Composition and dynamics of dissolved carbohydrates and lignin-degradation products in two coniferous forests, N.E. Bavaria, Germany

Abstract

In order to examine the biogeochemical control of dissolved carbohydrates and lignin-derived moieties at two Norway spruce sites, their chemical composition and dynamics were investigated in bulk precipitation, throughfall, forest floor solution, and in the mineral soil solution collected from two profile depths (30 and 90 cm). The series of sugars released by trifluoroacetic acid hydrolysis indicated that carbohydrates in the bulk and canopy precipitation were rich in hexoses and deoxysugars, and poor in pentoses. This indicates that microbial communities living on the needles and on or in the dust particles of the input are main sources of carbohydrates released in the forest canopy. In the forest floor preferentially carbohydrates rich in plant-derived pentoses were solubilized and translocated into the mineral soil. They are possibly of a lignocellulose nature. Annual flux of carbohydrates infiltrating the mineral soil amounted to ca 70kg ha −1. Carbohydrate flux in the mineral soil output was much lower ( ca 5 kg ha −1 yr −1), probably due to co-adsorption of carbohydrates associated with hydrophobic, recalcitrant compounds by the soil matrix, and biodegradation of free carbohydrates. CuO oxidation products served to characterize the lignin component. Compared with the remnant lignin in the forest floor preferentially highly carboxylated lignin-derived substances entered the forest floor solution. Thus, the solubilization of lignin-derived moieties is controlled by their degree of biooxidation. The amount of CuO-lignin percolating into the mineral soil was ca 30 kg ha −1 yr −1, and the annual CuO-lignin export from the solum amounted only to ca 0.2 kg ha −1. Adsorption, rather than biodegradation is more likely to be responsible for this low CuO-lignin flux in the mineral soil output.