Dissolved organic carbon concentrations in runoff from shallow heathland catchments: effects of frequent excessive leaching in summer and autumn

Abstract

Transport and turnover of dissolved organic carbon (DOC) is important in the C cycle of organic soils. The concentration of DOC in soil water is buffered by adsorption to the soil matrix, and has been hypothesized to depend on the pool size of adsorbed DOC. We have studied the effect of frequent artificial excessive leaching events on concentration and flux of DOC in shallow, organic rich mountain soils. Assuming a constant Kd value for DOC adsorption to the soil matrix, we used these data to assess the change in the pool of adsorbed (or potential) DOC in the soil. The study involved manipulation of precipitation amount and frequency in summer and autumn in small, heathland catchments at Storgama, southern Norway. The shallow soils (16–34 cm deep on average) limit the possibility for changes in water flow paths during events. The mini-catchments range in size from 75 to 98 m2. Our data show that after leaching of about 1.2 g DOC m−2 the DOC concentration in runoff declines by approximately 50%. From this we conclude that the pool size of adsorbed potential DOC in the shallow soils at any time is of the order 2–3 g m−2. Frequent episodes suggest that the replenishment rate, which depends on the decomposition rate of soil organic matter, is fast and the potential DOC pool could be fully restored probably within days during summer, but with some more time required in autumn, due to lower temperatures. Both pool size of potential DOC and replenishment rate are seasonally dependent. The pool of potential DOC, and thus the DOC concentration in discharge, is at their maximum in the growing season. However, under non-leaching conditions, the concentration of DOC in soil water and thus the pool size of potential DOC seems to level off, possibly due to conversion of DOC to less reversibly bound forms, or to further decomposition to CO2.