Soils Drowned in Water Impoundments: A New Frontier

Abstract

Water impoundments have major impacts on biogeochemical cycles at the local and global scales. However, although reservoirs flood soils, their biogeochemical evolution below water and its ecological consequences are very poorly documented. We took advantage of the complete emptying of the Guerlédan Reservoir (Brittany, France) to compare the composition of soils flooded for 84 years with that of adjacent non-flooded soils used as reference, in 3 situations contrasted by their soil type (Cambisol and Podzol) and initial land-use (forest or grassland). In the annual drawdown zone, upper horizons of submerged soils are eroded, especially near the upper shore and on slopes. In the permanently drowned area, silty sediments cover drowned soils. Compared to reference soils, forest soils drowned for 84 years maintain their original morphological differentiation, but colors are dull, and the humus (O horizons) have virtually disappeared. Spodic horizons are depleted in poorly crystallized iron minerals while the accumulation of amorphous aluminum compounds remains unchanged. Soil bulk density increases as well as pH while total phosphorus content is almost unchanged. On the other hand, the pH of drowned grassland soils is lower by almost one unit, and the total phosphorus content was halved compared to reference soils. In this context, in addition to the effects of flooding, differences are attributed to post-1950 changes in agricultural practices i.e., liming and fertilization. Organic matter stocks decrease by almost 40%. This rate is similar in Cambisols and Podzols. Assuming that carbon was lost as CO 2 and CH 4 , the corresponding flux averaged over the reservoir's life is close to global areal estimates of CO 2 emissions in temperate reservoirs and offsets a significant proportion of the carbon burial in reservoir sediments. Hence, flooded soils contribute significantly to the GHG budget of reservoirs, provide original long-term experimental sites to measure the effects of anoxia on soils and contain archives of past soil properties.

Understanding How Reservoir Operations Influence Methane Emissions: A Conceptual Model

“…Erosion and focusing of sediments in deeper areas of the reservoir is another mechanism by which sediment dynamics mediate the relationship between WLF and methane emissions. Drawdown zones of tropical water impoundments lost nearly 40% of organic soils over ~80 years due to erosion [110]. Worldwide, reservoirs are accumulating sediment at a rate of about 1% each year [111].…”

“…By concentrating sediments, WLF may create hotspots of methane emissions via ebullition [110]. However, this depends on whether the deposited sediments have sufficiently high organic content at a shallow depth and are deposited at a sufficiently slow rate.…”

Soil Bacterial and Archaeal Communities of the Periodic Flooding Zone of Three Main Reservoirs in the South Ural Region (Russia)

“…At the same time, the ground cover inside the area covered by the reservoir bed during the building process is not removed and stays under the water seasonally at levels between the normal and dead volume levels or permanently, during the entire operating period, Ecologies 2024, 5 234 at levels lower than the dead volume. In flooded water reservoir areas, while retaining the morphological profile of the soil under the reservoir drawdown area [2], silt sediment accumulation [3], and partial or total destruction of the soil profile in the littoral zone (at the development sites of wave extraction activities), specific soil hydromorphic processes are developing.…”

“…The soils drowned in water reservoirs are influenced by various processes affecting soil qualitative characteristics. Continuous [2] or seasonal [4] flooding adds to anoxic soil conditions. By accumulating sediments made up of native materials, particularly aquatic creature remnants [5], agricultural soil erosion products [6,7], and soil erosion products from banks and reservoir beds [8], new soil horizons are created.…”

Effects of hydropower management on the sediment composition and metabolism of a small Alpine lake

“…However, there is some confusion about the effects of impoundment and water level fluctuations, which commonly occur in reservoirs. Water level fluctuations may lead to soil and sediment erosion (Félix-Faure et al, 2019a, 2019b, littoral and benthic habitats degradation (Furey et al, 2004;Milbrink et al, 2011), sediment transfer (Blais & Kalff, 1995) toward the lake centre, and changes in lake metabolism (Houel et al, 2006) which also impacts the lake's trophic status (Milbrink et al, 2011;Hirsch et al, 2017).…”