Methane uptake responses to extreme droughts regulated by seasonal timing and plant composition

Increasingly frequent and severe droughts are occurring in multiple seasons of a year in many dryland ecosystems, with unknown impacts on the role of drylands in cycling of methane (CH4), a potent greenhouse gas. In particular, there is limited understanding of how drought occurring at different times within the growing season regulates biological CH4 uptake, and how these responses are mediated by plant community composition. Here, we quantify how drought timing and plant community composition regulate CH4 uptake in a semiarid grassland. We employ a field experiment in which droughts were imposed in early, middle, or late growing season in three different communities (two graminoids, two shrubs and their mixture), respectively. All three droughts increased CH4 uptake, but the effect size and pathway varied with seasonal timing. Early and middle drought increased CH4 uptake through increasing both soil pomA abundance and diffusivity resulting from reduced soil water content (SWC), while late drought increased CH4 uptake only by reducing SWC. Overall, early drought had the least positive effects on CH4 uptake because it excluded the least precipitation and therefore had smaller impacts on SWC. Besides, plant composition did not affect CH4 uptake under normal environment but regulated CH4 uptake in response to droughts due to different response of plant composition to droughts. Early and middle drought had larger positive effects on CH4 uptake in shrub communities than the other two communities, consistent with larger reductions in SWC and larger increases in pomA abundance, respectively. In contrast, late drought had consistent effects on CH4 uptake across three communities. Our results suggest that the magnitude and pathways of extreme drought effects on CH4 uptake are strongly co-regulated by seasonal timing and plant composition.