No abstract

Nitrous oxide contributes to the global greenhouse effect and affects the chemistry of O 3 in the upper troposphere and lower stratosphere. To define a relevant model for microbial NO 3 -and N 2 O reductions in soil and estimate the parameters involved, we propose a method combining measurements of anaerobic soil slurry and simulations of NO 3 -and N 2 O reductions, including non-enzymatic competition between NO 3 -and N 2 O as electron acceptors and the microbial dynamics of two denitrifier groups that are either able or unable to reduce N 2 O. Three models varying in the description of soil capability to reduce N 2 O through denitrification were assessed. The procedure was applied on an arable soil known for its small N 2 O emissions in situ during wet events.

Estuaries are important nutrient filters between rivers and coastal zones. However, the quantification of the nutrient mitigation capacity related to benthic diagenesis is still poorly quantified. In this paper, we investigated carbon mineralization and the contribution of benthic nitrate reduction in two macrotidal eutrophic estuaries (Elorn and Aulne, Brittany, France) during winter and spring. These anthropized estuaries exhibited large variations of bottom water nitrate concentrations from very high values in upstream waters (up to 500 µM) to low values downstream (less than 10 µM). Bottom water oxygen concentrations presented small gradients compared to nitrate concentrations gradients resulting in large variation in nitrate to oxygen ratios NO x-/O 2 between downstream and upstream (0.03-1.6). We combined the use of diagenesis modeling with field data (porosity, organic carbon and nitrogen, pore water profiles of dissolved oxygen, nitrogen, iron, manganese and sulfide concentrations, published in Khalil et al. 2013) and experimentally-determined nitrate reduction rates, in order to investigate the different organic carbon mineralization pathways in these estuarine sediments including denitrification and the contribution of benthic nitrate reduction to the estuarine N budget. Overall a good agreement between pore water data (organic carbon, oxygen, nitrate and ammonium) and model simulations was observed. The modeled organic matter mineralization rates were high in the upstream estuary and low in the saline estuary for the two estuaries Aulne and Elorn. This decrease may be related to the dilution and the trapping of allochtonous organic matter in estuarine sediments and its subsequent recycling in the upper estuary. Organic carbon mineralization rates were higher in the Elorn than in the Aulne estuary, which is most likely related to the labile character of the organic matter from urban origin exported from the Elorn watershed. The contribution of nitrate reduction to the total mineralization was generally high in upstream sediments (15-35%) of both Aulne and Elorn estuaries and decreased consistently downstream to 5-10%. The relative large contribution of nitrate reduction to organic matter degradation was to a large extent related to high bottom water nitrate concentrations that fueled 37-78% of total nitrate reduction in the upstream part of the estuaries.

No abstract

The Mediterranean Sea (MS) is a large oligotrophic sea whose productivity is sensitive to riverine nutrient inputs. More specifically, phosphorus (P) river supply is crucial for the MS, with an important role of the estuarine/deltaic filter especially for the storage and recycling in sediments. A benthic dataset from the Rhône River prodelta was used to derive P budgets, by means of an early diagenetic model including the benthic P cycle. The model was fitted to pore water profiles of oxygen, nitrate, sulfate, dissolved inorganic carbon, ammonium, oxygen demand units, dissolved inorganic phosphorus (DIP) and solid data (organic carbon (OC), Fe-bound P, Ca-bound P and organic P). Results indicated that the intensity of biogeochemical processes occurring below the sediment-water interface decreased from the river mouth to the adjacent continental shelf with decreasing integrated rates of OC mineralization (160-10 mmol m−2 day−1). The organic P mineralization was intense near the river mouth and decreased offshore (1196-80 μmol m−2 day−1). Its contribution to DIP release was large (> 90%). Fe-bound P had a key role in transferring P to deeper layers. These deltaic sediments played an important role as a source of regenerated DIP. A significant part of DIP was recycled to the overlying waters (72-94%), representing 25% of the riverine DIP discharge. Simultaneously, 6-28% of DIP produced in sediments was buried as Ca-bound P. Overall, this study highlighted the importance of deltaic sediments as an additional source of DIP to the coastal sea, and a minor but permanent sink of phosphorus as solid P burial.