Cover crops support the climate change mitigation potential of agroecosystems

was divided by the cultivated area. Since carbon opportunity costs factors published in Search- inger et al. [11] refer only to grain maize, the silage maize (SM) yields were transformed to grain maize (GM). It was assumed that 54% of the dry matter of silage maize is grain [19]. Thus, the fresh mass of silage corn was multiplied by a factor of 0.35 to calculate the dry matter and then multiplied by 0.54 [19] to generate the grain yield of SM (see the supplemental mate- rial for more detail). To obtain the grain yield based on the standard of 14% moisture, the result was divided by 0.86. The following formula shows the complete calculation to transfer SM to GM: GM ¼ SM � 0:35 � 0:54 0:86 : To quantify the effect of yield increases on climate change mitigation, we use the carbon land benefit approach of Searchinger et al. [11]. This approach quantifies the benefit of increas- ing yields on one piece of land through the carbon storage potential of native vegetation on other land that is not required to produce a certain quantity of product because of higher pro- duction on existing agricultural land. According to Searchinger et al. [11], the carbon land benefit carbon opportunity costs (COCs) is defined as follows: COCs ¼ Y � COC; where, COCs is the carbon land benefit, Y is the grain and biomass yield of an agricultural product, and COC is the carbon opportunity cost factor for the agricultural product. COC quantifies the forgone carbon storage potential of the native vegetation of land, which is glob-

ally used to produce an agricultural commodity and given in kg CO2e per kg of product. Searchinger et al. [11] use two methods to quantify the carbon opportunity cost. Here, we use the values of the so-called “loss-method.” To quantify the carbon land benefit caused by yield gains in maize through cover crops, the absolute yield increase of maize in kg ha-1 is multiplied by 2.1 kg CO2e kg-1, which is the carbon opportunity cost factor of grain maize published in Searchinger et al. [11]. 2.2. Carbon sequestration Carbon sequestration through cover crops is usually provided in units of soil organic carbon. This value was multiplied by a factor of 3.667 (i.e., the ratio of molar masses of CO2 and C) to obtain the value of CO2 stored per ha land. 2.3. Nitrogen fertilizer savings For nutrient savings, we considered only the main nutrient, namely nitrogen since its produc- tion is highly energy intensive. To quantify fertilizer savings, we only considered emissions of mineral fertilizers and not that of organic fertilizers. The overall nitrogen fertilizer savings were quantified as the sum of the mean literature values on nitrogen scavenging and the mean literature values of nitrogen fixation from cover crops. 2.4. Reduced indirect N2O emissions due to less leaching Reducing nitrogen leaching due to cover crops leads to lower indirect N2O emissions since less nitrogen is deposited to rivers and other water bodies. To quantify this effect, we use the approach suggested in the 2006 IPCC Guidelines for National Greenhouse Gas Inventories:

The amount of nitrogen leaching reduction is multiplied by the emission factor EF5 of 0.0075 [20] to obtain the reduction in indirect N2O emissions per hectare. This value is then con- verted from kg N2O-N ha-1 to kg CO2 -eq ha-1 using the conversion factor 273 and divided by 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 4 / 20