Journal
AUSTRALIAN JOURNAL OF SOIL RESEARCH
Volume 38, Issue 5, Pages 1005-1016Publisher
C S I R O PUBLISHING
DOI: 10.1071/SR99102
Keywords
charcoal; photo-oxidation; CP/MAS C-13 NMR
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The amount of organic carbon physically protected by entrapment within aggregates and through polyvalent cation-organic matter bridging was determined on non-calcareous and calcareous soils. The composition of organic carbon in whole soils and <53 m soil fractions was determined by C-13 NMR analysis. High energy photo-oxidation was carried out on <53 m fractions and results from the NMR spectra showed 17-40% of organic carbon was in a condensed aromatic form, most likely charcoal (char). The concept that organic material remaining after photo-oxidation may be physically protected within aggregates was investigated by treating soils with a mild acid prior to photo-oxidation. More organic material was protected in the calcareous than the non-calcareous soils, regardless of whether the calcium occurred naturally or was an amendment. Acid treatment indicated that the presence of exchangeable calcium reduced losses of organic material upon photo-oxidation by about 7% due to calcium bridging. These results have implications for N fertiliser recommendations based upon organic carbon content. Firstly, calcium does not impact upon degradability of organic material to an extent likely to affect N fertiliser recommendations. Secondly, standard assessment techniques overestimate active organic carbon content in soils with high char content.
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