Nitrogen source preferences in heathland plants
Plant growth is limited by N availability in high latitude and altitude ecosystems where soil N is held in organic matter due to slow rates of soil processes. Adaptations to stress tolerance in these ecosystems include mechanisms for increased N capture, including from organic sources. N uptake up heathland plants was examined, specifically with respect to; source effects, species differences, short-term uptake and ecological relevance. N capture was measured with 15N-labelled sources, including ammonium, glycine, leucine, fungal cell wall and powdered worm, over treatment durations between 2 and 168h. No consistent or predictable source preferences were found for Calluna vulgaris, mixed grasses, including Agrostis capilaris, Deschampsia flexuosa and Festuca ovina, and sedges (Carex spp.) between ammonium and glycine with only slightly less leucine-N captured. There was measurable uptake from the complex organic sources 2 and 6 hours after source addition. Calluna generally acquired more source-N but comprised a much greater proportion of combined biomass, consequently graminoids species captured more N per unit biomass. Over shorter treatment durations grasses were able to acquire more N in total than Calluna despite smaller biomass. Glycine-N turnover was measurable after 2 h with 6.5% in the ammonium pool. Subtle differences in mechanisms of plant stress tolerance in high latitude and altitude systems are suggested, with graminoids species able to rapidly capture N despite the non-mycorrhizal cluster root of Carex and the arbuscular mycorrhizal grass roots. In contrast the woody shrub Calluna is adapted to require less N.