| The poor quality of the seeding emergence in wheat is largely due to the short sowing period,the continuous rainy weather in the tillage and sowing process,and the difficulty of mechanical tillage and sowing because of the total amount of rice-straw returning to the field in the wheat following rice region,especially in the middle and lower reaches of the Yangtze River.In order to improve the quality of sowing and seedling emergence,a wet-resistant seeder was introduced from Sichuan Province.Under the condition of the rice-straw returning into the field with the full amount,the different mechanical tillage and sowing methods[No-tillage with strip shallow rotary sowing(ST),rotary tillage with shallow rotary sowing(RT)]and nitrogen-reduction modes were combined to investigate the nitrogen absorption and utilization,nitrogen migration and distribution in soil-plant system,yield formation and its physiological mechanism in wheat.According to the results,the best combination technology of the mechanical tillage and sowing methods and corresponding nitrogen reduction mode were recommended for high quality,high yield and high efficiency in wheat under different soil moisture conditions.This will provide theoretical and practical basis for green production,high-yield and high-efficiency cultivation of rice stubble wheat.The results are as follows:1.The highest yield was found in ST under the condition of suitable or wet soil moisture before sowing,which was owing to suitable dynamic changes of tillers,high percentage of effective tillers,and sufficient grains number.Compared with the treatment of nitrogen amount 270 kg·hm-2(N270),high photosynthetic area before anthesis was maintained under the ST mode,which also postponed the decline of leaf area index after anthesis and increased the dry matter accumulation even when nitrogen rate reduced to 225 kg·hm-2(N225).There was no significant difference in grain yield and net income between N270 and N225 treatment under ST.Although the excessive nitrogen reduction increased the percentage of effective tillers,the grain yield was lower due to the insufficient spikes.There was none significant difference between N225 and N270 using RT mode only under the suitable soil moisture conditions.However,the wheat yield continued reducing with the nitrogen application rate lower than 225 kg·hm-2.In the year with more raining before sowing,N270 was still needed to construct a high-quality population under RT method.2.The amount of NO3--N and NH4+-N remaining in the surface-soil layer(0-20 cm)was higher in the anthesis stage in the ST mode.More NO3-N remained in the surfacesoil layer at the mature stage,while more NO3--N and NH4+-N remained in the deep-soil layer under the RT mode.The nitrogen content of each soil layer in the anthesis and maturity stages showed a downward trend with the reduction of nitrogen application rate.There was no difference in the content of NH4+-N in each soil layer between the treatments of N225 and N270,and the residual amount of NO3--N and NH4+-N was low at maturity stage.3.Compared with RT,high nitrogen metabolism enzymes activities and nitrogen metabolism level in the middle and late growth stages were maintained under the ST mode.There was no significant difference in the activities of nitrate reductase(NR),glutamine synthetase(GS)and glutamate synthase(GOGAT)between N225 and N270 under ST,promising high nitrogen accumulation,nitrogen transport from vegetative organs to grains after anthesis,and nitrogen use efficiency.While the nitrogen accumulation in plants decreased significantly with the decrease of the nitrogen application amount.The nitrogen application rate 270 kg·hm-2 combined with RT maintained the high enzyme activity of nitrogen metabolism.The nitrogen reduction would lead to a decrease in nitrogen accumulation in each organ.4.The results of 15N tracing experiments indicated that high accumulation and residue of fertilizer nitrogen in the surface-soil layer(<20 cm)in jointing,flowering and maturity stages were maintained under the ST mode.The accumulation and residue amount of nitrogen in deep soil layer(>20 cm)were higher than that of RT.The absorption of fertilizer nitrogen(15N)in jointing and flowering stage under ST was significantly higher than those under RT by 16.51%～59.92%and 13.67%～14.02%,which mainly increased the accumulation of fertilizer nitrogen in stem sheath and leaves,especially the distribution of fertilizer nitrogen in leaves,and the distribution ratio of fertilizer nitrogen in grains at maturity stage.Compared with RT mode,the distribution of nitrogen was promoted,the absorption of nitrogen from fertilizer and soil by plants was also improved,thus the total nitrogen accumulation was increased in plant under the condition of soil with higher moisture before sowing.Further nitrogen reduction reduced not only the 15N residue in the jointing,flowering and maturity stages,but also the distribution of fertilizer nitrogen in each organ of the plant.When the nitrogen amount was reduced to 225 kg·hm-2 under ST,it could still maintain the same distribution level of fertilizer nitrogen as N270,stable proportion of fertilizer nitrogen and soil nitrogen to total nitrogen accumulation,and had the potential to coordinate the nitrogen absorption from fertilizer nitrogen and soil nitrogen.For RT method,225 kg·hm-2 nitrogen rate was suggested only under the suitable soil moisture condition,and nitrogen application rate of 270 kg·hm-2 were still needed to maintain in the high soil moisture condition.5.The optimal combination technology of mechanical tillage/sowing and nitrogen reduction mode of wheat following rice was suggested.When the relative water content of soil before sowing was high(84%),ST with 225 kg·hm-2 nitrogen reduction mode was recommended,which was conducive to maintaining the high enzyme activity of the nitrogen metabolism and nitrogen accumulation in the middle and late stages of wheat,transporting nitrogen to grains,improving nitrogen use efficiency and grain yield,and achieving high yield and high efficiency.Stable yield would be achieved under RT method together with 270 kg·hm-2 nitrogen,but the efficiency of nitrogen use was low.When the soil water content before sowing was suitable,ST and RT methods could both achieve high yield,high efficiency and high income under the nitrogen application rate of 225 kg·hm-2. |
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