| Suboptimal nitrogen(N)availability is a primary limitation to plant growth in terrestrial ecosystems.This is mainly because after irrigation and precipitation events,nitrate quickly moves with water to deeper soil strata,and N leaching has become a critical reason to limits the N acquisition and utilization of crops.How plant explore and exploit soil N pool by changing the traditional agricultural management measures to improve crop N uptake and utilization efficiency have become a hot issue in current agro-ecosystem research.Previous studies mainly focused on underlying mechanism changes of root architecture,mycorrhizal colonization,rhizosphere exudation,root turnover and N transfer of intercropped crops facilitated crop N uptake and utilization efficiency.In addition,root turnover also has a great effect on crop N uptake and utilization.In addition,the comprehensive effects of the above five aspects in the intercropping system for efficient N capture is still unclear.The field experiment was conducted with maize(Zea mays L.cv.Zhengdan 958)and alfalfa(Medicago sativa L.cv.Dongmu No.1),which was designed as a split-plot arrangement with four replicates.The main plot included two N treatments:without N fertilizer(NO)and with N fertilizer application(N1).The subplot was treated with the five cropping modes:(1)MM(monoculture maize),consisting of six maize rows;(2)MA(monoculture alfalfa),consisting of thirteen alfalfa rows;(3)IMA43,consisting of four maize rows intercropped with six alfalfa rows,with 30 cm intra-row distance between maize and alfalfa;(4)IMA42,consisting of four maize rows intercropped with six alfalfa rows,with 20 cm intra-row distance between maize and alfalfa;(5)IMA23,consisting of two maize rows intercropped with six alfalfa rows,with 30 cm intra-row distance between maize and alfalfa.The correlation of characteristics of root and rhizosphere microenvironment with crops N uptake and utilization were analyzed.Major results are showed as follows:(1)The competitiveness of alfalfa was 1.4-3.8 times stronger than maize for N capture in the maize/alfalfa intercropping.On four-year average,with N addition,N uptake of maize and alfalfa in IMA43 were significantly increased by 7%and 12%,N uptake of maize in IMA42 and IMA23 were slightly reduced by 12.3%and 22%,while N uptake of alfalfa were significantly increased by 105%and 94%.In addition,N fertilizer application improved N uptake of intercropping system.Total N uptake of intercropping were 118 kg N ha-1,113 kg N ha-1,269 kg N ha-1 and 213 kg N ha-1 from 2015 to 2018.(2)Intercropping significantly inhibited maize crown root growth,on four-year average,with N addition,the measured crown root parameters(crown root number,dry weight,surface area,volume and length),except of crown root growth angle,of intercropped maize in IMA43,IMA42 and IMA23 modes were significantly declined by 12%,27%and 45%relative to MM,respectively,which resulting in lower N uptake of intercropped maize.Roots growth of intercropped maize had no effect on N utilization efficiency.In addition,all the measured morphological parameters of maize crown root,except of the crown root growth angle,decreased with the reduction of rainfall in the growing season;while for alfalfa,as compared to MA,in the low-N soil,taproot roots of intercropped alfalfa in corresponding modes were significantly increased by 81%,28%and 49%,lateral roots were increased by 51%,80%and 17%respectively,leading to the increased N uptake of intercropped alfalfa.In addition,the values of root morphological traits of alfalfa improved with the planting years,and improved with increase of rainfall in the growing season.N fertilizer application significantly improved root growth and N uptake of both maize and alfalfa relative to that without N addition.(3)On four-year average,in the low-N soil,as compared to MA,rhizosphere pH of intercropped maize in IMA43,IMA42 and IMA23 were decreased by 10%,8%and 2%,total organic acid in the rhizosphere soil of intercropped maize were significantly increased by 58%,46%and 27%,148%,and soluble sugars concentrations in the rhizosphere soil of intercropped maize were significantly increased by 148%,128%and 84%.While rhizosphere pH of intercropped alfalfa in IMA43,IMA42 and IMA23 were decreased by 7%,4%and 3%,and total organic acid in the rhizosphere soil of intercropped alfalfa were decreased 2%,9%and 15%,soluble sugar content in the rhizosphere soil of intercropped alfalfa decreased 5%and 16%in the IMA43 and IMA23 treatments,respectively,while increased by 12%in the IMA42 treatment.In addition,N addition significantly decreased the pH in the rhizosphere of maize and alfalfa,and increased the total organic acid and soluble sugar content for them.Intercropping decreased the pH and increased total organic acid and soluble sugar content in the rhizosphere of maize,resulting the increase of maize N uptake,but for alfalfa,the decreased pH in the alfalfa rhizosphere led to an increase of alfalfa N uptake.(4)On four-year average,in the low-N soil,compared with monoculture maize,mycorrhizal colonization and hyphal length density in topsoil of maize in IMA43,IMA42 and IMA23 mode were increased by 11%and 1.8%,31%and 13.4%,2%and 12%,respectively.The root infection characteristics of alfalfa in corresponding modes increased by 12%and 14%,18%and 34%,12%and 1%,respectively.However,N addition significantly reduced the mycorrhizal colonization and soil hyphal length density in topsoil of maize and alfalfa.Moreover,the mycorrhizal colonization level of maize and alfalfa roots had the highest value in 2016.The correlation results showed that improvement of the mycorrhizal colonization and hyphal length density in topsoil of intercropping maize and alfalfa had little effect on crop N uptake and utilization.(5)On four-year average,in the low-N soil,as compared to MM,maize/alfalfa intercropping had 38%shorter mean lifespan,and 23%faster annual root turnover,when compared with MA,maize/alfalfa intercropping had 14%lower mean lifespan,and 11%faster annual root turnover.The improvement root turnover of intercropped crops facilitated N uptake,but had no significant effect on N concentration and utilization efficiency.The root turnover of monoculture and intercropping increased with the decrease of rainfall in the growing season.In addition,N addition significantly shortened average life span of intercropping crops,leading to an increase of annual root turnover rate.(6)N transfer amount ranged from 7 to 10 mg N plant-1 from alfalfa to maize and significantly decreased(by11%-22%)ith N fertilizer application.Comparatively,intercropping of 4 rows of maize and 6 rows of alfalfa with 30 cm intra-row spacing(IMA43)was the most optimal intercropping mode.Under no N addition,hyphal length density(HLD)of rhizosphere soil explained the largest significant amount(50%)of the variability in N transfer and crop yield.However,root exudates explained 77%of the variability in N transfer and crop yields with N fertilizer application.(7)Compared with MM and MA,the total yield of IMA43,IMA42 and IMA23 was increased,on average,by 113%,94%and 80%,and by 22%,11%and 3%.Output values per unit area were increased by 104%,79%and 70%,and by 25%,10.4%and 3.6%.Therefore,maize and alfalfa intercropping significantly improved the land use efficiency,leading to a significant increase in the total yield and economic benefits of the intercropping system.In addition,the advantage of maize/alfalfa intercropping varied clearly with interannual precipitation and intercropping years,which maintained relatively stable intercropping advantage and N uptake in both dry and wet years.Overall,maize/alfalfa intercropping had significant advantages in N uptake and yield production in different years.Especially,IMA43 was the most promising mode,annual yield variation ranged from 3.9 to 9.5 t ha-1,and annual economic benefits variation ranged from 1451 to 3442 USD t-1.The advantage of intercropping can be explained by the following reasons:intercropping increased alfalfa lateral root length,meanwhile increased root exudations(e.g.,soluble sugar,citric and acetic acid)and root turnover,leading to an increase of alfalfa N uptake.While maize root growth was inhibited by intercropping,resulting in a decrease of maize N uptake,maize may take a series of strategies by enhancing mycorrhizal colonization,increasing root exudation(e.g.,soluble sugars,formic acid and tartaric acid)in response to the reduction of N uptake.This study provides theoretical bases for the implementation and extension of maize/alfalfa intercropping in the farming and pastoral area of north-east China. |
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