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 horizon) 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 were 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 soil samples were collected; however, at sites II, III and IV annual burning to maintain grassland vegetation ceased about 20-30, 40-50 and more than 100 years ago, respectively, and the sites were left to return to forest. At site V, a mature, broad-leaf, deciduous forest established by natural regeneration existed. The influence of annual burning and vegetative cover on the chemistry of the organic materials contained in the whole soil, the < 53 mu m soil fraction, the residues remaining 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 solid-state C-13 NMR. On site I, where grasses were still burned annually, SOM and humic acid fraction contained a greater proportion of aromatic and carbonyl carbons compared to the other sites. Alkyl carbon made a relatively small (19%) contribution to the composition of SOM on site I. When grassland was invaded by forest, the chemical nature of the SOM and humic acid fraction changed. The greatest changes occurred during the first 20-30 years, after which changes in the chemistry of SOM 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 the total soil carbon. Application of a proton-spin relaxation editing (PSRE) procedure to the SOM of each site indicated heterogeneity within the SOM structures, and subspectra of carbon associated with slower- and faster-relaxing protons were derived. Subspectra of the slowly-relaxing fractions from sites I, II and III were similar and resembled spectra of partly decomposed plant materials. The fast-relaxing subspectrum 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 charcoal and charred residues. The fast relaxing fractions from other sites included less aromatic carbon and had some O-alkyl materials. The Bloch decay spectrum of SOM from site I showed more aromatic and carbonyl carbons than the CP/MAS spectrum and highlighted an important limitation of the CP/MAS technique when it is applied to SOM containing charcoals or charred plant residues.