STRUCTURAL AND DYNAMIC PROPERTIES OF SOIL ORGANIC-MATTER AS REFLECTEDBY C-13 NATURAL-ABUNDANCE, PYROLYSIS MASS-SPECTROMETRY AND SOLID-STATE C-13 NMR-SPECTROSCOPY IN DENSITY FRACTIONS OF AN OXISOL UNDER FORESTAND PASTURE

Changes in the content and isotopic composition of organic carbon as a consequence of deforestation and pasture establishment were studied i n three neighbouring areas on an Oxisol from Australia and used to mea sure the turnover of forest-derived carbon (C-3) under pasture (C-4) o ver 35 and 83 year time scales. The results indicated that the quantit y of forest-derived carbon declined rapidly during the first 35 years under pasture but the content remained nearly stable thereafter, sugge sting the presence of two pools of carbon with different turnover time s. The calculated values for turnover time of labile and resistant fra ctions of forest-derived carbon were 35 and 144 years respectively. Th e soil samples were separated into five fractions with densities <1.6 (free and occluded), 1.6-1.8, 1.8-2.0 and >2.0 Mg m(-3). Based on the spatial distribution of organic materials within the mineral matrix of soil, the soil organic matter contained in different density fraction s was classified as free particulate organic matter (<1.6 free), occlu ded particulate organic matter (<1.6 occluded, 1.6-1.8 and 1.8-2.0) an d clay associated organic matter (>2.0 Mg m(-3)). The C-13 natural abu ndance showed that the free particulate organic matter formed a signif icant pool for soil organic matter turnover when the forest was replac ed by pasture. Compared with free particulate organic matter, the orga nic materials occluded within aggregates had slower turnover times. Th e occluded organic materials were in different stages of decomposition and had different chemical stabilities. Comparison of the chemistry a nd isotopic composition of occluded organic materials indicated that t he O-alkyl C content of the occluded organic materials was inversely r elated to their stabilities whereas their aromatic C content was direc tly related to their stabilities. In soils under pasture, a considerab le amount of forest-derived carbon was associated with clay particles in the fractions >2.0 Mg m(-3). The rate of accumulation of pasture-de rived carbon was also rapid in this fraction, indicating the presence of two different pools of carbon (C-3 and C-4) associated with clay pa rticles. The forest-derived carbon had the highest stability in the fr actions >2.0 Mg m(-3), probably due to strong interaction with active aluminium or iron and aluminium oxides associated with clay surfaces.