Density effects on soil-water characteristics, soil-gas diffusivity, and emissions of N₂O and N₂ from a re-packed pasture soil

Abstract

Density-induced soil structural changes may potentially alter both soil total porosity and soil pore size distribution, and thus change the soil’s water retention characteristics, gas diffusion and transport properties, and subsequent greenhouse emissions. In this study, we characterized and parameterized water retention, pore size distribution, gas diffusivity and cumulative emissions of nitrous oxide (N₂O) and nitrogen (N₂) fluxes in a differently compacted silt-loam sampled from a grazed pasture in Lincoln, New Zealand. The soils used for the simulations were subjected to five different density treatments (1.1, 1.2, 1.3. 1.4, 1.5 Mg m⁻³), subsequently saturated and successively drained to 11 matric potentials at which water retention, gas diffusivity and flux measurements were performed. Results show strong correlations between best-fit soil-water characteristic parameters and the density levels. A recent predictive gas diffusivity model developed for undisturbed soils was modified to better characterize the measured gas diffusivity data in sieved-repacked pasture soils. Further, two exponential and linear parametric models were developed to adequately parameterize the observed fingerprints of cumulative (35-d) N₂O and N₂ fluxes, respectively. Results clearly distinguished the density-induced changes in pore structure, pore size distribution, gas diffusivity and emission of gas fluxes and hence provide useful implications for pasture management to reduce future emission of greenhouse gases. Results particularly highlighted the importance of ensuring a diffusivity ≥0.038 to limit extensive emission of N₂O and N₂ fluxes. The improved parametric models and parameter correlations provide valuable numerical insight to better characterize density-dependent behavior in soil physical properties and functions.