Inter-Specific Competition, but Not Different Soil Microbial Communities, Affects N Chemical Forms Uptake by Competing Graminoids of Upland Grasslands

Abstract

Evidence that plants differ in their ability to take up both organic (ON) and inorganic (IN) forms of nitrogen (N) has increased ecologists’ interest on resource-based plant competition. However, whether plant uptake of IN and ON responds to differences in soil microbial community composition and/or functioning has not yet been explored, despite soil microbes playing a key role in N cycling. Here, we report results from a competition experiment testing the hypothesis that soil microbial communities differing in metabolic activity as a result of long-term differences to grazing exposure could modify N uptake of Eriophorum vaginatum L. and Nardus stricta L. These graminoids co-occur on nutrient-poor, mountain grasslands where E. vaginatum decreases and N. stricta increases in response to long-term grazing. We inoculated sterilised soil with soil microbial communities from continuously grazed and ungrazed grasslands and planted soils with both E. vaginatum and N. stricta, and then tracked uptake of isotopically labelled NH4 + (IN) and glycine (ON) into plant tissues. The metabolically different microbial communities had no effect on N uptake by either of the graminoids, which might suggest functional equivalence of soil microbes in their impacts on plant N uptake. Consistent with its dominance in soils with greater concentrations of ON relative to IN in the soluble N pool, Eriophorum vaginatum took up more glycine than N. stricta. Nardus stricta reduced the glycine proportion taken up by E. vaginatum, thus increasing niche overlap in N usage between these species. Local abundances of these species in mountain grasslands are principally controlled by grazing and soil moisture, although our results suggest that changes in the relative availability of ON to IN can also play a role. Our results also suggest that coexistence of these species in mountain grasslands is likely based on non-equilibrium mechanisms such as disturbance and/or soil heterogeneity.

Contrasting growth responses of dominant peatland plants to warming and vegetation composition

“…Furthermore, while the warming treatment in this study only increased air temperature by 0.35 °C, we found no associations between Eriophorum growth and air temperature (range across all plots within the growing season: 13 °C). This suggests that microclimate is not a major factor regulating Eriophorum growth in this ecosystem, and is in agreement with previous studies examining Eriophorum growth, which have found it to be primarily nutrient limited (Kool and Heijmans 2009;Medina-Roldán and Bardgett 2012). The bryophyte plant functional type at our study site comprises 9 % P. schreberi (Ward et al, unpublished data), a feather moss that fixes atmospheric nitrogen (N) through symbiotic associations with cyanobacteria (DeLuca et al 2002).…”

Response of herbaceous wetland plant species to changing precipitation regimes

“…Additionally, in upland systems the intervening dry periods between events can lower microbial mineralization rates (Borken & Matzner, ; Fierer & Schimel, ). Our results suggest that N‐uptake in monocot wetlands species is not affected by precipitation frequency (Table ), perhaps because graminoids are also able to utilize soil organic‐N (Gao, Mo, Xu, Zhang, & Yu, ; Medina‐Roldan & Bardgett, ). In contrast, N‐content and C:N ratios were influenced by precipitation frequency and magnitude for two eudicots, E. perfoliatum and M. ringens (Table ).…”

Aridity Modulates N Availability in Arid and Semiarid Mediterranean Grasslands

“…Thus, it is important to develop a predictive understanding of the factors controlling the amount of available N within an ecosystem, which includes both the inorganic (nitrate and ammonium) and organic forms that can be assimilated by plants and microorganisms [3]–[4]. The amount of available N in an ecosystem is controlled by the cumulative effects of microbially-driven N inputs through N-fixation, by the mineralization of N from organic matter, and by N-losses through leaching and gas emissions [5]–[6].…”

“…Despite the emergence of new paradigms for soil N cycling that emphasize the importance of both organic and inorganic N forms [3]–[4], [18], and the growing literature on the topic, we lack an integrated understanding of the most important determinants of N availability. This is particularly true for N-poor ecosystems such as drylands (arid, semiarid and dry-subhumid ecosystems; [22]–[24]).…”