Role contribution of biological nitrogen fixation to future terrestrial net land carbon accumulation under warming condition at centennial scale

Abstract

Future changes in net land carbon (C) accumulation in the terrestrial ecosystem remain highly uncertain. This uncertainty is mainly due to nitrogen (N) availability not being included in model simulations. N availability is ultimately determined by the balance of N inputs and outputs for the terrestrial ecosystem. Here, as the largest N input pathway, the influence of biological N fixation (BNF) combined with increasing atmospheric carbon dioxide (CO2) concentration, climate change, and atmospheric N deposition on future terrestrial C sequestration is investigated to the year 2100 using the Community Atmosphere–Biosphere–Land Exchange (CABLE) global land surface model. Projections show a net land C accumulation increase of ∼20% by 2100 for the terrestrial ecosystem, relative to the pre-industrial (1901–1910) level. BNF will increase terrestrial C sequestration by ∼104 Pg C by the end of this century, accounting for ∼28% of the total increase in net land C accumulation due to increasing CO2, climate change, BNF, and atmospheric N deposition. Therefore, the findings highlight the fundamental importance of BNF in determining the size of future net terrestrial C accumulation, although there are still unresolved questions regarding BNF and its role in forecasting climate change. Furthermore, a climate warming-induced increase in soil N mineralization of ∼431 Tg N per °C is estimated, resulting in increased N uptake (∼17 Tg N yr−1) from soil to vegetation by the end of this century, relative to the pre-industrial period, which could offset soil carbon loss due to warming. This highlights the effect of warming-induced N mineralization on land net C accumulation. Thus, BNF variations and warming-stimulated soil mineralization should be included in model simulations of land C carbon sequestration to prevent the effect of N limitation on land C loss being overestimated.