Alpine meadows are currently threatened by both effects of climate change and increases in acid deposition from lowland industrialization and population growth. Nitrogen deposition in particular is poised to affect plant and soil communities in pristine alpine meadows of the Western United States. In order to study the effects of nitrogen deposition on alpine meadows, we simulated N deposition using fertilization at levels of 0, 0.3, 0.5 and 1g/m2 NH4NO3-N at three PNW alpine ecosystems: two mesic heath meadows and one dry mixed heath/graminoid meadow. As part of this study we measured levels of inorganic, organic and microbial N, and copy numbers of genes for nitrogen fixation and ammonia oxidation in the rooting zone soil of three target plant species that were most abundant at our sites (Cassiope mertensiana, Phyllodoce emptriformis, and Lupinus spp.).
Results/Conclusions
Soil inorganic N availability increased at all three sites, most strongly at the more established of the two heath meadows. We observed no effect of fertilization on copy numbers of genes for N fixation and ammonia oxidation. The only relationship between gene copy and plant species we found was an increase in genes for ammonia-oxidizing bacteria in the rooting zone of lupine. The greatest differences we observed were site-specific, not treatment-specific. We found that the dry meadow ecosystem had much higher levels of soil N mineralization and nitrification than the heath meadow ecosystems. Genes for N fixation were most abundant at the site with the lowest levels of ambient soil N. Gene copy number for ammonia oxidation in both bacteria and archaea were mostly quite low, in keeping with other studies of alpine microbiomes. However, we observed a much higher copy number for archaeal ammonia oxidation genes at the dry meadow site. Ammonia-oxidizing archaea, rather than bacteria, may drive nitrification in high-N alpine meadow ecosystems. This has significant implications for effects of N deposition on alpine soils, as the k-constant for many ammonia-oxidizing archaea is quite low, and increasing N deposition could cause a shift from ammonia-oxidizing archaea to bacteria.