Effects Of Nitrogen And Phosphorus Fertilizers On Soil-Plant Ecological Stoichiometric Characteristics Of Farmland In The Longzhong Loess Plateau

Ecological stoichiometry is an important indicator to measure the stability of the ecosystem.Nitrogen and phosphorus fertilizers have an impact on crop element cycle and soil nutrient management,and then affect the ecological stoichiometry characteristics of farmland soil and plants.Based on the experiment of adding nitrogen and phosphorus fertilizers at different levels(there were 4 levels for nitrogen and phosphorus,which were 0,75,115,190 kg·hm^-2,denoted as N0,N1,N2,N3;P0,P1,P2,P3,a total of 16 orthogonal treatments)in the arid agricultural area of the Longzhong Loess Plateau,the distribution characteristics of carbon（C）,nitrogen（N）and phosphorus（P）elements in different soil layers and organs of spring wheat were studied,the differences of soil nutrients and their limiting elements under the addition of nitrogen,phosphorus fertilizers and the interaction of nitrogen and phosphorus were analyzed combined with ecological stoichiometry.In order to reveal the stoichiometric characteristics of carbon,nitrogen and phosphorus in farmland soils and wheat plants,as well as nutrient regulation under different levels of nitrogen and phosphorus addition.It is expected to provide theoretical guidance for the determination of the optimal ratio of nitrogen and phosphorus in dryland spring wheat fields in the Loess Plateau,and then expand the research field of ecological chemometrics in dry farmland in the Loess Plateau.The main results are as follows:1.Soil ecological stoichiometry characteristics and nutrient had different responses to different levels of nitrogen and phosphorus fertilizers.（1）Soil organic carbon（SOC）,total nitrogen（TN）and total phosphorus（TP）of each treatment decreased with the deepening of soil layer.During the jointing,flowering and maturity periods of spring wheat,nitrogen application significantly increased SOC and TN of each soil layer,and decreased TP;phosphorus application significantly increased TN and TP,and decreased SOC.The addition of nitrogen and phosphorus fertilizers and their interaction effects could significantly affect soil C/N,C/P and N/P,with the range of 7.04-9.35,6.48-10.45 and 0.86-1.27 respectively.（2）Nitrogen fertilizer addition significantly increased soil nitrate nitrogen and ammonium nitrogen（NO₃^--N and NH₄⁺-N）,which reached the maximum under N3P0treatment,while phosphorus fertilizer addition significantly decreased soil NO₃^--N,which reached the minimum under N0P3 treatment while the combined application of nitrogen and phosphorus had no significant effect on soil NH₄⁺-N;phosphorus fertilizer addition significantly increased available phosphorus（AP）,but nitrogen fertilizer addition significantly reduced AP,combined application of nitrogen and phosphorus also significantly affected AP;soil p H decreased to varying degrees with the increase of nitrogen and phosphorus applications.（3）Further correlation and PCA analysis found,the main influencing factor of soil C/N and C/P was SOC,and the main influencing factor of soil N/P was soil TP.2.Nitrogen and phosphorus fertilizers addition had different effects on spring wheat C,N,P and their ecological stoichiometric ratio,changed the nutrient limiting elements of spring wheat farmland.Wheat had a greater coupling to N and P absorption,and C element was least affected by nitrogen and phosphorus.（1）With the addition of nitrogen and phosphorus fertilizers,grain C and root C did not change significantly in different periods.Nitrogen fertilizer addition significantly increased the N and P content of grains,leaves and roots;phosphorus fertilizer addition significantly increased the P content of roots,stems and leaves,and the N and P content of each organ reached the maximum during the flowering period.Phosphorus fertilizer addition significantly reduced the C/N and C/P of each organ,while nitrogen fertilizer addition significantly increased the N/P of each organ.The interaction of nitrogen and phosphorus had a significant impact on the C/N,C/P and N/P of each organ,all reaching the maximum at N2P2.（2）When the soil was not applied with nitrogen,the leaf N/P<14,indicating that the farmland was restricted by N,but after the application of nitrogen,N/P>16,the N restriction gradually changed to P restriction;and the addition of phosphorus did not change the farmland restricted element.（3）Regression analysis showed that under the regulation of nitrogen and phosphorus,the C,N,and P elements in roots,stems and leaves all possessed homeostasis,and there were differences among different organs,and the homeostasis of C was the greatest.3.The addition of nitrogen and phosphorus fertilizers could promote the growth of spring wheat and increase grain yield.The stoichiometric characteristics of spring wheat in this area dominated the distribution of biomass.（1）The addition of nitrogen and phosphorus significantly increased spring wheat aboveground biomass（AGB）,belowground biomass（BGB）,root/shoot（R/S）and grain yield,and R/S and yield reached the maximum at N2P2;（2）RDA analysis showed that:AGB and spring wheat TC,TN,TP and N/P were significantly positively correlated,and C/P and C/N were significantly negatively correlated,and the correlation coefficient with C/N was the largest;R/S was positively correlated with N/P and C/P,the correlation coefficient with P was larger than that with N/P,and it was negatively correlated with C/N.4.C,N,P performed different functions in spring wheat plants and soil,and C,N,P and ecological stoichiometry characteristics of each organ were not directly determined by soil nutrient content.Correlation analysis showed that:（1）Plant C had no significant correlation with soil C,N,P and its ecological stoichiometry;（2）Except for stems,N and C/N in the above-ground and underground parts were all affected by N and P of 0-10cm soil,and the above-ground part was also greatly affected by soil C;（3）The correlation between10-20cm soil and spring wheat organs C,N,P and their ratios was basically the same as 0-10cm soil layer,the difference was that the correlation between plants and soil N and P had been changed to different degrees;（4）Except for stems,there were different degrees of correlation between soil available nutrients and other organs.In summary,nitrogen and phosphorus fertilizers can significantly change C,N,P content and the ecological stoichiometric ratio of the dryland soil and plant in the Longzhong Loess Plateau,but the nutrient cycle of wheat plant and soil C,N,P is not completely synchronized.It is recommended that the combined fertilization of N115kg·hm^-2and P115·kg·hm^-2in this agricultural area can improve soil nutrients and increase wheat yield and promote local sustainable development of agriculture.