THE EFFECTS OF VEGETATION AND BURNING ON THE CHEMICAL-COMPOSITION OF SOIL ORGANIC-MATTER IN A VOLCANIC ASH SOIL AS SHOWN BY C-13 NMR-SPECTROSCOPY .1. WHOLE SOIL AND HUMIC-ACID FRACTION

Soil samples were collected from the surface mineral horizon (Ah horiz on) of four adjacent soils (sites I, II, III, IV) and one remote soil (site V) derived from volcanic ash in Japan. The four adjacent sites w ere managed as Miscanthus sinensis grassland for several hundred years by the use of annual burning to prevent the regrowth of native forest species. At site I, annual burning was still being practiced when soi l samples were collected; however, at sites II, III and IV annual burn ing to maintain grassland vegetation ceased about 20-30, 40-50 and mor e than 100 years ago, respectively, and the sites were left to return to forest. At site V, a mature, broad-leaf, deciduous forest establish ed by natural regeneration existed. The influence of annual burning an d vegetative cover on the chemistry of the organic materials contained in the whole soil, the < 53 mu m soil fraction, the residues remainin g after photo-oxidation of the < 53 mu m soil fraction, and the humic acid fraction present at each of the five sites was examined using sol id-state C-13 NMR. On site I, where grasses were still burned annually , SOM and humic acid fraction contained a greater proportion of aromat ic and carbonyl carbons compared to the other sites. Alkyl carbon made a relatively small (19%) contribution to the composition of SOM on si te I. When grassland was invaded by forest, the chemical nature of the SOM and humic acid fraction changed. The greatest changes occurred du ring the first 20-30 years, after which changes in the chemistry of SO M and humic acid fraction were of a smaller magnitude. The changes in SOM chemistry included a decrease in aromatic and an increase in alkyl carbon contents, indicating that SOM produced under forest was richer in alkyl carbon than that produced under grasses managed with annual burning. The SOM at the remote site, under deciduous forest (site V), was highly aliphatic in nature with alkyl carbon contributing 35% of t he total soil carbon. Application of a proton-spin relaxation editing (PSRE) procedure to the SOM of each site indicated heterogeneity withi n the SOM structures, and subspectra of carbon associated with slower- and faster-relaxing protons were derived. Subspectra of the slowly-re laxing fractions from sites I, II and III were similar and resembled s pectra of partly decomposed plant materials. The fast-relaxing subspec trum from site I contained a strong central resonance at 130 ppm and a small peak at 176 ppm, and was very similar to spectra obtained for c harcoal and charred residues. The fast relaxing fractions from other s ites included less aromatic carbon and had some O-alkyl materials. The Bloch decay spectrum of SOM from site I showed more aromatic and carb onyl carbons than the CP/MAS spectrum and highlighted an important lim itation of the CP/MAS technique when it is applied to SOM containing c harcoals or charred plant residues.