Modeling N2O, NH3 fluxes, and Nmin concentrations in agricultural soils treated with biogas digestate using a modified DNDC model

The positive and negative effects of animal manure application to agricultural soils on soil inorganic nitrogen content, crop yield, ammonia (NH₃), and nitrous oxide (N₂O) emissions are well known and integrated into biogeochemical models. However, it is unclear if the effects of using digestate from biogas plants as fertilizer can be described by biogeochemical process models too. Since in Germany, the number of biogas plants increased drastically in the last two decades, there is a need for an evaluation and calibration of biogeochemical models for the application of digestate on arable land. For this purpose, we used data from a field experiment consisting of a control without fertilization, 3 treatments with mineral fertilizer, and 3 treatments with biogas digestate application (each with 60%, 80%, and 100% of maximum required N) on two cereal/maize crop rotations. Digestate was applied using trailing hoses. Results from experiments are used to calibrate and improve the biogeochemical model DNDCv.Can. Starting from a simplified description of O₂ transport, a new sub-module quantifies O₂ concentration by coupling decomposition with a 1-dimensional diffusion approach. Since the size of the anaerobic balloon calculated by the model influences many processes occurring in the soil, such as nitrification and denitrification, we hypothesize that a more realistic description of O₂ concentration, together with a model calibration addressing the decomposition kinetics of digestate, will lead to a more precise process description and, thus, to a better estimation of N₂O and, indirectly, NH₃ gas fluxes, and to more reliable estimation of NO₃^- and NH₄⁺ contents in the topsoil.