High Carbon Stocks in Umbric Ferralsols Are a Function of A-Horizon Thickness.

The reason for the high C accumulation of umbric ferralsols was investigated. Because the qualifier ‘umbric' comes from dark color and thus suggests high carbon concentrations in the mineral soil, this could mean that umbric ferralsols have a mineral matrix with a particularly strong propensity to bind organic C. However, it is not known whether this assumption represents the true cause for the strong C accumulation in umbric ferralsols. We tested the two competing hypotheses that carbon stock would be either (i) a function of the depth to which the soil profile is C enriched (A-horizon thickness as the dominant variable) or (ii) a function of the extent to which the soil matrix has accumulated C (carbon concentration as the dominant variable). We used data from 203 published ferralsol profiles from across the world. C stocks in the A horizons were calculated using values of C concentration, bulk density, rock fragment, and horizon thickness. To accept or refute the competing hypotheses, linear regression analysis was performed with carbon stock as the dependent variable, while horizons thickness and C concentration as predictor variables. The A horizons of umbric ferralsols contain more C (23±12 kg m^-2 average, n=50) than that of non-umbric ferralsols (7±4 kg m^-2 average, n=153). Also, umbric ferralsols had considerably thicker A horizons (112±58cm average, n=50) than non-umbric ferralsols (34±21cm average, n=153). C-concentrations did not significantly vary between the two soil types. Linear regression (r²=0.6551) links the elevated C stocks in umbric ferralsols with the formation of thick A-horizons. This research suggests that a causal explanation for high C stocks in umbric ferralsols may be found by examining pedogenic processes involved in the formation of thick A-horizons. Therefore, further investigations should focus on processes that enable the formation of colluvial, anthropogenically altered or biologically reworked A horizons.