Effect of nitrogen fertilizers and moisture content on CH4 and N2O fluxes in a humisol: Measurements in the field and intact soil cores

Field and laboratory studies were conducted to determine effects of nitrogen fertilizers and soil water content on N₂O and CH₄ fluxes in a humisol located on the Central Experimental Farm of Agriculture Canada, Ottawa. Addition of 100 kg N ha^–1 as either urea or NaNO₃ had no significant effect on soil CH₄ flux measured using chambers. Fertilization with NaNO₃ resulted in a significant but transitory stimulation of N₂O production. Inorganic soil N profiles and the potential nitrification rate suggested that much of the NH₄⁺ from urea hydrolysis was rapidly nitrified. CH₄ fluxes measured using capped soil cores agreed well with fluxes measured using field chambers, and with fluxes calculated from soil gas concentration gradients using Fick's diffusion law. This humisol presents an ideal, unstructured, vertically homogeneous system in which to study gas diffusion, and the influence of gas-filled porosity on CH₄ uptake. In soil cores gradually saturated with H₂O, the relationship of CH₄ flux to gas-filled porosity was an exponential rise to a maximum. Steepening CH₄ concentration gradients partially compensated for the decreasing diffusion coefficient of CH₄ in soil matrix air as water content increased, and diffusion limitation of CH₄ oxidation occurred only at water contents > 130% (dry weight), or gas-filled porosities < 0.2.Corresponding author