The Effects of Decreasing Carbon Saturation Deficit on Temperate Forest Soil Carbon Cycling

Soil carbon (C) sequestration has been proposed as a tool for climate change mitigation and prevention. Understanding C stabilization processes and sequestration potential is crucial to the success of such strategies. However, Earth system models used to predict soil sequestration capacities do not account for carbon saturation and functional differences among soil carbon pools. It has been hypothesized and corroborated that soil has a finite capacity to sequester C, and that sequestration efficiency decreases as C content increases. The C saturation deficit is the difference between current C content and the point of C saturation. In this study, we examined carbon stabilization in a carbon-rich forest soil. Samples were taken from The Bousson Environmental Research Reserve in Pennsylvania, USA following 20 years of detrital input manipulation. Treatment had a highly significant (ANOVA, F=10.6, p<0.0001) effect on the soil C concentration in the 0-5 cm depth. Soil carbon pools were functionally disparate: the silt-and-clay-within-large-macroaggregates and the constituents of the micro-within-large macroaggregates (intra-microaggregate particulate organic matter and intra-microaggregate silt and clay) in plots receiving no detrital input had significantly lower C concentrations (1.1 g C kg-1 soil, 2.5 g C kg-1 soil, and 0.4 g C kg-1 soil, respectively) than the control plots, plots receiving double aboveground litter input, and plots with root barriers. Plots receiving no aboveground litter showed similar trends to plots receiving no detrital input in the same fractions. Unexpectedly, coarse-particulate-organic-matter-within-small-macroaggregates C concentrations were significantly lower in plots receiving double aboveground litter (4.5 g C kg-1 soil) and plots receiving no aboveground litter (6.6 g C kg-1 soil ) than all other treatments (15.3 - 28 g C kg-1 soil). The doubled aboveground litter treatment showed trends of continued sequestration; intra-microaggregate-particulate organic matter and intra-microaggregate-silt-and-clay within-large-macroaggregates C concentrations are increased 35% and 48% relative to the control. All bulk soil samples have a moderate (4-6%) carbon concentration. This suggests that the soil is not yet approaching C saturation. This study reinforces the functional disparity of soil C pools and the use of micro-within-large-macroaggregates as a diagnostic fraction for changes in soil C content. The results also provide novel information regarding the effects of varying organic matter qualities and quantities on soil C sequestration dynamics. Finally, this study demonstrates that even forest soils with relatively high C stores may still have a C saturation deficit and consequently maintain potential for increasing C storage. Consideration of these factors will enable more accurate assessment of soil C sequestration capacity and management options.