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Mr Clive Kirkby
School School of Agricultural & Wine Sciences
Location Charles Sturt University, Wagga Wagga
Phone 02 6246 5102
Influence of nutrient availability on changes in stable soil organic matter levels following crop stubble retention
Soils hold approximately 1,600 Gt of carbon, three times the amount in the terrestrial biomass and twice as much as in the atmosphere. Agricultural soils have lost approximately half (~ 50 t ha-1) of their organic carbon since the industrial revolution, a loss that has greatly contributed to soil degradation. The project has shown that humus, the most stable component of soil organic matter, contains, in addition to carbon, hydrogen and oxygen, substantial amounts of nitrogen, phosphorus and sulphur, in approximately constant ratios. It has also shown that the conversion of the straw into humus (which is much richer in N, P and S than are crop residues) is increased three to tenfold by augmenting the residues with supplementary nutrients according to the established ratios. These findings illuminate why the build-up of soil organic carbon in impoverished agricultural soils is often much less than expected from the amounts of carbon rich residues returned to them. It has important implications regarding global attempts aimed at increasing soil carbon sequestration to restore fertility and help mitigate climate change.
CSU Postgraduate Research Scholarship
Kirkby CA, Kirkegaard JA, Richardson AE, Wade LJ, Blanchard C and Batten G (2011). Stable soil organic matter: A comparison of C:N:P:S ratios in Australian and other world soils. Geoderma, 136, 197-208.