Belowground plant strategies for resource acquisition are an important factor in plant competitiveness in nutrient-limited Arctic tundra, now and in the future. For example, warming is predicted to increase the availability of nitrogen (N) in surface soils, while associated permafrost thaw and degradation may alter the thickness of the active layer and release previously inaccessible nutrients at the permafrost boundary. Species with root traits that allow them to compete for increased N in shallow soils, or with rooting depth distributions that provide better access to nutrients in deeper soil may gain a competitive advantage. However, little is known about the nutrient acquisition strategies of the dominant plant species in tundra ecosystems. We conducted a 15N isotope tracer experiment to assess the vertical distribution of soil nutrient acquisition among three dominant species representing important plant functional types on the Barrow Environmental Observatory in Alaska, USA. We injected a solution of 15NH4Cl into the soil in either the organic soil horizon, the mineral soil horizon, or at the permafrost boundary beneath plots located in homogeneous patches of Carex aquatilis (sedge), Eriophorum angustifolium (sedge), or Salix rotundifolia (deciduous shrub) and quantified the uptake and distribution of 15N one week later.
Results/Conclusions
The dominant plant species differed in their root traits and in their patterns of N acquisition. The ectomycorrhizal shrub acquired 15N primarily from the organic soil layer, while both non-mycorrhizal sedges acquired more 15N from the mineral soil layer. Across all three species, very little 15N was acquired from near the permafrost boundary. While most of the 15N acquired remained in the fine roots, aboveground 15N allocation was greatest at the depth at which N was preferentially acquired. The roots of the shrub were located almost exclusively in the organic horizon, which is consistent with the patterns of nutrient acquisition for this species. In contrast, the roots of the sedges extended throughout the soil profile, suggesting that root density does not entirely explain the patterns of nutrient acquisition for these species. Instead, a nutrient competition model (N-COM) that incorporated rooting depth distribution, as well as species-specific differences in nutrient uptake kinetics and the greater plant-microbe nutrient competition in surficial organic soils, was able to reproduce the observed patterns. Given limited access to warming-induced nutrient release from previously frozen soils, mechanisms that allow increased N acquisition in surface soils, such as association with ectomycorrhizal fungi partners, may provide a competitive advantage.