Baseline map of Australian soil organic carbon stocks and their uncertainty

Brief description

This is a fine spatial resolution (90 x 90 m pixels) baseline map of organic Carbon stocks at the continental scale. The uncertainty is described by the 95% confidence intervals and the standardised confidence intervals (range(95% CIs)/mean). Maps are provided in GeoTiff format with LZW compression.\nLineage: The data on the soil’s organic C content and BD recorded from 2000 to 2013 (median year 2009) came from three sources: Australia’s National Soil Carbon Research Programme (SCaRP) (reference); Spectroscopic estimates of organic C and BD made with the Australian visible–near infrared database (Viscarra Rossel and Webster, 2012) on soil samples collected for the National Geochemical Survey of Australia (NGSA) (de Caritat et al., 2008); and The Australian Soil Resource Information System (ASRIS), the Commonwealth Scientific and Industrial Research Organisation’s (CSIRO’s) central repository for soil data in Australia (Johnston et al., 2003). The combined data represent soil from all states and territories of Australia, all soil types present in the Australian Soil Classification and all land-use classes. It represents the most current dataset on the soil organic carbon stocks of Australia. \nThe map was derived by combining the use of a decision tree with piecewise regression on environmental variables and geostatistical modelling of residuals. The models were tested by (i) cross validation (ii) bootstrap validation and (iii) independent test set validation. The models were robust, accurate and unbiased. The concordance correlation coefficient for the test set validation was 0.81. The concordance correlation coefficient combines measures of both precision and bias to determine how far the observed data deviate from the line of perfect concordance, which is the 1:1 line. The concordance correlation coefficient ranges from -1 to +1. A value of +1 denotes perfect agreement, values >0.9 suggest near perfect agreement, values between 0.8 and 0.9 substantial agreement, between 0.65 and 0.8 moderate agreement and values <0.65 poor agreement.\nValues of stock were predicted at the nodes of a 3-arc-sec (approximately 90 m) grid and mapped together with their uncertainties. The uncertainty was calculated using 100 bootstrap realisations so that for each pixel there were probability density functions from which the computation of the best estimates and standard errors was possible. We calculated baselines of soil organic C storage over the whole of Australia, its states and territories, and regions that define bioclimatic zones, vegetation classes and land use.\nThe research is described in Viscarra Rossel, R. A., Webster, R., Bui, E. N. and Baldock, J. A. (2014), Baseline map of organic carbon in Australian soil to support national carbon accounting and monitoring under climate change. Global Change Biology. doi: 10.1111/gcb.12569\n