Cover crops support the climate change mitigation potential of agroecosystems

3.12. A factor not considered: Erosion reduction According to Lugato et al. [47], soil erosion can lead to a loss of soil carbon, lowering the car- bon sink capacity of soils. Additionally, soil erosion can disturb the soil structure and reduce soil fertility, which disconnects soil element cycles and further contributes to GHG emissions. However, the extent to which soil erosion leads to higher carbon fluxes out of the system depends on the specific context and interplay of many factors [48]. Cover crops protect soil erosion during the fall and winter and also after the establishment of the following crop, such as maize, due to remaining cover crop residues [49]. According to Laloy and Bielders [49] and Gentsch et al. [6], Cover crops increase the infiltration rate and improve soil structure, allowing the soil to absorb more water without eroding. According to Panagos et al. [15], cover crops reduce soil erosion by at least 20.0% in Europe and the United Kingdom. Machiwal et al. [50] found a reduction in soil loss of 33.0% to 77.0%, depending on the cover crop type for India. The reduction in soil erosion is highly dependent on the slope and management of the land. In addition, the soil type also plays a major role. Given the uncertainty between soil erosion and fluxes from the soil system as well as the varying factors leading to soil erosion, we have not included climate benefits of soil erosion prevention from cover crops in our calculations. 3.13. Net climate change mitigation impact (NCCMI) of cover crops

We calculate the summary of all climate benefits from cover crops as 4.23 Mg CO2e ha-1 a-1 (95%CI, 1.25, 7.49) and all climate costs from cover crops as 0.93 Mg CO2-eq ha-1 a-1 (95% CI 0.51, 1.34). The NCCMI of cover crops was calculated as 3.30 Mg CO2e ha-1 a-1 (Table 3). The NCCMI was then extrapolated to the maize harvest area for all EU-27 countries. Here, we assume a scenario in which cover crops are grown before maize on all of the EU-27’s harvested maize areas, which is 15,092 x 106 ha, on average [23]. Based on this calculation, planting cover crops before maize in the EU-27 results in a climate change mitigation potential of 49.80 mil- lion Mg CO2e a-1. This is equivalent to 13.0% of the EU-27’s agricultural GHG emissions [51]. 3.14 Sensitivity analysis based NCCMI scenarios The impact factors “carbon land benefit based on yield gain” and “carbon sequestration” have a high impact on NCCMI but also show a high uncertainty. To address this uncertainty, we use four scenarios for NCCMI to analyze the sensitivity of the results: A) Base scenario as shown in section 3.13 B) Base scenario less carbon land benefit based on yield gain C) Base scenario less carbon sequestration D) Base scenario less carbon land benefit based on yield gain and less carbon sequestration 4. Discussion 4.1. Review of results The results of this study indicate that the climate benefits of cover crops significantly exceed their climate costs. This resulted in a positive NCCMI of 3.30 Mg CO2e ha-1 a-1 that can be

achieved if cover crops were incorporated in crop rotations of maize production. The major contributor to climate benefits is through carbon land benefits based on yield gain for maize following a cover crop, which alone contributed to 33.8% of all climate benefits. Several meta studies [9,10] showed that cover crops increase soil organic carbon stocks, with an average organic carbon sequestration rate of 1.43 Mg ha-1 a-1. The most important mechanism behind this is the stimulation of microbial activity from cover crops’ organic matter input. Litter from 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 13 / 20