CHEMICAL DENITRIFICATION IN FROZEN SOILS

A series of experiments was carried out to determine the effect of low temperature on denitrification of nitrite in several Manitoba soils. In Wellwood soil at 20(DEGREES)C., the rate of formation of chemically produced N(,2) gas varied directly with initial nitrite concentration while nitrous oxide was inhibited at higher soil nitrite levels. When Wellwood soil was incubated with 100 ppm-N as nitrite over the temperature range of -20(DEGREES)C. to 20(DEGREES)C., the rate of N(,2) gas formation decreased steadily to a low at -1.8(DEGREES)C. As the temperature was lowered further, the rate of denitrification increased dramatically by approximately twelve times and then decreased as temperature was lowered further. Similar trends occurred at four other soil nitrite concentrations. This was attributed to freeze concentration of nitrite into the unfrozen water layer near the soil colloid. Arrhenius plots were approximately parallel above and below the freezing point. As the moisture content of the 100 ppm-N nitrite soil samples (on a soil basis) was changed from 11% to 50%, at 10(DEGREES)C., the rate of denitrification decreased due to nitrite dilution in the soil water. When soil moisture was added as an 800 ppm-N nitrite solution and the amount of moisture varied, little effect was noticed on denitrification rates between 7% and 22% moisture in the temperature range of 20(DEGREES)to 0(DEGREES)C. Upon freezing, soils with the highest moisture content showed the greatest jump in denitrification. Since unfrozen water content depends little on initial moisture content, the soils with initially the most nitrite solution had the highest nitrite concentration in the unfrozen water. Comparison of three slightly acid soil types, Keld (sandy clay loam), Wellwood (fine sandy loam) and Pineridge (loamy fine sand) at 100 ppm-N nitrite over 20(DEGREES)C. to -18(DEGREES)C. showed that soils with the coarsest structure had the greatest increase in denitrification rate upon freezing. Unfrozen water content was last in the coarse soil which caused a more pronounced freeze concentration effect upon freezing. Soils of similar textures at neutral pH showed little chemical denitrification at any temperature.