[Greenhouse gas emissions and net carbon sequestration of "Grain for Green" Program in China.]

Based on the estimation of carbon cost from afforestation in project boundary and carbon leakage out of boundary in the construction period of "Grain for Green" Program (GGP) (2000-2010), the annual variance and composition of the carbon cost and carbon leakage, as well as characters of variance of net carbon sequestration were analyzed for GGP and respective program regions. Results showed that the carbon costs in northwest region, southwest region, northeast region, north region and central south and east region were 3.38, 3.64, 1.03, 1.66 and 4.38 Tg C, respectively, totaling 14.09 Tg C. Meanwhile the carbon leakages of the above regions were 21.33, 4.60, 5.50, 1.32 and 3.78 Tg C, respectively, and 36.53 Tg C in total. The composition characters of the carbon costs of the GGP and the respective regions were similar. Carbon emissions from afforestation were the largest carbon cost, and afforestation on converted farmland was the main carbon emission source. Accordingly, among the materials consumed, fertilizer brought about the largest carbon cost, followed by building materials, while carbon emissions from fuels, irrigation, herbicides and pesticides only accounted for about 10% for respective regions. The carbon cost and carbon leakage of the GGP were 50.62 Tg C in total, which counteracted 19.9% of the sequestered carbon in the program. In northwest region, southwest region, northeast region, north region and central south and east region, carbon emissions (including cost and leakage) accounted for 38.9%, 10.4%, 26.1%, 8.9% and 15.5% of the carbon sequestration, respectively. The net carbon sequestration of the GGP was 203.50 Tg C with an annual average of 18.50 Tg C·a^-1. The carbon cost and leakage offset a minor part of the carbon sequestration of the GGP. Therefore, the GGP contributed significantly to greenhouse gas mitigation in China as well as global climate warming mitigation. Adopting precision fertilization in economic forest afforestation and supplying alternative livelihoods to farmers in the program could be the potential measures to reduce carbon cost and carbon leakage.