| The arid irrigation area in northwest China is one of the important pasture production areas.Because of the lack of management means and the blindness of fertilization,the soil nutrient elements was lack and imbalance,and the low utilization rate of nitrogen,phosphorus and potassium fertilizer caused the yield and quality of alfalfa(Medicago sativa)not to reach the standard of high-quality forage.So this study took the alfalfa(2 years in 2016 and 3 years in 2017)in the northwest arid irrigated area as the study object,the effects of different fertilization systems on alfalfa production performance,carbon and nitrogen metabolism,root nodule nitrogen fixation,soil nutrient characteristics and soil microbial characteristics were systematically studied by using the quadratic regression 3414 experimental design and single factor experiment design,and the relationship among fertilizer,crop,soil and microorganism was also analyzed.Moreover,the mechanism of nutrient absorption and utilization in high-yielding alfalfa field was revealed,and the fertilizer efficiency model of alfalfa(Gannong No.3)production in the northwest arid irrigated area was established,and the optimum amount of fertilizer was also obtained.The main results were as follows:The results showed that the contribution of N,P and K to the yield of two years old alfalfa was K > P > N,and the contribution of N,P and K to the yield of three years old alfalfa was P > K > N,and the interaction effect in two years old alfalfa was N-P > N-K > P-K,and the interaction effect in three years old alfalfa was N-K > N-P > P-K.The contribution of N,P and K to the total protein yield of two years old alfalfa was N > K > P,and the contribution of N,P and K to the total protein yield of three years old alfalfa was N > P > K.The effect of N-P interaction on the two years old alfalfa was better than that of N-K interaction and P-K interaction.The effect of N-P interaction and N-K interaction on the total protein yield of three years old alfalfa was significantly greater than that of P-K interaction.By using the method of frequency analysis,the optimal fertilization amount of N,P and K were 56.27~67.51 kg·hm-2,77.69~90.48 kg·hm-2 and 76.43~87.18 kg·hm-2 when the target yield of the two years old alfalfa was higher than the average yield of 17522.1 kg·hm-2,when the target yield of three years old alfalfa was higher than the average yield of 19234.1 kg·hm-2,the optimal fertilization amount of N,P and K were 46.75~57.66 kg·hm-2,80.15~92.28 kg·hm-2 and 57.79~69.74 kg·hm-2,respectively;when the total target protein yield of two years old alfalfa was larger than the average of 2115.7 kg·hm-2,the optimal fertilization amount of N,P and K were 66.35~77.48 kg·hm-2,79.34~ 98.87 kg·hm-2 and 73.68~85.38 kg·hm-2,respectively;when the total target protein yield of three years old alfalfa was larger than the average of 2656.0 kg·hm-2,the optimal fertilization amount of N,P and K were 68.44~79.50 kg·hm-2,72.74~85.96 kg·hm-2 and 50.68~61.61 kg·hm-2,respectively.Soil pH decreased after harvest of alfalfa without fertilization,soil organic matter content for all treatments increased,and increased by 9.31%~14.93%.Soil total nitrogen was not affected by fertilization and alfalfa planting years;N,P,K was benefitial for the increase for soil available nitrogen,available phosphorus and available potassium,respectively.The nitrogen uptake and utilization efficiency was the highest in N 51.75 kg·hm-2 level based on P 105 kg·hm-2,K 90 kg·hm-2,and reduced with the N level raised;It could achieve the purpose of nitrogen was promoted by phosphorous and nitrogen was promoted by potassium while reducing pollution in the range of N0~103.5 kg·hm-2 and P0~105 kg·hm-2;fertilizer application rates of P1K2N2(P 52.5 kg·hm-2,K 90 kg·hm-2,N103.5 kg·hm-2),P2K2N2(P 105kg·hm-2,K 90 kg·hm-2,N 103.5 kg·hm-2)and P2K1N2(P 105 kg·hm-2,K 45 kg·hm-2,N103.5 kg·hm-2)were higher than the national average level.The economic benefits of P2K2N1(P105 kg·hm-2,K90 kg·hm-2,N52.5 kg·hm-2),P1K2N1(P52.5 kg·hm-2,K90 kg·hm-2,N51.75 kg·hm-2)and P2K1N1(P105 kg·hm-2,K 45 kg·hm-2,N51.75 kg·hm-2,)treatments were higher,and increased by 9.31%~14.93% compared with CK treatment.It was beneficial to increase the nitrogen content,NR activity,GS activity,chlorophyll,RUBP carboxylase,photosynthetic capacity(Pn,Gs and Tr were increased significantly,Ci was the opposite),soluble sugar content and SPS activity in the range of N0~103.5 kg·hm-2,P0~105 kg·hm-2 and K0~90 kg·hm-2.However,the changes of GOGAT and GDH activity have no obvious regularity.The enzyme activity of alfalfa planted three years was higher than that of alfalfa planted two years,and the highest activity in the second stubble.Yield and nitrogen accumulation were positively correlated with soluble sugar content,RUBP carboxylase activity,NR,GS,chlorophyll content,Pn and Tr.By principal component analysis,the appropriate N,P,K ratio mainly strengthen their own nitrogen metabolism to promote the absorption and assimilation of nitrogen through the activation of NR and GS activity,photosynthetic capacity,RUBP carboxylase and SPS activity thereby enhancing yield,quality and fertilizer use efficiency in alfalfa.The ureide evaluation method,which a simple method to evaluate nitrogen fixation ability of alfalfa in the production practice.Ureide content(y)and nitrogenase activity(x)of stem was significantly positively related.The nitrogen fixation ability of alfalfa was not inhibited and increased with N in the range of 0~51.75 kg·hm-2 in Northwest drought irrigated area.It was beneficial to increase microbe quantity(bacteria,fungi and actinomycetes),microbial biomass(microbial biomass carbon,nitrogen and phosphorus)and soil enzyme activity(urease,invertase and alkaline phosphatase)in the range of N 0~103.5 kg·hm-2,however,the changes of catalase activity have no obvious regularity.All indexes in rhythm vertical decrease,most active in 0~10cm.Microbial biomass,urease and invertase showed seasonal,summer> spring> autumn.Yield,nitrogen accumulation and soil fertility were positively correlated with microbe quantity,microbial biomass and enzyme activity.By principal component analysis,the appropriate N supply level mainly strengthen their own nitrogen metabolism to promote the absorption and assimilation of nitrogen through the increase of bacteria,actinomycetes,microbial biomass and the activation of soil enzyme activity,thereby enhancing yield,quality and RFV. |
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