Winners and losers in response to nitrogen deposition: The roles of resource competition, abundance, and microbial interactions in diversity decline

Background/Question/Methods

Despite a large amount of work documenting the decline in plant diversity in response to nitrogen (N) enrichment, we are still largely unable to explain why these impacts occur. What factors cause plant species loss at high nitrogen availabilities? Here, we discuss three possibilities: 1) competitive exclusion due to a shift from belowground to aboveground light competition; 2) neutral loss of rare species caused by increased overall plant size and decreased density; and 3) changed plant-microbial interactions at elevated soil N. While these mechanisms can operate simultaneously, the first factor – plant competitive interactions – is widely assumed to be responsible for diversity decline despite limited experimental evidence. Here, we combine a cross-site synthesis of trait response to N fertilization experiments with a manipulative experiment at one of the sites to better understand the influence of all three factors.

Results/Conclusions

Evidence from N-fertilization experiments across a range of ecosystems suggest that traits related to the shift from belowground to aboveground competition are predictive of species loss; however most of this variation is explained by the increased susceptibility of N-fixing forbs and by increase in species with an acquisitive resource use strategy (annuals, non-natives). Specific leaf area and the ratio of carbon to nitrogen in leaf tissue, two traits that presumably should relate to light competitive ability, did not explain species loss.  Rare species were 6 times more likely to go extinct than abundant species, regardless of traits, indicating a large role of random loss. Effects of belowground microbial interactions are less studied; however, results from one of the sites (Niwot Ridge, Colorado USA), indicates that increased symbiont root parasitism rather than plant competitive interactions is linked to increased susceptibility to N enrichment. Thus while competitive mechanisms are commonly assumed, and trait-base patterns of species loss do provide limited support for this assumption, neutral loss and belowground microbial interactions can play as large, or in some cases, larger roles in species loss due to N enrichment.