Nitrogen critical loads for alpine vegetation and terrestrial ecosystem response: are we there yet?

• Published in: Ecological applications 2006-06, Vol.16 (3), p.1183-1193
• Main Authors: Bowman, William D., Gartner, Julia R., Holland, Keri, Wiedermann, Magdalena
• Format: Article
• Language: English
• Subjects: alpine; alpine dry meadows; alpine plants; application rate; atmospheric deposition; community change; diversity; Ecosystem; Ecosystems; environmental impact; grasslands; leaching; Meadows; mineralization; mountains; N cycling; N deposition; nitrate nitrogen; nitrification; Nitrogen; Nitrogen - analysis; nutrient availability; nutrient uptake; plant density; Plant Physiological Phenomena; Plant Physiology; Plants; pollution load; Rocky Mountains; Soil composition; Soil ecology; soil fertility; soil microorganisms; soil solution; Species; Species diversity; Terrestrial ecosystems; Vegetation
• ISSN: 1051-0761; 1939-5582
• DOI: 10.1890/1051-0761(2006)016[1183:NCLFAV]2.0.CO;2

Increases in the deposition of anthropogenic nitrogen (N) have been linked to several terrestrial ecological changes, including soil biogeochemistry, plant stress susceptibility, and community diversity. Recognizing the need to identify sensitive indicators of biotic response to N deposition, we empirically estimated the N critical load for changes in alpine plant community composition and compared this with the estimated critical load for soil indicators of ecological change. We also measured the degree to which alpine vegetation may serve as a sink for anthropogenic N and how much plant sequestration is related to changes in species composition. We addressed these research goals by adding 20, 40, or 60 kg N·ha⁻¹·yr⁻¹, along with an ambient control (6 kg N·ha⁻¹·yr⁻¹ total deposition), to a species- rich alpine dry meadow for an eight-year period. Change in plant species composition associated with the treatments occurred within three years of the initiation of the experiment and were significant at all levels of N addition. Using individual species abundance changes and ordination scores, we estimated the N critical loads (total deposition) for (1) change in individual species to be 4 kg N·ha⁻¹·yr⁻¹ and (2) for overall community change to be 10 kg N·ha⁻¹·yr⁻¹. In. contrast, increases in NO₃⁻ leaching, soil solution inorganic NO₃⁻, and net N nitrification occurred at levels above 20 kg N·ha⁻¹·yr⁻¹. Increases in total aboveground biomass were modest and transient, occurring in only one of the three years measured. Vegetative uptake of N increased significantly, primarily as a result of increasing tissue N concentrations and biomass increases in subdominant species. Aboveground vegetative uptake of N accounted for