Analysis Of The Nitrogen Uptake And Utilization, Rhizosphere Micro-Ecology In The "Wheat/Maize/Soybean" Relay-Cropping System

From 2005 to 2008 year,through ¹⁵N isotopic tracer method and root barriers technique,field experiment and ¹⁵N labeling pot experiment were carried out to investigate nitrogen source uptake and utilization mechanics,interspecific competition and facilitation for nutrients,and rhizosphere micro ecology effect in the relay-planting system of wheat/maize/soybean,contrasting by the relay-planting system of "wheat/maize/sweet potato".The results showed as fellows.1.Applying the double optimal value method,the annual optimal amount of nitrogen fertilizer and the ratio of nitrogen fertilizer distributed among crops in the relay-planting system of "wheat/maize/soybean" were studied.The results showed that the annual optimal total amount of nitrogen fertilizer applied in this system was 342.8kg.hm^-2 in the medium-yield cropland.The suitable distributive proportion of nitrogen fertilizer was 33.76:35.71:30.54 among the three crops,and the applying amounts for wheat,maize and soybean were 115.73kg.hm^-2,122.41kg.hm^-2,and 104.69kg.hm^-2 respectively.2.From 2006 to 2008 year,the methods of root barrier and ¹⁵N-isotope dilution were used to investigate the interspecific nitrogen competition and facilitation,nitrogen uptake and utilization in the relay-planting system of "wheat/maize/soybean".The results showed that there were coexisted the nitrogen competition and facilitation in the relay-planting system of "wheat/maize/soybean",wheat and maize had competing advantage,and soybean had competing disadvantage(Awc＞0,NCRwc＞1;Acs＞0, NCRcs＞1).There were strong interspecific nitrogen competition and infirm facilitation in the system of wheat/maize/sweet potato.Wheat had competing advantage(Awc＞0, NCRwc＞1),and maize and sweet potato had competing disadvantage(Acs=0.191＞0; NCRcs=1.01＞1).In the relay-planting systems of "wheat/maize/soybean" and "wheat/maize/sweet potato",comparing the no barrier with solid barrier,the grain yield,total biomass above ground,grain nitrogen uptake,the total nitrogen uptake above ground,¹⁵N grain uptake, ¹⁵N total uptake and ¹⁵N crop recovery rate of wheat increased remarkably,and ¹⁵N% abundance and N%content of soil reduced obviously.The grain yield increased by 25.594%and 24.811%respectively,¹⁵N grain uptake increased by 67.194%and 53.859% respectively,¹⁵N crop recovery rate increased by 38.069%and 38.727%respectively.In the relay-planting system of "wheat/maize/soybean",the grain yield,the total biomass above ground,grain nitrogen uptake,the total nitrogen uptake above ground,¹⁵N grain uptake,¹⁵N crop recovery rate of maize,and the ¹⁵N%abundance of soil and total N content increased by 14.402%,9.763%,44.926%,31.129%,37.8492%,25.157%,13.896 and 10.145 respectively.But in the "wheat/maize/sweet potato" system with no barrier, the value of above index reduced by 20.007%,10.094%,6.786%,2.978%,25.366%, 15.988%,17.366%and 5.195%respectively.For soybean,grain yield,the ¹⁵N crop uptake,¹⁵N grain uptake,and ¹⁵N crop recovery rate reduced,but the soil N%content increased by 7.28%.For sweet potato,earthnut yield,the ¹⁵N total uptake,and total N content of soil reduced,but ¹⁵N crop uptake and crop recovery rate increased by 47.958% and 47.957%.In the relay-planting system of "wheat/maize/soybean",the content of NO₃-N and the total amount of inorganic nitrogen(NO₃-N,NH₄-N) in soil reduced,but that of the wheat/maize/sweet potato system increased,and the values were higher than wheat/maize/soybean system.In the two relay-planting systems,the NO₃-N content of soil increased for soybean and sweet potato,and that of soybean was higher than sweet potato.Soybean or sweet potato utilized soil rudimental ¹⁵N from wheat,maize and soybean(sweet potato) stubble.There existed ¹⁵N transfer between crops in the two relay-planting systems,but the ¹⁵N uptake and recovery rate,and the intensity of ¹⁵N transfer in the relay-planting system of "wheat/maize/soybean" were higher than that of "wheat/maize/sweet potato".3.Pot experiment was carried out to study the nitrogen transfer occurring in the relay-planting system of "wheat/maize/soybean" at three different nitrogen fertilizer application rates(150kg/hm²,300kg/hm²,450kg/hm²) using ¹⁵N foliar labeling method. The results showed that,the grain yield,total nitrogen uptake above ground,and total N content of soil in the relay-planting system of wheat/maize/soybean,were higher than that of the relay-planting system of wheat/maize/sweet potato at the same nitrogen fertilizer application rate.The wheat/maize/soybean system had obvious advantage of yield and nitrogen nutrition,comparing for wheat/maize/sweet potato system.In the two systems,the grain(or earthnut) yield and nitrogen uptake of wheat and soybean(or sweet potato) increased first and then reduced with the nitrogen fertilizer application rate increasing,the indices values of the A3 treatment(300Nkg/hm²) was highest.The grain yield and nitrogen uptake of maize were increasing with the nitrogen fertilizer application rate increasing,the values of the A4(450Nkg/hm²) treatment was highest. The soil total nitrogen content of the wheat,maize and sweet potato was highest in A3 treatment.But the soil total nitrogen content of the soybean reduced with the nitrogen fertilizer application rate increasing,the value of Al(0Nkg/hm²) treatment was highest. NO₃-N content of crops in the two relay-planting systems increased with the nitrogen fertilizer application rate increasing,and the value of NO₃-N content of wheat/maize/soybean system was higher than that of wheat/maize/sweet potato system.There existed bi-directional N transfer in the two relay-planting systems,and the amounts of N transferred reduced with the soil fertility and nitrogen fertilizer application rate increasing.The amount of net ¹⁵N transferred and transfer intensity in the relay-planting system "wheat/maize/soybean" were higher than that of "wheat/maize/sweet potato".When wheat functioned as ¹⁵N donor plant,the ¹⁵N% abundance in maize shoot was consistently higher than the ¹⁵N%nature abundance.It indicated that the ¹⁵N transfered from wheat to associated maize,but the amounts of N transferred and the proportion of ¹⁵N transferred were small difference between the two relay-planting systems.When maize functioned as ¹⁵N donor plant,there existed the ¹⁵N transfer from maize to associated wheat and soybean(sweet potato).The amount of transferring N of A2(300Nkg/hm²) treatment was highest.In the two systems,the amounts of N transferred from maize to wheat was higher 290.2%～940.2%and 133.8%～218.7%than that of N transferred from maize to soybean(or sweet potato),but the amounts of N transferred from maize to sweet potato was higher 100.1%～220.3% than that of N transferred from maize to soybean.When soybean or sweet potato functioned as ¹⁵N donor plant,there existed the ¹⁵N transfer from soybean(sweet potato) to associated maize,and the proportion of ¹⁵N transferred from soybean to maize was higher 21.266%～52.003%than that of the ¹⁵N transferred from sweet potato to maize. The amount of net ¹⁵N transferred＞0 for wheat,but that of maize and soybean(sweet potato)＜0.Amounts of net ¹⁵N transferred of wheat,maize,and soybean in the wheat/maize/soybean system were higher 3.256%～12.083%,26.99%～166.209%,and 26.242%～78.694%than that of the wheat/maize/sweet potato system.4.Two-year field experiment was carried to study the effect of different planting systems on rhizosphere mic-ecology,and the relation between rhizosphere soil environment and nitrogen uptake and utilization.The results showed,comparing the single cropping condition and the relay-planting system of "wheat/maize/sweet potato"with the relay-planting system of "wheat/maize/soybean",wheat/maize/soybean system indicated obvious relay-planting advantage(LER＞1,Awc＜0,Acs＞0,NCRwc＜0, NCRcs＞1),soil PH of maize and soybean,and soil humidity of wheat reduced,but the soil humidity of maize and soybean increased.On the florescence date and maturity date of crops,the biomass,nitrogen uptake,root activity and root dry matter weight of wheat, maize,and soybean increased,and also the amounts of bacterium,the fungus,and ray fungi of soil increased.Result of DGGE showed,shanon-wiener index(H') of the relay-planting system of wheat/maize/soybean were higher than single cropping condition and the relay-planting system of wheat/maize/sweet potato,the similarity coefficient(Cs) between the relay-planting systems were higher than that of relay-planting system and single-planting system,and CS between fringe rows and single-planting system were low comparatively,but CS between middle rows and single-planting system were high comparatively.At the same time,the total N content, the activity of urease and proteinase enzyme of soil increased,and the nitrate reductase activity and NO₃-N content of soil reduced.Correlation analysis showed that the nitrogen uptake of wheat,maize and soybean(sweet potato) were significant positively correlated with root activity,and activity of urease and proteinase enzyme,and amounts of bacterium,fungus and ray fungi,but were significant negatively correlated with nitrate reductase activity,NO₃-N and NH₄-N content of soil.