Carbon Dioxide Emission and Soil Sequestration for the French Agro-Food System: Present and Prospective Scenarios

Julia Le Noë,Gilles Billen,Josette Garnier

doi:10.3389/fsufs.2019.00019

Abstract

France is a major agricultural power, characterized by a high degree of regional specialization, either in stockless cash crop farming, exporting most of its intensive cereal production, or in intensive livestock farming highly dependent on foreign feed imports. This agricultural model is characterized by wide nutrient and carbon cycle opening and severe environmental pollution. Based on the nutrient accounting GRAFS model, two contrasted scenarios for the French agricultural system at the 2050 horizon have recently been designed and evaluated for their capacity to meet both the national population's food demand and environmental standards in terms of water pollution. The first scenario (O/S, for opening and specialization) assumes the continuation of the current trends of intensification, specialization, and opening to international markets. The second one (A/R/D, for autonomy, reconnection, and demitarian diet) assumes a radical change toward organic farming with diversification of crop rotations, reconnection of crop and livestock farming, and reduction of the proportion of animal proteins in the human diet. Herein we calculate the budget of CO2 emissions and C sequestration in soils of these two scenarios compared with the current situation of the French agro-food system, by coupling the GRAFS and AMG models. These simulations reveal that the overall CO2 emissions balance of the O/S scenario is far higher than those of the A/R/D, namely because of the emissions associated with mineral fertilizer manufacture, and imported feed and mechanization of land management requiring a large amount of fossil fuel. As the organic carbon content of the soil is known to be highly path-dependent (in the sense that it is the inheritance of previous land use practices), we tested the effect of two rates of implementation of the two scenarios and evaluated the response time of the C soil store, which is of the order of two decades or more. This reveals that after about two-three decades following the implementation of a scenario, an equilibrium is reached with no more net soil C emission nor sequestration.

Highlights

The 4% initiative launched by France at the COP 21 in 2015 stressed the fact that carbon (C) sequestration in agricultural soil could be part of a mitigation strategy of greenhouse gas emissions
Both scenarios are able to feed the French population; the O/S scenario exports large amounts of cereals and other vegetal as well as animal products but depends on large imports of protein feed, while the A/R/D scenario is selfsufficient for animal products and feed, but can still export cereals to the international market at more than 40% the rate of current cereal exports. These prospective scenarios for French regional agro-food systems can be represented in terms of C fluxes with the nutrientaccounting GRAFS model (Billen et al, 2014; Le Noë et al, 2018, 2019) (Figure 1). It must be made clear from this figure that only the proportion of net primary production (NPP) that is neither exported nor respired by livestock and humans is brought to the soil as either crop residues or manure; of these only the fraction which is not mineralized can be considered as sequestered in the soil, in the end comprising a very tiny share of NPP
C concentrations in agricultural soils tend to increase due to a slow increase of humified C inputs, the reduction of agricultural areas, of grassland surfaces led agricultural soils to be net emitters of C over the 2004–2014 period (Figure 2; Table 2)

Summary

Introduction

The 4% initiative (http://www.4p1000.org/) launched by France at the COP 21 in 2015 stressed the fact that carbon (C) sequestration in agricultural soil could be part of a mitigation strategy of greenhouse gas emissions. The behavior of agricultural soils as a sink or source of C to the atmosphere results from the equilibrium between inputs and mineralization of humified C to the soil (Jenkinson and Rayner, 1977; Poulton et al, 2018). The former depends on the quantity and nature of fresh organic material inputs (Kong et al, 2005; Autret et al, 2016) and the latter on the pedo-climatic context (Kätterer et al, 1998; Stockmann et al, 2013). These conditions are themselves constrained by the agricultural practices such as fertilization and motorization of agriculture, the production patterns of food and feed, and the historical trajectories of agrofood systems, which determine the current C dynamic in soils (Aguilera et al, 2018; Le Noë et al, 2019)

Methods

Results

Conclusion