Effect of aggregate size distribution on soil moisture, soil-gas diffusivity, and N2O emissions from a pasture soil

Abstract

Grazed pastures rich in nitrogen (N) from ruminant urine and fertilizer inputs are significant sources of nitrous oxide (N2O), a highly potent greenhouse gas. Diffusion-controlled emission of N2O from pasture systems can be described by soil-gas diffusivity (Dp/Do), and its dependency on soil physical properties and soil moisture dynamics. But studies linking soil aggregation, soil moisture variation, Dp/Do and N2O emissions are lacking. Using coarse (2–4 mm) and fine (<0.2 mm) aggregates, and seven different combinations thereof, the effect of soil aggregate size distribution on soil–water characteristic (SWC), Dp/Do and N2O fluxes in a pastoral soil were investigated. Sieved-repacked samples, with varying fine aggregate fractions (F = 0, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 1.0) were saturated with KNO3 (1800 μg mL−1) solution and systematically drained to nine different matric potentials (−1 kPa to −10 kPa), followed by an air-dry step (−30 kPa). At each potential, Dp/Do and N2O fluxes were measured. The measured SWC and SWC-derived pore-size distributions showed clear bimodal pore structures in all combinations. The highest and lowest total porosities were observed with F = 0 and 0.7, respectively. The lowest N2O peak flux was observed with F = 0.7 which also had the lowest Dp/Do, while the highest flux among all combinations was observed in F = 1.0 at Dp/Do = 0.002. Peak N2O flux varied with Dp/Do dynamics that were in turn a function of inter-aggregate pore drainage. Initially increasing the fine fraction is speculated to have enhanced nitrifier-denitrification while further increases in the fine fraction, which lowered N2O peak emissions, were likely due to a shift from nitrifier-denitrification to denitrification and associated N2O consumption or entrapment.