Carbon stocks in submerged soils of a hydro-electric reservoir after 24 years of flooding

Global population growth and economic growth lead to increasing energy demand. This propels the construction of river dams and artificial reservoirs to produce hydropower. Ecological effects of water impoundments, such as the fragmentation of free-flowing rivers, habitat changes, loss of habitats and biodiversity, and changes in biogeochemical cycles have been addressed by researchers for several decades. Also, the influence of flooding on the soils within reservoirs and shore erosion have been studied in a variety of environments and soil types. The development of soil carbon stocks in the submerged soils and soil carbon mineralization upon flooding are of particular interest. Some studies observe a significant decrease of carbon stocks in submerged soil, whereas others report the opposite. Here, we present a study on the influence of 24 years of water impoundment on properties of organic and mineral constituents in submerged Andosols of the Blöndulón hydro-electric reservoir in the Icelandic highlands. Drowned soils are relatively enriched in carbon content, carbon densities and carbon stocks compared to the reference soils, while they are depleted in pedogenic minerals ferrihydrite and allophane. Depth patterns of carbon are rather uniform in the drowned soils in contrast to declining trends in the reference soils. Likely, movement of organic material from upper to lower horizons, and carbon additions from decaying vegetation in the years after the reservoir impoundment explain the carbon enrichment and altered depth distribution. While the drowned soils are enriched in carbon after a comparatively short inundation time of less than three decades, the stability of the soils carbon is uncertain. The apparent loss of mineral soil colloids will likely render the carbon more sensitive to oxidation in the coming decades, particularly during times of exposure of the inundated soils. Assessments of the consequences of water level fluctuations or potential future dam removal need to take the vulnerability of the exposed soils into account.