Cacl₂ extractable N fractions and K₂SO₄ extractable N released on fumigation as affected by green manure mineralization and soil texture

A soil incubation experiment was carried out over 70 days (aerobic, 15°C) using a clay and a sandy soil produced by amendment with bentonite and quartz sand respectively. Fresh endive leaves were added as green manure (109 mg N kg⁻¹ soil). Controls without green manure were also included. Samples were taken periodically in order to determine CaCl₂ extractable N fractions and N in microbial biomass by a fumigation-extraction method. With the green manured soils CaCl₂ extractable organic N was maximal at day 2 of incubation followed by an exponential decrease until constant levels were reached. At day 2 of incubation the apparent recovery rate of applied green manure as CaCl₂ extractable organic N was 44% (clay) and 46% (sandy soil). Without green manure the organic N extracted was only 0.3% (clay) and 0.5% (sandy soil) related to the total soil organic N. $NH_4^ - $ showed an intermediate increase (maximum at day 14) with green manured soils and was always negligible with controls. From day 14 onwards the $NO_3^ - $ accumulation was more pronounced with green manured soils compared with controls. The apparent extent of green manure mineralization at the end of incubation was 47.2 and 43.3% of the applied N in the clay and the sandy soil respectively. To summarize, bentonite amendment had no substantial effect on the rate and extent of N mineralization nor on the amount of CaCl₂ extractable organic N. The extractability of N released on fumigation appears to be restricted by bentonite. Green manure increased the amount of extractable N released on fumigation with both soils. It is concluded that CaCl₂ extractable organic N largely consists of organic substances which, in contrast to some constituents released on fumigation are only weakly adsorbed to clay surfaces and therefore are highly accessible to mineralization.