Response Of Nutrient Resorption Of Leymus Chinensis To Nitrogen And Phosphorus Addition In Songnen Grassland

In recent years,with the advancement of science and technology and the rapid development of industry,human activities such as the extensive use of fertilizers and the burning of fossil fuels have led to a sharp increase in atmospheric nitrogen deposition.Nitrogen deposition is one of the important factors that currently threaten grassland function and services,and it also affects the movement and storage of other key elements of the ecosystem?such as P?.Nutrient resorption is an important mechanism for the conservation of nutrients by plants.At present,there are many researches about effect of nitrogen addition on nutrient resorption of plant,however,changes in nutrient resorption traits under multi-gradient nitrogen and phosphorus addition treatments have not been deeply characterized.We conducted a 3-year field experiment about Leymus chinensis of meadow steppe.In this experiment,six nitrogen addition gradients(0,2.5,5,10,20,and 40 g N m^-22 yr^-1),three phosphorus addition gradients(0?5?10 g P m^-22 yr^-1)and their interactions?18 treatments in total?were performed on plots of temperate meadow grassland to analyze the response characteristics of plant biomass,nitrogen and phosphorus content of soil?including soil available nutrient content?and plant,N:P ratio and nutrient resorption to multi-gradient nitrogen and phosphorus addition treatments.The primary findings are as follows:i.Nitrogen and phosphorus additions increased the aboveground and belowground biomass.However,with the increase of the rate of nitrogen and phosphorus addition,the aboveground biomass increased at first and then tended to maintain at high level,while the belowground biomass showed the trend of increasing at first and then decreasing.ii.For the control plots,the resorption efficiency of nitrogen?NRE?and phosphorus?PRE?in the leaves of L.chinensis was 57.66±1.56%and 67.57±1.55%,respectively.The nitrogen and phosphorus resorption efficiency of the stem was 55.89±2.01%,54.26±2.74%,respectively.The nitrogen resorption efficiency of the stem of L.chinensis was similar to that of the global average value,leaf nitrogen and phosphorus resorption efficiency and stem phosphorus resorption efficiency were lower than the global average values.iii.Nitrogen addition increased nitrogen content of mature L.chinensis plants and reduced NRE,indicating that nitrogen deposition increased soil nutrient content and plants absorbed more nitrogen from the soil,hence reduced the proportion of nutrient by the resorption pathway.With the increase of nitrogen addition rate,the nitrogen content of senesced plant increased,which caused subsequent increase in the nitrogen content of litter,eventually nitrogen returned to the soil increased by the litter.The decomposition rate of plant litter is mainly affected by the quality of litter?i.e.nitrogen content?,and litter with high nitrogen content is easier to be decomposed.Nitrogen addition decreased the phosphorus content of mature and senesced plant and increased the PRE,indicating that the plant had a higher priority in optimizing their structure than phosphorus uptake,and then stored more phosphorus by nutrient absorption.Phosphorus addition had no significant effects on nitrogen content in mature and senesced plant and NRE,but increased the content of mature phosphorus and decreased the phosphorus resorption.There were negative correlations between nitrogen and phosphorus content of L.chinensis and their respective nutrient resorption efficiency.iv.The response of nutrient resorption to nitrogen and phosphorus addition was similar between leaf and stem in L.chinensis.v.The negative correlations between nitrogen and phosphorus concentration of mature plants and their respective nutrient resorption provided evidence for the effects of plant nutrient status on plant nutrient resorption in Songnen grassland ecosystem.In general,plant under nutrient-enriched conditions absorbed more nutrients from the soil than those under nutrient-poor conditions,thus nutrient abundance in soil increased plant nutrient concentration and reduced nutrient resorption.Because under nutrient-enriched conditions,the energy consumed by plant to absorb nutrients from the soil is less than that of nutrient resorption.The results of this study improved our understanding of the processes and functions of grassland ecosystem and the underlying mechanisms of nutrient cycling response to global change factors.It also provided important scientific proofs for grassland ecosystem management.