| Northwest area is the main forage production area in China.Because of the limited cultivated area,forage production is far from meeting the demand.Therefore,it is an urgent problem to find an efficient cultivation measure to deal with the shortage of forage in the development of animal husbandry.In the practice of forage production,the adoption of high-efficiency planting systems such as intercropping,interplanting and rotation between legumes and gramineaes can effectively improve the production potential and give full play to the ecological advantages.In this study,alfalfa?Medicago sativa?,the main legume forage widely planted in this area,was intercropped with 4 widely planted gramineae forages,including maize?Zea mays?,sorghum?Sorghum dochna?,oat?Avena sativa?and triticale?Triticale Wittmack?.In this experiment,3 consecutive years?2017,2018 and 2019?field experiment,as well as simulation intercropping experiment?soil culture in barrel planting method?and root interaction experiment?nutrient solution and sand culture method?were conducted to explore the intercropping advantages,nitrogen?N?efficiency mechanism and rhizosphere soil microecological effect of alfalfa and 4 gramineae forages in different years,different growth periods and different root interaction intensities etc.The results are as follows:1.Advantage of alfalfa/gramineous intercroppingThe hay yield and protein yield per unit area of 4 gramineae forages in 4 intercropping combinations were significantly higher than those in monoculture.Intercropping could reduce the crude protein content and relative feeding value of alfalfa and improve the nutritional quality and relative feeding value of 4 gramineae forages.The land equivalent ratio of alfalfa is smaller than that of gramineae,and the land equivalent ratios of 4intercropping combinations are greater than 1,that is to say,4 gramineae forages are in a competitive advantage position and alfalfa is in a competitive disadvantage position in intercropping systems.Among the different intercropping combinations,the population yield of alfalfa/sorghum intercropping(18700-19900 kg·hm-2)was the largest,and the population protein yield(2257-2356 kg·hm-2)of alfalfa/maize intercropping was the best.The net photosynthetic rate,stomatal conductance,transpiration rate and light utilization rate of 4 gramineae forages in intercropping were significantly higher than those in monoculture,while the photosynthetic characteristics and light utilization rate of alfalfa in intercropping were lower than those in corresponding monoculture.Intercropping not only promoted the chlorophyll content of gramineae,but also promoted the chlorophyll b content of alfalfa under low light,and then regulated the photosynthetic characteristics of alfalfa under low light.At the same time,the activities of RuBP carboxylase in the 4 gramineae forages were significantly higher than those in the monoculture;the activities of sucrose phosphate synthetase in the intercropping of maize,sorghum and oat were significantly higher than those in the corresponding monoculture;the activities of sucrose synthetase of alfalfa in the alfalfa/maize and alfalfa/sorghum intercropping systems were significantly lower than those in the corresponding monoculture;meanwhile,the carbohydrate content of 4gramineae forages in intercropping systems the were significantly higher than those in the monocultures.The results showed that intercropping could significantly improve the carbon metabolism activity and carbohydrate content of gramineae forages,but had little effect on alfalfa.Among them,alfalfa/sorghum intercropping was the highest.The competition ratio of N,phosphorus?P?and potassium?K?of 4 gramineous forages to alfalfa is greater than 1 in intercropping,that is to say,4 gramineous forages have stronger nutrient competition ability than alfalfa.Intercropping was beneficial to the nutrient competition and accumulation of gramineae.The contents of N,P and K of 4 gramineous forages in the intercroppings were higher than those in the corresponding monocultures,but alfalfa showed the opposite.In the 3-year field experiment,the N contents of oat and triticale in intercroppings were increased by 7.5-8.1%and 7.8-9.3%than corresponding monocultures,respectively.While the P and K contents of triticale increased by 21.7-26.3%and 3.0-4.9%,respectively.Under different growth stages,the differences of P and K contents and accumulations between intercropping and monoculture of alfalfa,and the differences of N,P and K contents and accumulations of 4 gramineous forages increased with the development of growth stage.At the same time,the closer the root system interaction,the higher the N,P and K contents in the stem and leave and root of the 4 gramineous forages,the lower those of alfalfa.In intercropping conbitions,alfalfa/sorghum intercropping had the highest competition rate for N and P,while alfalfa/triticale intercropping had the highest competition rate for K.2.Nitrogen metabolism and nitrogen efficiency mechanism of alfalfa/gramineous intercroppingsAlfalfa/gramineous intercroppings can improve the key enzyme activity of N metabolism of 4 gramineous forages.Among them,NR and GOGAT activities of sorghum,oat and triticale in intercropping were significantly higher than those in corresponding monocultures,GS activities of sorghum and oat in intercroppings were significantly higher than those of corresponding monocultures in the 3rd year of planting.Compared with monoculture,intercropping can significantly reduce the key enzyme activity of N metabolism in alfalfa.With the increase of root interaction,the activities of N metabolizing enzymes in gramineous forages were increased,while those in alfalfa were decreased.At the same time,the expressions of NR gene in root and GS gene in stem and leave of alfalfa,GOGAT gene in stem and leave and NR gene in root and stem and leave of 4 gramineous forages,NiR gene in stem and leave of oat and triticale,and GS gene in root of oat and triticale were similar to the change rules of N concentration.The root weights of 4 gramineous forages increased by 7.1-25.7%in intercropping compared with those in monoculture.Alfalfa changed greatly in intercropping with maize and sorghum,but little in intercropping with oat and triticale.The closer the root interaction between alfalfa and gramineae is,the better the growth and development of gramineae,the longer the root system length and the higher the root activity of alfalfa and gramineae,and the better the competition and absorption of nutrients.In the later period of planting,the root activity of alfalfa,maize,sorghum,oat and triticale of no barrier compared with those of plastic separation were increased by 6.7-13.8%,13.6-14.3%,8.7-12.5%,39.6-45.4%and17.3-19.0%,respectively.In this study,the total nodule number?TNN?,effective nodule number?ENN?,nodule weight per plant?PNW?,nitrogenase activity?NA?and nitrogen fixation potential per plant?PNF?of alfalfa in different intercropping combinations were significantly higher than those in monocultures,but there was no significant difference in nodule weight per plant?SNW?.The nodule number,nodule weight and nitrogen fixation ability of alfalfa were increased with the increasing of root interaction density,and the nodule number and nitrogen fixation ability with no barrier were significantly greater than those with plastic separation and single planting.The contents of 4 isoflavones in root under stress N level were higher than those under normal N level.The contents of anthocyanin and genistein were increased with the increased root interaction,and the contents of anthocyanin and genistein were significantly higher than those of plastic separation and single planting.IFS-1 and IFS-4 were up-regulated genes,IFS-2 and IFS-3 were down-regulated genes.The relative expressions of IFS-1 and IFS-4 with no barrier were significantly higher than those with nylon separation,and those of nylon separation were significantly higher than those of plastic separation and single planting.The expressions of IFS-2 and IFS-3 were opposite.NOD-1 and NOD-2 were up-regulated genes.The relative expressions of NOD-1 and NOD-2 were increased with the interaction of roots.At the same time,the relative expressions of NOD-1 and NOD-2 with mo barrier were significantly higher than those with plastic segregation and single planting.In the root system,IFS and NOD genes were significantly correlated with nodule and N fixation parameters except SNW.In the root system,daidzein,luteolin,anthocyanin and genistein were all positively correlated with the nodulation and N fixation parameters.3.Soil microecological effects of alfalfa/gramineae intercroppingsThe pH values of rhizosphere soil of alfalfa and gramineae forages in intercropping were lower than those in monoculture.The contents of organic matter in alfalfa and gramineae forages in intercropping were higher than those in monoculture,and the differences between monoculture and intercropping were significantly.In the rhizosphere soil of alfalfa,the differences of available N and P between intercropping and monoculture was small,but the content of available K in intercropping was significantly lower than that in monoculture.The contents of available N,P and K in rhizosphere soil of 4 gramineous forages in intercropping were significantly higher than those in the corresponding monoculture.Compared with monoculture,intercropping could improve the soil enzyme activities of alfalfa and gramineae forages.The activities of urease and sucrase in rhizosphere soil of alfalfa in intercropping with oat and sorghum,and the activities of alkaline phosphatase and catalase in rhizosphere soil of alfalfa in 4 intercropping systems were significantly higher than those in the corresponding monoculture.At the same time,the enzyme activities of the 4rhizosphere soils in intercropping were significantly higher than those in the corresponding monoculture.The numbers of bacteria and actinomycetes in rhizosphere soil of alfalfa and 4gramineous forages in intercropping were higher than those in monoculture,but the number of fungi was opposite.The sequence numbers of rhizosphere soil bacteria of alfalfa intercropped with oat and triticale,4 gramineae forages in intercropping were significantly higher than those in monoculture.At the same time,taxon,ACE index,Chao index and Shannon index in intercropping were higher than those in monoculture in rhizosphere soil of alfalfa and 4gramineous forages.At the level of phylum,sum of the relatively abundants of Proteobacteria,Bacteroidetes,Chloroflexi,Actinobacteria,Acidobacteria,Planctomycetes,Verrucomicrobia and Gemmatimonadetes is more than 84%.Proteobacteria,Bacteroidetes and Actinobacteria in alfalfa and gramineae forages rhizosphere soil in intercropping were larger than those in monoculture,and Chloroflexi showed that intercropping was smaller than monoculture.In intercropping,the soil enzyme and microbial community structure in forages rhizosphere soil affected and regulated each other with soil nutrient,and the abundance of Proteobacteria,Bacteroidetes and Actinobacteria were positively correlated with the content of organic matter and available N.In conclusion,intercropping can significantly improve the production performance and forage quality.This is because the above ground interaction in the intercropping promoted the photosynthetic performance and group light utilization rate of gramineae forages.At the same time,the underground interaction promoted the absorption and accumulation of gramineae nutrition.The high efficiency of N utilization between alfalfa and gramineae forage is mainly determined by N fixation,absorption and transformation.Intercropping can stimulate the increase of nodule number and N fixation ability of alfalfa.The mechanism is that intercropping can change the N concentration of alfalfa and then change the expression of isoflavone synthetase genes and isoflavone contents,so as to change the genes expression of N fixation signal pathway and the characteristics of nodule and N fixation.Intercropping stimulated the increase of total root length and root activity of alfalfa and gramineae forages to compete and absorb more N.At the same time,intercropping can promote the activity of N metabolizing enzyme,and then increase the transformation of N.The intercropping of alfalfa and gramineous forages can improve the rhizosphere soil nutrients and microbial diversity in Northwest China. |
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