The nature of organic carbon in the <2, 2-20, 20-53, 53-200, and 200-2000 pm fractions of four surface soils was determined using solid state 13C nuclear magnetic resonance (n.m.r.) spectroscopy with cross polarisation and magic angle spinning (CP/MAS). Analyses were repeated after high energy ultraviolet photo-oxidation was performed on the three finest fractions. All four soils studied contained appreciable amounts of physically protected carbon while three of the soils contained even higher amounts of charcoal. It was not possible to measure the charcoal content of soils directly, however, after photo-oxidation, charcoal remained and was identified by its wood-like morphology revealed by scanning electron microscopy (SEM) together with a highly aromatic chemistry determined by solid state 13C n.m.r. Charcoal appears to be the major contributor to the 130 ppm band seen in the n.m.r. spectra of many Australian soils. By using the aromatic region in the n.m.r. spectra, an approximate assessment of the charcoal distribution through the size fractions demonstrated that more than 88% of the charcoal present in two of the soils occurred in the <53 pm fractions. These soils contained up to 0.8 g C as charcoal per 100 g of soil and up to 30% of the soil carbon as charcoal. Humic acid extractions performed on soil fractions before and after photo-oxidation suggest that charcoal or charcoal-derived material may also contribute significantly to the aromatic signals found in the n.m.r. spectra of humic acids. Finely divided charcoal appears to be a major constituent of many Australian soils and probably contributes significantly to the inert or passive organic carbon pool recognised in carbon turnover models.
Five surface soils of differing chemical and mineralogical compositions were subjected to either a sequence of dithionitelcitrate extractions in which the soil: citrate ratio was varied or to a sequence of 1% or 2% HF extractions. The 2% HF treatment resulted in the removal of the highest Fe concentrations (79-95%) while the dithionitelcitrate extractions were less effective in removing Fe from the same soils (18-75%). The Fe remaining after HF treatment appeared to be mostly associated with ilmenite crystals which were only slowly attacked by the dilute acid. During the 2% HF treatments, some organic carbon was lost (8-17%). but this loss had no significant effect on the organic chemistry of the samples as determined by CP/MAS 13C n.m.r. spectroscopy.The total 13c signal recovered after the various treatments was found to be correlated, in order of decreasing significance, with the mineral fraction present in the sample, the organic carbon/Fe ratio and the mass magnetic susceptibility. The expression (organic carbon/Fe) -0.147(mineral fraction present in the sample) +0.043(l/mass magnetic susceptibility), accounted for 85.3% of the variation in the relative visibility of the 13C signal.Prior to solid state CP/MAS 13c n.m.r. analysis, the recommended pretreatment for surface soils containing Fe involves a sequence of 2% HF extractions in the order 5x1 h, 2x16 h and 1x64 h. For soils high in Fe, this procedure allows CP/MAS 13c n.m.r. spectra to be acquired that would otherwise be difficult to obtain. It also results in a significant increase in sensitivity and in resolution of the 13c n.m.r. spectra of soils with moderate Fe contents.
The biosphere plays an important role in determiniig the sources, sinks, levels and rates of change of atmospheric CO. , concentrations. Significant uncertainties remain in estimates of the fluxes of CO, from biomass buminhd deforestation, &d uptake and storage of C02 by the biosphere arising from in&eased atmospheric CO, concentrations. Calculation of probable rates of carbon sequestration for the major ecosystem compl&es and global 3-D tracer transp& model runs indicate the pc&ibility that a significant net C02 uptake (> 1 Pg C yfl), a C02 'fertilisation effect', may be occumng in tropical rainforests, effectively accounting for much of the 'missing sink'. This sink may currently balance much of the C 4 added to the atmosphere from deforestation and biomass burning. Interestingly, C02 released from biomass burning may itself be playing an important role in enhanced carbon storage by tropical rainforests. This has important implications for predicting future C02 concentrations. If tropical rainforest destruction continues then much of the C02 stored as a result of the C02 'fertilisation effect' will be rereleased to the atmosphere and much of the 'missing sink' will disappear. These effects have not been considered in the IPCC (Intergovernmental Panel on Climate Change) projections of future atmospheric C02 concentrations. Predictions which take account of the combined effects of deforestation, the return of carbon previously stored through the CO? 'fertilisation effect' and the loss of a large proportion of the 'missing sink' as a result of deforestation,-would result in much higher predicted concentrations and rates of increase of atmospheric C02 and, as a consequence, accelerated rates of climate change.
Summary. Non-living soil organic matter is a small but critical component of soils contributing to soil structure, fertility and a range of other chemical, physical and biological functions. Although considerable work has contributed to our knowledge of its distribution, chemical structure, mineral associations and turnover, there is still little information on which fractions or pools of non-living soil organic matter are implicated in various soil functions and to what extent. This review paper summarises some of what is known about the distribution, chemistry, mineral associations and soil structure, turnover and the measurement of non-living soil organic matter, with particular emphasis on Australia. It also discusses some of the difficulties in using current methods for describing the function of this material in soil.
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