Carbon footprint of cropping systems with grain legumes and cover crops: A case-study in SW France

Abstract

Agriculture contributes to a significant proportion of global emissions of greenhouse gases (GHG) but can also participate in climate change mitigation. The introduction of legumes in crop rotations reduces the dependence on N fertilizers and may mitigate the carbon (C) footprint of cropping systems. The aim of this study was to quantify the C footprint of six low-input arable cropping systems resulting from the combination of three levels of grain legumes introduction in a 3-yr rotation (GL0: no grain legumes, GL1: 1 grain legume, GL2: 2 grain legumes) and the use of cover crops (CC) or bare fallow (BF) between cash crops, covering two rotation cycles (6 years). The approach considered external emissions, on-site emissions and soil organic carbon (SOC) stock changes, and prioritized (i) field observations and (ii) simulation of non-measured variables with the STICS model, rather than default emission factors. As expected, fertilizers accounted for 80-90% of external emissions, being reduced by 50% and 102% with grain legumes introduction in GL1-BF and GL2-BF, compared to the cereal-based rotation (GL0-BF). Cover crops management increased machinery emissions by 24-35% compared to BF. Soil nitrous oxide (N2O) emissions were low, ranging between 205 and 333 kg CO2 eq. ha-1 yr-1 in GL1-BF and GL0-BF, respectively. Nitrate leaching represented the indirect emission of 11.6 to 27.2 kg CO2 eq. ha-1 yr-1 in the BF treatments and 8.2 to 10.7 kg CO2 eq. ha-1 yr-1 in the CC treatments. Indirect emissions due to ammonia volatilization ranged between 8.4 and 41.8 kg CO2 eq. ha-1 yr-1. The introduction of grain legumes strongly influenced SOC changes and, consequently, the C footprint. In the BF systems, grain legumes introduction in the rotations led to a significant increase in the C footprint, because of higher SOC losses. Contrarily, the use of cover crops mitigated SOC losses, and lowered the C footprint. These results indicated the need of CC when increasing the number of grain legumes in cereal-based rotations. Despite the multiple known benefits of introducing grain legumes in cropping systems our research highlights the need to consider soil organic carbon changes in environmental assessments.

The constructive suggestions of three anonymous reviewers greatly improved this manuscript. This research was supported by (i) the FP6 Grain Legumes Integrated Project (food-CT-2004-506223), (ii) the European Commission (REA) through the LEGATO project (FP7-613551) and (iii) the French National Research Agency (ANR) through the LEGITIMES French project (ANR-13-AGRO-0004). It also benefited from the support of the Climate-CAFE European project, which was selected by the European FACCE-JPI ERA-NET Plus program. The authors thank Jean-Marie Nolot for his key role in managing the cropping system experiment of Auzeville and Grégory Véricel for the database creation and maintenance. INS was funded thanks to a convention between the University of Lleida and INRA-Toulouse. DPB received a Juan de la Cierva postdoctoral grant from the Ministerio de Economía y Competitividad of Spain (IJCI-2016-27784).