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.