| Nutrient and leaf traits of plants can reflect their nutrient conservation and utilizing strategies,and thus play important roles in determining the competitive advantages of plants and forcing structuring plant communities.Exploring the response of plant nutrient and leaf characteristics to N deposition and precipitation variation will help to better understand the plant nutrient utilization strategies and the process of community structure regulation in the context of global climate changes.In the present study,the nutrient and leaf traits,as well as the important values of the dominant species along a multi-level N addition gradients(0,1.15,2.3,4.6,9.2,13.8 g N m-2 yr-1 6 levels)were examined in a typical grassland on the Loess Plateau;Nutrient and leaf traits along a multi-level N(ditto)and water addition(2 levels:long-term mean rainfall,1.3 times of long-term mean rainfall)gradients were also examined in a planting grass of edificator(Stipa bungeana),aiming to better understand the influence and mechanism of plant's nutrient and leaf traits to the plant community structure.The main conclusions are as follows:1)N:P(N:Pgr)ratio of the green tissues ranged between 13.91 to 15.94 with high homogeneity among the five species,which indicated that the plant growth were all limited by the N and P.The C:N and C:P ratio in green tissues were higher in graminaceae plants than that in non-graminaceae plants;the N and P resorption efficiency(NRE,78.52%;PRE,79.06%)were all higher in Stipa bungeana(gramineous)than that in the other species.The important values,as well as the stoichiometric characters for all of the five species were significantly and positively correlated with the nutrient resorption efficiencies(P<0.05),but showed no significant correlations with specific leaf area(SLA).These results demonstrated that nutrient traits played more important roles in determining the ecosystem structure than leaf traits.2)3 year's continued N addition changed the nutrient and leaf traits.N addition increased the N contents(Ngr and Nsen respectively)in the green and senescent tissues of all the species(except Lespedeza davurica).The Ngr of Stipa bungeana,Leymus chinensis,Cleistogenes squarrosa,Heteropappus altaicus and Lespedeza davurica were increased from 14.11,16.00,17.17,21,53 and 23.94 mg · g-1 in N0 treatment to 21.71,23.13,25.77,26.58 and 27.15 mg · g-1 in N13.8 treatment,respectively.The Nsen of Stipa bungeana,Leymus chinensis,Cleistogenes squarrosa and Heteropappus altaicus were increased from 3.05,9.28,11.18,and 11.36 mg · g-1 in N0 treatment to 7.17,13.84,14.32 and 15.83 mg · 9-1 in N13.8 treatment,respectively.N addition decreased the C:N ratios in senescent tissues of all species except Lespedeza davurica,but showed no effects on P contents and C:P ratios in the green and senescent tissues;However,N addition increased the N:P ratios in green tissues of all species,indicating that the limitation of P increased with N addition.The responses of N resorption efficiency(NRE)to N addition differed among the five plants.With the increases of N addition,the NRE of Stipa bungeana and Heteropappus altaicus decreased,the NRE of Lespedeza davurica increased,but the NRE of Leymus chinensis and Cleistogenes squarrosa did not change;The increased N had no effects on the P resorption efficiency of all species,but increased the SLA of all species and decreased the leaf life span of Stipa bungeana and Leymus chinensis.3)3 year's continued N addition shifted the plant community structure,increased the important values of Stipa bungeana and Leymus chinensis,decreased the important values of Cleistogenes squarrosa,Heteropappus altaicus and Lespedeza davurica;Along the N gradients,the important values of species exhibited significant and positive correlatioins with green and senescent tissues nutrient stoichiometry and resorption traits,but showed no correlatioins with SLA.The results demonstrated that N addition might regulate the plant community structure by altering plant nutrient traits rather than leaf traits,while the regulating effects of N and P contents in green and senescent tissues,as well as the C:N ratios to the plant community structure were strengthened with the increase of N deposition.4)The interaction of water and N addition altered the SLA,tiller number and biomass of the plant.The SLA of vegetative and reproductive tillers increased with the increase of N addition under the high water treatment.However,under the low water treatment,the significant higher SLA was only found in N9.2 treatment.Water and N addition treatment had significantly interactions to the vegetative tiller SLA.Under the high water treatment,the number and biomass of vegetative and reproductive tillers only increased significantly in N9.2 treatment,while below-ground biomass did not shift and root/shoot ratios decreased along the N gradients;Under the low water treatment,the number and biomass of reproductive tiller only increased significantly in N13.s treatment,but the number and biomass of vegetative tiller,below-ground biomass and root/shoot ratios did not shift.Under both of the high and low water treatment,the effects of N addition were greater on reproductive tiller than on vegetative tiller,thus increased the reproductive allocation,and the responses of plant to N addition were more sensitive in the reproductive stage than in the growing stage.The interactions of water and N addition significantly affected the biomass of vegetative tiller,reproductive tiller,above-ground and the whole plants.5)The interaction of water and N addition changed the nutrient traits.Under the high and low water treatments,N addition increased the C,N and P contents in above-ground green tissues,decreased its C:N and C:P ratios,and had no effect on its N:P ratios;N addition also increased C and N contents in above-ground senescent tissues and decreased its C:N ratios,however,it had no effects on its P contents,C:P and N:P ratios.Under the high water treatment,N addition had greater effects on N content and C:N ratio but less effect on P content and C:P ratios than those under the low water treatment.The interaction of water and N addition significantly affected all the nutrient traits except the green and senescent tissues P content and senescent tissues C:P and N:P ratios.6)The interaction of water and N addition affected the seed emergence and seedling growth.Under the high water treatment,N addition had no effects on the germination and accumulated germination rates.But under the low water treatment,N addition increased the germination and accumulated germination rates.Under both the high and low water treatments,N additions increased the seedling root length but had no effect on the seedling shoot length.Under N2.3 treatment,N addition increased the seedling root biomass and root/shoot ratio under the high water treatment,but had no effects on the shoot and whole seedling biomass;N addition had no effects on all the traits of seedling growth under the low water treatment.The interaction of water and N addition significantly affected the germination rates,shoot and whole plant biomass. |
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