Short-term effects of biogas digestate and cattle slurry application on greenhouse gas emissions affected by N availability from grasslands on drained fen peatlands and associated organic soils

摘要

A change in German energy policy has resulted in a strong increase inthe number of biogas plants in Germany. As a consequence, huge amounts ofnutrient-rich residues, the by-products of the fermentative process, areused as organic fertilizers. Drained peatlands are increasingly used tosatisfy the huge demand for fermentative substrates (e.g., energy crops,grass silage) 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 toincrease N₂O emissions from managed grasslands. Our study addressed thequestions (a) to what extent biogas digestate and cattle slurry applicationincrease N₂O and methane (CH₄) fluxes as well as the mineralnitrogen use efficiency (NUE_min) and grass yield, and (b) how differentsoil organic matter contents (SOMs) and nitrogen contents promote theproduction of N₂O. In addition NH₃ volatilization was determinedat one application event to obtain first clues with respect to the effectsof soil and fertilizer types. The study was conducted at two sites within agrassland parcel, which differed in their soil organic carbon (SOC) and Ncontents. At each site (named C_org-medium and C_org-high) threeplots were established: one was fertilized five times with biogas digestate,one with cattle slurry, and the third served as control plot. On each plot,fluxes of N₂O and CH₄ were measured on three replicates over 2years using the closed chamber method. For NH₃ measurements we used thecalibrated dynamic chamber method. On an annual basis, the application ofbiogas digestate significantly enhanced the N₂O fluxes compared to theapplication of cattle slurry and additionally increased the plant N-uptakeand NUE_min. Furthermore, N₂O fluxes from the C_org-hightreatments significantly exceeded N₂O fluxes from the C_org-medium treatments.Annual cumulative emissions ranged from 0.91 ± 0.49 to 3.14 ± 0.91 kg N ha?1 yr?1. Significantlydifferent CH₄ fluxes between the investigated treatments or thedifferent soil types were not observed. Cumulative annual CH₄ exchangerates varied between −0.21 ± 0.19 and−1.06 ± 0.46 kg C ha?1 yr?1. Significantly higher NH₃losses, NUE_min and grass yields from treatments fertilized with biogasdigestate compared to those fertilized with cattle slurry were observed. Thetotal NH₃ losses following the splash plate application were18.17 kg N ha?1 for the digestate treatments and 3.48 kg N ha?1 for theslurry treatments (36 and 15% of applied NH₄+–N). Theobserved linear increase of 16 days' cumulative N₂O–N exchange or annual N₂O emissions, with mean groundwater level and ammoniumapplication rate, reveals the importance of site-adapted N fertilization andthe avoidance of N surpluses in C_org-rich grasslands.