Estimating the Organic Carbon Stabilisation Capacity and Saturation Deficit of Soils.

The soil C saturation concept is based on the idea that each soil has a maximum SOC storage capacity, which is primarily determined by the characteristics of the fine mineral fraction (i.e.  <20 µm clay + fine silt fraction).  Early estimates of the SOC storage capacity were derived from the slope of the least-square linear regression (LSR) relating SOC content to the mass proportion of the fine mineral fraction in whole soil. We evaluated this and other approaches to quantifying the upper limit of C storage in New Zealand soils using the national soils database. Total SOC ranged from 0.65 to 138 mg C g^-1, with median values of 44.4 and 20.5 mg C g^-1 for the 0-15 and 15-30 cm depths, respectively.  Maximal C stabilisation (mg fine fraction C g^-1 soil) in the top 15 cm of non-allophanic soils was estimated as 0.62 ± 0.03, 1.23 ± 0.09, and 1.16 ± 0.11 using LSR, quantile regression, and boundary line methods, respectively. Allophanic soils had substantially higher stabilisation capacity than non-allophanic soils. A multi-variate regression that included mineral surface area, Aluminum and soil pH provided the “best fit” model for predicting SOC stabilisation.  The potential to store additional SOC (i.e. saturation deficit) was estimated from this equation as the difference between the median and 90^th percentile predicted SOC content of each soil.  There was strong evidence that nearly all soils had a saturation deficit greater than zero. The median saturation deficits for both Allophanic and non-allophanic soils in 0-15 and 15-30 cm soils were 12 and 15 mg C g^-1 of soil, respectively.  The implications of these predictions for the potential to sequester SOC in New Zealand soils are discussed.