Cover crops support the climate change mitigation potential of agroecosystems

3.9. Foregone benefits due to cover crop seed land use Land use for producing seeds for cover crops was calculated for the species phacelia, mustard, clover, and oat as representatives of the most important plant families used as cover crops. Their average was used for further calculations. The average land use requirement to produce one ha of cover crop seeds was calculated as 0.02 ha (95% CI, 0.00, 0.04). We assume that the land used for the production of cover crop seeds would otherwise be used for wheat production. Wheat production on land in the EU offers a carbon benefit that consists of: (1) the opportunity that the wheat output enables storing carbon elsewhere (yield × carbon opportunity cost); and (2) savings in global production emissions due to lower production emissions in the EU compared to the global average (here, the difference between Swedish and global wheat production emissions is based on Searchinger et al. [11]). Other fac- tors, like changes in soil organic carbon or bioenergy benefits are not considered here. The cli- mate benefit of wheat consisting of factors (1) and (2) is forgone due to land requirements for cover crops’ seed requirements and can be quantified accordingly to a mean value of 0.28 Mg CO2e ha-1 (95% CI, 0.27, 0.29). 3.10. Production emissions for cover crop seed production Since no specific literature sources for seed production emissions could be found for typical cover crop species, emissions from corn, spring wheat, and canola seed production are used in

this study. Hybrid corn seed production in China leads to emissions of 1,459.00 kg CO2e Mg-1, whereas in the U.S. it causes 2,250.50 kg CO2e Mg-1 [17,41] of emissions. Seed production of rapeseed in Poland causes slightly lower emissions of 1,014.00 kg CO2e Mg-1 [42]. In contrast, for spring wheat, it is only 580.00 kg CO2e Mg-1 in Southwest Finland and 680.00 kg CO2e Mg-1 in Northern Savonia (Finland) [43]. These emissions mostly depend on the amount of fertilizer used since fertilization is responsible for the majority of production emissions, espe- cially the production and application of nitrogen fertilizers [17]. The weighted average value used for our calculation is 1,066.26 kg CO2e Mg-1 (95% CI, 219.56, 1912.95). For the emissions of processing, packaging, and transporting cover crop seeds, only one literature source for corn seed could be found. The emissions here are 213.00 kg CO2e Mg-1 [44]. 3.11. Additional machinery operations for cover crop establishment We assume that cover crop establishment requires either tillage with a plow or cultivator fol- lowed by seeding. In addition, it is assumed that the only relevant emission from these machin- ery operations is from burning fossil diesel fuel. Based on data of an internet database from KTBL [45], plowing plus seeding is estimated at 55.00 liter diesel fuel ha-1, while cultivator till- age and seeding is estimated at 45.00 liter diesel fuel ha-1. The variability of the machinery operation intensity and corresponding fuel consumption was analyzed by Wittwer et al. [18].

In this study, diesel fuel consumption ranges from 22.00 l ha-1 for no-till systems to 61.00 l ha-1 for conventional intensive tillage systems. In organic farming, diesel fuel consumption ranges from 32.00 l ha-1 for reduced tillage to 55.00 l ha-1 for intensive tillage. The average diesel fuel consumption used in this study was calculated as 45.29 l ha-1 (95% CI, 32.43, 58.13). The emission factor per liter of diesel fuel ranges from 3.00 kg CO2e l-1 [44] to 3.31 kg CO2e l-1 [46]. The average emissions were calculated as 3.13 kg CO2e l-1 (95% CI, 2.92, 3.34) diesel fuel. The additional machinery operations thus cause emissions of 0.14 Mg CO2e ha-1 (95% CI, 0.10, 0.18). 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 12 / 20