On the Release of Soil Exchangeable Nitrogen: Effect of Pre-Treatment with Water at Different Temperatures.

Several tests have been developed to assess the potential of soils to provide plant available N. The most widely used tests involve either aerobic or anaerobic incubation in water at temperature up to 40°C, or extraction with hot KCl or hot alkali. The common assumption of those tests is that the mineral N accumulated during incubation, or released during extraction is essentially derived from biologically mineralized (incubation) or chemically hydrolyzed (hot KCl or alkali) organic N. Twenty-one different soils from New Zealand and Eastern Canada were extracted with water by shaking for 60 min. The slurry was centrifuged, decanted, and soil residue was incubated for 16 h in water at temperature from 20 to 80°C. The amount of mineral N recovered in water extracts and in the soil residue (extracted with 2 M KCl after incubation) was compared to mineral N in the initial soils. In all soils, the maximum amount of mineral N was found after incubation at 50-60°C, with subsequent decrease at 70°C. Amounts of mineral N released at 50-60°C were the highest in agricultural soils under permanent and rotated grasslands (38 to 86 mg N kg^-1), intermediate in arable soils (9 to 28 mg N kg^-1), and the lowest under native soils (6 to 15 mg N kg^-1). The peak amount of mineral N at 50-60°C is hardly explained by the mere mineralization or hydrolysis of soil organic N, as most microbes and enzymes are inactivated at this temperature and preliminary results indicated that little hydrolysis of amino sugars and amino acids occur. Yet, the amount of mineral N measured after the 16-h incubation at 50°C was 2 to 24 times higher than in the initial soils. Other possible sources of mineral N, such as recently fixed NH₄, could have contributed to the release of mineral N. The release of recently fixed NH₄ is likely confounded with N mineralization in soil N tests where the net release of mineral N is the key parameter.