mecHONO: a mechanistic model of reactive nitrogen gas emission from drying soils

Soil is a major source of reactive nitrogen (N) gases, such as nitric oxide (NO) and nitrous acid (HONO), that influence the atmospheric oxidative capacity by affecting near-surface hydroxyl radical (OH) and ozone (O₃) concentrations. Microbial N cycling activities in soil, particularly nitrification and denitrification, are recognised as significant biological sources of these gases, with emissions being especially notable after fertilizer application. Soil water content is the main variable determining the rates of nitrification (aerobic) and denitrification (anaerobic); however, calculations relating these processes to N gas emissions are often based on empirical relations without explicitly accounting for soil physical processes. Here, we introduce a mechanistic model, mecHONO, that integrates soil N transformation processes with soil pore drying dynamics, subject to physical constraints of mass conservation and liquid-gas interfacial transport. The model specifically accounts for evaporative concentration changes that impact pore chemistry, and, consequently, the transformation of microbial N products and their partitioning into NO and HONO. Through its application to controlled experiments, the model elucidates interactions between microbial activity and soil evaporation dynamics. Our results provide insights into effective nitrogen fertilizer application and land management to optimize nutrient utilization while simultaneously minimizing soil-derived NO_x and OH emissions.