Modeling coupled nitrification-denitrification in soil with an organic hotspot

The emission of nitrous oxide (N 2 O) from agricultural soils to the atmosphere is a significant contributor to anthropogenic greenhouse gas emissions. The recycling of organic nitrogen (N) in manure and crop residues may result in spatiotemporal variability in N 2 O production and soil efflux which is difficult to capture by process-based models. We propose a multi-species, reactive transport model to provide detailed insight into the spatiotemporal variability in nitrogen (N) transformations around such N 2 O hotspots, which consists of kinetic reactions of soil respiration, nitrification, nitrifier denitrification, and denitrification represented by a system of coupled partial differential equations. The model was tested with results from an incubation experiment at two different soil moisture levels ( - 30 and - 100 hPa) and was shown to reproduce the recorded N 2 O and dinitrogen (N 2 ) emissions and the dynamics of important carbon (C) and N components in soil reasonably well. The simulation indicated that the four different microbial populations developed in closely connected but separate layers, with denitrifying bacteria growing within the manure-dominated zone and nitrifying bacteria in the well-aerated soil outside the manure zone and with time also within the manure layer. The modeled N 2 O production within the manure zone was greatly enhanced by the combined effect of oxygen deficit, abundant carbon source, and supply of nitrogenous substrates. In the wetter soil treatment with a water potential of - 30 hPa, the diffusive flux of nitrate (NO 3 - ) across the manure-soil interface was the main source of NO 3 - for denitrification in the manure zone, while at a soil water potential of - 100 hPa, diffusion became less dominant and overtaken by the co-occurrence of nitrification and denitrification in the manure zone. Scenarios were analyzed where the diffusive transport of dissolved organic carbon or different mineral N species was switched off, and they showed that the simultaneous diffusion of NO 3 - , ammonium (NH 4 + ), and nitrite (NO 2 - ) was crucial to simulate the dynamics of N transformations and N 2 O emissions in the model. Without considering solute diffusion in process-based N 2 O models, the rapid turnover of C and N associated with organic hotspots can not be accounted for, and it may result in the underestimation of N 2 O emissions from soil after manure application. The model and its parameters allow for new detailed insights into the interactions between transport and microbial transformations associated with N 2 O emissions in heterogeneous soil environments.