Cover crops support the climate change mitigation potential of agroecosystems

crop species in the mixture. For example, red clover fixes 29.00 kg N ha-1 as a single species cover crop, but 52.00 kg N ha-1 in a mixture with ryegrass [13]. The same was observed with winter vetch, which fixes 85.00 kg N ha-1 as single species and 113.00 kg N ha-1 in a mixture with winter rye [27]. A much wider range from 50.00 kg N ha-1 to 300.00 kg N ha-1 was found by Kaye and Quemada [22]. Wittwer et al. [18] showed that legumes as a single species cover crop can increase nitrogen uptake of maize by 32.00 kg N ha-1 and as part of cover crop mix- tures by 28.00 kg N ha-1. The summarized literature resulted in a weighted mean of 52.49 kg N ha-1 (95% CI, 32.06, 72.92) that can be contributed by legumes from nitrogen fixation. 3.4. Reduced nitrogen leaching Cover crops prevent nutrient leaching over winter, especially of mineral nitrogen (Nmin) that otherwise would be leached from the soil profile. However, this leaching reduction strongly depends on the timing of when cover crop are planted and type of cover crop. Early sown oil radish reduces leaching in the fall and winter months [28]. Rye as an over-wintering cover crop can better protect nitrogen from leaching in the spring [28]. Non-legume cover crops and mixtures can protect up to 100.00 kg N ha-1 from leaching, while pure legumes can protect between 60.00 to 70.00 kg N ha-1 [6,22]. Abdalla et al. [25] show that legume cover crops pre- vent 18.00 kg N ha-1 from leaching, non-legume cover crops 38.00 kg N ha-1, and cover crop mixtures 19.00 kg N ha-1. A study by McCracken et al. [29] resulted in an average leaching

reduction of 35.80 kg N ha-1. The amount of scavenged nitrogen depends heavily on water availability and soil type. Early sowing and sufficient precipitation enable cover crops to develop faster, establish more biomass, and absorb more nitrogen in the fall. The tillage system also affects the amount of scavenged nitorgen. The faster mineralization in conventional tillage systems in autumn resulted in higher Nmin stocks and higher nitrogen uptake from cover crops than in no-till systems [30]. Literature research resulted in a weighted mean value of 38.82 kg N ha-1 (95% CI, 19.13, 58.51) of scavenged nitrogen available to the crop following the cover crop. 3.5. Fertilizer emissions Nitrogen fertilizer emissions are caused by different factors and vary between fertilizer types. About half of the CO2e emissions of nitrogen fertilizer is N2O emissions from soil after appli- cation and about one-third of all emissions occur during the production process of fertilizer itself. The remaining emissions are divided into N2O emissions from NH4+ and NO3- losses and CO2 from lime [31]. Total emissions per kg of nitrogen also vary between fertilizer types and the literature. For example, Hasler et al. [32] reported 8.69 kg CO2e kg-1 N for urea fertil- izer to 11.64 kg CO2e kg-1 N for CaH4N4O9 fertilizer. A much higher emission factor of 15.00 to 31.00 kg CO2e kg-1 N is used by Kahrl et al. [33]. The mean value of all studies considered was 11.23 kg CO2e kg-1 N (95% CI, 9.02, 13.43). Emissions from organic fertilizers were not considered.

3.6. Reduced indirect N2O emissions due to less leaching Leached nitrogen is deposited into aquatic ecosystems and represents a globally significant source of N2O through indirect emissions. These emissions can be calculated using the IPCC emission factor EF5, which incorporates three components: EF5g (groundwater and surface drainage), EF5r (rivers) and EF5e (estuaries) [34]. The IPCC emission factor EF5 amounts to 0.0075 and is the sum of EF5g, EF5r and EF5e with a value of 0.0025 each. Tian et al. [34] suggest a higher emission factor for EF5g of 0.0060, the other two factors are almost the same as those suggested by IPCC in 2006. Here, we use the 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 10 / 20