Substrate inputs, nutrient flows and nitrogen loss of two centralized biogas plants in southern Germany

In Germany, centralized biogas digestion plants (BGP) have been recently constructed. BGPs purchase the substrates from surrounding farmers and, in return, farmers receive the effluents. Substrate inputs, nutrient inputs and outputs were studied for two BGPs with effluent liquid–solid separation. Additionally, the path of the nitrogen (N) during manure handling was assessed. Silage maize (65–75% of the dry matter (DM) inputs) and grass (ca. 20% of the DM inputs) were the main inputs in both BGPs. During manure handling, it is estimated that 20–25% of the N in the effluents was lost via gaseous N emissions. From an environmental point of view the two main challenges are to reduce these gaseous N losses, and to provide N via the effluents mainly for spring manure application, and less so for autumn application. In solid effluents, gaseous N losses during storage are the main potential N loss pathway, whereas for liquid effluents gaseous N losses during and after field spreading are of great relevance. Current management indicated that approximately 50% of the N in the effluents was available for spring application and approximately 30% in autumn due to cleanout of stores before winter. Calculations show that the use of substrates with high DM content during autumn and winter would reduce the demand for storage capacity, thus reducing the demand for store’s cleanout in autumn. This leads to effluents with higher nutrient concentration that are very suitable for application to spring sown crops. Furthermore, some substrates like cereal grains and grass lead to effluents higher in N, whereas silage maize and other substrates lead to effluents low in N. An adapted substrate management would allow more N for spring application. The cycles of P and K are closed, enabling a complete replenishment of the P and K outputs.