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

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 ®tness, 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 in¯uence 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¯oatable (macro-organic matter), ®ne silt and clay fractions. Carbon turnover rates were determined on all fractions from AMS 14 C measurements. Solid state CPMAS TOSS 13 C 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¯oatable, ®ne 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¯oatable 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.

Conversion of a natural broad-leafed evergreen forest into pure plantation forests in a subtropical area: Effects on carbon storage

“…Many studies (e.g., [18,31,33,41]) have suggested that tree species have different impacts on soil C pools and dynamics. However, No significant differences in soil total C were observed between the plantations, indicating that tree species did not have significant impacts on soil total C, at least in the 33 years period after conversion.…”

Soil aggregate fraction-based 14C analysis and its application in the study of soil organic carbon turnover under forests of different ages

“…The disparity may be due to a number of factors, including sampling depth, regional climate, previous land use, tree species, and stand ages [23][24][25]. SOC turnover time, as a parameter for estimating soil carbon flux, is important for understanding the dynamics of SOC [26][27][28], but little has been done to evaluate the soil carbon pool change based on the SOC turnover times for the soils under planted forests. Therefore, systematic investigation on the turnover time of SOC in planted forests will be highly valuable for unraveling the mechanisms involved in the changes of the SOC pools.…”

Structural composition of organic matter in particle-size fractions of soils along a climo-biosequence in the main range of Peninsular Malaysia

“…The proportion of alkyl C increased, while the proportion of O-alkyl C decreased from 250-2000 to <53 µm fractions. A decrease in the O-alkyl C with decreasing particle-size indicated the preferential decomposition of carbohydrates in plant Unauthenticated Download Date | 5/10/18 7:48 AM [37]. The accumulation of alkyl C in the ne fractions of the studied soils could be due to both the selective preservation of aliphatic compounds in organic residues and in situ synthesis of alkyl-type products by microorganisms as stated by Baldock et al [14].…”

“…[14,18,19,37]. In each soil and from the same parent vegetation, di erences in structural composition of OM between fractions of physical fractionation were possibly ascribed to a di erent degree of decomposition as stated by Golchin et al [9].…”

“…The peak near 56 ppm corresponded to methoxyl carbon [36]. The resonance around 72 ppm was assigned to C2, C3, and C5 and that near 105 ppm to anomeric C1 in cellulose and hemicellulose, which were accompanied by the shoulders around 65 and 88 ppm corresponding to C6 and C4 carbons, respectively [37]. The signals near 130 and 150 ppm were attributed to C-or H-substituted and O-substituted (e.g.…”