Stabilized Carbon Pools in Forest Soils Below 20 cm Depth - Potential Sinks for Atmospheric CO2?.

The soil organic carbon (SOC) pool in mineral soil horizons plays an important role in the global C cycle. Any long-term change in forest SOC and, in particular, mineral soil SOC pools is a major uncertainty in credible terrestrial C budgets. Strategies are needed to increase the mineral soil SOC pool in forests because: (i) the turnover time of SOC increases with increase in soil depth, and (ii) the mineral soil SOC is biologically, chemically and physically stabilized. We compared the total SOC and TN pools, chemically and physically separated C fractions, and C and N pools in fine roots in a pedon under deciduous forest at the North Appalachian Experimental Watershed near Coshocton, Ohio, USA. The SOC pool (Mg ha^-1) was the highest in 0-20cm depth (47.4), and smaller in 20-30cm (6.9) and 30-46cm (6.7). The SOC and TN concentrations sharply decreased with depth. Fine root C and N pools (Mg ha^-1) were much larger in 0-20cm depth (0.71 and 0.025) than in underlying horizons. Although only 22% of the SOC pool was stored below 20-cm depth, 58% of the non-oxidizable, 49% of the mineral-associated and 31% of the clay-associated fractions of the entire SOC pool occur in the mineral soil horizons. Yet, in soils under coniferous forest mineral soil horizons contributed more to SOC stabilization than surface horizons. If this stabilized SOC pool can be managed as sink for atmospheric CO₂ by increasing C inputs to the mineral soil (e.g., by planting tree species with deep and extended root profiles and a high root turnover) needs to be shown as fresh C inputs may also stimulate the microbial mineralization (‘priming’) of stabilized SOC in deep soil horizons.