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 si tes (sites I and III) and one remote site (site V), derived from volca nic ash in japan, were collected and separated into fractions with den sities < 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 densit y fractions in which organic materials weakly associated with soil min eral particles resided external to or within soil aggregates, respecti vely. The studied sites were under different vegetative covers and had different burning histories. Sites I and III were managed as grasslan d for several hundred years by the use of annual burning to prevent th e regrowth of native forest. At site I, annual burning of Japanese pam pa grass (Miscanthus sinensis) was still occurring. However at site II I 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 C P/MAS, Bloch decay, and proton-spin relaxation editing (PSRE) NMR were applied to various density fractions to study the effect of vegetativ e 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 micro scopy. The C-13 CP/MAS NMR spectra obtained for the fractions < 1.0 fr ee 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 frac tions had more alkyl and less O-alkyl carbon than the corresponding fr actions from sites I and III, indicating an influence of vegetative co ver 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 d ominated 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 fract ions. The fractions < 1.6 Mg m(-3) occluded and 1.6-1.8 Mg m(-3), howe ver, had considerably less O-alkyl and more aromatic carbon contents t han the free light fractions. The aromatic carbon in these fractions w as suggested to originate in part from charcoal and charred plant resi dues resulting from burning. Lignin and its decomposition products wer e also another source of aromatic carbon for these fractions. Applicat ion of Bloch decay NMR to the fractions 1.6-1.8 Mg m(-3), confirmed th e presence of a source of aromatic carbon containing few protons simil ar to charcoal in all three sites studied and showed that the aromatic carbon content of the fractions were underestimated by the CP/MAS met hod. PSRE NMR separated the fractions 1.6-1.8 Mg m(-3) into two compon ents and subspectra for these components in site I resembled spectra o f slightly decomposed plant residues and charcoal.