The biogeochemistry of sulfur at Hubbard Brook

Abstract

A synthesis of the biogeochemistry of S was done during 34 yr (1964-1965 to 1997-1998) in reference and human-manipulated forest ecosystems of the Hubbard Brook Experimental Forest (HBEF), NH. There have been significant declines in concentration (0.44 mol/liter-yr) and input (5.44 mol/ha-yr) of SO4 in atmospheric bulk wet deposition, and in concentration (0.64 mol/liter-yr) an d output (3.74 mol/ha-yr) of SO4 in stream water of the HBEF since 1964. These changes are strongly correlated with concurrent decreases in emissions of SO2 from the source area for the HBEF. The con- centration and input of SO4 in bulk deposition ranged from a low of 13.1 mol/liter (1983-1984) and 211 mol/ha-yr (1997-1998) to a high of 34.7 mol/liter (1965-1966) and 479 mol/ha-yr (1967-1968), with a long-term mean of 23.9 mol/liter and 336 mol/ha-yr during 1964-1965 to 1997-1998. Despite recent declines in concentrations, SO4 is the dominant anion in both bulk deposition and stream water at HBEF. Dry deposition is difficult to measure, especially in mountainous terrain, but was estimated at 21% of bulk deposition. Thus, average total atmospheric deposition was 491 and 323 mol/ha-yr during 1964-1969 and 1993-1998, respectively. Based on the long-term 34 S pattern associated with anthro- pogenic emissions, SO4 deposition at HBEF is influenced by numerous SO2 sources, but biogenic sources appear to be small. Annual throughfall plus stemflow in 1993-1994 was estimated at 346 mol SO4 /ha. Aboveground litterfall, for the watershed-ecosystem averaged about 180 mol S/ha-yr, with highest inputs (190 mol S/ha-yr) in the lower elevation, more deciduous forest zone. Weathering release was calculated at a maximum of 50 mol S/ha-yr. The concentration and output of SO4 in stream water ranged from a low of 42.3 mol/liter (1996-1997) and 309 mol/ha-yr (1964-1965), to a high of 66.1 mol/liter (1970-1971) and 849 mol/ha-yr (1973-1974), with a long-term mean of 55.5 mol/liter and 496 mol/ha-yr during the 34 yrs of study. Gross outputs of SO4 in stream water consistently exceeded inputs in bulk deposition and were positively and significantly related to annual precipitation and streamflow. The relation between gross SO4 output and annual streamflow changed with time as atmo- spheric inputs declined. In contrast to the pattern for bulk deposition concentration, there was no sea- sonal pattern for stream SO4 concentration. Nevertheless, stream outputs of SO4 were highly sea- sonal, peaking during spring snowmelt, and producing a monthly cross-over pattern where net hydro- logic flux (NHF) is positive during summer and negative during the remainder of the year. No significant elevational pattern in streamwater SO4 concentration was observed. Mean annual, volume-weighted