Nitrogen Leaching Loss After Anaerobic Digestate Application to Biogas Cropping Systems.

The production of methane from anaerobic digestion of slurry and/or biomass for generating electricity/heat has substantially expanded in Germany, resulting in large amounts of biogas residues, which should be recycled to replace mineral fertilizer. Data on the N leaching risk after biogas residue application, however, is scarce. A 2-year field experiment (2007-2009) was conducted at two sites in Northern Germany to assess the nitrate leaching potential of biogas residues compared to animal manure and mineral N fertilizer when applied to grassland and different maize rotations. The work was conducted within the framework of the interdisciplinary Biogas-Expert project (www.biogas-expert.uni-kiel.de), initiated to contribute to a sustainable optimization of N flows in the soil-plant-fermenter system. Each N fertilizer type was applied in four levels from zero N to over-supply. Leachate was collected nearly all year round using ceramic suction cups installed at 60 cm depth. Water fluxes were simulated with a site-adopted model where simple logistic growth equations were fitted to experimental data of canopy height and LAI, while soil water balance calculation was based on well-accepted mechanistic approaches. Nitrate-N load was then obtained by combining the simulated leachate amount with measured nitrate-N con­cen­tration.The results revealed a similar nitrate-N leaching potential of biogas residues compared to liquid animal manure. Fertilizer ammonia-N share content turned out as a main driver of N leaching loss. Grassland caused substantially lower N leaching than maize monoculture (site 1). This advantage was partly offset when relating the N loss to potential methane yield, due to a higher maize methane hectare yield. A comparison of maize rotations (site 2) showed a 7 % lower nitrate-N loss per 1000 m³ methane for maize monoculture compared to a maize-whole crop wheat-Italian ryegrass rotation, caused by similar N leaching but higher methane hectare yield at optimal N input.