A simple model of the composition and decomposition of digestate produced by anaerobic digestion

Anaerobic digestion is a biological process that converts organic wastes into biogas and digestate. The digestate (or bioslurry) remaining after digestion is an effective organic fertiliser because it typically contains more stabilised organic matter and a higher nutrient concentration than the feedstock due to removal of carbon as methane. However, predicting its decomposition and nutrient release in the soil remains challenging, creating uncertainty around appropriate application rates to meet crop nutrient requirements. Here we develop a simple, but process-based, model of digestate decomposability that uses only readily available data from the biogas plant (feedstock type, biogas production). A sensitivity analysis demonstrated that model responses were consistent with expected trends. Model accuracy was evaluated by comparing to lab measurements of carbon dioxide emissions from three different soils amended with bioslurry at 120 and 480 kg ha−1 total nitrogen. The model successfully estimated annual organic fertilizer production from the digester and impacts on change in soil carbon content (R2 = 0981, root mean squared error = 1.5%). An example application demonstrates that including less decomposable feedstocks will retain more carbon in the soil, so improving soil structure and water retention over the long-term. However, since nitrogen rich slurries tend to be more decomposable, this could also limit plant available nitrogen. Therefore, the new model should be used to select optimum proportions of different feedstocks so that requirements for nitrogen can be balanced by the need to improve the organic matter content of soils at each specific location.