Optimal management of nitrogen fertilizer in the main rice crop and its carrying-over effect on ratoon rice under mechanized cultivation in Southeast China

This study attempted to clarify the carrying-over effect of different nitrogen treatments applied to the main crop on the crop population growth and yield formation of ratoon rice under mechanized cultivation in Southeast China.  Based on the constant total nitrogen application amounts (225.00 kg ha^–1) in the main crop, an experiment with different ratios of basal and topdressing nitrogen fertilizer (the ratio of basal fertilizer:primary tillering fertilizer:secondary tillering fertilizer:booting fertilizer at 3:1:2:4 (N1), 3:2:1:4 (N2), 3:3:0:4 (N3), and 4:3:0:3 (N4), respectively, and a control without nitrogen treatment (N0)) was set up across two consecutive years in field using hybrid rice variety Yongyou 1540 as the test materials.  The results showed that the total tiller number and effective tillering percentage increased in the main crop under the N1 treatment, more nitrogen fertilizer applied in late growth stage of the main crop, and its effective tillering percentage of the main crop was the highest at up to 70.18%, which was 9.15% higher than that of conventional fertilization treatment (N4), more nitrogen fertilizer applied in early growth stage of the main crop.  The same tendency was observed in leaf area index (LAI) value of the main crop and its subsequent ratoon rice, which were 16.52 and 29.87% higher, respectively, in the N1 treatment than that in the N4 treatment at the full heading stage.  The same was true in the case of the transport rates of stem and sheath dry mater and the canopy light interception rates in both the main and its ratoon crops.  The transport rate of stem and sheath in main crop rice under N1 treatment increased by 50.57% compared with N4 treatment.  The canopy light interception rate of N1 treatment increased by 5.07% compared with N4 treatment at the full heading stage of the ratoon crop.  Therefore, the total actual yield was the highest in the main and its ratoon crops under N1 treatment, averaging 17 351.23 kg ha^–1 in two-year trials, which was 23.00% higher than that in the conventional fertilization treatment (N4).  The results showed that appropriate nitrogen treatment was able to produce a good crop stand in the main crop, which was essential for producing a good ratoon crop population and high yield especially under mechanized cultivation with low stubble height of the main crop.  The study suggested that shifting the proper nitrogen application amounts to the late growth stage of the main crop, such as N1 treatment, not only had a higher productive effect on ensuring the yield of the main crop, but also had a positive effect on the axillary bud sprouts from the stubbles for ratoon rice, resulting in an increased percentage of productive panicles and achieving the goal of one planting with two good harvests under the conditions of our study.