Crop production for biogas and water protection—A trade-off?

In Germany the promotion of renewable energy has led to a rapid expansion of biogas production in recent years. Growing controversy surrounds this development due to potential environmental trade-offs caused by biomass production for co-fermentation, which is dominated by silage maize. The aim of the study was to evaluate the N-leaching potential following application of biogas residues and animal slurry to grassland and maize on a sandy soil in northern Germany.The study was based on a field experiment conducted in northern Germany over two consecutive leaching periods (2007/2008 and 2008/2009). Treatments included N-fertilizer type (co-fermented biogas residue, cattle slurry, calcium ammonium nitrate) and N amount (grass: 0, 160, 320 and 480kgNha−1; maize: 0, 120, 240 and 360kgNha−1). The fertilizer amount was split into two (maize) or four (grass) dressings. Leachate was collected using ceramic suction cups installed at 60cm depth. Water fluxes were simulated with a site-adapted model in which the soil water balance calculation was based on mechanistic approaches, while crop height and green area index were quantified by simple logistic growth equations. Nitrogen load was obtained by combining the simulated leachate amount with measured N concentration.Water fluxes, crop height and green area index were satisfactorily simulated. Nitrate-N loss in grassland generally was very low, staying below the critical nitrate-N load, according to the EU drinking water threshold. At optimal N input, i.e. N input required for maximum yield, maize caused a considerable nitrate-N load (48–67kgha−1), and it increased exponentially at higher inputs. Most of the variation of nitrate-N load among fertilizer types was explained by the proportion of fertilizer-N input as mineral N. For both grassland and maize, biogas residues had a similar nitrate leaching potential to animal manures. Maize achieved a substantially higher methane hectare yield (mN3 CH4 ha−1) than grassland. This could not overcompensate for its higher nitrate-N load, as indicated by the eco-efficiencies in terms of kg nitrate-N load per mega liter methane produced at optimal N input (maize: 7.7–14.7, grass: 0.3–2.9).