Dissolved Organic Carbon in Upland Forested Watersheds Underlain by Continuous Permafrost in Central Siberia

Abstract

Hydroclimatic variability and plant species ecology cause mosaics in forested watersheds in permafrost zones. Measurements of organic matter accumulation, stock of dissolved organic carbon (DOC), DOC concentrations in litter leachates, subsurface flow, stream and seasonal and annual export were made in two contrasting slopes and valleys in the northern taiga of Central Siberia. Increased organic carbon accumulation in litter was found in poor hydroclimatic conditions of the north-facing slope and bottom valleys. In contrast, DOC contents and its export to soil were almost two-fold higher in warmer well-drained sites of the south-facing slope. The overall DOC flux to mineral soil from the beginning of June to mid-September was 17 g C m−2 in the south-facing slope sites and only 9 g C m−2 in the north-facing slope sites. DOC export was positively correlated with precipitation stimulating leaching of mobilizable organic matter. Accordingly intra-seasonal and inter-annual variability of the DOC fluxes was tightly coupled with water input. Meanwhile DOC export in Sphagnum and feathermoss sites showed different behavior in dry and wet years. The presence of permafrost preventing deep seepage of organic solutes results in higher stream DOC fluxes compared to permafrost-free or island permafrost regions. However, thawing of seasonally frozen soil layer during the growing season led to the decreasing concentrations of DOC in the stream from June to September. For two seasons of continuous stream water sampling (June and August–September), the riverine DOC flux constituted about 14% of DOC entering mineral soil on both slopes. The ratio of hydrologic DOC loss to NPP of larch forests of the region was estimated to be 1.1%.