Sustainability of biogas production is strongly dependent on soil‐borne greenhouse gas (GHG) emissions during feedstock cultivation. Maize (Zea mays) is the most common feedstock for biogas production in Europe. Since it is an annual crop requiring high fertilizer input, maize cropping can cause high GHG emissions on sites that, due to their hydrology, have high N₂O emission potential. On such sites, cultivation of cup plant (Silphium perfoliatum) as a perennial crop could be a more environmentally friendly alternative offering versatile ecosystem services. To evaluate the possible benefits of perennial cup plant cropping on GHG emissions and nitrogen losses, an incubation study was conducted with intact soil cores from a maize field and a cup plant field. The ¹⁵N gas flux method was used to quantify N source‐specific N₂ and N₂O fluxes. Cumulated N₂O emissions and N₂+N₂O emissions did not differ significantly between maize and cup plant soils, but tended to be higher in maize soil. Soils from both systems exhibited relatively high and similar N₂O/(N₂+N₂O) ratios (N₂Oi). N₂O emissions originating from sources other than the ¹⁵N‐labelled NO₃ pool were low, but were the only fluxes exhibiting a significant difference between the maize and cup plant soils. Missing differences in fluxes derived from the ¹⁵N pool indicate that under the experimental conditions with high moisture and level, and without plants, the cropping system had little effect on N fluxes related to denitrification. Lower soil pH and higher bulk density in the cup plant soil are likely to have reduced the mitigation potential of perennial biomass cropping.

European policy recommends that biomass production occur on marginal land, such as poorly draining Stagnosols. Compared to annual cropping, perennial crops may better mitigate N₂O emissions at such sites, through more complete denitrification. To test that hypothesis, we compared N₂ and N₂O fluxes from the soils of a perennial crop (cup plant, Silphium perfoliatum L.) and an annual crop (silage maize, Zea mays L.). Intact soil columns (35 cm height, 14.4 cm diameter) were incubated for 37 days. The soils were fertilized with 60 or 120 kg N ha⁻¹ and exposed to successive phases of waterlogging: free drainage, waterlogging of 1/3-, and waterlogging of 2/3- of the column. Source-specific N₂O and N₂ fluxes were measured using the ¹⁵ N gas flux method. Denitrification was higher in cup plant than maize soil and total N losses from denitrification were dominated by emissions from the third phase. Cup plant soil emitted 33.6 ± 78.1 mg N m⁻² and 95.8 ± 64.4 mg N m⁻² more N₂O than maize soil in the low and high N treatments, respectively. The product ratio of denitrification (N₂Oi = N₂O/(N₂ + N₂O)) increased with waterlogging in maize soil, while remaining stable in cup plant soil. Emissions from the top 10 cm dominated the N₂Oi rather than N₂ fluxes from the saturated soil. This study did not show N₂O mitigation in cup plant soil, instead highlighting the complexity of plant-soil effects on denitrification. We clearly showed that the application of a general N₂Oi for agricultural soils across annual and perennial cropping is not recommended.

The prediction of liquid manure effects on N transformations in soils and the associated N₂O and N₂ fluxes is poor because previous investigations have mostly excluded N₂. The objectives of this study were thus to quantify N₂, N₂O and CO₂ fluxes, the source processes of N₂O, N₂O reduction and the depth distribution of moisture, NO₃⁻, NH₄⁺, water-extractable organic carbon concentration and pH in a laboratory incubation study with sandy arable soil using ¹⁵N tracing to quantify N processes and gaseous fluxes. The soil was amended with and without artificial slurry in various manure treatments (control, surface and injected) and incubated for 10 days at varying moisture levels, where the depth distribution of control parameters was determined twice during the experiment. Manure application was found to increase N₂ and N₂O fluxes from denitrification, with the highest fluxes occurring in the wet manure injection treatment (33 ± 32 mg N m⁻² d⁻¹ and 36.1 ± 39.1 mg N m⁻² d⁻¹, respectively), confirming that manure injection under wet conditions enhances denitrification and possibly also N₂O fluxes. This study concluded that the current dataset is suitable as a first step towards improving the capability of biogeochemical models to predict manure application effects, but further studies with more soils and refined experiments are needed.

Background: The sustainability of bioenergy is strongly affected by direct field‐derived greenhouse gas (GHG) emissions and indirect emissions form land‐use change. Marginal land in low mountain ranges is suitable for feedstock production due to small impact on indirect land‐use change. However, these sites are vulnerable to high N₂O emissions because of their fine soil texture and hydrology.Aims: The perennial cup plant (Silphium perfoliatum L.) might outperform silage maize (Zea mays L.) on cold, wet low mountain ranges sites regarding yield and ecosystem services. The aim of this study was to assess whether the cultivation of cup plant also provides GHG mitigation potential compared to the cultivation of maize.Methods: A t‐year field experiment was conducted in a low mountain range region in western Germany to compare area and yield‐scaled GHG emissions from cup plant and maize fields. GHG emissions were quantified using the closed chamber method.Results: Cup plant fields emitted an average of 3.6 ± 4.3 kg N₂O‐N ha^–1 year^–1 (–85%) less than maize fields. This corresponded to 74.0 ± 94.1 g CO_2‐eq kWh^–1 (–78%) less emissions per produced electrical power. However, cup plant had a significantly lower productivity per hectare (–34%) and per unit of applied nitrogen (–32%) than maize.Conclusion: Cup plant as a feedstock reduces direct field‐derived GHG emissions compared to maize but, due to lower yields cup plant, likely increases emissions associated with land‐use changes. Therefore, the increased sustainability of bioenergy from biogas by replacing maize with cup plant is heavily dependent on the performance of maize at these sites and on the ecosystem services of cup plant in addition to GHG savings.

Aims: Cup plant ( Silphium perfoliatum ) contributes to sustainable bioenergy production. However, little is known about the role of intensive litter fall and increased soil biodiversity on respiration and denitrification in its cultivation. This study aimed to assess CO ₂ , N ₂ O and N ₂ emissions of annual and perennial energy cropping systems affected by earthworms when supplied with senescent cup plant or maize litter. Methods: In a 32-day laboratory incubation, the ¹⁵ N gas flux method was applied in an N ₂ -reduced atmosphere to a repacked silt loam soil, inoculated with Lumbricus terrestris L., and amended with senescent maize and cup plant litter. During incubation, CO ₂ , N ₂ O and N ₂ emissions were measured. After incubation, NO ₃ ⁻ , NH ₄ ⁺ , total C, and total N was analysed in soil and casts. Litter removal from the soil surface and net nitrification rates were calculated. Results: Earthworms caused a fivefold increase in maize litter removal and a threefold increase regarding cup plant litter. Highest N ₂ O + N ₂ fluxes were observed in treatments with earthworms and cup plant litter, although the high CN ratio of senescent cup plant litter implied higher N immobilisation than the CN ratio of senescent maize litter. Earthworms increased CO ₂ , N ₂ O, and especially N ₂ emissions lowering the product ratio of denitrification ( N ₂ Oi ). Conclusions: Earthworm-controlled litter incorporation increased labile C from decomposition of recalcitrant litter as substrate for denitrification. Mineralisation patterns of senescent litter cannot be explained by CN ratio alone. The combination of both mechanisms affected denitrification, lead to elevated N ₂ O and N ₂ emissions and N loss from the soil.

Background: Incorporation of cover crop (cc) shoot and root biomass can have different effects on nitrogen (N) dynamics and the transformation of soil‐derived N and cc N.Aims: The objective was to determine the effects of different ccs, cc compartments (roots and shoots), and pretreatment of cc biomass (fresh vs. dried) on mineralization processes and on the transformation of soil and cc N following incorporation into a silty loam soil.Methods: Soil columns with incorporated ¹⁵N‐labeled root and shoot biomass of two cc species (winter rye and oil radish) and different pretreatments (dried and fresh) were incubated for 70 days at a constant temperature and soil moisture (8°C, 40% water‐filled pore space). Carbon and N transformation dynamics were determined repeatedly, distinguishing between N originating from cc biomass and from soil.Results: Net CO₂ emission was related to the amount of soluble cell components added with ccs. Net N mineralization was negatively related to the C:N ratio of cc biomass. The incorporation of dried cc biomass caused higher initial soil respiration and N immobilization than fresh biomass. All treatments with cc incorporation showed increased N₂O emission. Emitted N₂O‐N consisted mainly of cc N (55%–57%) in treatments with fresh shoot biomass, whereas soil N was the main source of N₂O (75%) in the treatment with fresh oil radish roots. Recovery of cc ¹⁵N was affected by crop compartment and pretreatment. At the end of the incubation, it was 17.5%–42.3% in soil NO₃⁻, 0.1%–8.1% in microbial biomass N, and less than 0.23% of cc N was found in cumulative N₂O emission.Conclusion: The incorporation of cc roots and shoots had different effects on N mobilization and immobilization processes and on the partitioning of cc N. These processes can be influenced significantly by pretreatment of the added plant biomass (dried vs. fresh).