DECOMPOSABILITY OF RECALCITRANT SOIL CARBON ASSESSED BY DENITRIFICATION

The decomposability of carbon compounds remaining in soil after decomposition during extended storage was studied by measuring denitrification in anaerobic soil slurries. The denitrification assay is not disturbed by heterogenous conditions of incubation and employs a physiologically rather homogenous group of respiring organisms. In five soils (alder, elm, meadow, barley and grass) all C mineralization occurred by oxidation and denitrification was the dominant process when NO3- was added. In beech soil, fermentation prevailed and the amount of C oxidized doubled when NO3- was added. The quality of soil C for respiration was low and surprisingly similar in soils from alder, beech and grass corresponding to 0.3 mg C g-1 soil C d-1. In meadow, barley and elm the quality of soil C for respiration was 2-3 times higher. The separation of the six soils based on C quality for denitrification was parallel to soil characteristics such as freeze/thaw effects and nutrient release from soil C and site characteristics such as earthworm biomass and plant cover.