Effects Of Soil Nitrogen And Phosphorus Levels And Mowing On The Rhizosphere Processes, Nitrogen And Phosphorus Nutrition Of Alfalfa

Nitrogen（N）and phosphorus（P）are essential elements for plant growth and development.The increase in atmospheric N deposition has greatly enhanced nitrogen availability,thus affecting soil nutrient status and vegetation productivity in terrestrial ecosystems.P availability in soil is low,and is one of the limiting factors for plant growth.Mowing can lead to compensatory growth of plants,stimulate the secretion of carbohydrates from plant roots in the community.In order to study the effects of mowing and nutrients on plant rhizosphere processes and N and P nutrition,a pot experiment was carried out by adding different levels of N fertilizer(0,25,50 mg N kg^-1)and P fertilizer(0,6.25,25 mg P kg^-1)to the soil to grow alfalfa.Shoot and root dry mass,N and P contentrations in shoot,the composition and amounts of root exudates,rhizosphere and bulk soil p H,alkaline phosphatase activity and Olsen-P concentration were measured by different mowing methods at different growth periods of alfalfa.The results showed that:（1）The addition of exogenous N and P increased the shoot and root dry mass of alfalfa.Mowing significantly increased the yield of alfalfa,but had an inhibitory effect on root growth.Shoot P concentration increased with the increase of soil P level,while the shoot N concentration decreased with the increase of soil P level.（2）The N-utilization efficiency of alfalfa shoots increased with the increase of soil P level,while the P-utilization efficiency of alfalfa shoots decreased with the increase of soil P level.（3）The amount of malonate,citrrate and soluble sugar exuded by roots decreased with the addition of exogenous N and P,and the amount of tartrate decreased by the addition of P,while the addition of N promoted the exudation of tartrate;48 h after mowing the shoots,the amount of root exudates decreased.（4）P addition increased rhizosphere and bulk soil p H,Olsen-P concentration,but decreased the activity of alkaline phosphatase and N:P ratio in the rhizosphere soil.N addition increased the activity of alkaline phosphatase in the bulk soil,and p H in both thed rhizosphere and bulk soil,but decreased Olsen-P concentration in the rhizosphere soil.48 h after mowing the shoots,the p H of the rhizosphere soil showed a downward trend,but Olsen-P concentration in the rhizosphere soil showed an upward trend.The activity of alkaline phosphatase in the rhizosphere soil was greater than that in the bulk soil,and the p H and Olsen-P concentration in the rhizosphere soil were lower than those in the bulk soil.N and P showed promoting effects on the growth of alfalfa.The effects of N and P addition and mowing on plant growth,N-and P-utilization efficiency,alkaline phosphatase activity,Olsen-P concentration,p H in rhizosphere and bulk soil and rhizosheath carboxylates,soluble sugars depended on the level of N and P addition and the method of mowing.