Greenhouse gas budget and net carbon sequestration of different afforestation types of the “Grain for Green” Project

To examine spatial patterns and heterogeneities in greenhouse gas (GHG) sequestration, mitigation and emission of different afforestation types (i.e., ecological, economic, timber and firewood forest) in the “Grain for Green” project (GGP), we estimated the GHG budgets of 30 typical tree species in afforested areas, established a carbon accounting and net mitigation (CANM-GGP) assessment framework, and determined the net carbon sequestration and mitigation rates of the four afforestation types in the GGP. From 2000 to 2020, total carbon sequestration and mitigation in GGP amounted to 5097 TgCO 2 , with 26.84% offset by emissions (1368 TgCO 2 -eqv). Economic forest had the greatest emissions at 1035 TgCO 2 , offsetting carbon sequestration by 281.2%. GHG emissions offset carbon sequestration and mitigation by 3.00%, 6.78%, and 11.89% in firewood, ecological, and timber forest, respectively. GGP net carbon sequestration and mitigation rate was 6.21 tCO 2 /ha/yr, highest in firewood forest (18.68 tCO 2 /ha/yr), followed by ecological forest (9.65 tCO 2 /ha/yr) and timber forest (3.52 tCO 2 /ha/yr). Economic forest showed a net GHG increase (6.67 tCO 2 /ha/yr). Overall, GGP achieved significant net carbon sequestration and mitigation benefits except economic forest. Ecological forest contributed most to net carbon sequestration due to the largest afforestation area and the second highest net carbon sequestration rate. Firewood forest had the highest net carbon sequestration and mitigation rate, and their potential mitigation benefits could be substantial. Our work provides a new approach for assessing afforestation as an important and promising nature-based climate solution and suggests multiple afforestation options for GHG mitigation.