Direct and indirect GHGs, SOC stock changes, net GHG balance, and GHGI. The annual direct GHGs (N 2 O + CH 4) emissions in the FN600 treatment were 1.88 Mg CO 2-eq ha −1 yr −1 , which was 26.2% higher than in DN600 (P < 0.05) and 44.1% higher than in DN420 (P < 0.05) (Fig. 2c,f). In contrast, the annual indirect GHGs emissions were 29.6% higher in DN600 than in FN600 (12.1 Mg CO 2-eq ha −1 yr −1), mainly because of the higher electricity consumption and polyethylene required for irrigation facilities in DN600 (Table 2). Because DN420 received 30% less N fertilizer than did DN600, its indirect GHGs emissions were 6.4% lower than those of DN600. Direct + indirect GHGs emissions were 23% and 9% higher in DN600 (17.1 Mg CO 2-eq ha −1 yr −1) than in FN600 and DN420, respectively. These results show that, in the life-cycle of wheat and maize production, indirect GHGs are main contributors to overall climate effects. After the 2-year experiment, the SOC content (0-30 cm) in DN420 and DN600 had increased by 0.52 and 0.41 g kg −1 , respectively, significantly more than in FN600 (0.22 g kg −1 , p < 0.05) (Table 2). That is, the increase in SOC stock (0-30 cm) for FN600 in the 2-year experimental period was 0.46 Mg C ha −1 yr −1 , significantly lower than that in DN420 (1.08 Mg C ha −1 yr −1) and DN600 (0.86 Mg C ha −1 yr −1). The annual net GHG balance values were greater than zero for all management systems, indicating that all systems were net GHGs sources (Table 2). Over the 2-year period, the total net GHG balance was 13.0% higher in DN600 (13.9 Mg CO 2-eq ha −1 yr −1) than in FN600. However, the total net GHG balance of DN420 was 15.8% and 4.9% lower than those of DN600 and FN600, respectively. The net GHG balance per unit grain yield is usually defined as GHGI. The GHGI and yield in DN600 (891 kg CO 2-eq Mg −1 grain) were 12.1% and 1.3% higher, respectively, than those in FN600. The GHGI and yield in DN420 were 15.7% lower and 3.2% higher, respectively, than those in DN600.
