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

摘要

In seven experiments conducted in late summer 1979 and 2000 extracts ofautumn-shed sugar maple leaves and spruce needles were added to two stream reaches ofthe Hubbard Brook Experimental Forest at concentrations similar to those foundduring peak leaf fall. Uptake rates for dissolved organic carbon (DOC) ranged from 7 to408 mg M-2 h-1. In Bear Brook uptake rates for spruce leachate were lower than for sugarmaple leachate in both 1979 and 2000. Uptake velocities tended to be higher for morebioavailable DOC fractions: in 1979, monomeric carbohydrates in DOM were removedfrom 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 bulkDOC in 2 of 3 cases. Both results suggest that biological assimilation is an importantcomponent of DOM uptake, in contrast to earlier reports that DOM uptake in HBEFstreams was primarily through abiotic sorption. In both years, uptake velocities for thesame forms of DOM were lower in the high DOC stream Cascade Brook than for the lowDOC stream Bear Brook. Experiments in 1979 found no change in inorganic nitrogenconcentrations during leachate additions, while all leachate releases in 2000 ledto reductions in stream NO3 concentrations.