4. Discussion
4.1. Effects of crop rotation and fertilization ondynamics of soil inorganic N
and plant N uptake
In the two rice seasons, CU treatments had higher soil inorganic N
content than BBU treatments under the three crop rotations at the middle
of the tillering and flowering stages because of the second and third
topdressings of N (Fig. 3). The plant N contents, in contrast, were not
significantly higher under CU treatments than BBU treatments at the
middle of the tillering stage. Hydrolysis of urea in the soil is a rapid
reaction, resulting in a sharp increase in the content of inorganic N in
the soil in a short period (Skiba & Wainwright, 1984). However, limited
by the N uptake rate of crops, only a small part of this
high-concentration N is assimilated, and the rest is discharged into the
environment through runoff, ammonia volatilization, etc. As shown in
Fig. 3a, the soil inorganic N contents in CU treatments were
significantly lower than that in BBU treatment at the elongation stage
in the Rice2019 season (Table S2). The CU treatments also resulted in
more inorganic N in the soil at the later periods of rice growth, which
might cause late ripening or lodging of rice. Advantageously, inorganic
N in the soils under BBU treatments was released slowly according to the
N uptake rate of rice, which maintained inorganic N in the soil at an
appropriate concentration. Therefore, application of BBU could increase
the N uptake of rice and reduce the loss of reactive N into the
hydrosphere and atmosphere, thereby
increasing the NAE. There was no significant difference in the soil
inorganic N between the single-dose and twice-split applications of BBU
(Table S2). However, twice-split application of BBU slightly increased
plant N content at seeding, tillering, and elongation stages. This
demonstrated that twice-split application of BBU also improved
suitability between the N demand and supply in rice, thereby increasing
the NAE. In this study, the release period of N was delayed for wheat
season, increasing soil inorganic N content in soil at the mature stage
under BBU treatment (Fig. 3a). Although there was no obvious difference
in soil inorganic N and plant N contents between different crop rotation
systems at various stages (Table S2), the dynamic pattern of soil
inorganic N significantly changed after the rotation season, especially
under the BBU treatments (Fig. 3a). Soil inorganic N in BBU treatments
under RG and RW rotations was higher than in CU treatment at the seeding
stage, because of the residual N from the wheat season and the
biologically fixed N from the green manure season. In addition, soil
inorganic N in RG and RW rotations still presented a high concentration
at the elongation stage. RG and RW rotations enhanced the retention
capacity of soil fertility to supply sufficient N for rice even at the
elongation stage. In addition, the residual N in RW rotation and
biologically fixed N in RG rotation were released and mineralized in the
Rice2020 season, respectively.