Twenty years apart: Comparisons of DOM uptake during leaf leachate releases to Hubbard Brook Valley streams in 1979 versus 2000

In seven experiments conducted in late summer 1979 and 2000 extracts of autumn‐shed sugar maple leaves and spruce needles were added to two stream reaches of the Hubbard Brook Experimental Forest at concentrations similar to those found during peak leaf fall. Uptake rates for dissolved organic carbon (DOC) ranged from 7 to 408 mg m−2 h−1. In Bear Brook uptake rates for spruce leachate were lower than for sugar maple leachate in both 1979 and 2000. Uptake velocities tended to be higher for more bioavailable DOC fractions: in 1979, monomeric carbohydrates in DOM were removed from the water column more rapidly than the bulk DOC pool in 4 of 4 cases; in 2000, uptake velocities for N‐containing DOC (DON) were higher than those measured for bulk DOC in 2 of 3 cases. Both results suggest that biological assimilation is an important component of DOM uptake, in contrast to earlier reports that DOM uptake in HBEF streams was primarily through abiotic sorption. In both years, uptake velocities for the same forms of DOM were lower in the high DOC stream Cascade Brook than for the low DOC stream Bear Brook. Experiments in 1979 found no change in inorganic nitrogen concentrations during leachate additions, while all leachate releases in 2000 led to reductions in stream NO3− concentrations.