Estimates of C stocks and pedogenic processes in the Russian Taiga

Soil organic carbon (OC) stocks are of utmost importance in the global carbon cycle, and a crucial factor in their evaluation is the soil depth to be taken into account in the estimate. Depending on the occurrence of pedogenic processes, the translocation of organic matter into deep soil horizons may lead to important OC stocks that are not taken into account if the estimate is limited to the first 30 cm. We evaluated the OC stocks down to different depths in 68 soil profiles of the Russian Middle (MT) and Southern Taiga (ST) zones, estimating the bulk density from a pedotrasfer function calculated on a subset of 79 soil horizons. The two Taiga zones differed in the type of horizons, being A horizons less frequent in MT than in ST. The thickness of soil horizons was also different, as were the OC contents: O and Bs horizons were thicker in ST than in MT (p<0.05 and p<0.01, respectively), but the C contents were higher in ST only in the E (p<0.05) and Bw or Bt horizons (p<0.01). The bulk density was found to depend on OC contents, and a logarithmic model well fitted the data (r2=0.899), allowing to calculate the carbon stocks. Because of the differences in thickness and OC contents, the stocks were higher in both the O and B horizons (Bs, Bw and Bt) from the ST zone. In MT, a greater translocation of organic matter was visible from the depletion in OC of E horizons, but the lower stocks in the Bs indicated a lack of accumulation during the chilluviation phase, and suggested an even deeper migration. When the stocks were calculated at fixed depths (0-10, 10-20, 20-30 and 30-70 cm), an almost regular decrease was visible in ST, while in MT the soil section located between 30 and 70 cm depth was richer in OC than the 10-20 and 20-30 sections, thus confirming the hypothesis of a deeper migration of organic matter. Because of this strong migration of organic matter, when the OC stocks are calculated down to a depth of 30 cm only, the most C depleted horizons are taken into account in the MT areas, leading to a pronounced underestimate. The 30 to 70 cm soil section contributed to total OC stocks by 20% in ST and by 30% in MT, thus the underestimate is greater in the zones where, due to climate and vegetation characteristics, organic matter is highly mobile and the accumulation in the Bs horizon does not fully compensate the eluviation.