| China is the country with the largest nitrogen(N)fertilizer production and unit area input in the world.Low nitrogen use efficiency(NUE),huge economic costs and serious environment damanges caused from excessive N application are over estimation.Crop straw is a kind of re-cycling and sub-product in agricultural,when returned to field,it can replenish soil nutrients,cut down the investments,and reduce environmental pollution.Before conducting the experiment,the field was managed as a pure N fertilizer gradient experiment under the summer cotton-winter wheat double-cropping system(2012-2014).The in-situ field experiment was carried out in the down reaches of the Yangtze River Valley of eastern China to study the effect of nitrogen(N)fertilization(0,75,150,and 300 kg N ha-1)and consecutive straw returning(2015-2018)on cotton canopy photosynthetic capacity,seedcotton yield,nitrogen use efficiency,and soil nitrogen supply capacity.CCRI 50 and Dong mai 3 were used as seasonal experiment metarial.Addition,a pot experiment was also conducted in 2016 at Pailou research station,Nanjing,China,four N fertilizer rates(0,75,150,and 300 kg N ha-1,suppled as 15N labeld urea)and three crop straw managements(no straw treatment,crop straw returning,and biochar addition)were set to study the straw returning and biochar addition on plant N uptake and the fate of 15N fertilizer in plant-soil system.The main results are as follows:1.Effect of straw returning under different nitrogen fertilizer rates on cotton canopy photosynthetic capacity and yield formationUnder 0 kg N ha-1,75 kg N ha-1 and 150 kg N ha-1 fertilizer rates,by increasing the leaf area index,specific leaf area,leaf nitrogen concentration,and canopy apparent photosynthesis of cotton with straw returning during the flowering and boll setting period,the cotton canopy photo synthetic capacity was improved,and then the accumulation of dry matter and the number of cotton bolls were increased,which laid the foundation for increasing cotton yield.While at 300 kg N ha-1 fertilizer level,too high nitrogen concentration in leaves treated with straw addition excessed nutrient growth and caused the significant reduction of boll weight were the important reasons for the failure of cotton yield to increase.Straw returning improved seedcotton yield by 3%,3%,5%and 1%,by 4%,7%,7%,and 3%,and by 8%,10%,12%,and 4%with 0,75,150,and 300 kg N ha-1,as compared with correspond straw removal from 2016-18,respectively.Validations shows CAP,LAI,LNC,and SLA at the peak flowering stage well-simulated seedcotton yield(R2=0.822-0.948).The results suggested that with approx.33%-44%of chemical N fertilizer reduction would not affect cotton yield with consecutive straw addition.2.Effect of straw returning under different nitrogen fertilizer rates on cotton nitrogen use efficiencyResults statistics showed that straw addition can improve the plant-soil N balance under 0 kg N ha-1,75 kg N ha-1 and 150 kg N ha-1 fertilizer rates,while straw addition caused N surplus(53 kg N ha-1 yr-1-95 kg N ha-1 yr-1)at 300 kg N ha-1.Based on the results of soil-crop-atmosphere nitrogen utilization calculation,the results of this study,considering soil microbial nitrogen fixation,atmospheric nitrogen deposition and straw nitrogen as total N input,showing that straw addition can effectively reduce the nitrogen utilization rate of soil-crop-atmosphere system under various nitrogen application levels,so as to maintain the system nitrogen use efficiency(NUE)within a desirable range(50%90%).We concluded that the desirable N ha-1,by considering seedcotton yield,apparent N balance,and NUE based on soil-crop-atmosphere system.3.Effect of straw returning under different nitrogen fertilizer rates on soil nitrogen supply capacitySoil NH4+,NO3-,microbial biomass nitrogen(MBN),organic nitrogen mineralization rate(NMR),nitrogen management index(NMI),urease and protease activity all increased with N fertilizer level increased,when the N fertilization over 150 kg N ha-1,soil NH4+,NO3-,NMR,urease and protease activity were stabilized,MBN and NMI were significantly decreased.Straw returning significantly increased soil NH4+,NO3-,MBN,NMR,NMI,intertase,urease,and protease activity in the 0-20 cm depth,where the combination with 150 kg N ha-1 showed the best results.The SON occurred mainly in the form of acid hydrolysable fractions(AHN).As the main form of SON fractions,a-AAN was significantly improved in the presence of straw under the given N fertilizer levels,followed by a-AN.Our study indicated that a-AN,a-AAN,NH4+,NO3-,NMR,NMI,urease and protease activities were the main organic N components and active N pools and were the key indicators of soil N supply capacity.4.Effect of straw returning and biochar addition on plant nitrogen uptake and nitrogen(15N)fertilizer use efficiencyStraw returning and biochar addition were beneficial to improve the soil nutrients content,enhance root growth and dry matter accumulation of cotton plants,and the positive effects of biochar returning were better than that of straw addition.By using the 15N isotopic method,our results indicated that the straw addition was beneficial to increase nitrogen absorption and recovery of fertilizer under various N levels.Under 75 kg N ha-1 fertilizer level,however,the proportion of increased plant nitrogen uptake with biochar addition from soil was increased by 19.1%,while the proportion from fertilizer nitrogen was decreased by 7.5%.Both straw returning and biochar addition significantly increased the 15N retention under the given N fertilizer rates,and the effect of biochar was higher than that of the straw incorporation.Our results indicated that N recovery efficiency calculated by non-isotopic method(67%-96%)was much higher than that calculated by isotopic method(31%-53%). |
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