Inter-specific competition, but not different soil microbial communities, affects N chemical forms uptake by competing graminoids of upland grasslands.

James F. Cahill,Eduardo Medina-Roldán,Richard D Bardgett

doi:10.1371/journal.pone.0051193

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.

Highlights

The ability of plants to directly take up organic nitrogen (ON) might constitute an important mechanism regulating plant species coexistence [1,2,3,4]
Unlike E. vaginatum, N. stricta shoot and root biomass were not affected by inter-specific competition nor inoculum source (F1,50 = 1.6, P.0.2; F1,48 = 0.6, P.0.4 for shoot and root biomass receptively, Table 2, Fig. 1b and d)
We investigated how functionally-distinctive soil microbial communities, as a result of long term differences in grazing management, and plant competition affected uptake of glycine and NH4+ by two coexisting graminoids, E. vaginatum and N. stricta

Summary

Introduction

The ability of plants to directly take up organic nitrogen (ON) might constitute an important mechanism regulating plant species coexistence [1,2,3,4]. If plants species show differential N uptake for ON or inorganic N (IN), niche overlap and competition intensity for N could decrease [2]. No differential uptake for N among plant species that differ in habitat might suggest weak niche differentiation, but other factors could alter patterns of plant N uptake. Inter-specific competition [11,12,13] and soil microbial community composition and activity [14,15] could alter N uptake among plant species. While the role of plant competition on N uptake patterns has recently been addressed [11], less is known about how soil microbes and changes in microbial communities affect plant uptake of different chemical forms of N [15], despite microbes being key agents in the N cycle

Methods

Results

Conclusion