CONTROLS ON THE SORPTION OF DISSOLVED ORGANIC CARBON BY SOILS

Abstract

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 (DOCnp), 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 (Kd) averaged 1.00 × 10−2 m3 kg−1, suggesting that DOC sorption by soils is of moderate strength compared with anions such as Cl−, NO3−, PO4− and SO42−. DOCnp 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 (R2 = 0.41–0.61, P 0.001). A multiple regression with these three variables explained 70% of the variance in DOCnp. 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 part of the DOC reactive pool.