SOIL FACTORS RELATED TO DISSOLVED ORGANIC-CARBON CONCENTRATIONS IN A BLACK SPRUCE SWAMP, MICHIGAN

Controls on dissolved organic carbon (DOC) concentrations were examined through field and laboratory measurements of a Typic Haplaquod in Michigan. Average DOC concentration in the soil solution at 30-cm depth was 32 mg/L, and groundwater DOC concentration at 2-m depth was 18 mg/L. Oxidation-reduction (redox) potentials measured in the upper 30 cm of soil ranged from -220 mV to +500 mV, indicating the presence of both reduced and oxidized conditions at the site. Mineral soil organic carbon (SOC) ranged from 0.47% in the Bg horizon to 2.70% in the Bhs horizon. Citrate-dithionite extractable iron (Fe(c/d)) ranged from 5.9 mug/g in the E horizon to 85.0 mug/g in the Bhs horizon. Citrate-dithionite extractable aluminum (Al(c/d)) ranged from 2.1 mug/g in the Bg horizon to 15.3 mug/g in the Bhs horizon. Mineral SOC, Fe(c/d), and Al(c/d) concentrations were generally in the low range of those reported for Spodosols in Canada and United States. DOC concentration in both the soil solution and groundwater was strongly negatively related to Al(c/d) and Fe(c/d). Null-point DOC (DOC(np)) concentrations (point at which no net DOC sorption occurs) obtained from sorption studies ranged from greater than 90 mg/L in the E horizon to 18 mg/L in the Bs horizon. This indicated that DOC sorption in the mineral subsoil potentially increased with depth. DOC(np) was also negatively related to Fe(c/d) and Al(c/d). Although DOC concentrations were strongly correlated with Fe and Al in the mineral soil, concentrations of those elements were so low that little DOC was retained in the mineral soil.