CONTROLS ON THE SORPTION OF DISSOLVED ORGANIC-CARBON BY SOILS

Dissolved organic carbon (DOC) sorption isotherms were, determined for 48 soil samples derived from Humaquepts, Inceptisols, and Spodosols using a DOC solution derived from a swamp peat (Hemist). Forty-six of the samples had DOC sorption adequately represented. by the linear initial mass isotherm, with initial DOC concentrations up to 81 mg L-1. Null-point DOC concentrations (DOC(np)), where there is zero net DOC sorption, ranged from 6.7 to 85.4 mg L-1, with an average regression coefficient of 0.45. Distribution coefficients (K(d)) averaged 1.00 x 10(-2) m3 kg-1, suggesting that DOC sorption by soils is of moderate strength compared with anions such as Cl-, NO3- , PO43- and SO42-. DOC(np) values were strongly related to contents of soil organic C (positive) and oxalate-extractable Al and dithionite-extractable Fe (both negative) using partial regression analysis (R 2 = 0.41-0.61, P < 0.001). A multiple regression with these three variables explained 70% of the variance in DOC(np). The sorption regression coefficient and reactive soil pool for DOC were also positively related to soil organic C content, with an average of 3.2% of the organic C in the soil being Dart of the DOC reactive pool.