FORMATION AND MOBILIZATION PATHWAYS OF DISSOLVED ORGANIC-MATTER - EVIDENCE FROM CHEMICAL STRUCTURAL STUDIES OF ORGANIC-MATTER FRACTIONS IN ACID FOREST FLOOR SOLUTIONS
G. Guggenberger et al., FORMATION AND MOBILIZATION PATHWAYS OF DISSOLVED ORGANIC-MATTER - EVIDENCE FROM CHEMICAL STRUCTURAL STUDIES OF ORGANIC-MATTER FRACTIONS IN ACID FOREST FLOOR SOLUTIONS, Organic geochemistry, 21(1), 1994, pp. 51-66
Dissolved organic matter (DOM) is well recognized to influence the geo
chemistry of ecosystems. This study was conducted to determine the pat
hways of DOC mobilization in the forest floor of coniferous forests. D
OM from the forest floor of two acid forest soils in the Fichtelgebirg
e (Germany) was fractionated into hydrophobic acids and neutrals, and
hydrophilic acids and neutrals. Carbon distribution at both sites was
similar: 53 and 52% hydrophobic acids, 23 and 22% hydrophilic acids, 1
0 and 11% hydrophobic neutrals, and 7 and 8% hydrophilic neutrals, res
pectively. Structural composition of the DOM fractions was determined
using chemical degradation, FT-IR and C-13 NMR spectroscopy, as well a
s pyrolysis-field ionization mass spectrometry. Results indicate diffe
rent chemical composition of the DOM fractions within each site. Hydro
phobic acids show high concentrations of carboxyl and hydroxyl groups,
and a high ratio of vanillic acid to vanillin (ca 1.0), both indicati
ng a high degree of biodegradation of plant-derived compounds. Carbohy
drates are covalently bound to apolar moieties and from carbohydrate c
omposition a lignocellulose nature of the hydrophobic acid fraction is
suggested. Hydrophilic acids can be differentiated from the hydrophob
ic acids by their higher degree of oxidative biodegradation. The hydro
philic acid fraction also exhibits a higher ratio of microbially relea
sed polysaccharides versus plant-derived polysaccharides. Hydrophobic
neutrals show the closest relationships to the refractory soil humin,
with less degraded dimeric 'condensed' lignin subunits and relatively
high contents of non-carbohydrate aliphatics. Hydrophilic neutrals are
enriched in carbohydrates mobilized by enzymatic cellulose and hemice
llulose breakdown, as well as from microbial origin. We conclude that
DOM release into forest floor solution is related to microbial activit
y by oxidative degradation of plant-derived organic matter (e.g. water
-soluble lignin and lignocellulose fragments) and by production of mic
robial metabolites (e.g. polysaccharides). Overall evidence suggests t
hat hydrophobic acids (i) represent intermediates in organic matter de
composition, which can be further degraded to hydrophilic acids and CO
2, and (ii) are precursors of humic substances in illuvial horizons af
ter precipitation/adsorption.