Effect of Acid Treatment on Dissolved Organic Carbon Retention by a Spodic Horizon

Abstract

AbstractProcesses involving the movement of organic substances in forest soils are not well understood. This study was conducted to examine the role of acidic inputs on dissolved organic carbon (DOC) mobility, processes affecting the retention of DOC by a B horizon, and SO2‐4 adsorption. Using O and B horizon samples from a Spodosol collected in a forested watershed in Maine, acid solutions leached through O, B, and O over B (O/B) soils in small vacuum extractor columns were analyzed for DOC, DOC fractions, and anions. For the O and O/B columns, DOC in the leachates decreased (7550 to 3350 and 2380 to 850 µmol C L−1, respectively) with an increase in acidic inputs; for the B horizon, the reverse was the case (435 to 1570 µmol C L−1). The DOC in leachates from the O horizon was dominated by hydrophobic and hydrophilic acids (68 and 20% of DOC with no acid addition), which were altered by acidic inputs (46 and 36% of DOC at p[H+] = 2). The hydrophobic acid percentage in leachates from O/B horizon columns decreased from 52 to 28%, whereas hydrophilic acids increased from 27 to 47% with H2O and p[H+] = 2 treatments, respectively. Ionic strength adjustment (I = 0.01) of treatment solutions reduced C solubilization in O and O/B horizon leachates, leading to lower DOC leaching. The SO2‐4 adsorption in O over variable B horizon columns decreased with greater B horizon mass due to pH dependent SO2‐4 adsorption processes. The B horizon adsorption of DOC from field collector leachates showed that hydrophobic acids were selectively adsorbed, as indicated by initial mass isotherm coefficients. The results indicated that acidity, salt effects, and B horizon mass are significant factors in controlling both the amount and composition of DOC in surface and groundwaters.