2.3 Experiment design
A randomized block design with 16 plots was used in this experiment, including four treatments with four replicate blocks. Sixteen 3 m × 4 m plots were built and a ridge of 50 cm width and 30 cm height was used to separate the adjacent plots. A thick plastic film was vertically buried along the ridge to a depth of 1 m between any two adjacent plots. The four treatments included control (CK, no N fertilization), NF1 (N fertilization with 150 kg N hm−2year−1), NF2 (N fertilization with 300 kg hm-2 year-1) and NF3 (N fertilization with 450 kg N hm−2year−1). Prior to the experiment, an amount of 2250 kg hm-2 calcium superphosphate was applied to all the plots as soil amendment measurement and phosphorus fertilization supply, according to the local management practice habits in coastal region (Wang et al., 2016; Yang et al., 2018). No potassium fertilization was used and urea (nitrogen content 46.4%) acted as base fertilizer and topdressing fertilizer during the experiment. Crop residues were removed from the plots after the harvest of each season. The amount of nitrogen rates in the paddy rice (Oryza sativa L.) season was equivalent to that in the winter wheat (Triticum aestivum L.) season. Before the sowing time of each crop season, the base fertilizer was applied and mixed uniformly with 0-15 cm soil layer by manual, and the topdressing fertilizer was added to soil by spraying manually in key stages of crop growth season. This work was repeatedly conducted from late May 2015 to early June 2018. Table 1 presents the scheme of nitrogen management in key stages of crop growth period.
Table 1