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Soil‐derived trace gas fluxes from different energy crops – results from a field experiment in Southwest Germany

Willow coppice, energy maize and Miscanthus were evaluated regarding their soil‐derived trace gas emission potential involving a nonfertilized and a crop‐adapted slow‐release nitrogen (N) fertilizer scheme. The N application rate was 80 kg N ha−1 yr−1 for the perennial crops and 240 kg N ha−1 yr−1 for the annual maize. A replicated field experiment was conducted with 1‐year measurements of soil fluxes of CH4, CO2 and N2O in weekly intervals using static chambers. The measurements revealed a clear seasonal trend in soil CO2 emissions, with highest emissions being found for the N‐fertilized Miscanthus plots (annual mean: 50 mg C m−² h−1). Significant differences between the cropping systems were found in soil N2O emissions due to their dependency on amount and timing of N fertilization. N‐fertilized maize plots had highest N2O emissions by far, which accumulated to 3.6 kg N2O ha−1 yr−1. The contribution of CH4 fluxes to the total soil greenhouse gas subsumption was very small compared with N2O and CO2. CH4 fluxes were mostly negative indicating that the investigated soils mainly acted as weak sinks for atmospheric CH4. To identify the system providing the best ratio of yield to soil N2O emissions, a subsumption relative to biomass yields was calculated. N‐fertilized maize caused the highest soil N2O emissions relative to dry matter yields. Moreover, unfertilized maize had higher relative soil N2O emissions than unfertilized Miscanthus and willow. These results favour perennial crops for bioenergy production, as they are able to provide high yields with low N2O emissions in the field.