Forests play a vital role in global carbon (C) cycling. Accordingly, afforestation engineering programsthat promote increased terrestrial C stocks are an important means to help gradually decrease atmo-spheric CO2emissions. China, however, had increased its afforested area bordering hydroclimatic zones to275.71 million hm2between 1949 and 2010. Ecosystem water use efficiency (EWUE) and plant water useefficiency (PWUE) provide data on ecosystem sensitivity to water availability across rainfall regimes. Thewater consumption cost of C sequestration (WCCC) is also an important parameter that gauges the costof C sequestration under afforestation. However, abrupt changes in EWUE and PWUE (threshold values of1.5 and 3.6 gC kg−1H2O, respectively) have been measured within the 400–500 mm precipitation climaticisoline boundary situated between semi-humid and arid zones. The threshold value of the correspondingWCCC was 1.0 kg H2O gC−1. Forest ecosystems in China typically generate high EWUE and PWUE values(2.80 ± 0.77 and 4.25 ± 1.02 gC kg−1H2O, respectively) but low WCCC values (0.52 ± 0.42 kg H2O g−1C),providing proof that afforestation is the best choice in increasing terrestrial C stocks. However, China’smajor afforestation engineering programs have concentrated efforts toward low EWUE and PWUE andhigh WCCC in the western region of the 400–500 mm precipitation isoline boundary, belonging to thearid and semiarid zones, which introduced potential environmental risks. Therefore, policies related tolarge-scale C sequestration initiatives under afforestation must first fully consider the statuses of WCCCand WUE.