Soil-derived trace gas fluxes from different energy crops – results from a field experiment in Southwest Germany MARTIN GAUDER*, KLAUS BUTTERBACH-BAHL † , SIMONE GRAEFF-HO ¨ NNINGER*, WILHELM CLAUPEIN* and REGINA WIEGEL † *Institute of Crop Science, University of Hohenheim, 70599, Stuttgart, Germany, †Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, 82467, Garmisch-Partenkirchen, Germany Abstract 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 appli- cation rate was 80 kg N ha À1 yr À1 for the perennial crops and 240 kg N ha À1 yr À1 for the annual maize. A repli- cated field experiment was conducted with 1-year measurements of soil fluxes of CH 4 , CO 2 and N 2 O in weekly intervals using static chambers. The measurements revealed a clear seasonal trend in soil CO 2 emissions, with highest emissions being found for the N-fertilized Miscanthus plots (annual mean: 50 mg C m À ² h À1 ). Signifi- cant differences between the cropping systems were found in soil N 2 O emissions due to their dependency on amount and timing of N fertilization. N-fertilized maize plots had highest N 2 O emissions by far, which accumu- lated to 3.6 kg N 2 O ha À1 yr À1 . The contribution of CH 4 fluxes to the total soil greenhouse gas subsumption was very small compared with N 2 O and CO 2 . CH 4 fluxes were mostly negative indicating that the investigated soils mainly acted as weak sinks for atmospheric CH 4 . To identify the system providing the best ratio of yield to soil N 2 O emissions, a subsumption relative to biomass yields was calculated. N-fertilized maize caused the highest soil N 2 O emissions relative to dry matter yields. Moreover, unfertilized maize had higher relative soil N 2 O emis- sions than unfertilized Miscanthus and willow. These results favour perennial crops for bioenergy production, as they are able to provide high yields with low N 2 O emissions in the field. Keywords: CH 4 , CO 2 , maize, Miscanthus, N 2 O, nitrogen fertilization, perennial crops, willow Received 17 May 2011; revised version received 14 September 2011 and accepted 18 September 2011 Introduction The agricultural sector is the second-largest emission source of greenhouse gases (GHG) in the European Union, including CO 2 , CH 4 ,N 2 O and fluorinated GHGs. More than half of these emissions originate from agricul- tural soils (UNFCCC, 2010b). The most important GHGs from terrestrial ecosystems are water vapour (H 2 O) and the trace gases carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) (Arneth et al., 2010). In agricul- tural soils, if plant respiration is disregarded, CO 2 is mainly produced by the decomposition of organic car- bon which is mostly derived from fresh plant litter or was accumulated in the soils over long time periods (Dolman, 2008). Carbon turnover in soils reaches equi- librium if external conditions remain stable, therefore notable amounts of CO 2 emissions exceeding carbon inputs by plant litter derive only from land use changes or changes in cropping management. The production of N 2 O is associated with the microbial soil N turnover processes of nitrification and denitrification, both requir- ing the presence of either nitrate (NO 3 À ) or ammonium (NH 4 + ) and organic N if heterotrophic nitrification pre- vails. Thus, N 2 O production is highly affected by N-fer- tilization. Methane fluxes from soils are the result of simultaneously occurring production (methanogenesis) or oxidation processes (methanotrophy), processes which are linked to predominantly anaerobic or aerobic conditions, respectively (Conrad, 1996). However, agri- cultural soils in Western Europe are considered overall as small sinks of CH 4 (Boeckx & Van Cleemput, 2001). Quantification of soil trace gas emissions on national and international scales is currently based on activity data, for example the amount of fertilizer use, which are multiplied with specific emission factors to derive national emission estimates (IPCC, 2006; UNFCCC, 2010a). Since the emission of trace gases from agricul- tural soils is strongly influenced by many factors, like crop, soil type, pH, carbon content, tillage practices, temperature and moisture, it is important to obtain quali- fied field measurements at as many sites as possible to further improve estimates of GHG emissions for a vari- ety of sites differing in environmental conditions and Correspondence: Martin Gauder, tel. + 49 071 145 922 379, fax + 49 071 145 922 297, e-mail: gauder@uni-hohenheim.de © 2011 Blackwell Publishing Ltd 289 GCB Bioenergy (2012) 4, 289–301, doi: 10.1111/j.1757-1707.2011.01135.x