DECOMPOSITION RATE CONSTANTS OF SIZE AND DENSITY FRACTIONS OF SOIL ORGANIC-MATTER

One of the main drawbacks of models describing soil organic matter tur nover is that most pools cannot be experimentally identified. The aim of this study was to define meaningful soil organic matter fractions t hat can be physically isolated and to determine their decomposition ra te constants so that they can be incorporated into models. This would be a major step toward verification of models. In this study, soil org anic matter was separated into five fractions: the light (density <1.1 3 g cm(-3)), intermediate (density 1.13-1.37 g cm(-3)), and heavy (den sity >1.37 g cm(-3)) fractions of macroorganic matter (>150 mu m) and the microaggregate fractions with particle sizes between 20 and 150 mu m and smaller than 20 mu m. The decomposition rate constants of the f ractions decreased in the order light, intermediate, and heavy macroor ganic matter (23.9, 9.8, and 3.9 x 10(-4) d(-1), respectively) and wer e lowest for C in the microaggregate fractions <20 and 20 to 50 mu m ( 0.5-0.7 x 10(-4) d(-1)). Since the rate constants of the fractions see med not to depend on soil texture, they can probably be applied widely . We propose to use these fractions as the principal pools in future s oil organic matter models.