ORGANIC AND INORGANIC NITROGEN LEACHING FROM INCUBATED SOILS SUBJECTED TO FREEZE-THAW AND FLOODING CONDITIONS

Freeze-thaw and flooding of usually well-drained soils occur in the spring in the prairie and boreal regions of Canada. We studied the impact of these conditions on nitrogen leaching in a Black Chemozemic soil (Udic Boroll). Soil samples, subjected to different treatments, were incubated for 12 wk in the laboratory and leached every 2 wk with 0.001 M CaCl2 solution. The cumulative leaching loss of total N (mg kg-1 soil) was reduced by freeze-thaw (76.0), flooding (41.4) and a superimposition of the two treatments (28.8) compared to the control (109). All treatments affected the distribution of the forms of N leached. The total loss of water soluble organic N (SON) and ammonium-N was in the order of flooded > flooded-freeze-thaw > freeze-thaw = control. In the leachates from the flooded treatments, SON accounted for 71.5-77.4 % of the total N leached. Nitrate- and nitrite-N dominated the total leachable N in the unflooded treatments following an order of control > freeze-thaw > flooded = flooded-freeze-thaw. During the incubation, the Eh of the flooded soils decreased from 344 to -46 mV, compare to a variation in Eh from 355 to 301 mV for the unflooded soils. The maximum rate of leaching of organic nitrogen from the flooded treatment (0.53 mg N kg-1 d-1) coincided with a sharp decrease in Eh, from 131 to 42 mV. It is concluded that climatic events will have a significant impact on the dynamics of soil nitrogen. Flooding, in particular, may promote the loss of N in water soluble organic matter.