Stressed crops emit more methane despite the mitigating effects of elevated carbon dioxide

Recent studies using single environmental variables show that under aerobic conditions terrestrial plants can emit methane (CH4). However, the effects of multiple environmental factors-as components of global climate change-on aerobic CH4 emissions have been little studied. We examined the combined effects of temperature, carbon dioxide (CO2) and watering regime on CH4 emissions from six commonly cultivated crop species: faba bean, sunflower, pea, canola, barley and wheat. Plants were grown from seeds in controlled-environment growth chambers under two temperature regimes (24 degrees C day/20 degrees C night and 30 degrees C day/26 degrees C night), two CO2 concentrations (380 and 760 mmol mol(-1)) and two watering regimes (well watered and water stressed). Plants were grown first under 24/20 degrees C for 1 week from sowing, and then placed under experimental conditions for a further week. After the specified time, plant growth, gas exchange and CH4 emission rates were determined. Our results revealed that higher temperature and water stress significantly enhance CH4 emissions from plants, whereas elevated CO2 had the opposite effect and partially reverses the promotive effects of these factors. We suggest that the despite the mitigating effects of rising atmospheric CO2, CH4 emission may be higher in the face of ongoing global climate change in warmer and drier environments.