Nitrogen Acquisition Strategies Of Typical Wetland Species Revealed By Stable Isotope Techniques

Global nitrogen deposition is becoming more and more serious,especially the global change trend of nitrogen deposition components shows huge regional differences.Shengjin Lake is a natural wetland,characteried by seasonally fluctuating water level,where wetland plant species may obtain different forms of nitrogen in response to the shift in nitrogen deposition composition and and thereby reduce competition for the same nitrogen source between coexisting species.However,it was unclear the way by which the change of the density of neighboring plants may affect the nitrogen acquisiiton.In spite of the fact the symbiosis of arbuscular mycorrhizal fungi（AMF）is beneficial to the utilization of nitrogen by plants,,the effect of AMF symbiosis on nitrogen utilization strategies of wetland plants under various nitrogen deposition composition and soil water conditions is also unclear.To this end,we investigated the effects of competition regime on the uptake of different N forms by two hygrophytes,Carex thunbergii and Polygonum cripolitanum,by conducting a hydroponic test of excised roots and an in situ experiment in a subtropical wetland ecosystem.The two species were grown either in monocultures or mixtures with various neighbour densities.Root functional traits and N uptake rates of different N forms were measured..At same time,we transplanted C.thunbergii to microcosms in a greenhouse experiment by manipulating both NH₄⁺:NO₃^-ratios（ammonium nitrogen:nitrate nitrogen 1:3,2:2 and 3:1）and amount of water supply（30%,50%and 70%）in field soil.The benomyl was applied to suppress AMF symbiosis in some pots and not in others.The main findings are as follows:（1）The absorption of nitrogen by C.thunbergii and P.cripolitanum is mainly affected by nitrogen form,and both can utilize organic nitrogen sources.Both plants prefer to absorb nitrate nitrogen regardless of the competition conditions.（2）In response to interspecific competitio,the total nitrogen absorption rate of each focal species has a tendency to decrease,but to different degree.When the competitor dominates the community,the total nitrogen uptake rate of the focal species decreases.Specifically,for the absorption rate of total inorganic nitrogen（nitrate nitrogen and ammonium nitrogen）,the ¹⁵N absorption rate of P.cripolitanum shows a downward trend as the density of competitors increases,whereas while the ¹⁵N absorption rate of C.thunbergii is generally stable expcet when competitor dominates（C.thunbergii:P.cripolitanum 1:3）.Similarly,the presence of C.thunbergii reduced the ¹⁵N recovery rate of P.cripolitanum.（3）Under competitive conditions,Carex thunbergii’s nitrogen acquisition strategy is relatively stable.As the density of competitor increases,P.cripolitanum tends to use ammonium and glycine,showing a more flexible nitrogen acquisition strategy.（4）AMF symbiosis promotes the growth of C.thunbergii.When the soil moisture is low（30%）,C.thunbergii will be under drought stress.Symbiosis with AMF will increase C.thunbergii’s tolerance to drought stress as shown by;when soil moisture is high（70%）,AMF symbiosis increases nitrogen uptake rate via promoting the belowground growth of C.thunbergii,mainly fine roots.（5）The increase of ammonium inhibit the growth of C.thunbergii,and symbiosis with AMF can reduce the inhibitory effect.In the presence of AMF,the specific leaf area is highly correlated with the nitrogen uptake rate of the host.（6）AMF status did not impact plant growth but did shift the biomass allocation and below-and aboveground functional traits which mediated nitrogen acquisition,depended on the availability of water and N composition.The results of this study elucide the competition and co-existence mechanism between C.thunbergii and P.cripolitanum,as well as the influence of AMF symbiosis on nitrogen acquisition of C.thunbergii,which provides insights into the response of typical wetland species to future nitrogen deposition changes in Shengjin Lake,and these results have significant implication for ecological restoration and conservation of wetland species diversity.