2.3. Soil organic carbon stock calculation
For each of the 96 INDY profiles the calculation of the SOC stock (also called soil organic carbon storage, soil organic carbon pool or soil organic carbon density) was performed in the layers of 0–30 and 0–100 cm depth. Commonly, these two depths are the most used in soil SOC studies (Batjes, 2000), although the IPCC (1997) methodology recommends SOC stock only at 0–100 cm depth.
The classical way of calculating C densities (C mass per area) for a given depth consists of summing C stocks by horizon, determined as a product of BD, SOC concentration, and horizon thickness, according to Bernoux et al. (2002):SOCstock=SOC∗BD∗Twhere:SOC stocksoil organic carbon stock (kg C m− 2);SOCsoil organic carbon (g kg− 1);BDsoil bulk density (Mg m− 3);Thorizon thickness (m).
In the soil survey, the soil profiles were divided into A-, B-, and C-horizons. In most cases, the calculations concerned two horizons where the first horizon typically was entirely above 30 cm, and the second one crossed this 30 cm or 100 cm depth. When a horizon crossed the 30 or 100 cm boundary, simply the portion of the horizon that was above that depth was used to calculate its SOC stock.