Some Controls on the Release of Dissolved Organic Carbon by Plant Tissues and Soils

Soil and plant tissues were used to examine the effect on the release of dissolved organic carbon (DOC) of rinsing over time at two temperatures and under oxic and anoxic conditions in a laboratory incubation. The release of DOC over 60 days of incubation ranged from 0.5 to 189 mg DOC g 1 oven-dry material and was correlated inversely with the degree of decomposition of the material: fresh maple leaves > old maple leaves > Sphagnum moss > fibric peat > hemic peat sapric peat > Inceptisol A horizon. Rates of DOC release were similar through the duration of the experiment, except for the fresh maple leaves, where release rates fell after 3 day. Rinsing, by the replacement of water in the incubating samples after 20 day, produced slower rates of DOC release, except in the Inceptisol A and sapric peat samples. There was no significant difference between DOC release under oxic and anoxic conditions, except for the Inceptisol A soil, where DOC release was greater under anoxic than under oxic conditions. The rate of DOC release at 22 C was an average of 2.4 times greater than at 4 C, translating into Q10 values of about 1.6. At 22 C under oxic conditions, DOC production accounted for 14 to 58% (average 24%) of the total C released as DOC CO2, with the highest proportion in the maple leaves. Under anoxic conditions, DOC production accounted for 63 to 95% (average 82%) of the total C released as DOC CO2 CH4. Production of CH4 under anoxic conditions was minor, accounting for <1% of the total C released. Under oxic conditions at 22 C, the incubations released between 2 and 107% of the organic C contained in the samples, the largest proportion of which was released from the plant tissues. Microbial utilization of DOC meant that some C was double-counted, both as DOC and as subsequently emitted CO2. Under anoxic conditions, 0.0 to 49% of the sample organic C was mineralized. The release of DOC represents the balance between production, adsorption, and desorption and microbial utilization. This release differs clearly among samples and among treatment effects. (Soil Science 2001;166:38–47)