East Asia (China, Japan, Koreas and Mongolia) has been the world’s economic engine over at least the past two decades, exhibiting a rapid increase in fossil fuel emissions of greenhouse gases (GHGs) and has expressed the recent ambition to achieve climate neutrality by mid-century. However, the GHG balance of its terrestrial ecosystems remains poorly constrained. Here, we present a synthesis of the three most important long-lived greenhouse gases (CO2, CH4 and N2O) budgets over East Asia during the decades of 2000s and 2010s, following a dual constraint bottom-up and top-down approach. We estimate that terrestrial ecosystems in East Asia is close to neutrality of GHGs, with a magnitude of between 196.9 ± 527.0 Tg CO2eq yr-1 (the top-down approach) and -20.8 ± 205.5 Tg CO2eq yr-1 (the bottom-up approach) during 2000-2019. This net GHG emission includes a large land CO2 sink (-1251.3 ± 456.9 Tg CO2 yr-1 based on the top-down approach and -1356.1 ± 155.6 Tg CO2 yr-1 based on the bottom-up approach), which is being fully offset by biogenic CH4 and N2O emissions, predominantly coming from the agricultural sector. Emerging data sources and modelling capacities have helped achieve agreement between the top-down and bottom-up approaches to within 20% for all three GHGs, but sizeable uncertainties remain in several flux terms. For example, the reported CO2 flux from land use and land cover change varies from a net source of more than 300 Tg CO2 yr-1 to a net sink of ~-700 Tg CO2 yr-1.

Afforestation has been suggested as an effective ecological engineering approach for carbon sequestration and environmental benefits. However, the impact of afforestation on soil inorganic carbon (SIC) is less clear and sometimes controversial. Here, we conducted a field campaign, with 2346 soil profiles from 619 afforested plots and 163 control plots, to investigate the relative and absolute changes of SIC between afforested and corresponding control plots in northern China. We found positive responses of SIC to afforestation in acidic soils, where afforestation increased soil pH. In contrast, in alkaline soil, afforestation caused soil acidification and thus negative SIC responses. Fitting a structure equation model (SEM) confirmed that afforestation-induced soil pH change (ΔpH) was the most significant factor regulated SIC responses to afforestation. In particular, we observed stronger SIC sensitivity to pH change in arid areas, where both soil pH and SIC stocks were high. Other factors indirectly affected SIC responses to afforestation through modulating soil pH and soil organic carbon (SOC) dynamics. Afforestation-induced SIC changes also varied considerably among different planted tree species and across different soil depths. Specifically, in Pinus sylvestris var. mongholica, Pinus tabuliformis and Populus spp. plantations, changes of SIC were large enough to be comparable to that of SOC. Our finding provides a data-based comprehensive understanding on the impact of afforestation on SIC and its underlying mechanisms. With increased uses of afforestation and reforestation as potential nature-based climate solutions, decisions need to consider potential associated SIC changes, especially in SIC-rich areas.