Objectives Straw returning methods affect rice yield and availability of soil and fertilizer nutrient. The study was to explore the difference of rice nitrogen uptake under different straw returning and transplanting methods to provide theoretical and practical basis for suitable straw returning methods under hand transplanting and machinery transplanting.

Methods A two-factor spilt-plot trial was conducted in Wenjiang of Chengdu, Sichuan Province from 2016 to 2018. The tested cultivar was F-you 498, a triple crossing hybrid of indica (Oryza sativa). The main plot was straw returning method: no straw returning (S0), straw mulching (S1) and straw overturned into soil (S2); the subplot was two rice seedling transplanting ways of by hand (HT) and by machine (MT). In all the plots, urea-N of 135 kg/hm² was applied in ratio of basal∶tilling∶booting∶flowering in 3∶3∶2∶2, and P₂O₅ of 90 kg/hm² from SSP was once basal applied, and K₂O of 150 kg/hm² from potassium chloride was applied in ratio of basal∶earing of 7∶3. Plant samples of stem-sheath, leaf and ear were collected in the stages of active tillering, jointing, full heading and maturity, respectively. The nitrogen contents were determined, and the nitrogen accumulation, transfer and utilization were calculated.

Results Straw returning methods had significant effects on rice yield, nitrogen accumulation and nitrogen utilization. 1) Compared with S0, both S1 and S2 improved rice yield, restrained nitrogen accumulation at active tillering stage, but promoted N accumulation in various organs of rice plant from the beginning of jointing to maturing stages, and S1 performed significantly better than S2 did. S1 promoted the nitrogen transfer from stem-sheath (44.1%) and leaf (23.5%), and increased the transfer rate of stem-sheath (10.2%), and N increment in panicle (21.2%). As for straw overturn, only the nitrogen transfer of stem-sheath (24.7%), and transfer rate of stem-sheath (6.5%) and N increment in panicle (16.7%) were increased. The N agronomic efficiency and N recovery efficiency were shown as S1 > S2 > S0, but the nitrogen grain production efficiency reduced after straw returning. 2) In active tillering and jointing stage by all treatments of straw returning, the nitrogen accumulation in each organ and rice plant, nitrogen transfer, N increment in panicle and rice yield of HT were higher than those of MT, but the N recovery efficiency of HT was significantly lower than that of MT. Straw returning had different effect on nitrogen accumulation and transfer under different transplanting ways. At active tillering stage, the rice nitrogen accumulation of HT was the lowest under the straw overturn, and under straw mulching, that of MT was the lowest. Straw mulching was more beneficial for the nitrogen accumulation and transfer, N agronomic efficiency and N recovery efficiency were higher in S1 under two transplanting ways from jointing stage to maturity stage.

Conclusions From the point of improving rice yield and nitrogen accumulation and transfer, straw mulching method is recommended for both the hand transplanting and machinery transplanting of rice, especially for hand transplanting rice.