Mechanisms Of Nitrogen And Phosphorus Utilization Among Invasive Composite Plants And Native Plants In Southwestern China

Exotic plant invasion has become one of the serious global environmental problems on the earth's surface,which aggravates the loss of biodiversity,affects the ecological security and causes huge economic losses.Therefore,the mechanism of exotic plant invasion is a common concern of geosciences,ecological environment and other disciplines,and is the key scientific basis for prevention and control of invasion and assessment of invasion impact.Plant nitrogen(N)and phosphorus(P)utilization,the influence of invasion on soil N and P cycle,and the feedback mechanism of plant community are important information for understanding the mechanism of exotic plant invasion and coexistence and disappearance of native plants.However,the methodology mainly depended on the control and culture experiments such as nitrogen addition and isotope labeling in previous studies,thus the mechanism of in-situ N and P utilization of invasive plants and its changes with soil N and P nutrients remaine unclear.Therefore,the ecosystem of invasive Compositae Plants in Southwest China is selected as the research object in this study.By analyzing the natural abundance of N,P,ions concentrations and stable N isotopes in plants and soil,the N and P utilization mechanism of invasive plants and native plants and its change mechanism with soil N,P nutrients are discussed.The main research results and their innovation,main understanding and significance are summarized as follows:(1)The concentration of N and P in the leaves of invasive plants was significantly higher than that in the leaves of coexisting native plants,and increased with the invasive pressure,which indicated that the ability of N and P absorption and enrichment of invasive plants was higher than that of native plants,and increased with the invasion pressure.When compared with the same native plants under invasion and no invasion,the N concentrations in the leaves of most native plants increased,while the P content decreased,which indicated that native plants under no invasion increased N uptake,but decreased P uptake.In addition,the isotope mass-balance method revealed that the preference of invasive plants and native plants to use dominant N source(NH₄⁺)in soil was the internal reason for the increase of N content in their leaves.(2)Leaf N concentration of invasive plant was significantly higher than that of native plant,but there was no significant difference between theie roots N,which indicated that invasive plants allocated more N to leaves than native plants.When compared with the same native plants under invasion and no invasion,most native plants under invaion increased leaf N and root N concentrations,but leaf N increased more than root N,which indicated that native plants under invasion increased leaf N allocation.In addition,based on the isotope mass-balance method,this study constructed a method to evaluate the contribution of NH₄⁺and NO₃^-to leaf N and root N,and further explored the allocation of NH₄⁺and NO₃^-between leaf and root.Results showed that more N was allocated to the leaves of invasive plants than that of native plants,and the increase of N allocatioin in the leaves of invasive plants was due to the assimilation of more NH₄⁺and NO₃^-.(3)The total N,NH₄⁺,net N mineralization rate,net nitrification rate and total P rhizosphere effect of invasive plants were all higher than 0,which indicated that the total N,NH₄⁺,total P content and N cycle rate of rhizosphere soil were improved by the rhizosphere effect of invasive plants.The net N mineralization rate,net nitrification rate and total P rhizosphere effect of local plants were higher than 0,but their available P rhizosphere effect was lower than 0,which indicated that native plants increased their N availability by accelerating the N cycling rate of rhizosphere soil,but decreased the effective P content.The total N,NH₄⁺,net N mineralization rate and net nitrification rate of invasive plants were higher than that of native plants,but their total P and available P had similar rhizosphere effect,which indicated that the improvement of N nutrient status of rhizosphere soil by invasive plants through rhizosphere effect was higher than that of native plants,but their effect on total P and effective P of rhizosphere soil was similar(4)With the increase of soil N and P content,plant height of invasive plants increased,while plant height of native plants unchanged;the concentrations of N and P in one or more organs in the leaves,stems and roots of invasive plants and native plants increased;the proportion of biomass allocation in invasive plants leaf increased,but the proportion of biomass allocation in the stems and roots of invasive plants did not change significantly;the concentration of N and P in Ageratina adenophora senescent leaves increased.These results showed that plasticity of N and P utilization with the change of soil N and P,which was helpful for the invasive plants to improve litter quality,increase N and P absorption,improve efficiency of N and P utilization,and thus maintain high productivity of the invasive plants.The important mechanism of exotic plant invasion is that invasive plants have stronger N and P uptake and accumulation capacity,advantage N allocation strategy,and improve soil N and P and the plasticity of N and P utilization.This study enriches the understanding of the mechanism of N and P utilization of invasive plants in in-situ invaded ecosystems,and fills the evidence of stem and roots N and P of invasive plants,which provides new insights for understanding the mechanism of exotic plant invasion and native plants coexistence,and provides useful scientific support for better evaluation and prediction of alien invasion and its ecological effects.