Soil-methane sink increases with soil age in forefields of alpine glaciers

In upland soils aerobic methane-oxidizing bacteria (MOB) catalyze methane (CH₄) oxidation, and thus regulate the sole terrestrial sink for atmospheric CH₄. While confirmed in mature upland soils, little is known about this important function in young mountainous soils in glacier forefields, which are progressively formed as a result of glacier recession. We assessed four attributes of the soil CH₄ sink, i.e., soil-atmosphere CH₄ flux (J_atm), CH₄ oxidation activity (k), MOB abundance and variation in community composition along the 6-120-yr soil chronosequence in two Alpine glacier forefields on siliceous and calcareous bedrock. At most sampling locations soil CH₄ profiles showed stable uptake of atmospheric CH₄, with J_atm in the range of-0.082 to-2.2mg CH₄m^-2d^-1. Multiple-linear-regression analyses indicated that J_atm significantly increased with soil age, whereas k did not. Instead, water content and CH₄ profiles in the youngest soils often indicated dry, inactive top layers with k<0.1h^-1, and active deeper layers (0.2h^-1≤k≤11h^-1) with more favorable water content. With increasing soil age the zone of highest CH₄ oxidation activity gradually moved upwards and eventually focused in the 10-40-cm layer (0.2h^-1≤k≤16h^-1). Copy numbers of pmoA genes significantly increased with soil age at both sites, ranging from 2.4×10³ to 5.5×10⁵ copies (gsoilw.w.)^-1, but also correlated with mineral nitrogen content. Terminal restriction-fragment-length-polymorphism and cluster analyses showed differences in MOB community composition apparently related to bedrock type rather than soil age. Yet, regardless of bedrock type, the soil CH₄ sink established within a few years of soil development, and J_atm increased to values comparable to mature soils within decades. Thus, young mountainous soils have the potential to consume substantial amounts of atmospheric CH₄, and should be incorporated into future estimates of global soil CH₄ uptake.