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

The future size of the terrestrial methane (CH4) 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 CH4 fluxes and laboratory soil CH4 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 CO2 concentration, and N deposition. Across all sampling dates and treatments, increased precipitation caused a 61% reduction in field CH4 uptake. However, this reduction depended quantitatively on other global change factors. Higher precipitation reduced CH4 uptake when temperature or N deposition (but not both) increased, and under elevated CO2 but only late in the growing season. Warming alone also decreased CH4 uptake early in the growing season, which was partly explained by a decrease in laboratory soil CH4 consumption. Atmospheric CH4 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 CH4 sink appears less likely than a reduced one.