Soluble organic carbon in a biosolids-amended mollisol

Organic matter can be mobilized from many sources and transported in soil by precipitation, snowmelt, and water table fluctuations. Knowledge of the potential fluxes of soluble organic C in different kinds of soil is essential to improve global C budgets. We characterized the soluble and suspendable organic matter in a biosolids-amended fine, smectitic, mesic Cumulic Vertic Endoaquoll under two vegetation treatments: switchgrass (Panicum virgatum L.) and cottonwood-poplar hybrid (Populus x euramericana-clone NC-5326), Soil organic C (OC) that was dispersed in 0.1 mM CaCl2 was fractionated according to molecular weight and polarity using dialysis membranes and XAD-8 resin chromatography. Under both types of vegetation, addition of biosolids increased the amount of soluble OC (SOC) in the soil, particularly under switchgrass. Biosolids application shifted the molecular-weight distribution of SOC to lower weights (<14 000 Da). The concentrations of hydrophilic OC fractions exceeded those of hydrophobic OC in all treatments, although biosolids application increased the levels of both fractions. As the rate of biosolids addition increased, the 0 to 5 cm zone yielded smaller concentrations of extractable colloidal organic compounds than the 5 to 25 cm zone, and we attributed this to the high amounts of soluble salts introduced by biosolids application. A sequential extraction experiment showed that large amounts of colloidal organic compounds could be mobilized from the soil only after removal of the soluble salts. Our results suggest that biosolids amendments could increase the mobilization of hydrophilic, low-molecular-weight compounds in a soil while at the same time limiting the mobility of organic colloids.