Assessing the fate of N deposition on the Colorado Plateau

Background/Question/Methods

Anthropogenic nitrogen (N) deposition is a growing concern in the western U.S., and is known to dramatically modify ecosystem composition and function. Although remote from major cites, Arches National Park is near an identified ‘hotspot’ of N pollution in the Four Corners region of the Southwest. Due to increasing N deposition rates, there is significant concern regarding how the area’s dryland ecosystems will respond to deposition. The National Park Service has classified Arches in their most severe risk category. Yet, our understanding of these potential responses remains notably poor. Due to the sandy, low N nature of the region’s soils, major 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 central questions: 1) does added N stay within soil and for how long; 2) are N additions primarily utilized by native grasses and/or the soil microbial community; and 3) is N is leaving the system as gas (NO_x, N₂O) or in leached forms? We measured 60 plots spanning three sites: each site with five 0, 3, 5, and 8 kg N/ha/yr plots.

Results/Conclusions

Our results show that 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. That said, significant amounts of N did indeed leave 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. However, while treatment effects were significant, soil moisture was the strongest control over rates of gas efflux, and we observed significant interactions between fertilization treatment and soil moisture at the time of sample collection. Taken together, 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-reaching influence on other biogeochemical dynamics. Taken together, data suggest that this arid, low N ecosystem passes a threshold of ecological change at relatively low levels of N, although much of the additional N is relatively quickly lost from the system.