Tracking methane fluxes using intact polar and core lipids in an aridity transect of the Okavango Delta (Botswana)

<p>Wetland methane (CH₄) emissions are the largest natural source in the global CH₄ budget, contributing to roughly one third of total natural and anthropogenic emissions. As the second most important anthropogenic greenhouse gas in the atmosphere after CO₂, CH₄ is strongly associated with climate feedbacks. The different pathways of biochemical cycling of CH₄, which exert a primary control on atmospheric CH₄ concentrations through its production and biological consumption, remain poorly constrained. It is therefore crucial to understand and, if possible, quantify these variable CH₄sources to natural climate variability.</p> <p>We studied a soil transect (up to seven sites, 250 m long) across a seasonal floodplain at Nxaraga on the south-west part of the Chief&#8217;s Island, Okavango Delta, Botswana, over three years (2018 &#8211; 2020, 50 samples in total). Previous studies showed a clear link between CH4 fluxes and soil water content in the area, with CH₄ fluxes in the seasonally flooded soils of up to 492 nmol m^-2 s^-1.</p> <p>To constrain biomass active in CH₄production (specifically, methanogenic archaea) intact and core isoprenoid lipids (and their stable carbon isotope signature) were quantified on a High Performance Liquid Chromatograph (HPLC) and on an high-resolution mass spectrometer ("Orbitrap"). To constrain biomass of CH₄oxidizers (i.e. bacterial methanotrophs), core (hopanol) and intact lipids (i.e., bacteriohopanepolyols (BHPs)) were analyzed non-derivatized on an Orbitrap. Confirming their proposed methanotroph source, BHP-aminopentol and methylcarbamate-BHP were detected and their variation correlated positively with those of hopanols and archaeol lipids. Methyl-amino BHPs however were not detected in the soils. In-depth study of their environmental variation points towards two bacterial communities depending on the pH, EC and water content of the soils. This will be confirmed or refuted by bacterial community profiling based on 16S RNA genes, and functional genes for methane oxidation</p>