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

Controls on dissolved organic carbon (DOC) concentrations were examine d through field and laboratory measurements of a Typic Haplaquod in Mi chigan. 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 bot h reduced and oxidized conditions at the site. Mineral soil organic ca rbon (SOC) ranged from 0.47% in the Bg horizon to 2.70% in the Bhs hor izon. Citrate-dithionite extractable iron (Fe(c/d)) ranged from 5.9 mu g/g in the E horizon to 85.0 mug/g in the Bhs horizon. Citrate-dithion ite extractable aluminum (Al(c/d)) ranged from 2.1 mug/g in the Bg hor izon 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 fo r 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 whi ch no net DOC sorption occurs) obtained from sorption studies ranged f rom 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) a nd Al(c/d). Although DOC concentrations were strongly correlated with Fe and Al in the mineral soil, concentrations of those elements were s o low that little DOC was retained in the mineral soil.