Study On The Mechanisms Of Ccarbon Assimilation And C/N Ratio In Alfalfa Responding To Nitrogen And Water Availabilities

Alfalfa (Medicago sativa) is the most widely distributed forage crop in China. It is cultivated on two million hectares nationally, making it the preferred legume to promote forage supply and the development of the livestock industry. As a legume, alfalfa can assimilate nitrogen through the process of biological nitrogen fixation, however this is often subject to soil nitrogen supply. Accordingly, the growth of alfalfa is regulated by soil nitrogen availability, particularly so in arid and semi-arid conditions. The process of biological nitrogen fixation is sensitive to drought stress and drought affects the soil nitrogen state of existence, influencing soil nitrogen availability. Photosynthetic carbon assimilation and nitrogen uptake in plants is coordinated and demonstrated in the C/N ratio. Therefore, in this study, field trials and greenhouse experiments were carried out to determine the physiological mechanisms of biomass accumulation, photosynthetic carbon assimilation and C/N ratio in the Longdong cultivar of alfalfa under different water and nitrogen treatments,The main results were as follows:(1) Under field conditions, the addition of nitrogen significantly improved the biomass of first year of alfalfa at the branching and early flowering stages (P<0.01) compared to the low nitrogen treatment (15kg N/ha). The stimulating effect of the high concentration of nitrogen treatment (100kg N/ha) on biomass was greater than the effect of moderate concentrations of nitrogen (50kg N/ha). Only the moderate concentration of nitrogen resulted in increased biomass at the flowering stage and high concentrations of nitrogen did not increase biomass. In the second year of the planting, the moderate concentration of nitrogen significantly improved aboveground biomass at the branching and early flowering stages (P<0.05), whereas moderate and high nitrogen treatments inhibited the accumulation of biomass at the stage of full bloom.(2) In the first year of the trial, adding a moderate concentration of nitrogen significantly reduced the C/N at the branching stage (P<0.05) whereas the opposite was true at the early flowering and full bloom stages. In the second year of planting, the addition of nitrogen promoted the accumulation of leaf N and reduced the C/N at the branching stage of the first crop. Conversely, nitrogen reduced the C/N at the branching stage in the second year. The greenhouse experiments showed that after nitrogen was added at5mmol/L concentration, the C/N ratio was increased during the early stage of exposure. The increase was greater under70%FWC treatment compared to35%FWC.(3) Under field conditions, nitrogen addition improved the net photosynthetic rate (Pn) at the branching and early flowering stages compared to low nitrogen growth conditions in the first year of planting, whilst it was inhibited at the stage of full bloom. The addition of a moderate concentration of nitrogen significantly improved water use efficiency (WUE) at the branching stage (P<0.05), however this was reversed at the early flowering and full bloom stages. In the second year of planting, the nitrogen addition improved WUE at the branching and early flowering stages of the first crop and the opposite was true in the second crop. Under greenhouse conditions, the addition of a moderate concentration of nitrogen did not increase Pn, however it did improve the WUE dynamic in the afternoon after water stress treatment (35%FWC).(4) In greenhouse experiments, the addition of the moderate concentration of nitrogen not only significantly improved alfalfa chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll (Chl (a+b) and carotenoids (Cx) content (P<0.05) compared to low nitrogen and high nitrogen treatments, it also increased the chlorophyll content of each component under water stress.(5) In greenhouse experiments under water stress, the addition of the high concentration of nitrogen (10mmol/L) and with the drought stress prolonged, the moderated concentration of nitrogen, significantly increased RWC in the leaves. Conversely, in conditions of adequate moisture (70%FWC), nitrogen addition did not significantly improve the RWC in leaves. Under water stress, nitrogen addition also resulted in improved alfalfa root activity (RA) and leaf nitrate reductase (NR) activity. Particularly, moderate concentration of nitrogen resulted in significantly increased RA and leaf NR activity (P <0.05).Based on the above results it was concluded that in alfalfa, moderate concentrations of nitrogen could improve leaf chlorophyll content, NR activity and root activity, leaf photosynthetic carbon assimilation, WUE C/N, and ultimately, improve biomass by optimizing the balance of carbon assimilation and nitrogen utilization. The addition of50kg N/ha can improve resource utilization efficiency by alfalfa and yield greater biomass in the Loess Plateau.