3.3 | Grain yields
Compared with CK, the NT and RT increased wheat grain yield by 21.1% and 14.2%, respectively (Fig. 2a; Table 2; P < 0.001). Straw return increased wheat grain yields by 18.5% under SR relative to the SR0 (P < 0.001). Significant interactions between tillage and straw managements on wheat grain yields were detected (P < 0.05).In SR plots, the NT and RT increased wheat grain yields by 17.9% and 9.5, respectively, relative to the CK, while the increase of wheat grain yields in SR0 plots were up to 25.2% and 20.0% under NT and RT, respectively (all P < 0.01). Compared with the SR0, the SR increased wheat grain yields by 14.0%, 17.6%, and 24.9% under RT, NT, and CK, respectively (allP < 0.01). Compared with the CK-SR0, wheat grain yields significantly increased by 47.2%, 36.8%, 25.1%, 24.9%, and 20.0% in NT-SR, RT-SR, NT-SR0, CK-SR, and RT-SR0, respectively (Fig. 2b; P < 0.01).
Similar trends existed for maize grain yields with wheat grain yields in different treatments (Fig. 3a~d). In most rotation cycle, grain yields of both wheat and maize in CK-SR were higher than in CK-SR0, except for wheat grain yields in 2011. In 2011–2013 and 2015-2016, wheat grain yields decreased under all treatments, but maize grain yields increased and then decreased substantially under all treatments. However, during 2013-2014, wheat grain yields increased substantially under all treatments. In 2012, all treatments reached their highest maize grain yields: 9.21, 8.52, 8.18, 8.17, 7.91, and 7.53 Mg ha−1 in NT-SR, RT-SR, NT-SR0, RT-SR0, CK-SR, and CK-SR0, respectively. However, the highest wheat grain yields of CK-SR0, NT-SR0 and RT-SR were 6.11, 7.40 and 8.21 Mg ha−1 in 2011, 6.97 and 7.18 Mg ha−1for RT-SR0 and CK-SR in 2014, and 8.42 Mg ha−1 for NT-SR, respectively.