Ecological Efficiency of Maize-Based Cropping Systems for Biogas Production

The design of site-adapted biogas cropping systems that combine high methane yield and low environmental impact represents a major challenge for biogas production. A 2-year field experiment was conducted at Hohenschulen, northern Germany, to quantify the nitrate leaching potential and the ecological efficiency, in terms of nitrate-N load per megaliter methane produced, of three maize-based biogas cropping systems: continuous maize (CS1), maize-whole-crop wheat followed by Italian ryegrass as catch crop (CS2), and maize-grain wheat followed by mustard as catch crop (CS3). Crops were fertilized with different levels of digestate, liquid pig manure, or mineral N fertilizer. Leachate was simulated using a plant-soil model, and the nitrate-N load was obtained as the product of measured nitrate-N concentration and the amount of simulated drainage. Regression functions quantifying the relations between nitrate-N load or ecological efficiency and total N input revealed a lower potential nitrate load for CS2 than CS1 and CS3. This indicates a less severe trade-off between methane yield and water pollution for CS2. At the N input required for maximum yield, however, CS1 resulted in lower or similar nitrate-N load than CS2 and CS3. A similar pattern was detected for ecological efficiency, which varied between 2.8 and 6.3 kg nitrate-N (megaliter CH4)(-1). Mineral N fertilizer resulted in higher nitrate-N load than digestate and liquid pig manure for a given total N input. Liquid pig manure tended to cause higher nitrate-N load and lower ecological efficiency than digestate, but differences were not significant. Potential environmental benefits from biogas production can be counteracted by adverse environmental impacts such as the pollution of water bodies. The results of the short-term study suggest that choosing the appropriate N source and N rate is more important than maize cropping system selection in maximizing ecological efficiency.