Elevated CO2 and warming effects on CH4 uptake in a semiarid grassland below optimum soil moisture

Semiarid rangelands are a significant global sink for methane (CH4), but this sink strength may be altered by climate change. Methane uptake is sensitive to soil moisture showing a hump-shaped relationship with a distinct optimum soil moisture level. Both CO2 and temperature affect soil moisture, but the direction of CH4 uptake response may depend on if the system is below or above the soil moisture optimum. Most climate change studies on CH4 uptake have been conducted in mesic environments with soil moisture levels typically above optimum, but little is known about responses in drier systems with suboptimal soil water. We studied effects of atmospheric CO2 (ambient versus 600 ppm), and temperature (ambient versus 1.5/3.0 degrees C warmer day/night) on CH4 uptake during two growing seasons in a full factorial semiarid grassland field experiment in Wyoming, United States. We observed typical hump-shaped relationships between CH4 uptake and water filled pore space. Averaged over a range of soil moisture conditions, CH4 uptake was not affected by elevated CO2, but significantly decreased with warming in both seasons (25% in the first and 13% in the second season). Warming showed the strongest reduction and elevated CO2 showed the strongest increase in CH4 uptake when soils were below optimum moisture, indicating that these effects are particularly strong when soils are dry. Thus, directional effects of elevated CO2 and warming on CH4 uptake in semiarid grasslands can be opposite to their effects in mesic ecosystems because semiarid grasslands are often below optimum soil moisture for methane uptake.