Nitrogen preferences and plant-soil feedbacks as influenced by neighbors in the alpine tundra

Abstract

Plant resource partitioning of chemical forms of nitrogen (N) may be an important factor promoting species coexistence in N-limited ecosystems. Since the microbial community regulates N-form transformations, plant partitioning of N may be related to plant-soil feedbacks. We conducted a (15)N tracer addition experiment to study the ability of two alpine plant species, Acomastylis rossii and Deschampsia caespitosa, to partition organic and inorganic forms of N. The species are codominant and associated with strong plant-soil feedbacks that affect N cycling. We manipulated interspecific interactions by removing Acomastylis or Deschampsia from areas where the species were codominant to test if N uptake patterns varied in the presence of the other species. We found that Deschampsia acquired organic and inorganic N more rapidly than Acomastylis, regardless of neighbor treatment. Plant N uptake-specifically ammonium uptake-increased with plant density and the presence of an interspecific neighbor. Interestingly, this change in N uptake was not in the expected direction to reduce niche overlap and instead suggested facilitation of ammonium use. To test if N acquisition patterns were consistent with plant-soil feedbacks, we also compared microbial rhizosphere extracellular enzyme activity in patches dominated by one or the other species and in areas where they grew together. The presence of both species was generally associated with increased rhizosphere extracellular enzyme activity (five of ten enzymes) and a trend towards increased foliar N concentrations. Taken together, these results suggest that feedbacks through the microbial community, either in response to increased plant density or specific plant neighbors, could facilitate coexistence. However, coexistence is promoted via enhanced resource uptake rather than reduced niche overlap. The importance of resource partitioning to reduce the intensity of competitive interactions might vary across systems, particularly as a function of plant-soil feedbacks.

Ecophysiological responses of two dominant subalpine tree species Betula albo-sinensis and Abies faxoniana to intra- and interspecific competition under elevated temperature

“…The results have indicated that the presence of neighbors changes plants' resource use traits Liancourt et al, 2013). Yet, relatively few studies have assessed how N acquisition (e.g., uptake of 15 NH 4 + or 15 NO 3 À ) is influenced by interactions with neighbors using stable N isotope analysis, which has proven to be a powerful tool for gaining insight into the foraging strategy of roots (Ashton et al, 2008).…”

Foliar and soil δ15N values reveal increased nitrogen partitioning among species in diverse grassland communities

“…Results from 15 Nlabeling with different N forms suggested that N allocation strategies play an important role (Kahmen et al 2006) or that N partitioning may allow for niche separation among subordinate plant species (McKane et al 2002;Aanderud & Bledsoe 2009;von Felten et al 2009). Moreover, recent studies indicated neighbour-identity effects on plant species capacity to take up different N forms or enhance the uptake of particular N forms (Miller, Bowman & Suding 2007;Ashton et al 2008).…”

Stable nitrogen isotope patterns of trees and soils altered by long-term nitrogen and phosphorus addition to a lowland tropical rainforest

“…For example, plant d 15 N values reflected a community-wide switch between two mineral N sources along a precipitation gradient in Hawaii (Houlton et al 2006;Schuur and Matson 2001) and among spatially distinct N sources in litter and mineral soils of French Guiana (Schimann et al 2008). Experimental additions of 15 N-enriched tracers have shown that plants prefer mineral over organic forms of N, even in N limited ecosystems (Harrison et al 2007;Ashton et al 2008), and that local adaptations to growth conditions in part determine N uptake rates and preferences for ammonium versus nitrate (Wang and Macko 2011;Andersen and Turner 2013).…”