No abstract

Peatlands are an important global carbon reservoir. The continued accumulation of carbon in peatlands depends on the persistence of anoxic conditions, in part induced by water saturation, which prevents oxidation of organic matter, and slows down decomposition. Here we investigate how and over what time scales the hydrological regime impacts the geochemistry and the bacterial community structure of temperate peat soils. Peat cores from two sites having contrasting groundwater budgets were subjected to four controlled drought-rewetting cycles. Pore water geochemistry and metagenomic profiling of bacterial communities showed that frequent water table drawdown induced lower concentrations of dissolved carbon, higher concentrations of sulfate and iron and reduced bacterial richness and diversity in the peat soil and water. Short-term drought cycles (3–9 day frequency) resulted in different communities from continuously saturated environments. Furthermore, the site that has more frequently experienced water table drawdown during the last two decades presented the most striking shifts in bacterial community structure, altering biogeochemical functioning of peat soils. Our results suggest that the increase in frequency and duration of drought conditions under changing climatic conditions or water resource use can induce profound changes in bacterial communities, with potentially severe consequences for carbon storage in temperate peatlands.

processes. Peat soils have a significant impact on water quality, ecosystem productivity and 3 greenhouse gas emissions. However, the extent of peatlands is decreasing across the world, 4 mainly because of anthropogenic activities such as drainage for agriculture or groundwater 5 abstractions in underlying aquifers. Potential changes in precipitation and temperature in the 6 future are likely to apply additional pressure to wetland. In this context, a methodology for 7 assessing and comparing the respective impacts of groundwater abstraction and climate 8 change on a groundwater-fed wetland (135 km²) located in Northwest France, is presented. A 9 groundwater model was developed, using flexible boundary conditions to represent surface-10 subsurface interactions which allowed examination of the extent of the wetland areas. This 11 variable parameter is highly important for land management and is usually not considered in 12 impact studies. The model was coupled with recharge estimation, groundwater abstraction 13 scenarios, and climate change scenarios downscaled from 14 GCMs corresponding to the 14 A1B greenhouse gas (GHG) scenario over the periods 1961-2000 and 2081 -2100 show that climate change is expected to have an important impact and reduce the surface of 16 wetlands by 5.3 % to 13.6 %. In comparison, the impact of groundwater abstraction (100 % 17 increase in the expected scenarios) would lead to a maximum decrease of 3.7 %. Results also 18show that the impacts of climate change and groundwater abstraction could be partially 19 mitigated by decreasing or stopping land drainage in specific parts of the area. Water 20 management will require an appropriate compromise which encompasses ecosystem 21 preservation, economic and public domain activities. 22