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.