Effects Of Sea-level Rise And Nitrogen Enrichment On Fluxes And Radiation Balance Of Greenhouse Gases In An Estuarine Brackish Marsh

Sea-level rise and nitrogen enrichment are two major environmental problems faced by estuarine wetlands.They profoundly affect the carbon and nitrogen cycle process of estuarine wetland ecosystem.However,there are few reports about how sea-level rise and the combination of sea-level rise and nitrogen enrichment affect greenhouse gases fluxes,radiation balance,and decomposition of organic matter in soil carbon pool in estuarine wetlands.In this study,the passive weir method was used to simulate the sea-level rise and nitrogen enrichment in situ.From July 2019 to June 2021,the effects of sea-level rise,nitrogen enrichment and their combination on greenhouse gases fluxes,radiation balance and the decomposition of buried plant residues in the Minjiang Estuary wetland were studied.The main conclusions are as follows:（1）Throughout the study period,the weir itself had no significant effects on net ecosystem exchange of CO₂（NEE）,gross primary production（GPP）,ecosystem respiration（ER）,N₂O flux,greenhouse gases radiation balance and the decomposition of buried plant residues in the Cyperus malaccensis marsh.However,in the first year of the experiment,the presence of a weir significantly reduced the annual CH₄ emission by 51%from the C.malaccensis marsh.In general,it is feasible to use the passive weir method to study the effects of sea-level rise on the carbon and nitrogen cycles in coastal wetlands,but when studying the scientific issues related to CH₄ in coastal wetlands,a no-weir control should be added to reveal the effects of the weir itself.（2）The effects of sea-level rise and nitrogen enrichment on NEE,GPP,and ER in the C.malaccensis wetland were related to the time scale chosen for the study.The effects of sea-level rise and nitrogen enrichment on the NEE at noon had significant seasonal variation.On the whole,they have significantly promoted the NEE at noon（because the promotion on the GPP at noon was greater than that on the ER at noon）,which was greater in the active growth season（May to October）and smaller or even less significant in the inactive growth season（November to April）.The impacts of sea-level rise and nitrogen enrichment on monthly NEE also had significant seasonal variation,which significantly promoted the monthly NEE in the active growth season（because the promotion on monthly GPP exceeded the impact on monthly ER）,however,in the inactive growth season,they significantly inhibited NEE（because the promotion effect on the monthly scale ER was greater than that on the monthly scale GPP）or had no significant impact on NEE（because the impact on the monthly scale GPP and ER were roughly the same）.On an annual scale,sea-level rise and nitrogen enrichment significantly promoted NEE（because the promotion on annual GPP exceeded that on annual ER）,and the average promotion degrees were 78%and 62.5%,respectively.At all scales,the combined effects of sea-level rise and nitrogen enrichment on NEE,GPP,and ER were additive（1+1=2）.In addition,sea-level rise and nitrogen enrichment did not change the seasonal dynamics of NEE,GPP,and ER.Sea-level rise had no significant effect on the temperature sensitivity coefficient Q₁₀ of ER,while nitrogen enrichment significantly reduced the Q₁₀value.During the two years of the experiment,the annual scale NEE of the C.malaccensis wetlands in Minjiang Estuary changed significantly,and the NEEs of the weir control plots were 2.8±0.55 and 0.55±0.38kg CO₂ m^-2 yr^-1,respectively.（3）The influence of sea-level rise on CH₄ flux（instantaneous and monthly scale）of the C.malaccensis wetland had significant seasonal variation,which had a significant promotion effect in the early（May-June）and middle（July-August）of the active growing season,and a significant inhibition effect in the rest of the time.The comprehensive effect made the sea-level rise had no significant influence on the annual CH₄ flux of the C.malaccensis wetland.Nitrogen enrichment continuously inhibited CH₄ emission from the C.malaccensis wetland,and the inhibition was greater in the active growing season and smaller in the inactive growing season.As a whole,the annual CH₄ emission from the C.malaccensis wetlands was significantly reduced,with an average reduction of 39.5%in two years.The combined effects of sea-level rise and nitrogen enrichment on CH₄ emission fluxes from the C.malaccensis wetland were additive.During the two years of the experiment,the annual CH₄ emission flux of the C.malaccensis wetland in Minjiang Estuary changed significantly,and the CH₄ emission fluxes of the control plots were18.4±1.54 and 4.8±0.55 g m^-2 yr^-1,respectively.（4）Sea-level rise had no significant effect on N₂O fluxes（instantaneous and cumulative fluxes）,mainly because sea-level rise had no significant effect on pore water nitrate-nitrogen concentrations;nitrogen enrichment significantly promoted N₂O emission（instantaneous and cumulative fluxes）,and the average promotion degree of annual N₂O flux was 6866.5%.The reason was that nitrogen enrichment significantly increased pore water nitrate-nitrogen concentrations and provided abundant available reactive nitrogen for microorganisms such as denitrifying bacteria.The impact of nitrogen enrichment on N₂O emissions had significant seasonal changes,with a small promotion in the active growth season and large promotion in the inactive growth season.This may be due to different levels of competition between plants and microorganisms for active nitrogen.At all scales,the combined effects of sea-level rise and nitrogen enrichment on N₂O flux were additive.During the two years of the experiment,the N₂O fluxes of the weir control plots were0.088±0.16 and 0.092±0.021 g m^-2 yr^-1,respectively.（5）Sea-level rise significantly increased the radiation balance（total CO₂-eq flux of greenhouse gases）of the C.malaccensis wetland,with an average promotion degree of 143%in two years,mainly because sea-level rise significantly enhanced the net CO₂ absorption capacity of the C.malaccensis wetland.Nitrogen enrichment had no significant impact on the radiation balance of the C.malaccensis wetland.The main reason was that nitrogen enrichment significantly promoted N₂O emissions,resulting in significant positive radiative forcing,which largely offset the negative radiative forcing caused by increased net CO₂ absorption and reduced CH₄ emissions.The combined effect of sea-level rise and nitrogen enrichment on the radiation balance of the C.malaccensis was additive.During the two years of the experiment,the interannual variation of greenhouse gas radiation balance in the C.malaccensis wetland in Minjiang Estuary was significant,and the radiation balances of the weir control plots were 1950±515 and 312±370 g CO₂-eq m^-2yr^-1,respectively.（6）Individual sea-level rise and nitrogen enrichment treatment significantly promoted shoot decomposition by promoting plant growth,but not for roots and rhizomes because of their inability to increase porewater NH₄⁺levels.However,combined sea-level rise and nitrogen enrichment significantly promoted both shoot and root and rhizome decomposition.The combined effects of sea-level rise and nitrogen enrichment on the decomposition of both plant residues were not additive（1+1≠2）,with the specific type of effect depending on the type of residue（i.e.,substrate quality）,antagonistic（1+1<2）on shoot residues,and synergistic（1+1>2）on root and rhizome residues.