Gorse is a 'facultative' N2 fixer

Abstract

Many legumes reduce their atmospheric N2 fixation per unit biomass in response to increased soil N availability but there are reports that some maintain a constant rate of N2 fixation per unit biomass regardless of soil N levels. These different responses to soil N availability have been described, respectively, as 'facultative' and 'obligate' N2 fixation strategies. Views in the literature differ if gorse is a facultative or obligate N2 fixer. Here, firstly, the proportion of N derived from the atmosphere (%Ndfa) was assessed for mature gorse plants mainly in hedges bordering intensive agricultural land at different sites in the Selwyn district, Canterbury, New Zealand using the 15N natural abundance technique. Secondly, the effect of nitrate (NO3 -) supply on %Ndfa was determined for gorse seedlings under glasshouse conditions using 15NO3 -. Under field conditions, values ranged from 14.7-88.0 %Ndfa. In the glasshouse, %Ndfa values decreased from 97 when no N was supplied to 24 %Ndfa when N supply was increased to the equivalent of 200 kg N/ha. It is concluded that gorse shows a facultative N2 fixation strategy. Keywords: legume, nitrate, 15N natural abundance, nitrate reductase activity, gorse, Ulex europaeus