N dynamics during a 3-year crop rotation fertilized with digestates and cattle effluents

Abstract. On-farm anaerobic digestion is used as a means of producing biogas, with the resulting digestates serving as organic fertilizers. However, such digestates have different fertilizer properties than undigested animal effluents, and are associated with different degrees of N loss. We conducted a field experiment in which cattle slurry and farmyard manure were co-digested with urban and agro-industrial wastes, which represented slightly more than two-thirds of the total. We managed a three-year crop succession (wheat – rapeseed – wheat) with five fertilization systems: no fertilization, mineral fertilizers, cattle manure and slurry, raw digestate, or separated solid and liquid digestates. An exogenous organic matter (EOM) (cattle slurry, liquid or raw digestates) or mineral fertilizer was applied five times in winter and spring. A different type of EOM (cattle manure, solid or raw digestates) was applied twice in summer. After each fertilizer application, we measured ammonia volatilization and N₂O emissions, along with crop N uptake and soil mineral N stocks. Across the three-year rotation, the NH₃ volatilization rate was the lowest in the plot treated with mineral fertilizers (2 % of applied total N, TN), followed by cattle effluents (7 % TN), liquid and solid digestates (9 % TN), and raw digestate (18 % TN). Seasonal cumulative N₂O emissions were similar between mineral fertilizers and digestates, and were lower with cattle effluent, mainly because of lower ammoniacal N inputs. Compared to unfertilized crops, the surplus crop N uptake strongly reflected the mineral N content of fertilizers, ammonia volatilization, and the decomposition of EOM in the soil. Liquid digestate and cattle slurry had similar N use efficiencies (37 % to 60 % depending on the cropping season), while values for raw digestate were lower (25 to 41 %), likely due to its larger NH₃ volatilization. Overall, digestates served as an effective N fertilizer but require particular attention to NH3 volatilization.