Extreme drought boosts CO₂ and CH₄ emissions from reservoir drawdown areas

Abstract

This is the accepted version of a paper published in INLAND WATERS. This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination.

Sediment drying-rewetting cycles enhance greenhouse gas emissions, nutrient and trace element release, and promote water cytogenotoxicity

“…We observed a significant increase of CO 2 and CH 4 emissions resulting from the drying and rewetting of freshwater sediments, similar to what has been found in previous studies [21,[35][36][37][38]. The observed pattern is attributed to a phenomenon generally known as the "Birch effect", in reference to studies carried out by H.F. Birch in the 1950s and 1960s on the effects of droughts and rewetting events on the carbon and nitrogen cycle in agricultural and forest soils [39,40].…”

“…Following the experimental setup of Kosten et al (2018) [21], we distinguished four different periods in our experiment that were described as: (i) "flooded period", which corresponds to the period when we added distilled water daily to the induced-to-drought cores during the first week of the experiment plus the second week when we stopped adding distilled water and the overlying water, corresponding to the period that the sediments were inundated, started to evaporate (days 0-14); (ii) "drying period", which started when the sediment from all inducedto-drought cores was directly exposed to the air, i.e. all overlying water had evaporated and the sediment moisture content started to decline (days 15-31); (iii) "dry period", when the moisture content was considered as zero, i.e.…”

“…Subsequently, during the entire drying period, the magnitude of CO 2 and CH 4 emissions from the induced-to-drought cores remained substantially higher than the emissions observed in the permanently flooded cores (4 and 1000 times higher for CO 2 and CH 4 , respectively, after 15 days of drying) (Fig 1). When sediments are exposed to direct contact with the atmosphere, we may expect a series of changes ultimately affecting the CO 2 and CH 4 dynamics, as follows: (i) the now exposed sediment has direct contact with atmospheric oxygen stimulating the decomposition of the usually high amount of labile organic matter in the surface sediment [43] and resulting in substantial carbon losses to the atmosphere [21]; (ii) as drought persists, sediment may crack causing the penetration of oxygen into deeper layers, which triggers more organic matter mineralization [35]; (iii) the cracks formed by the sediment desiccation may also favor the release of gases, that were retained in lower parts of the sediment, to the atmosphere, especially CH 4 , which tends to form gas bubbles (not accounted here) in the sediment due to its low solubility [21,44]; (iv) the gas-exchange velocity between the sediment-air interface is much faster…”

“…Some studies describe that 24 h of wet conditions are enough to stimulate heterotrophic microbial activities in the surface of sediments, promoting shifts in the microbial communities and increasing their biomass [35,48]. Borken et al (2003) [37] and Kosten et al (2018) [21] showed that rewetting events on sediments induced to drought triggered instantaneous CO 2 emissions to the atmosphere, which remained increasing as the water content was also increasing. The same pattern was also observed in situ by Fromin et al (2010) [35], who reported peaks of CO 2 emissions after a rain event on dry sediments from a Mediterranean pond.…”

“…The incidence of extreme droughts decreases reservoir water levels, exposing large areas of previously submerged aquatic sediment to direct contact with the atmosphere. Exposed aquatic sediments have been reported as significant sources of greenhouse gases (GHG) to the atmosphere, especially carbon dioxide (CO 2 ) and methane (CH 4 ) [21][22][23][24]. However, when precipitation levels return to normal and the reservoir water level rises, these exposed sediments become submerged again.…”