Nitrogen And Phosphorus Utilization Strategy In Leymus Chinensis And Artemisia Frigida

Long-term overgrazing is the primary reason for grassland degradation succession.The patterns of degradation succession in typical steppe vegetation types of Inner Mongolia shows convergence,i.e.,typical steppes will change into A.frigida steppe if sustained grazing pressure is imposed.Artemisia frigida steppe stage,as a threshold stage of regressive succession of degraded communities,plays an important role in preventing grassland degradation.Nutrient are the main factor for the formation of plant productivity and affect the succession of vegetation degradation,but understanding about grassland degradation mechanism in the perspective of nutrients is still limited.Leymus chinensis is the dominant plant in the typical steppe,and A.frigida is the dominant plant in the degradation succession of the typical steppe.In 2017,the long-term enclosed plot and adjacent grazing plot in Xilinhaote were selected to study the response of nutrient utilization and characters in organs of L.chinensis and A.frigida to grazing.On the basis of above mentioned design,we have studied the effects of nitrogen and phosphorus on biomass and interspecific relationships in L.chinensis and A.frigida by conducting pot experiment under greenhouse.To study the nutrient strategies of L.chinensis and A.frigida,the stoichiometric ratio,nutrient accumulation,nutrient uptake efficiency and nutrient utilization efficiency were analyzed.The mechanism of adaptation of L.chinensis and A.frigida to nutrients was studied based on the morphological and physiological functional traits of the plants.Thus,we provide a new advancement in the understanding the mechanisms involved in grazing-resistance within the plant-soil system.The specific results of this study are as follows:(1)In our study,grazing decreased total nitrogen,total phosphorus,and available nitrogen,while it increased available phosphorus in the soil.Grazing significantly increased the concentrations of N and P in the leaves and stems of L.chinensis,and in the stems and roots of A.frigida,respectively.The C:N and C:P decreased as a result of grazing.Leymus chinensis promoted photosynthesis and nitrogen utilization by increasing chlorophyll content in the leaves,and Rubisco activity and GS activity in the roots,but less than the biomass consumed by livestock overfeeding.However,A.frigida increased the activities of NR and ACP in the roots to promote the uptake and utilization of nitrogen and phosphorus under overgrazing,promoted the growth of the stems and roots,increased the formation of adventitious roots,and expanded the population space.(2)The biomass and interspecific relationship of L.chinensis and A.frigida were significantly affected by nitrogen,phosphorus and their interaction.The organ and total biomass of L.chinensis and A.frigida increased exponentially with the increase of nitrogen or phosphorus supply.The limiting effect of low nitrogen on L.chinensis was higher than that of A.frigida,and the promoting effect of high nitrogen on L.chinensis was higher than that of A.frigida.The limiting effect of low phosphorus on L.chinensis was lower than that of A.frigida.The interaction of nitrogen and phosphorus can affect the competitive ability of L.chinensis and A.frigida.Artemisia frigida showed strong competitive ability in low N + medium P and medium N + high P treatments,while L.chinensis showed competitive advantage in other treatments.(3)Leymus chinensis and A.frigida had different nutrient utilization strategies.Nutrient concentrations,stoichiometric ratio,nitrogen and phosphorus accumulation in organs of L.chinensis and A.frigida were significantly correlated with nutrient supply.Comparatively,the total N accumulation of L.chinensis was higher than that of A.frigida,and the two species were mainly distributed in the roots.The total P accumulation in L.chinensis slightly less than that in A.frigida and was mainly distributed in the roots,whereas A.frigida was mainly distributed in the leaves.The N and P uptake efficiency in the roots of L.chinensis was higher than that of A.frigida.The N utilization efficiency in the leaves of L.chinensis was lower than that of A.frigida,but the P utilization efficiency of L.chinensis was higher than that of A.frigida.Thus,L.chinensis adopted the acquisitive resource-use strategy,but A.frigida showed the conservative resource-use strategy.(4)The higher root biomass,root length and root surface area of L.chinensis enabled it to acquire resources rapidly,and the higher plant height,specific leaf area,chlorophyll content,soluble sugar and starch content promoted its rapid growth.The response of A.frigida in the corresponding index was beneficial to its nutrient conservation and nutrient utilization efficiency.