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 (CO₂) and methane (CH₄) emissions from drawdown sediments of Brazilian semiarid reservoirs submitted to a prolonged drought period. The observed average CO₂ and CH₄ 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 CO₂ emissions may be considered low, but CH₄ 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 CO₂ emission rates, whereas dissolved oxygen concentration in the overlying water appears to be controlling CH₄ 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 CO₂ and CH₄ to the atmosphere after reservoirs re-flooding.

由于气候变化，半干旱地区的长期干旱有所增加，边缘水库沉积物更频繁地暴露在大气中。最近的研究表明，干燥和再淹没沉积物之间的交替可能会增加有机物 (OM) 矿化，从而增加向大气中排放的温室气体 (GHG)。然而，人们对干燥和再洪水对温室气体排放的影响知之甚少，尤其是在位于半干旱地区的水库中，这些水库偶尔会经历暴雨再灌。在这里，我们通过实验评估了沉积物再泛滥对巴西半干旱水库沉没沉积物中二氧化碳 (CO ₂ ) 和甲烷 (CH ₄ ) 排放的影响，这些沉积物处于长期干旱期。观察到的平均 CO₂和CH ₄排放范围分别为3至78 mg C m ^-2  day ^-1和0.4至13.5 mg C m ^-2  day ^-1。与文献中报告的其他下降排放值相比，我们的 CO ₂排放可能被认为是低的，但 CH ₄排放与干燥-再润湿循环实验报告的值一致。长时间的沉积物干燥可能是我们降低 CO ₂排放率的关键，而上覆水中的溶解氧浓度似乎控制着 CH ₄排放。长期干旱使暴露的沉积物与大气中的氧气接触，促进更高的 OM 分解，主要是高度难降解的 OM 在重新淹没后分解。尽管先前的研究表明再润湿沉积物的温室气体排放量可能很大，但我们的研究结果表明，长期暴露于大气中的巴西半干旱水库的压降沉积物是水库重新淹没后大气中 CO ₂和 CH _{4 的}低来源。