Nitrogen leaching losses after biogas residue application to maize

The production of methane from anaerobic digestion of slurry and biomass for generating electricity/heat has expanded significantly in Germany following the introduction of high subsidy payments. This results in large amounts of biogas residues, which should be used in a sustainable manner to replace fossil fuel-based fertilizer. Data on the nitrogen leaching risk after biogas residue application, however, are scarce.A two-year field experiment (2007–2009) was conducted at two sites in northern Germany to assess the nitrogen leaching potential of biogas residues compared with animal slurry (pig and cattle) and mineral N fertilizer when applied to silage maize grown in monoculture. Each fertilizer type was applied at four levels, 0, 120, 240 and 360 kg N ha−1, split into two dressings. Leachate was collected using ceramic suction cups installed at 60 cm depth. Water fluxes were simulated with a site-adapted model where simple logistic growth equations were fitted to experimental data of canopy height and green area index, while the soil water balance calculation was based on mechanistic approaches. Nitrogen load was obtained from the simulated leachate amount and measured nitrogen concentration.The nitrogen content of the leachate was dominated by nitrate-N, whereas ammonium-N and dissolved organic-N were of minor importance. Nitrate-N losses ranged between 3 and 289 kg N ha−1 a−1, depending on year, fertilizer type and location, showing an exponential increase in the amount of N applied. The mineral share of N input explained most of the variation found in the nitrate-N load. Despite distinct differences in chemical properties compared to animal slurry, especially with respect to the ammonia-N content, pH value and C/N ratio, the application of biogas residue resulted in a similar nitrate leaching potential to that of animal slurry.

► Nitrogen leaching evaluated for biogas residues (BR) applied to maize over 2 years.
► Leaching was affected by site, year, N fertilizer type and N rate.
► Digestate had similar nitrate leaching potential to animal slurries.
► Mineral share of fertilizer nitrogen explained most of the variation in nitrate-N load.
► High density of biogas plants could increase risk for nutrient management problems.