Effect Of Nitrogen And Phosphorus Addition On Productivity Of Alfalfa-grass Mixed Grassland

In In recent years,there is a large forage deficit in China,which has become a constraint to the development of China’s agriculture and livestock industry.Therefore,finding a planting system with small environmental cost and high and stable yield is an urgent problem in China.The forage production can be ensured in poor environment and the sustainability of artificial grassland can be effectively improved.Fertilizer application will improve the yield and quality of mixed grasses,but it is not clear whether fertilizer application in this environmental context will improve the productivity of mixed grasses and affect the quality and phenotypic plasticity of forage in mixed grasses.In this thesis,we used Medicago sativa and Leymus chinensis as research objects to investigate the effects of nitrogen and phosphorus addition on productivity,quality and phenotypic plasticity of alfalfa and sheepgrass mixed planting,to clarify the changes of productivity and quality of forage grasses in this system,the sensitivity of phenotypic plasticity,and to elucidate the evolution of forage grass communities in this system.The main progress and conclusions of this study are as follows1.Nitrogen and phosphorus additions significantly increased the aboveground biomass of alfalfa and grass in the mixed grasses.In the field trial the annual yield of bean-grass mixed grassland in 2021 and 2022 was highly significant(P<0.01)higher under the fertilizer treatment than under the CK treatment,and both had the highest annual yield under the NP treatment.7.68 t/hm~2 and 8.67 t/hm~2,respectively,which were 41.17%and 67.68%higher than the CK treatment.In the pot experiment,the total biomass of beangrass under the NP treatment was significantly higher than that of the CK and P treatments when the harvest time was 40 d and 60 d.The total biomass under the NP treatment reached a maximum of 21.57 g/pot at 80 d,which was significantly higher than that of the other three treatments(P<0.05).The total biomass increased by 74.60%,29.40%and 49.80%compared to CK,N and P treatments,respectively.Nitrogen and phosphorus addition significantly increased the crude protein content of alfalfa and sheepgrass in the mixed grass,and the crude protein content of alfalfa was about 25.00%higher than that of sheepgrass between treatments.2.Nitrogen and phosphorus addition improved alfalfa and sheepgrass forage production traits.In the field trial,fertilization treatments significantly increased alfalfa and sheepgrass plant height,branching(singeing),and stem thickness(P<0.05),all reaching a maximum under NP treatment in September 22.Nitrogen and phosphorus additions significantly increased plant height,branching(branching),and stem thickness(P<0.05)in alfalfa and sheepgrass in the pot experiment,all reaching a maximum under NP at 80 d.The correlation analysis revealed that the biomass of alfalfa and sheepgrass in mixed grasses was significantly and positively correlated with their growth indices.Moreover,the plant height,branching(branching),stem thickness,root-to-crown ratio and biomass of both were plastic phenotypic traits that were easily disturbed by NP addition.3.Fertilizer application and mowing caused changes in the relative proportions of each plant within the community in terms of community productivity.In the field experiment,the aboveground biomass share of sheepgrass and alfalfa was 56.00%higher than that of alfalfa under the N treatment and 56.00%and 54.00%higher than that of sheepgrass under the P and NP treatments,respectively,in June 2021.As the number of mowings increased in September 2022,the percentage of aboveground biomass of alfalfa exceeded 90.00%under both the CK and P treatments.Mowing changes the relative proportions of alfalfa and sheepgrass in community productivity.In the pot experiment,the percentage of biomass of sheepgrass was higher than that of alfalfa in all treatments at 60 d of harvest,reaching a maximum of 63.00%in the CK treatment,while the percentage of aboveground biomass of alfalfa showed a slow increase in the N,P and NP treatments.Under different fertilization treatments at harvest time of 80 d,sheepgrass reached a maximum of 60.00%under the CK treatment,while the percentage of biomass of alfalfa increased with the addition of N and P.Fertilizer application changed the relative proportion of productivity in alfalfa and sheepgrass communities.In conclusion:alfalfa-goatgrass mixed grasses with the highest yield and crude protein content could be harvested from artificial mixed grasses under the conditions of nitrogen and phosphorus mixture.The effect of fertilization treatments on forage productivity was a combined result of the causes through changing its growth indicators.Fertilization and mowing caused changes in the relative proportions of each plant in the community in terms of community productivity,and by the trait sensitivity to mowing with sheepgrass made alfalfa the dominant species in this mixed grassland and the dominant factor in the yield of the mixed grassland.