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.