Exploring the terrestrial carbon sink at a global scale: the effects of nitrogen fixation, nitrogen deposition and CO2 fertilization

Abstract

Rising atmospheric CO2 concentrations have reached 51% above the pre-industrial level. The terrestrial carbon sink performs a critical role in sequestering atmospheric CO2, thereby mitigating the rate of climate change. The terrestrial carbon sink is primarily driven by CO2 fertilization, but the extent of this driver is constrained by nitrogen limitation. However, biological nitrogen fixation can fuel CO2 sequestration and relieve nitrogen limitation. Exploring global change drivers, CO2 fertilization, and nitrogen deposition, provides crucial insight into how biological nitrogen fixation will respond to future climate scenarios. Therefore, we conducted an analysis of terrestrial biosphere model outputs was also conducted to compare the influence of CO2 fertilization and nitrogen deposition on biological nitrogen fixation rates at a global scale. We found a significant increase in biological nitrogen fixation rates from elevated CO2 concentrations. A global meta-analysis of terrestrial biological nitrogen fixation in response to elevated CO2 concentrations across various ecosystems was also conducted to evaluate the model. We found that the empirical effect of elevated CO2 (0.00393 ppm-1) closely aligns with the modelled effect of elevated CO2 on the rate of biological nitrogen fixation (0.00156 ppm-1). In addition, our study found that global biological nitrogen fixation mean attribution to CO2 concentrations was 19.7 Tg N yr-1, and -1.63 Tg N yr-1 for nitrogen deposition. Overall, insights from this study contribute to our understanding of how the terrestrial carbon sink and nitrogen cycling processes may respond to future environmental variability, particularly in the context of rising atmospheric CO₂ concentrations.