Evidence for phosphorus limitation in high-elevation unvegetated soils, Niwot Ridge, Colorado

A key challenge to understanding the effects of climate change and nutrient deposition on ecosystem functioning is our lack of knowledge about nutrient limitations of heterotrophic and phototrophic microbial communities. This is especially true in high elevation ecosystems where it has been shown that earlier melt-out of snow beds and glacial retreat is allowing photosynthetic microbes and plants to move into previously unvegetated areas. We used landscape-level analyses of microbial enzyme stoichiometries combined with soil microcosm fertilization studies to determine which nutrients are limiting to microbes in plant-free or sparsely vegetated, snow bed areas of the Colorado Front Range. Both of these independent approaches indicated that the ultimate limiting nutrient in unvegetated and sparsely vegetated soils is phosphorus (P) for phototrophic microbes, with co-limitation by carbon (C) for the entire microbial community. In contrast, vegetated soils in the same watersheds showed more balanced nitrogen (N), P and C co-limitation similar to patterns seen in other plant-dominated ecosystems. In microcosm experiments, P additions resulted in increased growth rates and percent cover by phototrophs, whereas N additions decreased the relative abundances of phototrophs. Taken together, our findings indicate that the colonization of high elevation ecosystems being impacted by N deposition and climate warming will likely be constrained by P limitation of both heterotrophic and phototrophic microbes and by negative impacts of N on microbial phototrophs. These effects may in turn limit the ability of these fragile ecosystems to immobilize inputs of atmospheric N causing increased runoff of excess N to downstream ecosystems.