Effects Of Nitrogen And Phosphorus Addition On The Carbon Cycle Of Spartina Alterniflora Salt Marsh

Due to the development of industry and agriculture,the content of nitrogen and phosphorus released into the ocean has been increasing,and ocean eutrophication would be a long-standing environmental problem.Eutrophication will change the ecosystem structure and biogeochemical cycling such as plant photosynthetic capacity,soil respiration,microbial community structure and metabolic activities,which may change the carbon cycle in saltmarsh.In this study,we conducted microcosm of Spartina alterniflora saltmarsh culture system.We designed nitrogen and phosphorus addition experiment of three treatments and five-level including N-addition(N1,N2,N3,N4,N5),P-addition(P1,P2,P3,P4,P5)and NP-composite addition(NP1,NP2,NP3,NP4,NP5).We used stable isotopic carbon(¹³C)labeling method to analyze the photosynthetic carbon distribution in the plant-soil system.We measured soil respiration and soil microbial diversity.We attempted to study the effects of nitrogen and phosphorus addition on carbon cycling in saltmarsh.The results show:(1)The fixed amount of ¹³C in each part of the system generally showed a trend of leaf,stem>spike,root>rhizosphere soil>bulk soil,and the fixed amount of ¹³C was mainly concentrated in the above-ground biomass.The addition of low nitrogen,high phosphorus and high nitrogen and phosphorus is more conducive to the transfer of S.alterniflora photosynthetic carbon from the above-ground biomass to the underground biomass.(2)Low N-addition(N1,N2)and low P-addition(P1)promoted soil respiration on the 1^st and 10^thday after ¹³C pulse-labeling,and medium and high P-addition increased the soil respiration rate on the 20^th and 30^thday after ¹³C pulse-labeling.The soil respiration CO₂ flux of S.alterniflora saltmarsh is significantly positively correlated with soil temperature,total biomass of S.alterniflora,and underground biomass.(3)The addition of nutrients generally reduced the relative abundance of Proteobacteria in rhizosphere soil and soil.Under the addition of low N-addition(N1),high P-addition(P4,P5)and the low NP-composite addition(NP2),the total number of bacteria in the rhizosphere soil of S.alterniflora was higher.Under the medium N-addition(N3),low P-addition(P1,P2)and high NP-composite addition(NP5),S.alterniflora soil species diversity is greater.The dominant phyla of S.alterniflora rhizosphere soil and soil fungi is Ascomycota,followed by Basidiomycota.Under low and medium N-addition,low P-addition,and high NP-composite addition,S.alterniflora rhizosphere soil and soil fungi have greater species diversity and uniformity.Salt marsh eutrophication significantly affects the photosynthetic carbon allocation strategy,soil respiration flux,soil physical and chemical properties,and soil microbial diversity of S.alterniflora.Thereby further affects the dynamic balance of input,output,decomposition,and accumulation of soil organic carbon pool.And also affects the fixation potential of soil organic carbon.Understanding the photosynthetic carbon distribution process of S.alterniflora,soil respiration and soil microbial diversity response mechanism to eutrophication provides theoretical support for the scientific use of vegetation to regulate the function of soil carbon sink.