Nitrous oxide emissions and nitrogen use efficiency of manure and digestates applied to spring barley

Digestates produced by anaerobic treatment of manure and organic wastes for biogas production are rich in nutrients which should be recycled to agricultural land to sustain crop growth. However, digestate properties are highly variable, complicating the prediction of agronomic value and environmental impact. The objective of this study was to assess nitrous oxide (N2O) emissions and nitrogen use efficiency during growth of spring barley (Hordeum vulgare L.) when fertilized with digestates and untreated manure, and to relate emission patterns to properties of soil and manure materials. Experimental treatments received 100 kg NH4+-N ha−1 in either pig slurry (PS), cattle slurry (CS), a slurry-based digestate from Maabjerg Bioenergy (MBD), or cattle slurry mixed with digested and dewatered sewage sludge (CS + DDS). Ammonium sulfate (MIN) and unfertilized soil (Ctrl) served as references. Cumulative N2O-N emissions at harvest ranged from 0.02 to 1.97 kg ha−1, and net emissions corresponded to 0.10–0.41% of total N input. According to a graphical model N2O emissions were related to soil NO3−, rather than NH4+ availability, indicating that denitrification was the main source of N2O. When observations were fitted to an empirical model of cumulated N2O emissions, nitrification in manure hotspots was identified as a main driver of N2O emissions, but low soil NO3− availability suggested that the role of nitrification was indirect, via coupled nitrification-denitrification. Yield-scaled emissions ranged from 0.04 to 0.39 g N2O-N kg−1 grain yield. The emissions intensity of MBD was lower than that of untreated manure, and similar to mineral fertilizer. In contrast, treatment CS + DDS containing also digested organic material had the highest total and yield-scaled N2O emissions. Thus, agronomic and environmental performance of digestates can not be predicted from management, but must take specific soil and manure properties into account.