Dissolved Organic Carbon Production and Flux from Long Term Litter Manipulations in a Pacific Northwest Old-Growth Forest

Dissolved organic carbon (DOC) flux is an important mechanism to convey soil carbon (C) from aboveground organic debris (litter) to deeper soil horizons and can influence the formation of stable soil organic carbon (SOC) compounds. Aboveground litter quantity and quality was manipulated for 20-years in an old-growth Douglas fir forest under six treatments to study these relationships. DOC concentrations were measured in surface and subsurface horizons using tension lysimeters, and a hydrologic model was created to quantify water flux through the soil profile. This model, coupled with lysimeter measurements, was used to estimate annual DOC flux under the different treatment scenarios. DOC concentrations ranged from 3.0-8.0 and 1.5-2.5 mgC/L between treatments 30 and 100cm below the soil surface respectively. Aboveground detrital inputs did not have a direct linear effect on soil solution DOC; doubling the mass of aboveground leaf litter decreased observed DOC concentrations by 41%. The energetically high-quality litter accelerated metabolic rates, resulting in a “priming” effect that led to this net decrease. In contrast, the addition of aboveground woody debris increased observed DOC concentrations by 58% relative to the control. DOC temporal trends revealed that decaying root debris, coupled with the exclusion of an active rhizosphere, doubled DOC concentrations over 20 years. This debris may be a long-term source of C that is not metabolized when live roots are excluded and therefore may be a potential mechanism for the formation of stable SOC. Annual DOC flux into groundwater was small (2.7-3.7 gC/m2/year) and accounts for only 0.03-0.09% of estimated litter C at the site. Therefore, direct DOC flux from surface litter to groundwater supplies is relatively negligible in the soil C budget. However, DOC flux into surface horizons was significantly greater (73-210 gC/m2/year), equivalent to 1.2-1.9% of aboveground litter C. Therefore, DOC transport may be an important mechanism for C accumulation in shallow horizons. A greater proportion of litter C was transported annually as DOC with the addition of woody debris (1.9%), and leaf litter (1.5%), relative to the control (1.2%). This suggests that an increase in litter accumulation accelerates DOC flux relative to the total C supply, and therefore may be an important negative feedback loop on aboveground litter C storage.
Key Words: Dissolved organic carbon, soil organic carbon, litter inputs, carbon cycling.