Soil organic matter processes: characterization by C-13 NMR and C-14 measurements

Soil organic matter (SOM) is a central contributor to soil quality as it me diates many of the chemical, physical, and biological processes controlling the capacity of a soil to perform successfully. SOM properties (e.g. C/N r atio, macro-organic matter) have been proposed as diagnostic criteria of ov erall soil fitness, but their use is hampered by a poor understanding of th e basic biochemical principles underlying SOM processes. The objective of t his project was to determine the influence of scrub oak. (Quercus dumosa Nu tt.) and Coulter pine (Pinus coulteri B. Don) vegetation on decomposition a nd SOM formation processes in a lysimeter installation constructed in 1936 in the San Gabriel mountains of southern California. Soil samples archived during construction of the installation, and A horizons sampled in 1987, we re fractionated according to density and mineral particle size to isolate t he water floatable (macro-organic matter), fine silt and clay fractions. Ca rbon turnover rates were determined on all fractions from AMS C-14 measurem ents. Solid state CPMAS TOSS C-13 NMR was used to semiquantitatively charac terize the chemical structure of organic matter on fresh litter and soil fr actions. For the two soils, there was a progressive decrease in O-alkyl C, and an increase in alkyl and carbonyl C from the litter to the floatable, f ine silt and clay fractions. These compositional differences were due to th e oxidative degradation of the litter material, with preferential decomposi tion of the cellulose and hemicellulose entities and selective preservation of recalcitrant waxes and resins. in all soil fractions, turnover rates of carbon were longer for the pine than for the oak lysimeter (up to 10 times longer). Also under pine, there was a gradual increase in turnover rate pr ogressing from the floatable to the clay fraction, and differences in turno ver rates among fractions may be explained based on differences in carbon c hemistry. In contrast, under oak, rapid carbon turnover for all fractions s uggested intense biological activity in this soil. (C) 2000 Published by El sevier Science B.V.