ASSESSMENT OF NITRATE ADSORPTION IN SOILS BY EXTRACTION, EQUILIBRATION AND COLUMN-LEACHING METHODS

The possibility of nitrate adsorption in 18 samples representing 11 so il types from the lower Fraser Valley of British Columbia was examined by differential extraction, equilibration and column leaching methods . Contrary to what was expected if nitrate was adsorbed by the soil, m ore nitrate was extracted by water than by 2 M KCl from some of the sa mples. Observations in related studies of greater microbial growth in 0.1 and 1.0 than in 2.0 M KCl extracts after more than 1 wk of storage and of different equilibrium results when conducted with and without toluene supported the conclusion that microbial or enzyme activity cau sed the larger amount of nitrate to be extracted by water than by 2 M KCl. Both equilibration and column leaching methods measured adsorptio n in some of the soil samples, but the amounts in the various samples by the two methods were not always the same. The equilibration method was analytically more precise than the column leaching method because it was simpler and required fewer measurements, but the column leachin g method was considered to match more closely the soil to water ratio that would occur in the field. The equilibrium method found from 0 to 34% adsorption of the nitrate when added at a concentration not exceed ing 50 mu g N g(-1). Further work is required to develop a practical m ethod to meaningfully quantify nitrate adsorption in soils. The presen ce of nitrate adsorption has important implications for the interpreta tion of soil nitrogen research data and should possibly be included in nitrogen simulation models. The observation of microbial or enzyme ef fects on extraction of nitrate from soil shows the importance of using extraction solutions (e.g., those of high salt concentration or that contain a microbial inhibitor) that eliminate that possibility.