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Estimating the size of Australia’s soil carbon sinks

M.M. Barson1, K. Malafant2, J.O. Skjemstad3, S. Royle1, L.J. Janik, L.R. Spouncer3 and Richard H. Merry3

1Bureau of Rural Sciences, GPO Box 858, Canberra ACT 2601. Australia. Email: michele.barson@brs.gov.au
2
Complexia, PO Box 3011, Belconnen, ACT 2617, Australia.
3
CSIRO Land and Water PMB Glen Osmond, SA 5064, Australia.

Abstract

A data base of 11,980 soil profiles was used to estimate the size of the continental soil carbon pool, and to quantify the effects of charcoal, gravel and bulk density determinations on soil carbon inventory. Profiles with 3 or more observations of carbon with depth were used to fit a linearised version of an exponential decay model to describe the distribution of % TOC with depth for each profile. The summarised values for each profile were combined with explanatory variables, including climatic factors and soil properties. A Generalised Linear Model was used to provide independent predictive models based on the explanatory variables for variation in surface % TOC and % TOC with depth across the sites. These predictive equations were combined with continental surfaces for bulk density, clay content, pH, elevation and climatic parameters to predict %TOC for each 5 km grid cell across the Australian continent, and TOC t/ha stored at 0-30, 0-50 and 0-100 cm depths was estimated for the continent. The results provided useful surfaces for 0-30 and 0-50 depths, however, storage below 50cm was significantly overestimated. The lack of bulk density, charcoal and gravel content information for most Australian soils was shown to be a significant source of uncertainty in estimating soil carbon stores.

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