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.