The use of legumes as a biogas substrate - potentials for saving energy and reducing greenhouse gas emissions through symbiotic nitrogen fixation

Table 2 Energy consumption [kWh ha ⁻¹ ] and greenhouse gases emissions [kg CO ₂ eq. ha ⁻¹ ] of N fertilizers

Fertilizer type	Non-renewable energy input per kg of produced nitrogen [kWh/ kg N]	Greenhouse gases emissions during production [t CO ₂ eq. ha ⁻¹ ]	N ₂ O emissions after application [kg CO ₂ eq. kg ⁻¹ of applied fertilizer]	N ₂ O emissions after application [kg CO ₂ eq. kg ⁻¹ of inorganic N fertilizer equivalent]	Sum of greenhouse gas emissions [kg CO ₂ eq. kg ⁻¹ of inorganic N fertilizer equivalent]
Digestate, N derived from SNF^a	-	-	2.98^b	3.73	3.73
Inorganic nitrogen	16.17	6.032	2.98^b	2.98	9.01

^aFuel and energy for biogas plant operation belongs to methane production. ^bAccording to the IPCC scheme [10,12], the N₂O emissions after application are similar but due to the lower efficiency, a higher amount is needed. So, the relevant value is the emission per kg of replaced mineral fertilizer (next column).