High methane concentrations in tidal salt marsh soils: Where does the methane go?

Tidal salt marshes produce and emit CH4 . Therefore, it is critical to understand the biogeochemical controls that regulate CH4 spatial and temporal dynamics in wetlands. The prevailing paradigm assumes that acetoclastic methanogenesis is the dominant pathway for CH4 production, and higher salinity concentrations inhibit CH4 production in salt marshes. Recent evidence shows that CH4 is produced within salt marshes via methylotrophic methanogenesis, a process not inhibited by sulfate reduction. To further explore this conundrum, we performed measurements of soil-atmosphere CH4 and CO2 fluxes coupled with depth profiles of soil CH4 and CO2 pore water gas concentrations, stable and radioisotopes, pore water chemistry, and microbial community composition to assess CH4 production and fate within a temperate tidal salt marsh. We found unexpectedly high CH4 concentrations up to 145,000 μmol mol-1 positively correlated with S2- (salinity range: 6.6-14.5 ppt). Despite large CH4 production within the soil, soil-atmosphere CH4 fluxes were low but with higher emissions and extreme variability during plant senescence (84.3 ± 684.4 nmol m-2  s-1 ). CH4 and CO2 within the soil pore water were produced from young carbon, with most Δ14 C-CH4 and Δ14 C-CO2 values at or above modern. We found evidence that CH4 within soils was produced by methylotrophic and hydrogenotrophic methanogenesis. Several pathways exist after CH4 is produced, including diffusion into the atmosphere, CH4 oxidation, and lateral export to adjacent tidal creeks; the latter being the most likely dominant flux. Our findings demonstrate that CH4 production and fluxes are biogeochemically heterogeneous, with multiple processes and pathways that can co-occur and vary in importance over the year. This study highlights the potential for high CH4 production, the need to understand the underlying biogeochemical controls, and the challenges of evaluating CH4 budgets and blue carbon in salt marshes.

Integrated effects of Ulva prolifera bloom and decay on nutrients inventory and cycling in marginal sea of China

Ulva prolifera blooms occur annually in the Yellow Sea. Most studies focus on how U. prolifera blooming is influenced by nitrogen chemical forms and concentrations, while little concern goes to how U. prolifera bloom-decay cycle would impact local seawater nutrients structure. Therefore, we use ¹⁵N-labeled NO₃ tracers and transcriptome analysis to determine N uptake, metabolism, and interconversion during U. prolifera growth and decay, so that we can quantify the conversions rate and fluxes of different nitrogen chemical forms. U. prolifera absorbes 17.37 μmol g^-1·d^-1 NO₃-N during growth. NO₃-N predominates (73.75-92.15%) in the dissolved inorganic nitrogen (DIN) in U. prolifera. During decay, NH₄-N accountes for 60.87-92.13% of the in-cell DIN. The decomposing U. prolifera releases considerable amounts of NH₄-N and dissolved organic nitrogen (DON) (63.8-98.2% < 1 kDa fraction and 1.8-36.2% is > 1 kDa fraction) into the ambient environment. The high DON release rate (59.57 μmol g^-1 d^-1) indicates active DON biosynthesis in U. prolifera. The isotope ¹⁵NO₃-N tracer showes that 73.6% of the ¹⁵NO₃-N is transformed to DON. The <1 kDa and the >1 kDa fractions account for 67.46-90.86% and 9.14-32.54% of the DON, respectively. The high efficiency of U. prolifera in utilizing NO₃-N is explained by the responsive nitrate/nitrite transporter in cell membrane, and the DON biosynthesized capability is attributed to the up-regulated glutamine synthetase. Our study highlights the unique role of U. prolifera as a "Nitrogen-Pump" in converting nitrogen chemical forms during its bloom-decay cycle and quantifies its impacts on local N-nutrients inventory.

Environmental Conditions and Species Identity Drive Metabolite Levels in Green Leaves and Leaf Litter of 14 Temperate Woody Species

Research Highlights: Leaf chemistry is a key driver of litter decomposition; however, studies directly comparing metabolites that are important for tree growth and defence across different woody species are scarce. Background and Objectives: Choosing 14 temperate woody species differing in their growth rates, nutrient demand, shade tolerance, and drought sensitivity, we hypothesized that the species would group according to their metabolite profiles based on their ecological background. Materials and Methods: We analysed total N and C, soluble amino acid, protein, and phenolic levels in green leaves and leaf litter of these species, each in two consecutive years. Results: Metabolite levels varied significantly across species and between the sampling years which differed in temperature and precipitation (i.e., colder/drier vs warmer/ wetter). Conclusions: The 14 woody species could not be grouped according to their green leaf or leaf litter metabolite profiles. In litter leaves, most of the variation was explained by total phenolics and total nitrogen levels, and in green leaves by total phenolics and total soluble amino acid levels. Local climate variation between the two consecutive years for green leaves or leaf litter led to significant differences in metabolite levels, although some of them were species-specific.

Release and microbial degradation of dissolved organic matter (DOM) from the macroalgae Ulva prolifera

Release and microbial degradation of dissolved organic matter (DOM) and chromophoric dissolved organic matter (CDOM) from the macroalgae Ulva prolifera were studied in laboratory incubation experiments. The release of DOM and CDOM from Ulva prolifera was a rapid process, and hydrolysis played an important role in the initial leaching of the organic compounds from the algae. Bacterial activity enhanced the release of DOM and CDOM during degradation of the algae and utilization of the released organic compounds. It is calculated that 43±2% of the C and 63±3% of the N from Ulva prolifera's biomass were released during the 20-day incubation, and 65±3% of the released C and 87±4% of the released N were utilized by bacteria. In comparison, only 18±1% of the algae's C and 17±1% of its N were released when bacterial activities were inhibited. The fluorescence characteristics of the CDOM indicate that protein-like DOM was the major organic component released from Ulva prolifera that was highly labile and biodegradable. Bacteria played an important role in regulating the chemical composition and fluorescence characteristics of the DOM. Our study suggests that the release of DOM from Ulva prolifera provides not only major sources of organic C and N, but also important food sources to microbial communities in coastal waters.