Soil pore network effects on the fate of nitrous oxide as influenced by soil compaction, depth and water potential

Soil physical properties may determine the fate of nitrous oxide (N2O) in soil, but little is known about how soil compaction affects specific properties and their interactions. This study aimed to assess the impact of compaction on the soil pore functionality and architecture, and the effects on N2O diffusion. Intact soil cores were sampled from lysimeters previously subjected to induced topsoil or subsoil compaction, as well as from uncompacted lysimeters. The soil cores were drained, sequentially, to -30, -50, and -100 h Pa to examine gas phase characteristics, each time followed by N2O diffusion measurements after injecting N2O at the bottom of the soil cores to simulate hotspots. Pore architecture was determined with X-ray microtomography. Results showed that soil compaction decreased pore volume, gas flow (convection and diffusion), and pore connectivity, and increased water-filled pore space, isolated pore ratios, and solid-to-pore distance, with a concomitant effect on N2O diffusion. Changes in soil matric water potential did not influence the N2O diffusion ratio (N2O in the headspace/ N2O injected into the reservoir). The algorithmic evaluation of interacting effects revealed that pore connectivity was the best predictor for N2O diffusion. In hierarchical order, the N2O diffusion ratio could be explained by air permeability, pore connectivity and relative gas diffusivity. Multivariate analysis of functional and architectural pore characteristic parameters provided a comprehensive selection of factors driving N2O diffusion within the soil layers. This is essential to understand the contribution of N2O produced in agricultural soil to atmospheric emissions under climate change scenarios.