Potential roles of CO2 fertilization, nitrogen deposition, climate change, and land use and land cover change on the global terrestrial carbon uptake in the twenty-first century

Anthropogenic influences and global climate change are expected to alter the land carbon stocks in the future. In this modeling study, using the NCAR Community Earth System Model (CESM), we assess the relative importance of CO2 fertilization, nitrogen deposition, climate change, and land use and land cover changes (LULCC) on the land carbon uptake in three future scenarios used in phase 5 of the Coupled Model Intercomparison Project (CMIP5). Our simulations show that CO2 fertilization is the primary driver of the increase in net primary production (NPP) and total ecosystem carbon (TEC) in the representative concentrations pathway 2.6 (RCP2.6), RCP4.5, and RCP8.5 scenarios. The effect of nitrogen deposition on NPP and TEC in the future scenarios is small. Climate warming causes increases in NPP in the RCP4.5 and RCP8.5 scenarios, but it leads to loss of TEC in the future scenarios because of increased heterotrophic respiration. LULCC leads to an enhancement of NPP in the future scenarios due to post-harvest regrowth in the RCP2.6 and RCP8.5 scenarios, and due to afforestation in the RCP4.5 scenario. We find that land is a source of carbon in the RC8.5 and RCP2.6 scenarios mainly because of LULCC and climate change, but afforestation and CO2 fertilization in the RCP4.5 scenario facilitate the land to be a sink. Our findings, albeit from a single model, are in broader agreement with other studies that highlight the need for better land management practices and moderation in climate change for a continued land carbon sink.