil moisture was the strongest control over gas efflux rates, and we observed significant interactions between fertilization treatment and soil moisture at the time of sample collection. These data support the idea that N strongly interacts with rainfall patterns to regulate pulse-driven losses of excess N. There was no treatment effect in foliar N concentrations, but treatment effects on soil P concentrations and microbial biomass stoichiometry suggest that N addition has a broad influence on other biogeochemical dynamics. Data suggest that this arid, low N ecosystem passes a threshold of ecological change at low levels of N, although much of the additional N is relatively quickly lost from the system.