The biogeochemistry of chlorine at Hubbard Brook, New Hampshire, USA

Chlorine is a minor constituent of most rocks and a minor (although essential) element in plants, but it cycles rapidly through the hydrosphere and atmosphere. In forest ecosystem studies, chloride ion (Cl^-) is often thought to be conservative in the sense that the sources and sinks within the ecosystem are assumed negligible compared to inputs and outputs. As such, Cl^- is often used as a conservative tracer to assess sources and transformations of other ions. In this paper we summarize research on chloride over the course of 36 years (1964-2000) at the Hubbard Brook Experimental Forest (HBEF) in central New Hampshire, USA. Evidence presented here suggests that in the 1960s and 1970s the dominant source of atmospheric Cl^- deposition was from pollutant sources, probably coal burning. In the 1970s the Cl ^- inputs in bulk deposition declined, and the lower Cl^- deposition in the last two decades is dominated by marine sources. Between 1964 and 2000 there was no significant trend in Cl^- export in stream flow, thus the net hydrologic flux (NHF = bulk deposition inputs - streamflow outputs) has changed over this period. Early in the record the NHF was on average positive, indicating net retention of Cl^- within the system, but since about 1980 the NHF has been consistently negative, indicating an unmeasured input or source within the ecosystem. Dry deposition can account for at least part of that unmeasured source, and it appears that release of Cl ^- from mineralization of soil organic matter (SOM) may also play an important role. We believe that accumulation of Cl^- in vegetation during the 1960s and 1970s offset the unmeasured source and resulted in net ecosystem retention. Accumulation of vegetative biomass has ceased since about 1982, leading to the apparent net export (negative NHF) since that time. Although we have no direct measurements of Cl^- accumulation in vegetation, our estimates suggest that an aggrading forest could sequester about 32 mol Cl ha^-1 year^-1, or about a third of the annual average bulk deposition flux to this ecosystem. Experimental additions of Cl^- to the forest floor cause increases in Cl^- concentration in foliage, throughfall, and soil solution. Manipulations of vegetation also affect the Cl^- cycle. Harvesting or devegetation of watersheds causes an increase in the Cl^- concentration and flux in stream water for several years after the disturbance. This period of release is followed by a period of reaccumulation of Cl^- that may last more than 15 years. In this respect, the behavior of Cl^- after disturbance parallels that of NO₃ ^-, for which export increases after disturbance due to reduced plant nitrogen uptake and mineralization of nitrogen from detritus, rather than SO₄ ^2-, for which export decreases after disturbance due to pH-dependent adsorption onto mineral soils. The interannual pattern of Cl^- export from the system primarily reflects the atmospheric inputs, but the net retention and cycling of Cl ^- within the system appears to be largely under biological, rather than geochemical, control.