Cover crops support the climate change mitigation potential of agroecosystems

some authors for separating supply and demand side efficiencies and including supply and demand side interactions [61,62]. Despite this criticism, carbon opportunity cost is a widely used approach to quantify changes in efficiency of land use for mitigating climate change. Given that land use change is still the highest source of emissions in the global agri-food sector [2], carbon opportunity cost based on Searchinger at al. [11] is an adequate land use change quantification approach for changes in maize yield. For the climate impact of land use for cover crop seed production, we use the forgone cli- mate benefit of wheat based on Searchinger et al. [11]. Here, the same arguments discussed above for using carbon opportunity costs are relevant. We assume wheat as the displaced crop when cover crop seeds are produced since wheat is the crop with the largest acreage in the European Union [23]. For additional machinery operations to establish cover crops, we only consider diesel fuel emissions. There are also emissions for producing, maintaining, and repairing agricultural machinery, but in the majority of cases, existing machinery is used for cover crop establishment, i.e., no additional machinery is produced to establish cover crops. In this study, we only evaluated maize as the crop to follow cover crops. Maize is the largest spring crop by acreage in the European Union [23] and the most investigated crop in the scientific lit- erature with a good data base. Despite this, other spring crops like oats, spring barley, sun-

flower, or sugar beet are investigated for the benefits of winter cover crops in the European Union. More data is needed to complete the view on climate benefits of cover crops based on different main crops. 4.4. Conclusion and policy recommendations This is the first study to include all relevant climate change impacting effects of cover crops in a comprehensive calculation and to extrapolate the NCCMI of cover crops to the whole maize cropping area of the EU-27. We show that cover crops are powerful tools to mitigate climate change impacts from European agriculture, which make our findings very relevant for political decisions in the EU. If all maize cropping acreage in the EU-27 were to include cover crops, GHG emissions from EU-27 agriculture could be reduced by 13.0%. Based on this result, we recommend that the Common Agricultural Policy (CAP) of the European Union should continue to accelerate the integration of cover crops into farming. The cur- rent CAP (2023–2027) does not include regulations that make cover crops before spring crops mandatory or provide incentives so the area devoted to cover crops in the EU is maxi- mized. There are conditionality requirements, such as obligatory soil cover in the winter for 80% of a farm’s arable land. Therefore, if a farm has 80% winter crops and 20% spring crops in its rotation, there is no requirement to grow cover crops under the current CAP for this farm [63]. Our results recommend a stronger commitment from the EU’s agricultural policy to make

cover crops before spring crops a common practice on all farms. This will not only help to achieve the EU’s climate protection goals, but also global targets in the Paris Agreement. 5. Statistics The weighted mean was calculated from study values using the number of observations as the weighting factor. Mean values are followed by the 95% confidence intervals (CI) and are shown in brackets (lower CI limit, upper CI limit). All data collected from the literature as well as the calculations shown in Table 3 are provided as supplementary material (S1 File). Forest plots were produced with the R package [64] with R version 4.3.2 [65]. Detailed description of the statistic parameters are outlined in [66]. PLOS ONE Cover crops support the climate change mitigation potential of agroecosystems PLOS ONE | https://doi.org/10.1371/journal.pone.0302139 May 8, 2024 16 / 20

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