DOI: 10.5061/dryad.qv9s4mwfz
title: Riparian cottonwood trees and adjacent river sediments have different microbial communities and produce methane with contrasting carbon isotope compositions

description: Rivers and their adjacent riparian forests are intimately linked by the
 exchange of water, nutrients, and organic matter. Both riparian cottonwood
 trees and adjacent river sediments host microbial communities including
 archaeal methanogens, supporting methane production and emission to the
 atmosphere. Here we combine microbial community and stable isotope
 analyses to characterize the drivers of methane cycling in distinct anoxic
 habitats (river sediments versus riparian cottonwood stems) in the Oldman
 River, southern Alberta (Canada). We demonstrate that, differences in the
 chemical characteristics of organic matter support divergent microbial
 communities that generate methane from distinct metabolic pathways.
 Organic matter in river sediments had C/N ratios approximately 50-fold
 lower than in tree stems and had more diverse dissolved organic
 components. Contrasting substrate availability between river sediment and
 tree stems was likely the primary mechanism for the observed differences
 in bacterial and methanogen community compositions, and greater microbial
 diversity in river sediments than in tree stems. The methane carbon
 isotope composition (δ13C values) differed for the tree stem (-103.6 to
 -70.6‰) and river sediment (-55.1 to -48.4‰) environments, suggesting that
 methane was primarily produced via CO2-reduction in tree stems by
 Methanobacteriales, while river sediments produced more methane through
 acetate fermentation primarily by Methanosarcinales. This study
 demonstrates the importance of organic matter quality and microbial
 community composition in driving metabolic processes contributing to
 methane production and emission in rivers and adjacent riparian forests.
description: Tree stem, river sediment and alluvial groundwater samples were
 collected and used for DNA extraction and Illumina sequencing of bacterial
 and methanogen communities. Sequence data were processed using the DADA2
 package in R to create amplicon sequence variants. Tree stem and river
 sediment samples were sealed in mason jars and flushed with nitrogen gas
 to provide an anaerobic environment where methane production could occur,
 and these were left to incubate in the dark at room temperature. At
 intervals, gas samples were removed from the headspace and measured
 for methane and carbon dioxide gas concentrations and stable carbon
 isotope composition using cavity ring-down spectroscopy. Measured values
 were adjusted based on repeated measures (n=2) of reference gases measured
 prior to, and following, sample measurements. Tree stem and river sediment
 samples were also dried, and used to measure the organic carbon and
 nitrogen and their associated isotopic compositions by Continuous
 Flow-Elemental Analysis-Isotope Ratio Mass Spectrometry. Tree stem and
 river sediment samples were also used for leachates and analysis of
 dissolved organic matter. Chromophoric dissolved organic matter was
 characterized by UV-Vis absorption, while additional dissolved organic
 matter properties were determined using a spectrofluorometer. All
 additional methodology for these analyses is present within the ReadMe
 file
 "Smits_Nov2021_riparian_methane_production_ReadMe.txt"
description: Values presented in the datasets are final and have undergone
 normalization methods to standards or controls. Any missing values are
 presented as "%". Samples across datasets were collected from
 the same trees, groundwater well and river sediment locations, and have
 matching sample names in the files. Additional data
 collection/processing, equipment and variables involved in these analyses
 are discussed in more depth in the ReadMe file
 "Smits_Nov2021_riparian_methane_production_ReadMe.txt"