Greenhouse gas fluxes from soils fertilised with anaerobically digested biomass from wetlands

Riverine wetlands play important roles at local and global scales by contributing to climate regulation, biodiversity maintenance, flood attenuation and water purification. They also supply large amounts of biomass that can be used as a substrate for the production of biogas, while post-fermentation material can be used as an organic fertiliser, although the latter may constitute a source of greenhouse gases (GHG). The aim of the study was to determine the intensity of GHG fluxes from soils enriched with anaerobically digested plant biomass sourced from the wetlands of the Narew River Valley. The study has shown that anaerobically digested wetland biomass is a source of easily biodegradable organic compounds and its mineralisation in the soil results in the release of GHG to the atmosphere. Fluxes of carbon dioxide (CO2) and methane (CH4) were short-term, while nitrous oxide (N2O) fluxes remained at elevated levels for a period of two weeks after fertilisation. During the course of the experiment, carbon (C) losses were much higher than nitrogen (N) losses and amounted to 8-50 % of the amount supplied by digestate. The analysed digestates clearly differed in terms of the intensity of changes that occurred after application to the soil. The lowest GHG fluxes and the lowest amount of C mineralised (8 % of the amount supplied) were observed with the digestate of common reed (Phragmites austral's), which would indicate that it should be primarily considered as a substrate for biogas production. The relatively small amounts of C and N in particular, released in gaseous form from the digestates indicate that GHG fluxes from agricultural soils may be mitigated if digestates are used in place of inorganic fertilisers. However, it should be noted that the obtained results should be treated as approximates, as due to analytical limitations, they were carried out without replications. However, these results indicate that common reed could be used as a substrate for biogas production, its digestate applied on arable land as a fertiliser and should be examined in more detail.