CO2-adapted legumes ameliorate but do not prevent the negative effect of elevated CO2 on nitrogen fixation

Abstract:

The response of biological nitrogen fixation (BNF) to climate and elevated CO₂ (eCO₂) is a key uncertainty in modelling C cycle projections. In addition, as BNF provides 50% of the nitrogen (N) input to agricultural production and as ecosystem responses to eCO₂ are strongly influenced by N availability then the eCO₂ impact on BNF is central to modelling legume-based system responses to climate change. Greater photoassimilate production under eCO₂ should lead to enhanced BNF and this response is a feature of ecosystem models thus providing the N inputs necessary to provide continuing stimulation of NPP. FACE experiments provide a ‘realistic’ environment for eCO₂ studies; however, even if run for multiple years, they still may not capture adaptation to eCO₂ particularly in ecosystems dominated by perennial species. We tested the effect of eCO₂ on BNF and the potential importance of adaption by growing legumes that had been exposed to high or ambient CO₂ concentrations at a natural CO₂ spring in a long-running (16 year) FACE experiment on grassland. BNF was significantly lower under eCO₂ but the reduction was less marked where plants had originated in a high CO₂ environment. An ecosystem model run with reduced BNF proved a better fit to the experimental data for the FACE experiment than where BNF was enhanced or unchanged under eCO₂.