Abstract: The Mollisol belt of Northeast China is currently experiencing problems such as deterioration of soil structure, thinning of topsoil layer, and decline in fertility, which may have an adverse impact on soil nitrogen transformation. We investigated the effects of soil bulk density and aggregate size on nitrogen cycling and gaseous nitrogen losses in Mollisols from Northeast China. Two groups of treatments were established using a Roflow automated helium-based soil incubation monitoring system: one manipulating bulk density (1.0, 1.2, 1.4, and 1.6 g·cm^-3), and the other varying soil aggregate sizes (unfractionated, <1, 1-2, and 2-4 mm). Emissions of N₂O and N₂ were continuously monitored under both aerobic (20% O₂ + 80% He) and anaerobic (100% He) conditions. Coupling these observations with changes in inorganic nitrogen and the N₂O/(N₂O+N₂) ratio, we explored how soil physical structure regulates the emission pathways of N₂O and N₂. Results showed that both bulk density and aggregate size significantly affected N₂O and N₂ emissions. Under aerobic conditions, the 1.4 g·cm^-3 treatment exhibited the highest N₂O emission. Under anaerobic conditions, the 1.6 g·cm^-3 treatment had 55.1% higher cumulative N₂O emission than other treatments. The intermediate aggregate size (1-2 mm) treatment resulted in the highest N₂O emission peaks and cumulative emission and inhibited the reduction of N₂O to N₂. These findings suggest that moderate bulk density promotes nitrification-derived N₂O emissions, whereas high bulk density enhances denitrification-derived N₂O production under anaerobic conditions. Coarser aggregates (2-4 mm) and unfractionated soil (intact structure) exhibited better aeration and higher net nitrification rates, while fine aggregates showed the weakest nitrification potential. There was a significant positive correlation between N₂O emissions and net nitrification rates under varying bulk density treatments, while there was a significant negative correlation under different aggregate size treatments (P<0.01). Those results indicate that micro-environmental changes induced by soil physical structure play a critical role in modulating nitrification and N₂O reduction processes. Overall, our results elucidate the mechanistic differences in nitrogen transformation and N₂O emission pathways regulated by soil structure in the Mollisols of Northeast China. These insights provide a theoretical basis for optimizing soil physical properties to mitigate gaseous nitrogen losses and improve nitrogen use efficiency.

Key words: black soil, bulk density, particle size, N₂O, product ratio