High NH 4+ efflux from roots of the common alpine grass, Festuca nigrescens, at field-relevant concentrations restricts net uptake

Abstract

Alpine meadows of high ecological value could be severely endangered by anthropogenic N enrichment, modifying the relationships between species and the environment. While a constraint exerted by N availability on alpine plant development has been demonstrated by some fertilization experiments, in others no effect was observed. Basically, the problem is that mineral N absorption has not been characterized in alpine plants. In growth chamber experiments, we investigated the component fluxes of 15NO 3 − and 15NH 4 + uptake in a tussock grass ( Festuca nigrescens) very common and representative of the dominant plant growth form in European alpine meadows. Rates of influx supported data already published for low elevation herbaceous species. These rates were up to ten times higher for NH 4 + than for NO 3 − but rates of net uptake were similar for both ions demonstrating the occurrence of elevated NH 4 + efflux (80% of primary influx). An increase in external N in the range of field-relevant concentrations did not substantially enhance net uptake. Thus, the alpine plant which is assumed to be adapted to relatively high soil NH 4 + responded like an NH 4 +-sensitive species: as if it was unable to use the incoming nitrogen. It is suggested that the ability of this typical alpine grass to respond to increasing N availability due to global changes is limited.