Abstract. The change in the German energy policy resulted in a strong development of biogas plants in Germany. As a consequence, huge amounts of nutrient rich residues remain from the fermentative process, which are used as organic fertilizers. Drained peatlands are increasingly used to satisfy the huge demand for fermentative substrates and the digestate is returned to the peatlands. However, drained organic soils are considered as hot spots for nitrous oxide (N2O) emissions and organic fertilization is additionally known to increase N2O emissions from managed grasslands. Our study addressed the questions (a) to what extent biogas digestate and cattle slurry application increase N2O, methane (CH4) and ammonia (NH3) fluxes as well as the mineral nitrogen use efficiency (NUEmin), and (b) how different soil organic matter contents (SOM) promote the production of N2O. The study was conducted at two areas within a grassland parcel, which differed in their soil organic carbon (SOC) contents. At each area (named Corg-medium and Corg-high) two sites were established, one was fertilized five times with biogas digestate and one with cattle slurry. For each treatment, fluxes of N2O and CH4 were measured over two years using the closed chamber method. For NH3 measurements we used the calibrated dynamic chamber method. On an annual basis the application of biogas digestate significantly enhanced the N2O fluxes compared to the application of cattle slurry and additionally increased the NUEmin. Furthermore, N2O fluxes from the Corg-high site significantly exceeded N2O fluxes from the Corg-medium sites. Annual cumulative emissions ranged from 0.91 ± 0.49 kg N ha−1 yr−1 to 3.14 ± 0.91 kg N ha−1 yr−1. Significantly different CH4 fluxes between the investigated treatments or the different soil types were not observed. Cumulative annual CH4 exchange rates varied between −0.21 ± 0.19 kg C ha−1 yr−1 and −1.06 ± 0.46 kg C ha−1 yr−1. Significantly higher NH3 losses from treatments fertilized with biogas digestate compared to those fertilized with cattle slurry were observed. The total NH3 losses following splash plate application were 18.17 kg N ha−1 for the digestate treatments and 3.48 kg N ha−1 for the slurry treatments (36% and 15% of applied NH4+-N). The observed linear increase of 16 days cumulative N2O-N exchange or rather annual N2O emissions, due to a higher mean groundwater level and a higher application rate of NH4+-N, reveal the importance of site adapted N fertilization and the avoidance of N surpluses in Corg rich grasslands.
The Multiscale Monitoring of Peatland Ecosystem Carbon Cycling in the Middle Taiga Zone of Western Siberia: The Mukhrino Bog Case Study
