Modeling coupled nitrification-denitrification in manure-amended soil

The flux of nitrous oxide (N₂O) from the soil to the atmosphere is an important contributor to the global greenhouse effect. Spatial variation in the distribution of factors that drive N₂O producing processes often creates hotspots within soil that are difficult to quantify and model, and this is particularly the case after manure amendment. In the stagnant soil matrix, solute diffusion is crucial in supplying the nitrate (NO₃^-)to nearby zones, i.e., hotspots, to maintain a locally high NO₃^- reduction. To provide detailed insight into the spatiotemporal variability of nitrogen (N) transformations around N₂O hotspots, we propose a multi-species, reactive transport model containing kinetic reactions of soil respiration, nitrification, nitrifier denitrification, and denitrification, built on a system of partial differential equations. The model was used to simulate the amount of N₂O, dinitrogen (N₂) and carbon dioxide (CO₂) emitted from a 10 cm soil profile with time, and concentration profiles of crucial intermediates. The measured N₂O and N₂ fluxes correlate well with the model simulations, with a simulated stratification of growing nitrifying and denitrify activities in and around the manure hotspot which is consistent with previous incubation experiments. N₂O evolution was sensitive to the initial setup of oxygen (O₂) availability, and anaerobic condition was maintained in the saturated manure zone throughout the simulation period, demonstrating the necessity of simulating the heterogeneity within soil in N₂O models. Simulation experiments will be conducted to assess the effects of solute diffusion on N transformations. We anticipate that diffusive NO₃^- transport to be crucial to facilitate the coupled nitrification-denitrification around the manure zone. Neglecting such a process in process models may make it difficult to reflect the rapid turnover of nitrogen pool around organic hotspots and underestimate N₂O emissions. The model and its parameters allows for new detailed insight into N₂O formation processes in heterogeneous environments.