Agriculture specialization influence on nutrient use efficiency and fluxes in the St. Lawrence Basin over the 20th century.

Increasing the overall use efficiency of nitrogen (N) and phosphorus (P) resources in food production while minimizing losses to the environment are required to meet the dual challenge of food security and sustainability. Yet studies quantifying the overall performance of different agro-system types and how these have changed over time remain rare, although they are essential to propose solution pathways. Here, we reconstructed fluxes of N and P within 78 watersheds of the St. Lawrence Basin (SLB) of eastern Canada between 1901 and 2011, using the Generalized Representation of Agro-Food System model (GRAFS). This analysis allowed us to classify different agro-food system types and to evaluate how agricultural specialization influenced nutrient efficiencies and potential losses to the environment over time. Using a cluster analysis, we identified four agro-food system types with different overall outcomes in efficiencies and losses. We show that agricultural practices in the SLB were similar until the 1950's and deemed unsustainable in several watersheds by depleting agricultural soils of their nutrients (particularly N). With the advent of manufactured fertilizers and the intensification of livestock farming, the SLB then rapidly shifted through the 1970s and 1980s to more intensified and highly unsustainable agro-food system types, where, in 2011, ~77 % of N and ~ 94 % of P inputs were lost to the environment. We also show that nutrient pollution continued to increase despite gains in the nutrient use efficiency of animal farming due to higher nutrient throughput from intensive production. The increased proportion of confined animals, disconnected from croplands, indeed resulted in inefficient nutrient recycling. While nutrient use efficiency may mitigate nutrient pollution, reducing the absolute nutrient flux through agro-food systems should be a priority, likely through a reconnection of crop and animal farming and an overall reduction of meat production, specifically from concentrated, intensive livestock systems.

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).

A long-term view of nutrient transfers through the Seine river continuum.

A model (the Riverstrahler model) is used to describe nutrient transfer and transformation at the scale of the whole drainage network of the Seine based on information concerning the basic mechanisms governing N, P and Si inputs to the drainage network and in-stream transformation and retention. It was used to calculate the budget of these nutrients through the whole river continuum from land to sea. With the help of historical documents, the constraints used as forcing function in this model were reconstructed to express the changing conditions of land-use and urban population over the last five hundred years. The corresponding scenarios were run for different hydrological regimes including dry, mean and wet conditions. The results were validated on the long-term series of nutrient measurements spanning more than a century available at some stations on the Seine, upstream and downstream of the city of Paris. The model was also used to explore past and future trends in nutrient loading, retention and delivery to the coastal zone, in response to human management of the terrestrial watershed. Beside the initial pristine state, used as an idealized reference state (with N, P and Si delivered fluxes of about 45-110 kg N km(-2) yr(-1), 2-5 kg P km(-2) yr(-1), 510-1325 kg Si km(-2) yr(-1)), four periods were distinguished. The first one is that of the traditional cottage economy which prevailed, with quite a constant total population, until the end of the 18th century. N, P and Si fluxes were about 235-750 kg N km(-2) yr(-1), 15-60 kg P km(-2) yr(-1) and 425-1280 kg Si km(-2) yr(-1), depending on hydrological conditions. The second period, from the beginning of the 19th century to about the 1950's, corresponded to rapid increase in the total and urban population with a corresponding increase of point sources of N and P. From 1950 onwards, modern farming practices resulted in a dramatic increase in diffuse sources of nitrogen and to a lesser extent phosphorus: riverine N and P export reached 1320-2800 kg N km(-2) yr(-1), and 310-340 kg P km(-2) yr(-1): silica export remained fairly constant at around 410-1260 kg Si km(-2) yr(-1) depending on the hydrological conditions. In the 1990's, the fourth period is represented by a stabilized population and improved wastewater treatment, when the export of phosphorus is reduced to values as low as 40-60 kg P km(-2) yr(-1), but without as effective a reduction of nitrogen export. This represents an unprecedented situation for the marine coastal system, i.e. a shift from nitrogen to phosphorus limitation, as nitrogen is still delivered far in excess of the amount of silica available for diatom blooms.