Making up biomass: Environmental effects of nitrogen fertilization in sagebrush communities on the Pinedale Anticline in Wyoming

Background/Question/Methods

Nitrogen additions have been shown to elicit variable responses in semi-arid ecosystems, with responses increasing with precipitation. These responses to nitrogen are important to understand given the spatial extent of semi-arid ecosystems worldwide, and the global trend of increasingly available nitrogen. In this study, we evaluate the impact of nitrogen additions on semi-arid sagebrush (Artemisia spp.) ecosystems for two reasons: First, sagebrush ecosystems are poorly understood, despite their prevalence in the western US. Second, large-scale nitrogen additions have begun on sagebrush landscapes in Wyoming in order to mitigate population declines in mule deer (Odocoileus hemionus). We sought to fill a knowledge gap with implications for large-scale landscape manipulation.  The objectives of our study were to (1) evaluate the effectiveness of nitrogen fertilization in increasing sagebrush biomass and forage quality, and (2) assess effects of nitrogen addition on soil biogeochemistry and vegetation community structure. We fertilized 15 plots across 5 locations in western Wyoming using nitrogen (5.47g N m^-2), in the form of urea. In addition, we immobilized available nitrogen through surface hay treatments (254g hay m^-2). We measured mineralized soil nitrogen and soil gas emission (CO₂, CH₄, & N₂O), plant species cover, sagebrush new growth (“leaders”), and sampled for forage quality.

Results/Conclusions

Nitrogen additions failed to increase growth of sagebrush and caused shifts in the above ground plant community. There were no significant increases in either length or density of sagebrush leaders as a result of nitrogen addition. We also found no significant growth response of the ephemeral branches. The only significant change in growth was seen between the hay and nitrogen treatments, where the hay-treated sagebrush leaders grew 0.73cm less. The vegetation community shifted; nitrogen-fertilized plots were only 62% similar to the controls (Bray-Curtis). We found increases in mean cover for Erigonium umbellatum and Opuntia polyacantha in the nitrogen-fertilized plots. Over the two years of this study, we did not find indications of nitrogen limitation. These findings are despite a wet growing season in 2014; NOAA reports that this region received 15.7” of rain, 2” above average. Consistent with other studies examining nitrogen fertilization, we found shifts in plant species abundance. Given the negligible effect of nitrogen fertilization on sagebrush biomass, forage quality must be improved (i.e. higher nitrogen concentration) to mitigate population declines in mule deer. Thus, we have not yet found positive or negative effects of nitrogen addition to this system, only a lack of response.