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 ni trogen fertilizers and soil water content on N2O and CH4 fluxes in a h umisol located on the Central Experimental Farm of Agriculture Canada, Ottawa. Addition of 100 kg N ha(-1) as either urea or NaNO3 had no si gnificant effect on soil CH4 flux measured using chambers. Fertilizati on with NaNO3 resulted in a significant but transitory stimulation of N2O production. Inorganic soil N profiles and the potential nitrificat ion rate suggested that much of the NH4+ from urea hydrolysis was rapi dly 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. Thi s humisol presents an ideal, unstructured, vertically homogeneous syst em in which to study gas diffusion, and the influence of gas-filled po rosity on CH4 uptake. In soil cores gradually saturated with H2O, the relationship of CH4 flux to gas-filled porosity was an exponential ris e to a maximum. Steepening CH4 concentration gradients partially compe nsated for the decreasing diffusion coefficient of CH4 in soil matrix air as water content increased, and diffusion limitation of CH4 oxidat ion occurred only at water contents > 130% (dry weight), or gas-filled porosities < 0.2.