The Effects of Decreasing Carbon Saturation Deficit On Temperate Forest Soil Carbon Cycling.

As anthropogenic derived atmospheric CO₂ begins to visibly impact the global climate, society looks increasingly toward forest soils as a carbon sink that could potentially curb further change.  However, one caveat in current Earth System models used to predict soil carbon sequestration capacity is that they assume a linear relationship between carbon inputs and sequestration, failing to account for soil carbon saturation.  Previous studies of agricultural and grassland soils suggest that the carbon sequestration efficiency of a soil decreases as the carbon content of the soil increases, i.e., approaches its saturation level.  Nevertheless, data for forest soils has not yet been reported. 

 The carbon deficiency of a soil also influences into which pools additional carbon input is sequestered.  Previous studies have indicated that as an initially high carbon deficit decreases, chemical sequestration is followed by physical sequestration, first in microaggregates and then in macroaggregates.  Where carbon is subsequently stored, as the carbon saturation deficit approaches zero, is not well understood. 

In this study, we seek to expand the understanding of soil sequestration behaviors in very carbon-rich forest soils, where the carbon saturation deficit likely approaches zero.  This research employs soil fractionation procedures to determine the aggregate size distribution and carbon content of the aggregate fractions of LTER forest soil plots that have received varied detritus input and removal treatments over a 20-year period.  Preliminary results indicate a direct relationship between increasing litter input and percent macroaggregates, with a concomitant decrease in percent microaggregates and silt and clay fractions.  Within the macroaggregate fraction, percent large macroaggregates increases as litter input increases.  Further analyses will elucidate if and how soil carbon sequestration rates change as the carbon saturation deficit approaches zero and which carbon pools are active in sequestering carbon as the carbon saturation deficit approaches zero.  The results of this study will speak to the implications of soil carbon saturation on the potential of soils, especially forest soils, to serve as a significant global carbon sink.