CAN FLOOD

River ecosystems play a key role in the transport and transformation of carbon and nutrients. While material is being transported downstream, organic matter is produced and degraded. This matter carries the fingerprint of human activities along its entire course. Microbial community features such as composition and activity are the major biotic component in all processes, especially in nitrogen cycling. Within the riverine landscape these processes are strongly associated with the availability of retention zones such as floodplain, riparian and instream zones. The processes related to nitrogen and organic matter cycling are basically controlled by the hydromorphology. Thus, at the landscape scale, three fundamental principles regulate the cycling and transfer of carbon and nitrogen in river ecosystems: i) The mode of carbon and nitrogen delivery affects ecosystem functioning - connectivity patterns; ii) Increasing contact between water and soil or sediment increases nitrogen retention and processing - geomorphology; iii) Floods and droughts are natural events that strongly influence pathways of carbon and nitrogen cycling. These three principles can be strongly affected either by natural disturbances or anthropogenic impacts, which involve altered water regimes or a change in the geomorphologic setting of the river valley. An altered natural water regime will affect the biogeochemistry of riparian and instream zones as well as their ability to cycle and mitigate nutrient fluxes originating from upstream and upslope. This calls for a more integrated approach including restoration of landscape dynamics and key ecosystem processes such as carbon and nutrient retention. In this context the objectives of our project are i) to understand the consequences of changes in flow regimes on the functioning of river ecosystems and, more specifically, on their nitrogen cycling capacity, and ii) to accurately estimate the rates of these biogeochemical processes under hydrological changes. The following 3 hypotheses based on the 3 above-mentioned fundamental principles are the methodological approach to investigate the regulation of nitrogen and carbon cycling and transfer at the sediment/water interface in retention areas of river ecosystems: H1: The hydromorphic structures of retention areas affect the nitrogen cycling: High surface water connectivity levels and high sediment to water ratios in retention areas increase potential denitrification rates and the N 2 /N2 0 ratio. H2: The mode of organic carbon supply to retention areas controls denitrification potential because carbon availability directly affects microbial nitrogen processing at the sediment surface H3: Past water regime patterns control the resistance and the resilience of the nutrient cycling processes to restoration and rehabilitation measures because they have shaped the current geomorphological setting of retention areas at the habitat and at the reach scale.

River ecosystems play a key role in the transport and transformation of carbon and nutrients. While material is being transported downstream, organic matter is produced and degraded. This matter carries the fingerprint of human activities along its entire course. Microbial community features such as composition and activity are the major biotic component in all processes, especially in nitrogen cycling. Within the riverine landscape these processes are strongly associated with the availability of retention zones such as floodplain, riparian and instream zones. The processes related to nitrogen and organic matter cycling are basically controlled by the hydromorphology. Thus, at the landscape scale, three fundamental principles regulate the cycling and transfer of carbon and nitrogen in river ecosystems: i) The mode of carbon and nitrogen delivery affects ecosystem functioning - connectivity patterns; ii) Increasing contact between water and soil or sediment increases nitrogen retention and processing - geomorphology; iii) Floods and droughts are natural events that strongly influence pathways of carbon and nitrogen cycling. These three principles can be strongly affected either by natural disturbances or anthropogenic impacts, which involve altered water regimes or a change in the geomorphologic setting of the river valley. An altered natural water regime will affect the biogeochemistry of riparian and instream zones as well as their ability to cycle and mitigate nutrient fluxes originating from upstream and upslope. This calls for a more integrated approach including restoration of landscape dynamics and key ecosystem processes such as carbon and nutrient retention. In this context the objectives of our project are i) to understand the consequences of changes in flow regimes on the functioning of river ecosystems and, more specifically, on their nitrogen cycling capacity, and ii) to accurately estimate the rates of these biogeochemical processes under hydrological changes. The following 3 hypotheses based on the 3 above-mentioned fundamental principles are the methodological approach to investigate the regulation of nitrogen and carbon cycling and transfer at the sediment/water interface in retention areas of river ecosystems: H1: The hydromorphic structures of retention areas affect the nitrogen cycling: High surface water connectivity levels and high sediment to water ratios in retention areas increase potential denitrification rates and the N 2 /N2 0 ratio. H2: The mode of organic carbon supply to retention areas controls denitrification potential because carbon availability directly affects microbial nitrogen processing at the sediment surface H3: Past water regime patterns control the resistance and the resilience of the nutrient cycling processes to restoration and rehabilitation measures because they have shaped the current geomorphological setting of retention areas at the habitat and at the reach scale.