Soils Drowned in Water Impoundments: A New Frontier

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 CO2 and CH4, the corresponding flux averaged over the reservoir's life is close to global areal estimates of CO2 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

Les temps de résidence du carbone et le potentiel de stockage de carbone dans quelques sols cultivés français

International audienceAfin de prévoir la capacité des sols à séquestrer du carbone, en réponse aux changements climatiques ou aux changements d’utilisation, nous avons estimé les flux et la distribution des temps de résidence du carbone dans les sols cultivés français. Nous avons utilisé les abondances naturelles en isotopes 13C et 14C pour mesurer cette distribution dans des expérimentations agricoles en maïs de longue durée en France. Les trois quarts du carbone du sol y ont un temps moyen de résidence de 40 ans. Les fractions granulométriques grossières contiennent la majeure partie du carbone à durée de vie plus courte. La datation au carbone 14 des matières organiques permet d’introduire un compartiment de carbone sable. Les restitutions végétales souterraines stockent autant de carbone que les restitutions aériennes. L’effet de la température sur la minéralisation du carbone du sol se traduit uniquement par une modification des constantes de vitesses, avec un Q10 de 3.1 constant dansl’intervalle 1–25°C. Ces données, synthétisées dans un modèle simple, permettent de prévoir un effet faible ou nul des changements climatiques globaux sur le stockage net de carbone dans les sols. Le mode d’occupation du sol aura en France une influence sur la séquestration de carbone plus importante que celle des changements atmosphériques.In order to predict the potential of soils to store carbon in response to land use or climate changes, we measured the fluxes and distribution of residence times of C in French cultivated soils. We used the natural abundances in 13C and 14C to measure this distribution in long-term experiments of maize cultivation in France. 75% of the topsoil carbon had a mean residence time of 40 yr. Coarse particle-size fractions contained most of the younger carbon. A compartment of stable C was estimated using radiocarbon dating. Belowground plant material inputs stored as much as C as aboveground inputs. The effect of temperature on soil carbon mineralization affected only rate constants, with a Q10 = 3.1 constant in the range 1–25°C. The data were summerized in a simple simulation model, which predicted a nil or low effect of climatic change on soil carbon storage in the next50 yr. In France, land use changes will have more influence than atmospheric changes on C storage

Sols et service de régulation du climat

National audienc

Cycle du carbone dans l'écosystème forestier : impact du milieu et de la sylviculture sur les immobilisations et les flux

National audienc

Approaching "functional" soil organic matter pools through particle-size fractionation: examples for tropical soils

International audienc

Dynamics of soil carbon and moder horizons related to age in pine and beech stands

International audienc

Dynamics of soil carbon and moder horizons related to age in pine and beech stands

International audienc

Dynamique des matières organiques particulaires en fonction de leur localisation dans la structure du sol. Exemple d'une chronoséquence de mise en culture

National audienc

The age distribution of carbon in world soil profiles and its drivers revealed by C stable isotopes

International audienc