The effects of vegetation and burning on the chemical composition of soil organic matter of a volcanic ash soil as shown by C-13 NMR spectroscopy .2. Density fractions

Soil samples from the surface mineral horizons (Ah) of two adjacent sites (sites I and III) and one remote site (site V), derived from volcanic ash in japan, were collected and separated into fractions with densities < 1.0 free, < 1.6 free, < 1.6 occluded, 1.6-1.8, 1.8-2.0 and > 2.0 Mg m(-3). The terms free and occluded were used to indicate density fractions in which organic materials weakly associated with soil mineral particles resided external to or within soil aggregates, respectively. The studied sites were under different vegetative covers and had different burning histories. Sites I and III were managed as grassland for several hundred years by the use of annual burning to prevent the regrowth of native forest. At site I, annual burning of Japanese pampa grass (Miscanthus sinensis) was still occurring. However at site III vegetation burning was stopped more than 100 years ago and the site was left to return to forest. At site V a mature, broad leaf deciduous forest maintained by natural regeneration existed. Solid state C-13 CP/MAS, Bloch decay, and proton-spin relaxation editing (PSRE) NMR were applied to various density fractions to study the effect of vegetative cover and burning on the chemical composition of soil organic matter (SOM) associated with different density separates. The components of SOM contained in density fractions were also studied using light microscopy. The C-13 CP/MAS NMR spectra obtained for the fractions < 1.0 free and < 1.6 free density fractions from sites I and III were similar to spectra of plant material and litter. However, at site V these fractions had more alkyl and less O-alkyl carbon than the corresponding fractions from sites I and III, indicating an influence of vegetative cover and/or extent of decomposition of plant material on the chemistry of SOM contained in these fractions. Using light microscopy, the < 1.0 Mg m(-3) free and < 1.6 Mg m(-3) free fractions were observed to be dominated by large, undecomposed root and shoot fragments and included charcoal and charred plant residues. The chemistry of the 1.8-2.0 Mg m(-3) fractions was comparable to that of the < 1.6 Mg m(-3) free fractions. The fractions < 1.6 Mg m(-3) occluded and 1.6-1.8 Mg m(-3), however, had considerably less O-alkyl and more aromatic carbon contents than the free light fractions. The aromatic carbon in these fractions was suggested to originate in part from charcoal and charred plant residues resulting from burning. Lignin and its decomposition products were also another source of aromatic carbon for these fractions. Application of Bloch decay NMR to the fractions 1.6-1.8 Mg m(-3), confirmed the presence of a source of aromatic carbon containing few protons similar to charcoal in all three sites studied and showed that the aromatic carbon content of the fractions were underestimated by the CP/MAS method. PSRE NMR separated the fractions 1.6-1.8 Mg m(-3) into two components and subspectra for these components in site I resembled spectra of slightly decomposed plant residues and charcoal.