|Other Abstract||Due to the high primary productivity, low decomposition rate of soil organic carbon (SOC) and high efficiency in sedimentation of salt marshes, they have great potential in capturing and burying SOC. Thus, these ecosystems are generally considered as important "Blue carbon sinks" on Earth. However, in recent years, under the combined effect of large-scale production and application of chemical fertilizer, fossil fuel combustion and intensive management of animal husbandry, more and more nitrogen (N), especially nitrate nitrogen, enters the marine ecosystem through surface runoff, resulting in the increasing eutrophication of coastal water. N carried by the tide input into a salt marsh ecosystem, which have great uncertainty of the influence on the biogeochemical cycle of organic carbon in a salt marsh ecosystem. Therefore, in this study, we took a salt marsh ecosystem in the Yellow River Delta (YRD) as the research object, through conducting a field experiment of N input and a laboratory experiment of N input coupled with tidal flooding, combining with 13C-CO2 labelling technic. We mainly discussed the responses of key processes of carbon cycle, such as net ecosystem CO2 exchange (NEE), photosynthetic carbon allocation, soil CO2 and CH4 emissions, and dissolved organic carbon (DOC) loss of a salt marsh to different N input levels in the Yellow River Delta, the effects of environmental and biological factors on each process of carbon cycle under N input conditions, and the effect of N input on the control of the inundation frequency on SOC loss. The main results were as follows:
(1) N input significantly increased CO2 uptake of ecosystem by 84.06%-220.54%, mainly contributed to the significant increase of CO2 uptake in the growing season with the increasing N input. The increase of CO2 uptake caused by N input was mainly due to the increase of aboveground biomass, coverage, average height and frequency of Suaeda salsa. N input increased the contents of inorganic N in soil, alleviated nutrient stress, thereby stimulated plant growth and improved the ability of ecosystem to absorb CO2. The seasonal variation of NEE was mainly related to soil temperature. NEE decreased significantly with the increasing soil temperature, and N input strengthened the effect of temperature on NEE in a salt marsh.
(2) N input significantly increased the input of photosynthetic carbon and net carbon assimilation. The treatment of +20 g N m-2 y-1 increased the net photosynthetic 13C by over 200%. The increased photosynthetic carbon mainly existed in the form of plant aboveground biomass and SOC in the vegetation-soil system. With the increase of N input, the total biomass of plant and the fraction of aboveground biomass increased, while the fraction of belowground biomass and root/shoot ratio decreased. The N response efficiency of plant aboveground and underground biomass was decreased rapidly when the N input level lower than 5 g N m-2 y-1. The increase of plant biomass caused by nitrogen input increased SOC content, especially DOC and microbial biomass carbon (MBC) contents.
(3) N input increased soil CO2 and CH4 emissions in a vegetated salt marsh ecosystem. The increase of CO2 emission flux mainly occurred in the growing season, while the increase of CH4 emission flux mainly occurred in the flooding period. N input stimulates plant growth by increasing soil nutrient content (ammonium N nitrate N etc.), so as to transfer more active organic carbon, such as DOC, to the soil, and then stimulate microbial metabolic activity and produce more CO2 and CH4. The seasonal variation of soil CO2 emission flux was mainly controlled by temperature. With the increase of soil temperature, soil CO2 emission flux increased exponentially, and Ninput inhibited the temperature sensitivity Q10 of soil CO2 emission. The seasonal variation of soil CH4 emission flux was mainly controlled by soil water content, and soil CH4 emission flux increased exponentially with the increase of soil water content. Nitrogen input increased the sensitivity of soil CH4 emission flux to water.
(4) Inundation frequency is the main factor controlling SOC loss in a salt marsh, while Ninput weakens the control of inundation frequency on the production and emission of soil CO2 and CH4 and DOC loss. With the increasing frequency of inundation, the emission of CO2 was significantly inhibited and the emission of CH4 was stimulated. In addition, the increase of inundation frequency stimulates the loss of DOC by increasing tidal flush. N input inhibited the loss of SOC in this experiment conducted in the salt marsh wetland without vegetation, which was different from that in the salt marsh ecosystem with vegetation, mainly due to the lack of active organic carbon input by plants. There was a positive linear correlation between the decomposition of SOC and the loss of soil DOC under the lunar tide treatment, but there was no significant correlation between the two under the semidiurnal tide treatment. This was due to the different ways of soil DOC loss under different inundation frequencies. DOC was mainly in the form of decomposition and leaching under low and high frequency flooding, respectively.
In summary, N input significantly changed the key processes of carbon cycle, including carbon input, carbon allocation and carbon emission. Meanwhile, Nnput also affected the relationships between the carbon cycle and soil temperature, dynamics of soil water and salinity, which was mainly owing to the changes of soil nutrient content and vegetation productivity. In the future, when evaluating the carbon budget and carbon sink function of a salt marsh ecosystem, we should consider the impact of large amount of N nput caused by coastal eutrophication on the key process of carbon cycle, as well as the coupling effect of periodic tidal inundation and N input. This will help to predict the ecological function of "Blue Carbon" sink of salt marsh under global change in the future and put forward reasonable management measures for salt marshes.|