Abstract
Soil organic matter (SOM) is a central contributor to soil quality as it mediates many of the chemical, physical, and biological processes controlling the capacity of a soil to perform successfully. SOM properties (e.g. C/N ratio, macro-organic matter) have been proposed as diagnostic criteria of overall soil fitness, but their use is hampered by a poor understanding of the basic biochemical principles underlying SOM processes. The objective of this project was to determine the influence of scrub oak ( Quercus dumosa Nutt.) and Coulter pine ( Pinus coulteri B. Don) vegetation on decomposition and 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, were fractionated according to density and mineral particle size to isolate the water floatable (macro-organic matter), fine silt and clay fractions. Carbon turnover rates were determined on all fractions from AMS 14C measurements. Solid state CPMAS TOSS 13C NMR was used to semiquantitatively characterize the chemical structure of organic matter on fresh litter and soil fractions. 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, fine silt and clay fractions. These compositional differences were due to the oxidative degradation of the litter material, with preferential decomposition 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 progressing from the floatable to the clay fraction, and differences in turnover rates among fractions may be explained based on differences in carbon chemistry. In contrast, under oak, rapid carbon turnover for all fractions suggested intense biological activity in this soil.
Highlights
Soil organic matter (SOM) mediates many of the chemical, physical, and biological processes controlling the capacity of a soil to perform successfully; as such, an assessment of SOM properties is critical to de®ning overall soil quality
Quideau et al / Forest Ecology and Management 138 (2000) 19±27 framework is a valuable step towards identi®cation of a quality index that correlates with dynamic SOM processes, but it is hampered by major de®ciencies
Results from our NMR analyses, such as the observed decrease in O-alkyl from the litter to the macro-organic matter to the more decomposed organic materials associated with the mineral soil fractions, are in general agreement with changes noted in other studies where 13C NMR was used to characterize decomposition processes (Oades et al, 1987; Preston et al, 1989; Guggenberger et al, 1995)
Summary
Soil organic matter (SOM) mediates many of the chemical, physical, and biological processes controlling the capacity of a soil to perform successfully; as such, an assessment of SOM properties is critical to de®ning overall soil quality. 0378-1127/00/$ ± see front matter # 2000 Published by Elsevier Science B.V. PII: S 0 3 7 8 - 1 1 2 7 ( 0 0 ) 0 0 4 0 9 - 6. S.A. Quideau et al / Forest Ecology and Management 138 (2000) 19±27 framework is a valuable step towards identi®cation of a quality index that correlates with dynamic SOM processes, but it is hampered by major de®ciencies. To allow a more effective interpretation of measurable SOM properties as diagnostic criteria of soil quality, there is a need to better understand the fundamental biochemical processes governing the decomposition and formation of organic matter in soils
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