Interactive effects of soil moisture and temperature on methane uptake and microbial community dynamics in an upland forest soil

Aerated soils, especially forest soils, are a sink for atmospheric methane, oxidizing an average of 30 to 40 Tg (bottom-up and top-down estimates) of this powerful greenhouse gas each year. Methane-oxidizing microorganisms, i.e., methanotrophs, mediate this methane sink. Soil methane uptake (SMU) depends primarily on environmental factors, such as soil water content and temperature. Climate change is expected to alter these soil properties, affecting SMU, but more research is needed to understand how SMU will respond to combined changes in temperature and precipitation.

To investigate how soil water content and temperature interact to regulate SMU and methanotroph abundance, we established a rain exclusion experiment in an upland forest soil on the Telegrafenberg campus, Potsdam (Germany), and began monitoring SMU and microbial community composition and abundance at the 0-10 cm soil depth. Soil methane uptake is measured biweekly using a chamber-based method, while microbial abundances are assessed monthly by qPCR (pmoA, mcrA, and 16S rRNA gene) and 16S rRNA gene sequencing. Additionally, we measured methane uptake and microbial gene abundances of soil samples from the same location in controlled laboratory incubations at varying water contents and temperatures.

The incubation experiment revealed that SMU was highest at 35% and 65% of the maximum water-holding capacity. The incubations with 100 and 130% WHC even switched to methane production. The community composition shifted along the moisture gradient and differed significantly across water levels. Regarding the methane-cycling community, there was an increase in Methylocystis, Methanobacteria and Methanocella, and in the high-water content treatments. The community composition of methanotrophs was dominated by Methylocapsa and Methylocella. The differences in methane uptake were accompanied by differences in the abundances of microbial genes (mcrA and pmoA). So far, all forest plots show high methane uptake, and the methanotroph community is dominated by type II methanotrophs. Our research will provide valuable insights into how climate change may impact SMU and the associated microbial community in upland forest soils.