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.