Gilles Billen

Nitrogen fluxes from the landscape are controlled by net anthropogenic nitrogen inputs and by climate

The flux of nitrogen (N) to coastal marine ecosystems is strongly correlated with the “net anthropogenic nitrogen inputs” (NANI) to the landscape across 154 watersheds, ranging in size from 16 km2 to 279 000 km2, in the US and Europe. When NANI values are greater than 1070 kg N km−2 yr−1, an average of 25% of the NANI is exported from those watersheds in rivers. Our analysis suggests a possible threshold at lower NANI levels, with a smaller fraction exported when NANI values are below 1070 kg N km−2 yr−1. Synthetic fertilizer is the largest component of NANI in many watersheds, but other inputs also contribute substantially to the N fluxes; in some regions, atmospheric deposition of N is the major component. The flux of N to coastal areas is controlled in part by climate, and a higher percentage of NANI is exported in rivers, from watersheds that have higher freshwater discharge.

Mortality rates of autochthonous and fecal bacteria in natural aquatic ecosystems.

Bacterial mortality has been investigated in freshwater (River Seine) and in marine (North Sea) systems using a method based on following the disappearance of radioactivity from the DNA of assemblages of bacteria previously labeled with tritiated thymidine. Measurement of bacterial mortality of autochthonous and various types of fecal bacteria allowed direct comparisons between their respective first-order mortality rates. Mortality rates obtained for the different types of bacteria in the River Seine were, respectively, 7.9-33.9 x 10(-3)h(-1) for Escherichia coli, 12.2-29.2 x 10(-3)h(-1) for S. faecium and 7.0-18.3 x 10(-3)h(-1) for the autochthonous bacteria. In the Belgian coastal waters, these rates were 4.6-27.3 x 10(-3)h(-1) for E. coli, 6.0-22.0 x 10(-3)h(-1) for S. typhimurium, 10.0-18.9 x 10(-3)h(-1) for S. faecium and 1.0-13.9 x 10(-3)h(-1) for autochthonous bacteria. In both environments, the overall mortality rates of autochthonous and the different fecal bacteria were in the same order of magnitude and overall mortality rates of E. coli were on average about twice as high for autochthonous bacteria. Grazing by protozooplankton was the dominant process of mortality for fecal and autochthonous bacteria in both environments. Except in a few situations, grazing by protozooplankton was responsible for more than 90% of the overall mortality rate of fecal and autochthonous bacteria in the river and in the coastal area.

Distribution of nitrifying activity in the Seine River (France) from Paris to the estuary

The distribution of nitrification has been measured with the H14CO3− incorporation method in the Seine River and its estuary during summer conditions. The Seine River below Paris receives large amounts of ammonium through wastewater discharge. In the river itself, this ammonium is only slowly nitrified, while in the estuary nitrification is rapid and complete. We show that this contrasting behavior is related to the different hydrosedimentary conditions of the two systems, as nitrifying bacteria are associated with suspended particles. In the river, particles and their attached bacteria either rapidly settle or have a sestonic behavior. Because of the short residence times of the water masses, the slow growing nitrifying population has no time to develop sufficiently to nitrify the available ammonium. The estuary is characterized by strong tidal dynamics. Particles settle and are resuspended continuously with the strong current inversions of ebb and flood. As a result of these dynamics, particles and their attached nitrifying bacteria experience longer residence times in a temporary suspended state than the water masses themselves, providing to slow growing nitrifying bacteria the opportunity to develop a large population capable of nitrifying all the available ammonium.

How the structure of agro-food systems shapes nitrogen, phosphorus, and carbon fluxes: The generalized representation of agro-food system applied at the regional scale in France

The aim of the study was to develop a conceptual framework to analyze the agro-food system of French agricultural regions from the angle of N, P and C circulation through five major compartments (cropland, grassland, livestock biomass, local population and potential environmental losses). To reach that goal we extended the Generalized Representation of Agro-Food System approach to P and C and applied it to French regions. Using this methodology we analyzed the relation between production pattern and N surplus, P budget, and efficient organic carbon inputs (OCeff), assuming these three indicators to be good proxies for (i) N losses to waterbodies and the atmosphere, (ii) P accumulation or depletion in soils, and (iii) potential additional C sequestration in soils, respectively. A typology was then established, allowing for comparison between five types of agricultural systems. This made it possible to highlight that intensive specialized agricultural systems generate high environmental losses and resource consumption per unit of agricultural surface and present a very open nutrient cycle due to substantial trade flows. Conversely, mixed crop and livestock farming and extensive cropping systems had more limited N and P consumption and led to lower potential water and air contamination. However, this trend was reversed when expressing resource consumption and N and P budget on a pro rata basis of vegetal and animal product unit, reflecting the better nutrient use efficiency of specialized regions in their respective field of specialization. This study demonstrates the systemic impact of production patterns on environmental and agronomic performances at the regional scale.

Impact of hydro-sedimentary processes on the dynamics of soluble reactive phosphorus in the Seine River

This paper focuses on soluble reactive phosphorus (SRP) dynamics along a 225xa0km stretch of the Seine River, including the Paris urban area, for the 2007–2011 period. The impact of hydro-sedimentary processes on SRP concentrations and fluxes is estimated under various hydrological conditions. Sorption interaction parameters between SRP and suspended matter are experimentally determined on river water samples and are included in a hydro-ecological model. Simulated concentrations are compared to weekly measurements at 11 monitoring stations. The introduction of sorption in the model reduces the root mean square error of simulated SRP concentrations by 20xa0% and allows the simulation of particulate inorganic P (PIP) accumulation in the system. With these ameliorations, the model constitutes a reliable management tool, which is compatible with the requirements of new regulations as the European Water Framework Directive. P mass balances are assessed upstream and downstream the major waste water treatment plant of the Paris urban area. P fluxes in the system are mainly driven by hydrological conditions and sediment-related processes. While SRP is the predominant P form during low flow, PIP accounts for more than 70xa0% of the total P during high flow. Moreover, SRP sorption fluxes are of the same order of magnitude as biotic fluxes affecting SRP concentrations. According to the model, and based on all the available data, 75xa0% of the SRP release by the river bed sediments occurs during high flow periods, and PIP exchanges at the sediment–water interface are more than 4 times higher during high flow periods than during low flow periods.

Reducing marine eutrophication may require a paradigmatic change

Marine eutrophication in the North-East Atlantic (NEA) strongly relies on nutrient enrichment at the river outlets, which is linked to human activities and land use in the watersheds. The question is whether human society can reduce its nutrient emissions by changing land use without compromising food security. A new version of Riverstrahler model (pyNuts-Riverstrahler) was designed to estimate the point and diffuse nutrient emissions (N, P, Si) to the rivers depending on land use in the watersheds across a large domain (Western Europe agro-food systems, waste water treatment). The loads from the river model have been used as inputs to three marine ecological models (PCOMS, ECO-MARS3D, MIRO&CO) covering together a large part of the NEA from the Iberian shelf to the Southern North Sea. The modelling of the land-ocean continuum allowed quantifying the impact of changes in land use on marine eutrophication. Pristine conditions were tested to scale the current eutrophication with respect to a natural background (sensu WFD), i.e. forested watersheds without any anthropogenic impact. Three scenarios representing potential management options were also tested to propose future perspectives in mitigating eutrophication. This study shows that a significant decrease in nitrogen fluxes from land to sea is possible by adapting human activities in the watersheds, preventing part of the eutrophication symptoms in the NEA rivers and adjacent coastal zones. It is also shown that any significant achievement in that direction would very likely require paradigmatic changes at social, economic and agricultural levels. This requires reshaping the connections between crop production and livestock farming, and between agriculture and local human food consumption. It also involves cultural changes such as less waste production and a shift towards lower-impact and healthier diets where half of the animal products consumption is replaced by vegetal proteins consumption, known as a demitarian diet (http://www.nine-esf.org/node/281/index.html).

Phosphorus management in cropping systems of the Paris Basin: From farm to regional scale

The sustainability of phosphorus (P) fertilization in cropping systems is an important issue because P resources on earth are limited and excess P in soils can lead to ecological damage such as eutrophication. Worldwide, there is an increasing interest in organic farming (OF) due to its good environmental performance. However, organic cropping systems are suspected of generating negative P budgets, which questions their ability to provide sustainable P management. The design of agricultural systems at a broader scale also largely influences the shape of the P cycle and the possibility of its recycling to cropland. In this context, the aim of this study was to assess the relative influence of (i) OF versus conventional farming (CF) practices and (ii) the structure of agro-food systems at the regional scale, on P cycling and availability on cropland. For this purpose, we examined P budgets and soil P status of 14 organic and conventional cropping systems in commercial farms located in the Paris Basin. Available P was analyzed using two different methods: resin P and Olsen P. The results revealed no significant differences between CF and OF in available P stocks. Phosphorus budgets were always negative and significantly lower in CF systems, indicating that P was mined from soil reserves. In parallel, we estimated P budgets over cropland in all French regions for two distinct periods, 2004-2014 and 1970-1981, and showed that specialized intensive cropping systems in the Paris Basin led to a high, positive P budget in the latter period. However, this trend was reversed in the 2004-2014 period due to a sharp reduction of the mineral fertilizer application rate. The shift from very high P budgets to much lower and sometimes negative P budgets would not be a threat for agriculture due to the current high level of Olsen P in these regions, which was consistent with our measurements at the plot scale. Overall, these results suggest that OF would not lead to more P deficiency than CF. Instead, they emphasize that sustainable P management not only depends on farmers choices but mainly on the structure and specialization of agro-food systems.

Organic carbon transfers in the subtropical Red River system (Viet Nam): insights on CO2 sources and sinks

The Red River, draining a 169,000xa0km2 watershed, is the second largest river in Viet Nam and constitutes the main source of water for a large percentage of the population of North Viet Nam. Here we present the results of an investigation into the spatial distribution and temporal dynamics of particulate and dissolved organic carbon (POC and DOC, respectively) in the Red River Basin. POC concentrations ranged from 0.24 to 5.80xa0mgxa0Cxa0L−1 and DOC concentrations ranged from 0.26 to 5.39xa0mgxa0Cxa0L−1. The application of the Seneque/Riverstrahler model to monthly POC and DOC measurements showed that, in general, the model simulations of the temporal variations and spatial distribution of organic carbon (OC) concentration followed the observed trends. They also show the impact of high population densities (up to 994xa0inhabxa0km−2 in the delta area) on OC inputs in surface runoff from the different land use classes and from urban point sources. A budget of the main fluxes of OC in the whole river network, including diffuse inputs from soil leaching and runoff and point sources from urban centers, as well as algal net primary production and heterotrophic respiration was established using the model results. It shows the predominantly heterotrophic character of the river system and provides an estimate of CO2 emissions from the river of 330xa0Ggxa0Cxa0year−1. This value is in reasonable agreement with the few available direct measurements of CO2 fluxes in the downstream part of the river network.

Nitrate retention at the river–watershed interface: a new conceptual modeling approach

Denitrification in riparian wetlands plays a major role in eliminating nitrate coming from agricultural watershed uplands before they reach river water. A new approach was developed for representing this process in the biogeochemical Riverstrahler model, using a single adjustable parameter representing the potential denitrification rate of wetland soils. Applied to the case of three watersheds with contrasting size, land-use and hydro-climatic regime, namely the Seine and the Loir rivers (France) and the Red River (Vietnam), this new model is able to capture the general level of nitrate concentrations as well as their seasonal variations everywhere over the drainage network. The nitrogen budgets calculated from the results show that riparian denitrification eliminates between 10 and 50% of the diffuse sources of nitrogen into the hydrosystem coming from soil nitrate leaching.

Nutrient inputs and hydrology together determine biogeochemical status of the Loire River (France): Current situation and possible future scenarios

The Grafs-Seneque/Riverstrahler model was implemented for the first time on the Loire River for the 2002-2014 period, to explore eutrophication after improvement of wastewater treatments. The model reproduced the interannual levels and seasonal trends of the major water quality variables. Although eutrophication has been impressively reduced in the drainage network, a eutrophication risk still exists at the coast, as shown by the N-ICEP indicator, pointing out an excess of nitrogen over silica and phosphorus. From maximum biomass exceeding 120u202fμgChlau202fl-1 in the 1980s, we observed decreasing maximum values from 80 to 30u202fμgChlau202fl-1 during the period studied. Several scenarios were explored. Regarding nutrient point sources, a low wastewater treatment scenario, similar to the situation in the 1980s, was elaborated, representing much greater pollution than the reference period (2002-2014). For diffuse sources, two agricultural scenarios were elaborated for reducing nitrogen, one with a strict application of the agricultural directives and another investigating the impact of radical structural changes in agriculture and the populations diet. Although reduced, a risk of eutrophication would remain, even with the most drastic scenario. In addition, a pristine scenario, with no human activity within the basin, was devised to assess water quality in a natural state. The impact of a change in hydrology on the Loire biogeochemical functioning was also explored according to the effect of climate change by the end of the 21st century. The EROS hydrological model was used to force Riverstrahler, considering the most pessimistic SRES A2 scenario run with the ARPEGE model. Nutrient fluxes all decreased due to a >50% reduction in the average annual discharge, overall reducing the risk of coastal eutrophication, but worsening the water quality status of the river network. The Riverstrahler model could be useful to help water managers contend with future threats in the Loire River, at the scale of its basin and at smaller nested scales.