Potential short-term losses of N<sub>2</sub>O and N<sub>2</sub> from high concentrations of biogas digestate in arable soils

Abstract

Abstract. Biogas digestate (BD) is increasingly used as organic fertilizer, but has a high potential for NH 3 losses. Its proposed injection into soils as a countermeasure has been suggested to promote the generation of N 2 O, leading to a potential trade-off. Furthermore, the effect of high nutrient concentrations on N 2 losses as they may appear after injection of BD into soil has not yet been evaluated. Hence, we performed an incubation experiment with soil cores in a helium-oxygen atmosphere to examine the influence of soil substrate (loamy sand, clayey silt), water-filled pore space (WFPS; 35, 55, 75 %) and application rate (0, 17.6 and 35.2 mL BD per soil core, 250 cm 3 ) on the emission of N 2 O, N 2 and CO 2 after the usage of high loads of BD. To determine the potential capacity for gaseous losses, we applied anaerobic conditions by purging with helium for the last 24 h of incubation. Immediate N 2 O and N 2 emissions as well as the N 2 / (N 2 O+N 2 ) product ratio depended on soil type and increased with WFPS, indicating a crucial role of soil gas diffusivity for the formation and emission of nitrogenous gases in agricultural soils. However, emissions did not increase with the application rate of BD. This is probably due to an inhibitory effect of the high NH + 4 content of BD on nitrification. Our results suggest a larger potential for N 2 O formation immediately following BD injection in the fine-textured clayey silt compared to the coarse loamy sand. By contrast, the loamy sand showed a higher potential for N 2 production under anaerobic conditions. Our results suggest that short-term N losses of N 2 O and N 2 after injection may be higher than probable losses of NH 3 following surface application of BD.

Soil incubation study showed biogas digestate to cause higher and more variable short‐term N2O and N2 fluxes than mineral‐N

“…Despite slightly different boundary conditions (different biogas digstate, slightly higher temperature with 15°C), a subsequent laboratory study by Fiedler et al. () showed higher N 2 O fluxes overall but compared to Gülzow significantly higher N 2 O fluxes again for the Dornburg soil cores. On the other hand, Heintze et al.…”

“…Differences in N 2 O fluxes between the three related studies might be due to the well‐known spatial and temporal variability of N 2 O, particularly because samples were taken in different years and soil cores were treated differently [ i.e ., drying and rewetting for WFPS adjustment in Fiedler et al. ()]. However, N 2 fluxes from the BD treatment between the two subsequent studies were found to be similar for the two sites (Dornburg and Gülzow).…”

“…Hence, the N 2 ‐free incubation technique provides an alternative for the investigation of a large number of undisturbed soil cores from different sites, where the actual N 2 O and N 2 release can be measured. Generally, the initial phase, i.e ., the first days after fertilizer application, is crucial for N 2 O emissions ( Senbayram et al., ; Köster et al., ) and previous BD experiments have indicated a similar immediate reaction also for N 2 ( Köster et al., ; Fiedler et al., ). Since such measurements on BD are up to now not available under field conditions due to the issues described above and still rare under laboratory conditions, a short‐term laboratory incubation experiment would provide reliable data on N 2 O and N 2 fluxes with respect to fertilizer type and soil conditions from various field sites, which might be further used to supplement field measurements.…”

“…Moreover, as BD contain mostly water (up to 97% moisture content) ( Möller , ; Charles et al, ), their addition rapidly saturates micro‐pores, which then enhances the potential for N 2 O reduction to N 2 ( Butterbach‐Bahl et al, ). However, there is no clear picture about the impact of BD application on N 2 formation up to now ( Köster et al., ; Fiedler et al., ).…”

“…N 2 O and N 2 fluxes are significantly influenced by a range of factors, e.g ., availability of N, C, and O 2 in the soil ( Weier et al., ; Conrad , ; Senbayram et al., ), as well as soil moisture, soil type, soil texture, soil pH, climate, microbial community structure, fertilization, and management ( Butterbach‐Bahl et al., ; Saggar et al., ). In particular, soil moisture might be the most important driving factor during BD application in spring which has not been widely investigated with respect to N 2 fluxes ( Fiedler et al., ).…”