Effect Of Different Nitrogen Forms On The Alfalfa-Soil Feedback

The input of exogenous nitrogen affects the feedback of 'plant-soil' system,and then affects the structure,function and process of ecosystem.Plant-soil feedback is composed of multiple levels,such as root surface,rhizosphere,individual,community or larger space and time scale.The study on the influence of nitrogen input of different forms on plant-soil mutual feedback is helpful to consolidate the theoretical basis of above-underground mutual feedback in the ecosystem and explore the methods and approaches of rational use of nitrogen fertilizer.Alfalfa is a kind of high-quality forage and an important forage resource for the development of animal husbandry which has a wide planting area in China.In alfalfa as the experimental material,this study set three experiments,included the quartz sand pot(soilless),soil pot,and field plot experiment.From the perspective of 'plants-soil'mutual feedback research different form of nitrogen ratio on alfalfa nitrogen uptake,soil nitrogen transformation,the influence of soil enzyme activity and microbial nitrogen cycle function,analysis of the law of feedback between plants and soil.The relationship between alfalfa and the elements involved in the nitrogen cycle in the soil under the treatment of different nitrogen proportions was expounded,which laid a theoretical foundation for the above-ground and underground feedback interaction in the ecosystem and provided a scientific basis for the rational nitrogen management in the ecosystem The main results are as follows(1)The results of quartz sand pot experiment showed that the application of nitrate nitrogen and ammonium nitrogen significantly increased alfalfa plant height and aboveground biomass.Compared with ammonium nitrogen,nitrate nitrogen was more beneficial to increase alfalfa plant height and nitrate nitrogen content in the above mention part.However,the ammonium nitrogen was beneficial to the growth of alfalfa roots,increasing the underground biomass and the content of ammonium nitrogen in the underground roots.The mixture of nitrate nitrogen and ammonium nitrogen in equal proportion can improve the aboveground biomass and plant height of alfalfa,promote its absorption and utilization of different forms of nitrogen in alfalfa,has the best effect on the growth of alfalfa(2)The results of soil pot experiment showed that nitrate nitrogen was more beneficial to promote tillering and increase above-ground biomass of alfalfa,while ammonium nitrogen was more beneficial to the elongation of alfalfa root system and increase the underground biomass.The activities of soil amylase,nitrate reductase,nitrite reductase and hydroxylamine reductase were significantly increased when the two forms of nitrogen were mixed.The abundances of functional genes of ammonia-oxidizing microorganisms,nitrogen-fixing microorganisms and denitrifying microorganisms in soil were significantly increased in the same proportion of nitrate nitrogen and ammonium nitrogen,but no significant effect was found by applying nitrate alone.The content of nitrate,ammonium and total nitrogen in soil had positive feedback relationship with the growth of alfalfa.(3)The results of field experiment showed that the aboveground biomass of alfalfa increased with different nitrogen treatments,which was consistent with the results of pot experiment.The nitrogen absorption efficiency and nitrogen utilization efficiency of alfalfa were improved by applying nitrate nitrogen and ammonium nitrogen in equal proportion.With the progress of alfalfa growth and the increase of cutting times,the activities of neutral protease,urease and nitrite reductase in soil decreased gradually,while the activities of hydroxylamine reductase in soil increased gradually.Single ammonium nitrogen treatment had the best effect on improving nitrate reductase activity.The application of nitrate nitrogen and ammonium nitrogen in equal proportion could improve the activity of soil neutral protease,urease and hydroxylamine reductase.The application of ammonium nitrogen is beneficial to increase the abundance of ammonia-oxidizing microorganisms,while the single application of nitrate nitrogen has no significant effect.Application of nitrate nitrogen and ammonium nitrogen in equal proportions can improve the abundance of ammonia-oxidizing microorganisms and denitrifying microorganisms in soil in different degrees(4)The different exogenous forms of nitrogen additions not only improved alfalfa yield and nitrogen accumulation,but also changed the abundance of soil microbial functional groups and soil enzyme activity,and promoted the transformation process of soil nutrients.The abundance of soil microbial functional groups and the activity of soil enzymes showed the same positive growth trend as those of alfalfa in the early growth period.With the increase of cutting times,the abundance of soil microbial functional groups and the soil enzyme activities involved in the nitrogen fixation,nitrification and denitrification processes in the soil fluctuated,which promoted the conversion of different forms of nitrogen in the soil and the absorption and utilization of nitrogen in alfalfa.The positive feedback relationship between alfalfa and soil maintains the stability of individual plant response to different forms of nitrogen.