Labile fractions of soil organic matter, their quantity and quality

Abstract

The objective of the present paper is to contribute to the evaluation of quantity and quality of non-humified part of soil organic matter (SOM). In samples of soil organic matter from the humus profile of Šumava forest soils and forest meadows, taxonomically designated as mor and moder forms, the fractions of labile soil carbon C<sub>cws</sub>, C<sub>hws</sub>, C<sub>PM</sub> and fraction of stable carbon represented by carbon of humus acids C<sub>HA</sub> and C<sub>FA</sub> were determined. Organic matter of samples was fractionated according to the degrees of hydrolyzability by two different methods in particle-size fractions of 2.00–0.25 mm and < 0.25 mm. The quality of labile fraction C<sub>hws</sub> was expressed on the basis of reaction kinetics as the rate constant of biochemical oxidation K<sub>bio</sub> and rate constant of chemical oxidation K<sub>chem</sub> of the first order reaction from a reduction in the concentration of C-compounds. The highest values of labile forms of carbon were determined in samples with the least favorable conditions for transformation processes of SOM, and these samples also had the highest content of labile forms in hydrolyses by both methods and the most labile fractions at the same time. The degree of SOM humification was strictly indirectly proportional to the lability of SOM and its hydrolyzability. The quality of labile fraction C<sub>hws</sub> can be expressed by both K<sub>bio</sub> and K<sub>chem</sub> while the sensitivity of K<sub>bio</sub> is higher but the reproducibility of K<sub>chem</sub> is better. K<sub>bio</sub> corresponds with the degree of SOM transformation, K<sub>chem</sub> with the proportion of C<sub>PM</sub> in total C<sub>ox</sub>.