Warmer Boreal Forest Organic Soil Horizons are Associated with Larger Fluxes of Dissolved Organic Carbon than their Cooler Climate Counterparts.

Abstract:

Boreal forest soils are characterized by large stocks of carbon associated with relatively slow decomposition and deep organic horizons. Dissolved organic carbon (DOC) loss from organic soil horizons occurs through hydrologically-mediated leaching processes, and contributes to downstream carbon both in deeper mineral soils and also in connected aquatic systems. However, the amount of DOC that leaves organic horizons and the environmental controls on this flux are poorly understood and are likely to be affected by climate changes. We sampled zero-tension lysimeters along 3 forested sites of a boreal climate transect to estimate DOC export from organic soil horizons of mesic boreal podzols (spodosols), and to investigate the climatic drivers of this flux. The sites are part of the Newfoundland and Labrador Boreal Ecosystems Latitudinal Transect (NL-BELT) and span approximately 5°C in mean annual temperature.

Lysimeters were sampled over 4 years and DOC flux was calculated for each seasonal period (summer, fall, and winter) of each year. DOC flux was greatest in the warmest site (114 mg C day^-1 m^-2), with the two cooler sites having lower flux rates (40 and 36 mg C day^-1 m^-2 in the intermediate and coolest sites respectively). Seasonal variation was most pronounced in the warmest site where more DOC exited the organic soil horizons during fall than during summer or winter. DOC flux was correlated with the volume of soil solution collected in the lysimeters (R² = .46), however the larger sample volumes collected in the warmest climate do not reflect greater overall precipitation. During the 4-year period of this study, similar amounts of precipitation were recorded in all regions, and the number of days with >10mm precipitation did not differ. The greater DOC flux in the warmer climate site may be due to higher rates of both litterfall and decomposition, contributing to increased labile DOC sources in the warmer climate. Analyses of the carbon quality of these samples and a better understanding of the seasonal hydrological drivers of leaching are being pursued to further investigate the potential climate influence on DOC export from these boreal organic soil horizons.