Estimating the organic carbon stabilisation capacity and saturation deficit of soils: a New Zealand case study

The capacity of a soil to sequester organic carbon can, in theory, be estimated as the difference between the existing soil organic C (SOC) concentration and the SOC saturation value. The C saturation concept assumes that each soil has a maximum SOC storage capacity, which is primarily determined by the characteristics of the fine mineral fraction (i.e. < 20 A mu m clay + fine silt fraction). Previous studies have focussed on the mass of fine fractions as a predictor of soil C stabilisation capacity. Our objective was to compare single- and multi-variable statistical approaches for estimating the upper limit of C stabilisation based on measureable properties of the fine mineral fraction [e.g. fine fraction mass and surface area (SA), aluminium (Al), iron (Fe), pH] using data from New Zealand's National Soils Database. Total SOC ranged from 0.65 to 138 mg C g(-1), median values being 44.4 mg C g(-1) at 0-15 cm depth and 20.5 mg C g(-1) at 15-30 cm depth. Results showed that SA of mineral particles was more closely correlated with the SOC content of the fine fraction than was the mass proportion of the fine fraction, indicating that it provided a much better basis for estimating SOC stabilisation capacity. The maximum C loading rate (mg C m(-2)) for both Allophanic and non-Allophanic soils was best described by a log/log relationship between specific SA and the SOC content of the fine fraction. A multi-variate regression that included extractable Al and soil pH along with SA provided the "best fit" model for predicting SOC stabilisation. The potential to store additional SOC (i.e. saturation deficit) was estimated from this multivariate equation as the difference between the median and 90th percentile SOC content of each soil. There was strong evidence from the predicted saturation deficit values and their associated 95 % confidence limits that nearly all soils had a saturation deficit > 0. The median saturation deficit for both Allophanic and non-Allophanic soils was 12 mg C g(-1) at 0-15 cm depth and 15 mg C g(-1) at 15-30 cm depths. Improving predictions of the saturation deficit of soils may be important to developing and deploying effective SOC sequestration strategies.