4.4. Effects of crop rotation and fertilization on rice grain yield and NAE
In this study, crop rotation and fertilization affected grain yield and NAE directly by regulating N supply in the soil on the one hand and indirectly by altering soil chemical property on the other. In general, fertilization showed more direct effects through regulating N supply while crop rotation had more indirect effects by altering soil chemical property. By regulating the release of N, BBU maintained the soil inorganic N at a high and stable level during the period of rice growth, especially during the elongation stage, which not only improved the N uptake of the rice plant but also effectively reduced the losses of reactive N into the environment. As a result, the grain yield and NAE were improved. Twice-split application of BBU improved grain yield and NAE because it was better to balance the N requirement of rice and supply, especially in the soil with poor fertility retention. However, reduction of N fertilization rate was adverse to rice growth and resulted in unsatisfactory grain yield because of the relatively low basic soil fertility. Crop rotation had no direct effect on grain yield and NAE but showed significant indirect impacts on the changes in soil properties. Under RG rotation, changes in soil pH and POXC had significant negative effects on grain yield and NAE (Fig. 7a). As mentioned above, RG rotation increased soil pH and the labile fraction of SOC most and decreased the stable SOC. In other words, the strong increase in pH and degradation of stable SOC were adverse to improving grain yield and NAE under RG rotation. RR rotation indirectly affected grain yield and NAE by changing soil properties as strong correlations between grain yield and NAE and changes in soil pH, SOC, and POXC were observed. Only the change in SOC showed a significant effect on NAE under RW rotation, indicating the weakly indirect impact of RW rotation on grain yield and NAE. On the whole, application of BBU had direct positive effects on improving grain yield and NAE under RG and RW rotations but direct negative effects under RR rotation.