Long-term changes in greenhouse gas emissions from French agriculture and livestock (1852-2014): From traditional agriculture to conventional intensive systems.

France was a traditionally agricultural country until the first half of the 20th century. Today, it is the first European cereal producer, with cereal crops accounting for 40% of the agricultural surface area used, and is also a major country for livestock breeding with 25% of the European cattle livestock. This major socioecological transition, with rapid intensification and specialisation in an open global market, has been accompanied by deep environmental changes. To explore the changes in agricultural GHG emissions over the long term (1852-2014), we analysed the emission factors of N2O from field experiments covering major land uses, in a gradient of fertilisation and within a range of temperature and rainfall, and used CH4 emission coefficients for livestock categories, in terms of enteric and manure management, considering the historical changes in animal excretion rates. We also estimated indirect CO2 emissions, rarely accounted for in agricultural emissions, using coefficients found in the literature for the dominant energy consumption items (fertiliser production, field work and machinery, and feed import). From GHG emissions of ~30,000 ktons CO2 Eq yr-1 in 1852, reaching 54,000 ktons CO2 Eq yr-1 in 1955, emissions more than doubled during the 'Glorious thirties' (1950-1980), and peaked around 120,000 ktons CO2 Eq yr-1 in the early 2000s. For the 2010-2014 period, French agriculture GHG emissions stabilised at ~114,000 ktons CO2 Eq yr-1, distributed into 49% methane (CH4), 22% carbon dioxide (CO2) and 29% nitrous oxide (N2O). A regional approach through 33 regions in France shows a diversity of agriculture reflecting the hydro-ecoregion distribution and the agricultural specialisation of local areas. Exploring contrasting scenarios at the 2040 horizon suggests that only deep changes in the structure of the agro-food system would double the reduction of GHG emissions by the agricultural sector.

Seasonal and spatial variability of the partial pressure of carbon dioxide in the human-impacted Seine River in France.

Carbon evasion from rivers is an important component of the global carbon cycle. The intensification of anthropogenic pressures on hydrosystems requires studies of human-impacted rivers to identify and quantify the main drivers of carbon evasion. In 2016 and 2017, four field campaigns were conducted in the Seine River network characterized by an intensively cropped and highly populated basin. We measured partial pressures of carbon dioxide (pCO2) in streams or rivers draining land under different uses at different seasons. We also computed pCO2 from an existing data set (pH, water temperature and total alkalinity) going back until 1970. Here we report factors controlling pCO2 that operate at different time and space scales. In our study, the Seine River was shown to be supersaturated in CO2 with respect to the atmospheric equilibrium, as well as a source of CO2. Our results suggest an increase in pCO2 from winter to summer in small streams draining forests (from 1670 to 2480 ppm), croplands (from 1010 to 1550 ppm), and at the outlet of the basin (from 2490 to 3630 ppm). The main driver of pCO2 was shown to be dissolved organic carbon (DOC) concentrations (R2 = 0.56, n = 119, p < 0.05) that are modulated by hydro-climatic conditions and groundwater discharges. DOC sources were linked to land use and soil, mainly leaching into small upstream streams, but also to organic pollution, mainly found downstream in larger rivers. Our long-term analysis of the main stream suggests that pCO2 closely mirrors the pattern of urban water pollution over time. These results suggest that factors controlling pCO2 operate differently upstream and downstream depending on the physical characteristics of the river basin and on the intensity and location of the main anthropogenic pressures. The influence of these controlling factors may also differ over time, according to the seasons, and mirror long term changes in these anthropogenic pressures.

Carbon dioxide, methane and nitrous oxide emissions from the human-impacted Seine watershed in France.

Greenhouse gas (GHG) emissions from rivers and lakes have been shown to contribute significantly to global carbon and nitrogen cycling. In temperate and human-impacted regions, simultaneous carbon dioxide, methane and nitrous oxide emissions from aquatic systems are poorly documented. We estimated carbon dioxide (CO2) concentrations in the Seine hydrosystem (71,730 km2, France) using direct measurements, and calculations of CO2 partial pressures from 14 field campaigns conducted between 2010 and 2017, and compared them to methane (CH4) and nitrous oxide (N2O) concentrations. In the main stem of the Seine River, CO2 showed the same spatial gradient as N2O and CH4 with peaks in concentration downstream from the arrival of effluents from wastewater treatment plants enriched in organic matter, thus favoring mineralization. It is likely that high CO2 concentrations upstream were due to organic carbon inputs from soils and enriched CO2 groundwater discharges, whereas high N2O and CH4 upstream values were likely due to denitrification in riparian wet areas and anoxic decomposition of organic matter-rich wetlands, respectively. In addition, seasonal variations in all three GHGs were observed with higher concentrations in summer when higher temperatures promote mineralization and low water reduces the dilution of organic matter mainly originating from WWTP effluents. GHG emissions were calculated and compared with agricultural and nonagricultural (urban, transport) fluxes in the basin. In the Seine River network, CO2 emissions dominated riverine GHG emissions, reaching 95.3%, while N2O and CH4 emissions accounted for 4.4% and 0.3%, respectively. These indirect emissions from the hydrosystem were estimated to account for 3.7% of the total GHG emissions from the basin that amounted to 61,284 Gg CO2eq yr-1. Comparatively, direct agricultural and nonagricultural GHG emissions were estimated at 23.3% and 73.0%., respectively.