Title: Impacts of atmospheric nitrogen deposition on vegetation and soils in Joshua Tree National Park
Author: Allen, E.B.; Rao, L.; Steers, R.J.; Bytnerowicz, A.; Fenn, M.E.;
Source: In: Webb, R.H.; Fenstermaker, L.F.; Heaton, J.S.; Hughson, D.L.; McDonald, E.V.; Miller, D.M. eds. 2009. The Mojave Desert: Ecosystem Processes and Sustainability. University of Nevada Press, Las Vegas: pp. 78-100
Publication Series: Book
Description: The western Mojave Desert is downwind of nitrogen emissions from coastal and inland urban sources, especially automobiles. The objectives of this research were to measure reactive nitrogen (N) in the atmosphere and soils along a N-deposition gradient at Joshua Tree National Park and to examine its effects on invasive and native plant species. Atmospheric nitric acid (HNO3) and ozone (O3) were elevated in western Joshua Tree National Park, and there were some high levels of atmospheric ammonia (NH3) in the east that may be related to local sources. The central areas of Joshua Tree National Park were lowest in reactive atmospheric N; HNO3 was higher in summer, while NH3 was higher in winter. Extractable soil N was generally higher in sites that had higher atmospheric reactive N.
Invasive grasses and forbs, such as Mediterranean split grass (Schismus barbatus), red brome (Bromus madritensis), and stork’s bill (Erodium cicutarium) have become more productive and widespread in the last two decades. To test the hypothesis that elevated N may be related to the success of invasive species, N fertilizer experiments were done at four sites in Joshua Tree National Park at levels of 5 and 30 kg N/ha for each of two years. Sites with higher and lower atmospheric N deposition were selected, as well as low-elevation sites with creosote bush scrub and high-elevation sites with pinyon-juniper woodland. Non-native grass biomass increased significantly with 30 kg N/ha at three of the four sites but not with 5 kg/ha. The response of native forbs to N fertilizer was related to the amount of non-native grass present. The species richness of native forbs declined with fertilization at a site with high non-native grass cover, but native richness and cover increased with fertilization at a site with low non-native grass cover. Non-native grass cover was not related to atmospheric reactive N concentrations but is likely controlled by soil conditions such as soil texture and soil N-supply rate, as well as anthropogenic N inputs. This study provides evidence that atmospheric N deposition over time will increase the soil N to levels that may shift the community to a species-poor vegetation dominated by non-native grasses.
Keywords: Colorado Desert, diversity, invasive species, Joshua Tree National Park, Mojave Desert, native forbs, nitrogen deposition, nitrogen gradient
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Allen, E.B.; Rao, L. E.; Steers, R.J.; Bytnerowicz, A.; Fenn, M.E. 2009. Impacts of atmospheric nitrogen deposition on vegetation and soils in Joshua Tree National Park. In: Webb, R.H.; Fenstermaker, L.F.; Heaton, J.S.; Hughson, D.L.; McDonald, E.V.; Miller, D.M. eds. 2009. The Mojave Desert: Ecosystem Processes and Sustainability. University of Nevada Press, Las Vegas: pp. 78-100
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