Soil-gas diffusivity and soil-moisture effects on N₂O emissions from repacked pasture soils

Abstract

Grazed pasture constitutes a major source of agriculturally derived nitrous oxide (N₂O), which is a potent greenhouse gas. Soil texture and structure, soil moisture, and soil-gas diffusivity are considered to be major soil physical drivers controlling pastoral N₂O emissions. Research gaps exist regarding their combined roles on N₂O emissions dynamics. This study used 2-mm-sieved and repacked soil samples, retrieved at three depths (0, 10, and 15 cm) from three grazed pasture sites in New Zealand, to investigate the combined effects of soil-water characteristic (SWC) and soil-gas diffusivity on N₂O emissions. The existing and modified parametric functions were used to numerically characterize and parameterize measured particle size distribution (PSD), SWC, and soil-gas diffusivity. We observed distinct PSDs within the three soils with little variation across depths. Distinct fingerprints were observed for SWC and gas diffusivity in the three pasture soils, suggesting clear effects of soil type on diffusion-controlled gas emissions. The soil moisture retention above approximately −10 kPa decreased with increasing depth and showed clear soil type effects. Soil-gas diffusivity, on the other hand, showed pronounced depth-wise variation below approximately −1000 kPa. Pore tortuosity was found to be nonlinearly correlated to air-filled pore space as well as the PSD. The measured N₂O fluxes peaked around a diffusivity window of 0.005 to 0.01 for all soil types, and the corresponding water-filled pore space ranged from 0.80 to 0.95. The results provide guidance for managing pasture soils to reduce high N₂O fluxes through reductions in compaction and excess irrigation such that the critical diffusivity window, where peak N₂O emissions occur, is avoided.