New paper finds Mediterranean cover crops are a net source of CO2 to the atmosphere

A new paper published in Science of the Total Environment finds cover crops in the Mediterranean act as a net source of CO2 to the atmosphere, and as a net sink of CH4 [methane]. The paper adds to several other peer-reviewed papers finding various agricultural and ocean sites act as net sources of CO2 rather than sinks as previously thought.

Do cover crops enhance N₂O, CO₂ or CH₄ emissions from soil in Mediterranean arable systems?

• A. Sanz-Cobena^a, , 1, , 
• S. García-Marco^a, 1, 
• M. Quemada^a, 
• J.L. Gabriel^b, 
• P. Almendros^a, 
• A. Vallejo^a

• ^a Technical University of Madrid, School of Agriculture, Avd. Complutense s/n, 28040 Madrid, Spain
• ^b Dpt. Environmental Sciences (ELI-e), Earth and Life Institute, Université Catholique de Louvain, Croix du Sud 2 bte L7.05.02, 1348 Louvain-la-Neuve, Belgium

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Highlights

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Differences in GHG emissions between CCs were only significant in the fallow period.

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Vetch was the only CC significantly enhancing N₂O losses.

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There were CC specific effects over direct N₂O emission.

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Negative CH₄ fluxes were measured in all incorporated CCs.

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Planting CC may be a good strategy to decrease direct N₂O losses in these rotations.

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Abstract

This study evaluates the effect of planting three cover crops (CCs) (barley, Hordeum vulgare L.; vetch, Vicia villosa L.; rape, Brassica napus L.) on the direct emission of N₂O, CO₂ and CH₄ in the intercrop period and the impact of incorporating these CCs on the emission of greenhouse gas (GHG) from the forthcoming irrigated maize (Zea mays L.) crop. Vetch and barley were the CCs with the highest N₂O and CO₂ losses (75 and 47% increase compared with the control, respectively) in the fallow period. In all cases, fluxes of N₂O were increased through N fertilization and the incorporation of barley and rape residues (40 and 17% increase, respectively). The combination of a high C:N ratio with the addition of an external source of mineral N increased the fluxes of N₂O compared with − Ba and − Rp. The direct emissions of N₂O were lower than expected for a fertilized crop (0.10% emission factor, EF) compared with other studies and the IPCC EF. These results are believed to be associated with a decreased NO₃⁻ pool due to highly denitrifying conditions and increased drainage. The fluxes of CO₂ were in the range of other fertilized crops (i.e., 1118.71–1736.52 kg CO₂–C ha^− 1)  [positive values indicate a net source]. The incorporation of CC residues enhanced soil respiration in the range of 21–28% for barley and rape although no significant differences between treatments were detected. Negative CH₄ fluxes were measured and displayed an overall sink effect for all incorporated CC (mean values of − 0.12 and − 0.10 kg CH₄–C ha^− 1 for plots with and without incorporated CCs, respectively).

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