Cover crops support the climate change mitigation potential of agroecosystems

potential, a systematic literature review was conducted. The research platforms "Web of Sci- ence" and "Google Scholar" were used to explore the literature. Based on the initial research, we defined five factors that support climate change mitigation (“climate benefits”) and five fac- tors leading to additional GHG emissions (“climate costs”) due to cover cropping (Table 1). The individual research terms of the individual impact factors are described below in detail. To calculate the NCCMI of cover crops, the values of all impact factors were converted to the same unit, namely megagram of CO2 equivalents per hectare and year (Mg CO2e ha-1 a-1). From the values in the literature, a weighted average value that takes into account the number of measurements in the specific study was calculated. Only data from the literature that indi- cated the number of measurement points were considered in the calculation of the NCCMI. Thus, the climate benefits and climate costs could be added together in each case before sub- tracting the costs from the benefits. This resulted in a value for the NCCMI per hectare of cover crops given in Mg CO2e ha-1 a-1. The 95% confidence interval was calculated from all study means and presents a range of estimates. To extrapolate the results to the EU-27 maize area the respective impact factors and the climate balances were subsequently multiplied with the maize acreage in the EU-27. The result was then divided by the total emissions from agri- culture in the EU-27. This calculation resulted in the share of the NCCMI of cover crops before

maize relative to all EU-27 agricultural emissions. The average maize acreage in 22 of the EU- 27 countries was calculated as an average based on the years 2011 to 2022 (five of the EU-27 countries do not grow relevant quantities of maize and were thus not considered.) The following search terms were used in Web of Science: Cover crop & albedo, Cover crop & C sequestration, Cover crop & Yield corn, Cover crop & Yield maize, Cover crop & life cycle assessment, Cover crop & N-fixation, Cover crop & N- leaching, Fertilizer & Greenhouse gas emissions, Greenhouse gas emissions & fertilizer pro- duction, Cover crop & N2O, Catch crop & N2O. To enable a phrase search, all search terms were used with quotation marks to obtain search results exactly in this word order. A total of 883 publications were extracted and checked for their suitability. Criteria for the required data were an absolute value from the respective study and the number of measure- ments. Only 44 articles met these criteria and were finally included in the review. In addition, we used 31 publications from Google Scholar and data from 26 other Internet sources that were selected based on the above criteria. In selecting sources, cover crops were considered both consisting of individual components and mixtures, overwintering as well as freezing, and legumes as well as non-legumes. All data sources are presented in the supplemental material (S1 File) and sorted based on the subsections below. 2.1. Carbon land benefit based on yield gain

The relative yield gain of maize following cover crops is based on literature research. The abso- lute yield gain was calculated by multiplying the relative yield gain with the average maize yield in the EU-27. For this purpose, the harvest volume of silage and grain maize in Europe Table 1. Calculation for mean land use for cover crops’ seed production. Species Seed yield in kg ha-1 Seeding rate in kg ha-1 Seed land use in ha seed ha-1 for cover crops Phacelia 350.00 10.50 0.03 Mustard 1,738.00 20.00 0.01 Clover 732.00 29.17 0.04 Oats 4,869.50 87.50 0.02 Mixture 0.03 https://doi.org/10.1371/journal.pone.0302139.t001 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 3 / 20