nitrogen agronomic efficiency
Under the RG rotation, RG-CB1 and RG-CB2 treatments only increased inappreciable grain yields in both Rice2019 and Rice2020 seasons compared with RG-CU (Fig. 2a). RG-RB1 treatment decreased the grain yields in both Rice2019 and Rice2020 seasons, while the RG-RB2 treatment increased the grain yield in the Rice2019 season. Under the RR rotation, RR-CB2 treatment significantly increased the grain yield in the Rice2019 season compared with RR-CU. Both RR-RB1 and RR-RB2 treatments had significantly lower grain yields than RR-CU in both Rice2019 and Rice2020 seasons. Under the RW rotation, RW-CB2 treatment showed significantly highest rice grain yield in Rice2019 season and wheat grain yield in Wheat season. Grain yield was significantly affected by fertilization in the Rice2020 season (Table S1, supplementary materials). Crop rotation and the interaction between crop rotation and fertilization had no significant effects on grain yield.
RG-RB2 and RR-RB2 treatments had the highest NAE in the Rice2019 season (Fig. 2b). Under the RW rotation, both the RW-CB2 and RW-RB2 treatment increased the NAE of rice than other treatments in the Rice2019 season. Both the RW-RB2 and RW-RB2 treatments showed significantly higher NAE of wheat than RW-CU. Crop rotation had a significant effect on rice NAE in the Rice2020 season, while fertilization and the interaction between crop rotation and fertilization had no significant effects (Table S1).
3.2.Dynamics of soil inorganic nitrogen and plant nitrogen
Soil inorganic N increased to the highest at the middle of the tillering stage, then decreased to stability in the Rice2019 season (Fig. 3a). At the middle of the tillering stage, RG-CU, RR-CU, and RW-CU had higher soil inorganic N contents than other treatments, respectively (Table S2). The rice plant under the treatments of BBU also had higher plant N contents at tillering and elongation stages in the Rice2019 season (Fig. 3b, Table S4). In wheat season, soil inorganic N under RW-CU treatment increased to the highest at the middle of the tillering stage, while that under BBU treatments increased to the highest at flowering and mature stages (Fig. 3a). Reduction of N decreased the plant N contents of wheat at seeding stages (Fig. 3b). The dynamics of soil inorganic N were significantly changed after crop rotation (Fig. 3a). Soil inorganic N contents in all N treatments remained at a high level at seeding, tillering, elongation, and flowering stages. The plant N of rice in all N treatments were considerable at seeding, tillering, elongation, flowering, and mature stages (Fig. 3b). Soil inorganic N contents were significantly affected by crop rotation at elongation and flowering stages and significantly affected by fertilization at seeding, tillering, and mature stages in the Rice2020 season (Table S3). Crop rotation, fertilization, and interaction between them both had significant effects on rice plant N content at tillering and elongation stages in the Rice2020 season (Table S5).