Aggregate Stability and Carbon Distribution as a Function of Soil Physiochemical Variables.

The recent rise in atmospheric CO2 has focused attention on the mechanisms of carbon stabilization in soils. Within soils, soil organic matter is stabilized through interactions with the mineral phase. These interactions then form aggregates that serve to protect soil organic matter from rapid degradation. The stability of the soil aggregates serves as an indicator of how soil systems respond to environmental forcings. In this study, four soils formed under Ponderosa Pine forests were sampled in a lithosequence including rhyolitic, granitic, basaltic, and limestone parent materials. These soils possess different mineral/metal(oid) assemblages and varying organic carbon content. We specifically examined variation in aggregate stability as a means to elucidate physical mechanisms regulating abundance of carbon occluded in aggregates and carbon preserved on mineral surfaces. This study employed an energy-resolved sonication-dispersion method to quantify aggregate stability of the fine earth fraction. The information gathered was then used to establish aggregate stability based on change in percentage of sand-sized aggregates. The results indicate that while granitic soils contain few aggregates, the aggregates are more stable than the aggregates present in the basaltic soils. Limestone and rhyolitic soils fall within these two endpoints. Results also indicate significant variation in aggregate stability and carbon distribution among parent materials with both parameters exhibiting significant dependence on soil physiochemical and mineralogical variables. Regression analysis of the carbon fractions indicates that occluded carbon is correlated positively with non-crystalline Fe and negatively with crystalline Fe. Mineral carbon is correlated positively with crystalline Fe and exchangeable acidity and negatively with Al-humus complexes. Regression analysis also suggests the amount of water stable aggregate increases with clay content, non-crystalline Fe and Al-humus complexes. This study indicates iron plays a more important role than clay in stabilizing both water stable aggregates and carbon in the occluded and mineral fractions.