Assessment of nitrate adsorption in soils by extraction, equilibration and column-leaching methods

The possibility of nitrate adsorption in 18 samples representing 11 soil 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, more nitrate was extracted by water than by 2 M KCl from some of the samples. 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 caused the larger amount of nitrate to be extracted by water than by 2 M KCl. Both equilibration and column leaching methods measured adsorption 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 leaching 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 exceeding 50 mu g N g(-1). Further work is required to develop a practical method to meaningfully quantify nitrate adsorption in soils. The presence of nitrate adsorption has important implications for the interpretation of soil nitrogen research data and should possibly be included in nitrogen simulation models. The observation of microbial or enzyme effects 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.