Cover crops support the climate change mitigation potential of agroecosystems

mean of two results: One using the IPPC factor which results in 0.08 Mg CO2 -eq ha-1 and the other one using the factor of Tian et al. [34] which results in 0.12 Mg CO2 -eq ha-1. The mean of both values leads to an average climate benefit caused by reduced indirect N2O emissions due to less leaching of 0.10 Mg CO2 -eq ha-1 (95% CI, -0.15, 0.35). 3.7. Albedo change When soil is covered with cover crops, less solar radiation reaches the soil surface compared to fallow land. Thus, the albedo value of plants is higher than that of fallow soil. Due to this reduced net radiation, there is a cooling effect that impacts the global climate. However, the albedo value of fallow soil strongly depends on soil type and condition. In particular, the color and moisture content of the soil play important roles. Dark and moist soils have a lower albedo than light dry soils, so there is a wide variation in the albedo value of fallow land. Since there is often a high soil moisture level during the growth period of cover crops (in fall and winter) the albedo effect of covering the soil with cover crops is stronger than for the yearly average [14]. Unfortunately, research on albedo effects of cover crops is in its infancy and only two adequate publications were found. The albedo effect of cover crops ranges from -4.00 to 10.00 W m-2 was calculated by Kaye and Quemada [22]. Converted to CO2e, this corresponds to a climate benefit of 0.25 Mg CO2e ha-1 a-1. Carrer et al. [14] show that three months of cover cropping

in Europe results in a climate benefit of 0.16 Mg CO2e ha-1 a-1. Similarly, this study found that longer time periods with cover cropping would increase the mitigation potential by 27.0%. Overall, a mean climate benefit of 0.20 Mg CO2e ha-1 a-1 (95% CI, -0.37, 0.78) was identified in the considered literature for the albedo change of cover crops. 3.8. Nitrous oxide emissions Cover cropping can increase N2O emissions under certain weather conditions compared to fallow land depending on the fertilizer N rate, soil incorporation, and the period of measure- ment and rainfall [35].There are many factors that influence the level of N2O emissions which leads to high variability from this GHG emission source. A prerequisite for cover crops is high soil moisture and water saturation, or at least water saturation of the upper soil layers. Another factor is the C/N ratio of the cover crop, which influences how fast cover crop residues decom- pose. Cover crops with low C/N ratios emit more N2O than cover crops with high C/N ratios [36,37]. Another factor influencing N2O emissions is the treatment of cover crop residues. Incorporating residues into the soil leads to higher N2O emissions as compared to leaving cover crop residues decaying on the soil surface [36]. Air temperature also effects N2O emis- sions. As temperatures increase, N2O emissions from cover crops also increase. Overall, air temperature accounts for 22.0% of the variability in N2O emissions [38]. In a meta-analysis, Basche et al. [35] found that N2O emissions increased in 60.0% and decreased in 40.0% of the

publications when cover cropping was practiced. The overall conclusion of the meta-analysis is that cover cropping increases N2O emissions only to a very small extent and that the vari- ability is very large. For example, compared to a chemical fallow system, Olofsson and Ernfors [39] identified the highest value with 1.80 kg N2O-N ha-1 for oilseed radish, whereas Preza- Fontes et al. [40] identified the lowest emissions with -0.54 kg N2O-N ha-1 for a sorghum- sudan-grass-mixture. The result of our calculation based on a systematic literature search also shows only a very small increase in N2O emissions from cover cropping. The determined weighted mean value of emissions, as displayed in Table 2, is 0.13 kg N2O-N ha-1, which is equivalent to 0.04 Mg CO2e ha-1 (95% CI, -0.02, 0.09). 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 11 / 20