Towards an integrated view on microbial CH <subscript>4</subscript> , N <subscript>2</subscript> O and N <subscript>2</subscript> cycles in brackish coastal marsh soils: A comparative analysis of two sites.

Coastal ecosystems, facing threats from global change and human activities like excessive nutrients, undergo alterations impacting their function and appearance. This study explores the intertwined microbial cycles of carbon (C) and nitrogen (N), encompassing methane (CH 4 ), nitrous oxide (N 2 O), and nitrogen gas (N 2 ) fluxes, to determine nutrient transformation processes between the soil-plant-atmosphere continuum in the coastal ecosystems with brackish water. Water salinity negatively impacted denitrification, bacterial nitrification, N fixation, and n-DAMO processes, but did not significantly affect archaeal nitrification, COMAMMOX, DNRA, and ANAMMOX processes in the N cycle. Plant species age and biomass influenced CH 4 and N 2 O emissions. The highest CH 4 emissions were from old Spartina and mixed Spartina and Scirpus sites, while Phragmites sites emitted the most N 2 O. Nitrification and incomplete denitrification mainly governed N 2 O emissions depending on the environmental conditions and plants. The higher genetic potential of ANAMMOX reduced excessive N by converting it to N 2 in the sites with higher average temperatures. The presence of plants led to a decrease in the N fixers' abundance. Plant biomass negatively affected methanogenetic mcrA genes. Microbes involved in n-DAMO processes helped mitigate CH 4 emissions. Over 93 % of the total climate forcing came from CH 4 emissions, except for the Chinese bare site where the climate forcing was negative, and for Phragmites sites, where almost 60 % of the climate forcing came from N 2 O emissions. Our findings indicate that nutrient cycles, CH 4 , and N 2 O fluxes in soils are context-dependent and influenced by environmental factors and vegetation. This underscores the need for empirical analysis of both C and N cycles at various levels (soil-plant-atmosphere) to understand how habitats or plants affect nutrient cycles and greenhouse gas emissions.

Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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