Effects of interactive global changes on methane uptake in an annual grassland

Abstract

The future size of the terrestrial methane (CH 4 ) sink of upland soils remains uncertain, along with potential feedbacks to global warming. Much of the uncertainty lies in our lack of knowledge about potential interactive effects of multiple simultaneous global environmental changes. Field CH 4 fluxes and laboratory soil CH 4 consumption were measured five times during 3 consecutive years in a California annual grassland exposed to 8 years of the full factorial combination of ambient and elevated levels of precipitation, temperature, atmospheric CO 2 concentration, and N deposition. Across all sampling dates and treatments, increased precipitation caused a 61% reduction in field CH 4 uptake. However, this reduction depended quantitatively on other global change factors. Higher precipitation reduced CH 4 uptake when temperature or N deposition (but not both) increased, and under elevated CO 2 but only late in the growing season. Warming alone also decreased CH 4 uptake early in the growing season, which was partly explained by a decrease in laboratory soil CH 4 consumption. Atmospheric CH 4 models likely need to incorporate nonadditive interactions, seasonal interactions, and interactions between methanotrophy and methanogenesis. Despite the complexity of interactions we observed in this multifactor experiment, the outcome agrees with results from single‐factor experiments: an increased terrestrial CH 4 sink appears less likely than a reduced one.