Fig. 7
There was a significant quadratic relationship between SOC concentration and maize yield (R2 = 0.549, P < 0.001) (Fig. 7a) and between SCS and maize yield (R2 = 0.513, P = 0.005) (Fig. 7b) under NT. A quadratic relationship was also observed between RT and SCS, however, this was not significant (R2 = 0.531, P = 0.15). For most experiments conducted in NEC where SOC concentration and stock were under 20 g kg-1 and 50 Mg ha-1, respectively, maize yield showed a positive correlation with SOC concentration and SOC stock, which stressed the importance of SOC concentration and restoration in maintaining the yield of the crop in NEC. Because of the lack of data, yield of soybean and other crops are not shown.
3.4. Factors affecting Yield and SOC
There were no significant relationships between yield effect and other site characteristics for any of the tillage practices (Table 2). SOC (0-20cm) was significantly correlated with SOC (0-5 cm) and SCS under NT (R2 = 0.788, R2 = 0.998, P < 0.01) and RT (R2 = 0.965, R2 = 0.900, P < 0.01), and ST showed a significant positive correlation (P < 0.01) with SCS. Significant correlations (P < 0.01) were also observed between SOC effect and TN (R2 = 0.882), C/N (R2 = 0.843), and BD (R2 = 0.844) under NT. Under NT and ST, pH effects were negatively but not significantly correlated with the SOC effect. NT and RT all showed a non-significant negative correlation between water content and SOC, while ST showed a positive one.