Nitrogen management in machinery transplanted japonica rice under different rotation systems for stable grain yield and higher nitrogen use efficiency

Objectives The soil fertility after the harvest of pre-crop affects the growth and yield of the following crops in a rotation system. We investigated the soil fertilities in the main rotation systems, and the suitable nitrogen fertilizer rates and application times for the following rice production, in order to achieve the goal of reducing nitrogen fertilizer input and maintain high yield of rice.

Methods Field experiments were conducted in the japonica rice region of Yunnan Province, using rice cultivar of ‘Longke16’ as tested materials. The tested three rotation systems were oilseed rape-rice, wheat-rice, and broad bean-rice. Six nitrogen rates of 0, 90, 120, 150, 180 and 210 kg/hm² (expressed as N0, N90, N120, N150, N180, N210, respectively) were topdressed as spikelet-promoting and spikelet-sustaining fertilizer in ratio of 6∶4. Local recommended high-yield N management (N 285 kg/hm² in total, evenly divided and applied as basal, tillering, spikelet-promoting and spikelet-sustaining fertilizer) was used as control. The 0–20 cm soil fertility were measured before rice seeding, and the rice yields and nitrogen utilization were investigated at harvest.

Results Among the three rotation systems, the field after oilseed rape had the highest soil N content, followed by that after wheat, and the lowest in that after broad bean. As a result, the yields and N utilization efficiencies of following rice were significantly different. All the yield, N uptake, apparent N use efficiency of the rotation systems were in descent order of oilseed rape-rice, wheat-rice and broad bean-rice system. The highest yield was achieved at N rate of 180 kg/hm² in all the three rotation systems. Compared with CK, nitrogen input in N180 was reduced by 36.84%, while the rice yield kept stable in the oilseed rape-rice and wheat-rice system , and was increased by 3.94% in broad bean-rice system. For two consecutive years of reduced nitrogen cultivation, the rice yields reduction of 1.07, 0.30, and 0.29 t/ hm² occurred in oilseed rape-rice system, wheat-rice and broad bean-rice system. The application of panicle fertilizer alone increased the number of effective panicles. Although the increase in the number of effective panicles was slightly lower than that of the control, there was no significant difference in the total number of spikelets. The highest N agronomic efficiency was achieved when the panicle fertilizer application rate was 180 kg/hm² in the oilseed rape-rice rotation system.

Conclusions In the paddy-upland rotation systems, the fertility of field after oilseed rape is higher than taht after wheat and broad bean. For rice after wheat and broad bean, base and tiller fertilizers are not necessary, top-dressing N 180 kg/hm² as panicle fertilizer (spikelet-promoting to spikelet-sustaining in ratio of 6∶4) will increase the rice yield and N use efficiencies within two years. However, the sustainability of the N management needs further study for rice after oilseed rape.