Recycling energy and nutrients from anaerobic co-digestion of swine manure with crop residues in soybean-winter cover crop-corn rotations.

Cover crops have been widely adopted on farmlands between corn and soybean growing seasons to reduce soil erosion. This study investigated the potential of using anaerobic digestion (AD) to recycle energy and nutrients from crop biomass from a soybean-winter cover crop-corn rotation with incorporated swine manure. Soybean, corn, and winter cover crops including annual ryegrass, cereal rye, and a mix of Pea, Clover, Radish, and Oat (PCRO), were planted in 2023-2024. Results from AD experiments using benchtop reactors showed that the highest methane yields were achieved at a 60:40 (soybean stalk: swine manure) mixing ratio for soybean stalk, 80:20 for the three cover crops, and 40:60 for corn stover. These optimal mixing ratios were then applied in pilot-scale reactors, which produced 271-361 L/kg-VS (volatile solids) of methane. A total of 350,203 MJ of energy was estimated from the co-digestion of swine manure with one hectare of biomass in the soybean-cereal rye-corn rotation. However, when accounting for the effects of ensiling and storage, energy output decreased approximately 10-15 %. Based on biomass yield and composition, an estimated 422, 74, and 545 kg of N, P, and K, respectively, were present in one hectare of crop biomass in the soybean-cereal rye-corn rotation. Analysis of the digestate showed that over 86 % of the NPK in feedstocks remained available, and the amount of NH-N, which is readily available for plant uptake, increased significantly following AD. This study shows that a three-crop rotation can offer significant bioresources for energy production and nutrient recycling.