5. Conclusions
The BBU could controlla bly release N to improve the N uptake according to the requirement of rice during various growth stages. Accordingly, fertilization of BBU with twice-split application increased both the rice grain yield and NAE in comparison to conventional urea. At the same time, crop rotation combined with fertilization significantly increased the soil pH, SOC, TN, and POXC, resulting in the enhancement of soil fertility retention. The RG rotation showed a higher increase in soil pH and TN than other crop rotations, which benefited from the biological nitrogen fixation of Chinese milk vetch. FTIR spectra indicated that RG rotation combined with fertilization significantly increased the aliphatic and methyl compounds which were considered as the labile fraction of SOC and reduced the relative abundance of carboxylic acids, amides, and aromatics which were regarded as the stabled fraction of SOC. Crop rotation also significantly reduced the soil carbonate. The rice grain yield and NAE were influenced more by the changes in soil properties under RG and RR rotations than that under RW rotation.