High Temporal and Spatial Variability of Atmospheric-Methane Oxidation in Alpine Glacier Forefield Soils

Abstract

Glacier forefield soils can provide a substantial sink for atmospheric CH₄, facilitated by aerobic methane-oxidizing bacteria (MOB). However, MOB activity, abundance, and community structure may be affected by soil age, MOB location in different forefield landforms, and temporal fluctuations in soil physical parameters. We assessed the spatial and temporal variability of atmospheric-CH₄ oxidation in an Alpine glacier forefield during the snow-free season of 2013. We quantified CH₄ flux in soils of increasing age and in different landforms (sandhill, terrace, and floodplain forms) by using soil gas profile and static flux chamber methods. To determine MOB abundance and community structure, we employed pmoA gene-based quantitative PCR and targeted amplicon sequencing. Uptake of CH₄ increased in magnitude and decreased in variability with increasing soil age. Sandhill soils exhibited CH₄ uptake rates ranging from -3.7 to -0.03 mg CH₄ m^-2 day^-1 Floodplain and terrace soils exhibited lower uptake rates and even intermittent CH₄ emissions. Linear mixed-effects models indicated that soil age and landform were the dominating factors shaping CH₄ flux, followed by cumulative rainfall (weighted sum ≤4 days prior to sampling). Of 31 MOB operational taxonomic units retrieved, ∼30% were potentially novel, and ∼50% were affiliated with upland soil clusters gamma and alpha. The MOB community structures in floodplain and terrace soils were nearly identical but differed significantly from the highly variable sandhill soil communities. We concluded that soil age and landform modulate the soil CH₄ sink strength in glacier forefields and that recent rainfall affects its short-term variability. This should be taken into account when including this environment in future CH₄ inventories.IMPORTANCE Oxidation of methane (CH₄) in well-drained, "upland" soils is an important mechanism for the removal of this potent greenhouse gas from the atmosphere. It is largely mediated by aerobic, methane-oxidizing bacteria (MOB). Whereas there is abundant information on atmospheric-CH₄ oxidation in mature upland soils, little is known about this important function in young, developing soils, such as those found in glacier forefields, where new sediments are continuously exposed to the atmosphere as a result of glacial retreat. In this field-based study, we investigated the spatial and temporal variability of atmospheric-CH₄ oxidation and associated MOB communities in Alpine glacier forefield soils, aiming at better understanding the factors that shape the sink for atmospheric CH₄ in this young soil ecosystem. This study contributes to the knowledge on the dynamics of atmospheric-CH₄ oxidation in developing upland soils and represents a further step toward the inclusion of Alpine glacier forefield soils in global CH₄ inventories.

Keywords: atmospheric-methane oxidation; glacier forefield soil; high-affinity MOB; methane flux; methanotroph; pmoA; proglacial landforms.