Effects Of Simulated Desertification On Dominant Plant Species Growth And Nutrient Uptake In Horqin Grassland

Grassland accounts for nearly 40%land area of China,and 78%of which is distributed in the temperate semi-arid areas of the northern China.Climate change such as drought and sandstorms,as well as human grazing and cultivation,have caused desertification in large grassland areas.Grassland desertification has become one of the serious environmental problems for China.Ecosystem nutrient loss caused by desertification can affect the nutrient uptake and growth of plants.Due to the fact of over-exploitation and utilization of grassland resources and progressive desertification in Horqin grassland,we established a field experiment mimicking desertification by adding different mass proportion of sand into soils and assembled plant community on the experimental platform.Two dominant plant species of C.duriuscula and P.tanacetifolia were chosen as model plants to investigate different desertification intensity on plant growth and nutrient uptake.Nitrogen stable isotope(¹⁵N)was utilized to study plant nitrogen uptake preference of two inorganic N forms ammonium nitrogen(¹⁵NH₄⁺)and nitrate nitrogen(¹⁵NO₃^-)and to determine the ¹⁵N recovery of the two N forms in both plants and soil.We also analyzed the concentrations of C,N,S,P,Ca,Mg,K,Na,Fe,Mn,Cu,Zn and their stoichiometric traits in the model plants as affected by desertification,and the consequential responses of plant growth characteristics,e.g.plant biomass,plant abundance and height.The relationships between plant nutrient traits and plant growth parameters were also discussed.The main results from this study are as follows:?1?The N uptake intensity(as represented by ¹⁵N enrichment factor)significantly increased and the recovery rate of ¹⁵N significantly decreased with the increasing level of sand addition for both ammonium(¹⁵NH₄⁺)and nitrate nitrogen(¹⁵NO₃^-)in C.duriuscula;however,the P.tanacetifolia showed no significant response.The ¹⁵N enrichment factor was significantly higher for C.duriuscula than that of P.tanacetifolia,indicating higher N uptake capacity of C.duriuscula.Both plants prefer to absorb NO₃^-.The ¹⁵N recovery rate of soil was much higher than that of C.duriuscula and P.tanacetifolia,indicating that the soil is the main sink for exogenous N source in the Horqin grassland ecosystem.The N recovery rate of soil decreased with the increasing desertification level,indicating decline of soil N retention capacity and increasing risk of N lost.?2?The concentrations and stoichiometric ratios of C and N showed no response to desertification for both plant species.With the increase of sand addition,the concentrations of S,P of C.duriuscula significantly decreased resulting in significant increase of C:P,N:P and S:P ratios.However,elemental concentrations of P.tanacetifolia showed no response to desertification,excepting P which significantly increased and contributed to the decline of C:P,N:P and S:P ratios.The N:P ratios of both plant species are both lower than fourteen,indicating that C.duriuscula and P.tanacetifolia are both limited by N availability.?3?With the increase of desertification intensity,Ca concentration of C.duriuscula significantly increased but Mg,K,Na,Cu decreased leading to significant increases in Ca:Mg,Ca:K,Fe:Mn,Fe:Cu and Mn:Cu ratios.For P.tanacetifolia,the concentrations of K and Zn and ratio of Ca:Mg significantly decreased with increasing intensity of sand addition.Therefore,the elemental concentration and stoichiometric ratios of P.tanacetifolia is more stable than C.duriuscula suggesting the stronger resistance and the better adaption of P.tanacetifolia to grassland desertification for maintaining plant growth and productivity.?4?Desertification reduces the biomass,abundance the coverage of C.duriuscula,but it showed no impact on P.tanacetifolia.Desertification altered biomass and abundance of C.duriuscula through influencing N absorption.Furthermore,significant correlations were found between plant growth characteristics and stoichiometric ratios of metal nutrients,e.g.correlations of Ca:Mg and Ca:K with plant biomass,biomass per plant individual,abundance,coverage and height.The results indicate that plant N uptake preference and absorption traits and stoichiometric ratios of metal nutrient are the main factors for plant adaptive growth under grassland desertification scenarios.This study took Horqin grassland as the research object which is a typical desertification area in northern China.We investigated the relationships of plant N uptake preference as well as concentration and stoichiometry of nutrients with plant growth traits and the underpinning mechanisms for the interactive relationships.The results would be helpful for enriching the database of this typical wind-erosion and desertified area,and it is expected to provide scientific basis and theoretical support for clarifying causes and processes of desertification and the related strategies for solving the problem.