Response Of Typical Grassland Community And Its Dominant Species To Nitrogen And Phosphorus Addition At Semiarid Loess Hilly-gully Region

The problems of sluggish restoration succession,simplistic community structure and poor ecological function coexist in the farming-withdrawn grassland on semiarid loess hilly-gully region.Grassland production and ecosystem services are widely co-limited by soil N and P availability in this region because of high nutrient loss through soil erosion and intensive cultivation.Rational N and P combination can not only efficiently improve grassland productivity,but also optimize community structure and promote grassland restoration.Three grassland communities dominated by annual forbs(AF),perennial grass(PG)and perennial forb(PF)were selected.A split-plot N and P addition experiment(main-plot:0,25,50,and100 kg N ha^-1 yr^-1;subplot:0,20,40 and 80 kg P₂O₅ ha^-1 yr^-1)was conducted on three grassland communities during 2017-2019.We systematically examined the effects of N and P addition on leaf functional traits and photosynthetic physiology,stoichiometry characteristic,nutrient use and resorption efficiency of different dominant species and functional group composition and its relationship with community productivity,species diversity and community stability.The main results are as follows:(1)N addition alone significantly increased area-based leaf N content and N:P ratio of Stipa bungeana only,while light-saturated net photosynthetic rate(PNmax)was increased significantly in S.bungeana,Bothriochloa ischcemum and Lespedeza davurica.Area-based leaf N content,P_Nmax and photosynthetic nitrogen use efficiency(PNUE)increased significantly only in L.davurica.N and P combined addition caused significantly lower area-based leaf N content but higher stomatal conductance,PNUE and P_Nmax in B.ischcemum and S.bungeana compared to N addition alone.The stomatal conductance,area-based leaf N and P content,P_Nmax and PNUE of L.davurica were increased significantly under N and P combined addition.The LNC_a/LPC_a for optimum P_Nmax and PNUE was changed with species.The P_Nmax and PNUE of B.ischaemum and S.bungeana had a peak when LNC_a/LPC_a attained?11 and?20.L.davurica tended to retain greater P_Nmax and PNUE at lower LNCa/LPCa.B.ischaemum achieved higher photosynthetic capacity through the improvement of nutrient use efficiency rather than leaf nutrient content,while the increased P_Nmax in S.bungeana might be due to improved stomatal conductance and increased leaf N content and nutrient use efficiency.L.davurica tended to retain greater photosynthetic capacity and nutrient use efficiency at higher leaf P content and stomatal conductance.(2)B.ischaemum and S.bungeana possess relatively higher N and P resorption and use efficiency,but relatively lower leaf N and P under unfertilized treatment.Leaf N content,N:P,P resorption efficiency and aboveground biomass(AGB)of B.ischaemum and S.bungeana increased significantly under N addition alone.P addition alone significantly increased leaf P,but had no effect on AGB.Under 50 and 100 kg N ha^-1 yr^-1 combined with P,N:P of B.ischaemum and S.bungeana decreased significantly compared to N addition alone,but AGB exhibited further increases.L.daurica possessed had relatively higher leaf N and N:P,but relatively lower leaf P,N and P resorption and use efficiency under unfertilized treatment.Leaf N and P,N and P resorption efficiency and aboveground biomass of L.daurica increased significantly after P alone and 25 kg N ha^-1 yr^-1 combined with P addition.Artemisia sacrorum and Artemisia scoparia had relatively higher leaf P and P resorption efficiency,but lower N:P and N and P use efficiency.N or P addition alone had no significant effect on leaf N,N and P resorption efficiency and AGB of A.sacrorum and A.scoparia.Their AGB increased significantly only under 50 and 100 kg N ha^-1 yr^-1 combined with P.The species-specific nutrient limitation model is the key mechanism to predict species composition and community structure changes in response to N and P addition.(3)Stoichiometric homeostasis for leaf N and N:P was consistently stronger than leaf P under N and P addition across all species.Stoichiometric homeostasis differed largely among species under N and P addition(N homeostasis:2.70-9.10,P homeostasis:0.25-1.28,N:P homeostasis:1.33-7.70).L.daurica,B.ischaemum and A.sacrorum perform stronger stoichiometric homeostasis than other species.Leaf N:P showed significantly positive relationship with stoichiometric homeostasis across species.L.daurica and B.ischaemum with higher stoichiometric homeostasis had higher dominance under unfertilized condition and 25kg N ha^-1 yr^-1 combined with P addition,which contributed to maintain high community homeostasis.Whereas the dominance of Heteropappus altaicus,Artemisia capillaris and Artemisia scoparia with lower stoichiometric homeostasis increased notably under N50 and N100 combined with P addition,which caused significant decline in community homeostasis.(4)AGB of the three grassland communities showed interannual variation in response to N and P addition.AGB of three communities increased under N and P combined addition due to deficient growing season precipitation in 2017.In 2018 and 2019,N or P addition alone caused significant increases in AGB of three communities,and AGB showed further increases under N and P combined addition.The increased AGB of AF community under N alone or N and P combined addition was mainly caused by the increase of tall annual and biennial.Under N addition alone,the increased AGB of PG and PF communities was mainly due to significant increase of tall perennial grass and forb.Under P alone and 25 kg N ha^-1 yr^-1 combined with P addition,the increased AGB were mainly benefit from perennial legumes.Under 50 and 100kg N ha^-1 yr^-1 combined with P addition,the increases in AGB of PG and PF communities were mainly driven by pronounced increases in tall clonal or annual forbs.Significant diversity decline occurred only in AF and PF communities,which was the consequence of strong light availability reduction induced by increased tall annual and biennial(light availability of soil surface reduced to?5%).Tall perennial species and tall annual and biennial are key functional groups that drive productivity and diversity changes in the three communities.(5)Tall perennial forbs or grasses had higher temporal stability than short perennial legumes or tall annuals and biennials under N and P addition.Decreased stability of PG and PF communities under N addition alone was primarily due to reduced species asynchrony,while it attributed to increased dominance of short perennial legumes after P addition alone.50 and 100 kg N ha^-1 yr^-1 combined with P addition notably increased dominance of annuals and biennials but decreased stability,which caused declines in stability of the three communities.Nutrient-induced shifts of functional groups,particularly increased dominance of less-stable functional groups,is the main driver reducing perennial grassland stability independent of diversity.Whereas N and P addition decreased annual grassland stability via additive effect of increased dominance of annuals and biennials and diversity decline.The trade-off analysis found that 25 kg N ha^-1 yr^-1 and 20 kg P₂O₅ ha^-1 yr^-1 was the optimum N and P combination to promote annual grassland restoration.50 kg N ha^-1 yr^-1 and 20 kg P₂O₅ ha^-1yr^-1 was the optimum combinations for perennial grassland restoration.The interspecific differences of physiological adaptation,stoichiometric characteristics and nutrient use under N and P addition on loess hilly region are the physiological manifestation of asynchrony of different functional groups in response to environmental changes,which determined the niche separation of different functional groups.The trade-off between high stoichiometric homeostasis species and low stoichiometric homeostasis species under N and P addition is the key mechanism to predict species composition and stability.Rational and targeted N and P combination was essential to maintain high productivity,high diversity and high stability for different grasslands by controlling the response of key functional groups when applying fertilization to promote grassland restoration.