Out of gas: re-flooding does not boost carbon emissions from drawdown areas in semiarid reservoirs after prolonged droughts

Prolonged droughts in semiarid regions have increased due to climate change and marginal reservoir sediments have been more frequently exposed to the atmosphere. Recent studies suggest that an alternation between dry and re-flooded sediments may increase organic matter (OM) mineralization, boosting greenhouse gas (GHG) emissions to the atmosphere. However, the impact of drying and re-flooding on GHG emissions is poorly understood, especially in reservoirs located in semiarid regions that experience torrential refilling occasionally. Here, we experimentally evaluated the influence of sediment re-flooding on carbon dioxide (CO 2 ) and methane (CH 4 ) emissions from drawdown sediments of Brazilian semiarid reservoirs submitted to a prolonged drought period. The observed average CO 2 and CH 4 emissions ranged from 3 to 78 mg C m −2 day −1 and from 0.4 to 13.5 mg C m −2 day −1 , respectively. Compared with other drawdown emission values reported in the literature, our CO 2 emissions may be considered low, but CH 4 emissions are in line with values reported for drying–rewetting cycle experiments. The long period of sediment desiccation may be the key to our low CO 2 emission rates, whereas dissolved oxygen concentration in the overlying water appears to be controlling CH 4 emissions. Long periods of drought keep the exposed sediment in contact with atmospheric oxygen, promoting higher OM decomposition, lasting mainly highly refractory OM for decomposition after re-flooding. Although previous studies indicate that GHG emissions from rewetted sediments may be substantial, our findings suggest that drawdown sediments of Brazilian semiarid reservoirs submitted to long atmospheric exposure are low sources of CO 2 and CH 4 to the atmosphere after reservoirs re-flooding.