| Nitrogen is one of the important nutrient elements for crop growth and development. The scientific and reasonable application of nitrogen is of great significance to increase crop output and guarantee food safety. Both the production and consumption of nitrogen fertilizer in China ranks first in the world, but the nitrogen use’ efficiency is only about 35%, which is lower than that in developed countries. Excessive application of nitrogen may pollute soils, water bodies and atmospheric environment and affect the production efficiency of farming land. So researches should be carried out to improve the efficiency of nitrogen. The present study was carried out from 2013 to 2015 on the Experimental Farm of Jiangsu Key Laboratory of Crop Physiology and Genetics of Yangzhou University using soil pools. Wheat was planted after the harvest of rice in the autumn of 2013 and after both rice and maize in the autumn of 2014. A 15N-microplot experiment was done to investigate the translocation and fate of nitrogen that was applied on wheat in the soil-crop system. The similarities and differences in the translocation and fate of nitrogen applied on wheat grown in the fields with rice and maize stubbles were also compared. The objective of this study was to investigate the characteristics of absorption, utilization and distribution of 15N, the regulation mechanism of photosynthesis and N metabolism-related enzymes in the wheat plants grown in the fields with rice and maize stubbles. This study was critical importance for exploring the ways of improving nitrogen use efficiency of wheat following rice and thus providing a theoretical and practical basis for reducing N rate and improving N use efficiency of wheat following rice in the Middle and Lower Reaches of the Yangtze River. The main results are as follows:1. As nitrogen rate increased within a certain scope, the grain yield of wheat following rice and wheat following maize was increased. The grain yield achieved the highest level when the nitrogen application amount was 240 kg per hectare. The grain yield was increased insignificantly or even decreased when the nitrogen rate was increased further. At the same nitrogen rate, previous crops had significant effects on the number of productive ear, number of grain per ear, thousand-kernel-weight and grain yield. The number of productive ear, number of grain per ear and grain yield of wheat following maize were all higher than those of wheat following rice. For this reason, nitrogen rate should be higher for wheat following rice than for wheat following maize to achieve the equal grain yield per unit area.2. The earing percentage of main stems and tillers, LAI and ear-bearing tiller percentage were increased significantly with increased nitrogen rate in both wheat following rice and wheat following maize. The earing percentage of main stems and tillers, rate of LAI and ear-bearing tiller.percentage in wheat following maize were slightly higher than those in wheat following rice. At the same nitrogen rate, the dry matter accumulation amount at each growth period and after anthesis in wheat following maize was higher than that in wheat following rice.3. Under the two conditions of previous crops, the N amount needed to produce 100 kg of grain in high nitrogen treatments (240 and 300 kg N per hectare, abbreviated as N240 and N300) was higher than that in NO (0 kg N per heactare, abbreviated as NO) treatments, the N amount needed to produce 100 kg of grain in low nitrogen treatments (180 kg N per heactare, abbreviated as N180) was lower than that in NO. At the same nitrogen rate, the capacity of increasing production (in terms of grain yield increased by the application of 1 kg nitrogen) in wheat following rich was declined as compared with wheat following maize. At all the three nitrogen rates, the growth rate of partial factor productivity (PFP) in wheat following maize were 0.23,0.47, and 0.48 kg·kg-1 higher than in wheat following rice, respectively.4. With the increase in nitrogen rate, the NR, GS and GOGAT activities in the flag leaves of wheat following rice and wheat following maize were increased significantly from anthesis to the 28th day after anthesis. The GS, GOGAT, GOT and GPT activities of grain yield had the similar trends. At the same nitrogen rate, the NR, GS and GOGAT activities in the flag leaves of wheat following maize from anthesis to the 28th day after anthesis were all higher than those of wheat following rice at the same period. In the meantime, the decreasing rate of the NR, GS and GOGAT activities from the 14th day to the 28th day after anthesis were lower than those of wheat following rice. The GS, GOGAT, GOT and GPT activities of grain yield in wheat following maize from anthesis to the 28th day after anthesis were higher than those of wheat following rice, but the decreasing rate of the GS, GOGAT, GOT and GPT activities of grain yield from day 14 to day 28 after anthesis were lower than that in wheat following rice. Compared to wheat following rice, wheat following maize kept both high capacity of nitrogen assimilation and high capacity of nitrogen translocation after anthesis.5. The results of 15N-isotope tracing showed that about 55%～65% nitrogen assimilated by wheat following rice and wheat following maize was from soil, about 35%～45% was from fertilizer application. The nitrogen assimilated in the treatments of N180 and N240 of wheat following maize was 1.96% and 0.64% higher, respectively, than that in the same treatments of wheat following rice, and the contribution efficiency of nitrogen was 0.97% and 1.74%, respectively higher than that of wheat following rice. In the treatment of N240 of wheat following maize, the contribution efficiency of basal dressing and topdressing fertilizer nitrogen was 5.29% and 0.69%, respectively higher than that of wheat following rice. Nitrogen assimilated rate by basal dressing fertilizer was lower than that of top dressed nitrogen. The nitrogen derived from both soil and fertilizer was detected in the different organs of wheat plants, with a descending order of nitrogen assimilation amount of grain, stem and sheath, leaf, and glume and spike stalk at maturity. With the increase in nitrogen rate, there was a declined trend in the proportion of nitrogen absorption from soil and an increased trend in the proportion of nitrogen absorption from fertilizer and the residual rate of nitrogen in soil. This indicated that the increased rate of nitrogen enhanced the proportion of nitrogen from fertilizer in wheat plants and decreased the absorption of nitrogen from fertilizer. The highest nitrogen translocation efficiency and contribution efficiency by each organ was found in leaf, followed by glume and spike stalk, and stem and sheath. In terms of nitrogen contribution efficiency to grain, fertilizer application was lower than soil nitrogen. Nitrogen translocation efficiency and contribution efficiency to grain of topdressing fertilizer were higher than those of basal dressing fertilizer.6. The results 15N-isotope tracing showed that the utilization rate of fertilizer was between 38% and 40%, the rate of fertilizer N loss was between 33% and 35%, and the ratio of soil residual was between 26% and 27%. The fate of nitrogen distribution rate on per unit area performance followed the descending order of plant assimilation, loss and soil residue. The utilization rate and residual rate in the treatment of N180 of wheat following maize was 2.80% and 0.68% higher than that in the treatment of wheat following rice, while the loss rate was 3.47% lower in the treatment of wheat following rice. The utilization rate and residual rate in the treatment of N240 of wheat following maize was 1.86% and 1.70% higher than that in the treatment of wheat following rice, while the loss rate was 3.56% lower in the treatment of wheat following rice. With the increase in nitrogen rate, the nitrogen assimilated by wheat (including basal dressing fertilizer and topdressing nitrogen), the residual rate of nitrogen in soil and the rate of fertilizer N loss were remarkably increased. The N loss rate of basal dressing fertilizer was higher than that of topdressing fertilizer. The nitrogen assimilated by wheat and the rate of soil residue of basal dressing nitrogen were lower than those of topdressing fertilizer. |
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