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

Abstract

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 À ) 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 À 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 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 À deposition was from pollutant sources, probably coal burning. In the 1970s the Cl À inputs in bulk deposition declined, and the lower Cl À deposition in the last two decades is dominated by marine sources. Between 1964 and 2000 there was no significant trend in Cl À export in stream flow, thus the net hydrologic flux (NHF = bulk deposition inputs À streamflow outputs) has changed over this period. Early in the record the NHF was on average positive, indicating net retention of Cl À 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 À from mineralization of soil organic matter (SOM) may also play an important role. We believe that accumulation of Cl À 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 À accumulation in vegetation, our estimates suggest that an aggrading forest could sequester about 32 mol Cl ha À1 year À1 , or about a third of the annual average bulk deposition flux to this ecosystem. Experimental additions of Cl À to the forest floor cause increases in Cl À concentration in foliage, throughfall, and soil solution. Manipulations of vegetation also affect the Cl À cycle. Harvesting or devegetation of watersheds causes an increase in the Cl À 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 À that may last more than 15 years. In this respect, the behavior of Cl À after disturbance parallels that of NO 3 À , for which export increases after disturbance due to reduced plant nitrogen uptake and mineralization of nitrogen from detritus, rather than SO 4 2À , for which export decreases after disturbance due to pH-dependent adsorption onto mineral soils. The interannual pattern of Cl À export from the system primarily reflects the atmospheric inputs, but the net retention and cycling of Cl À within the system appears to be largely under biological, rather than geochem...

Experimental Evidence of Large Changes in Terrestrial Chlorine Cycling Following Altered Tree Species Composition

“…34 It is sometimes assumed that Cl leaching from canopies is negligible and that throughfall represents one of the best estimates of stand-specific dry deposition. 35,36 However, the Cl − levels in (throughfall and stemflow) from oak and beech were almost equal to the estimated total deposition (Figure 1 (Figure 1), despite having a larger standing foliage biomass (31 t dry mass ha −1 versus 18 t dry mass ha −1 , respectively). 37 Thus, throughfall was independent of foliage biomass, indicating that speciesdependent leaching could be more important for the observed differences between stands than possible differences in capturing dry deposition.…”

What can flux tracking teach us about water age distribution patterns and their temporal dynamics?

“…Further, plants require Cl − and thus take up Cl − stored in the root zone at varying rates (e.g. Kauffman et al, 2003;Lovett et al, 2005;Van der Velde et al, 2010). Here these combined imbalances in catchment Cl − budgets were accounted for by rescaling the input concentrations using lumped adjustment factors as successfully applied in earlier studies (e.g.…”

“…Note that, although Cl − turnover by plants can under certain circumstances play a significant role (cf. Lovett et al, 2005), it was in the cool Scottish climate, also in the absence of suitable data warranting the inclusion of this process, considered negligible as individual process in the models (cf. Kirchner et al, 2000;Page et al, 2007;Shaw et al, 2008) due to low transpiration rates (and thus limited sap flow) and limited organic matter turnover in the Heather moorland vegetation and the coniferous forests of the study catchments (cf.…”

Is chloride a conservative ion in forest ecosystems?

“…nuclear waste (Campbell et al 2003). The few studies that have focused specifically on Cl -transport do indicate that there can be imbalances in catchment Cl -budgets, which are typically attributed to unmeasured dry deposition (Juang and Johnson 1967), mineral weathering (Peters 1991) or interactions with vegetation and soil (Lovett et al 2005;Öberg et al 2005a;Öberg and Sandén 2005b;Svensson et al 2007). …”

“…Chloride can be immobilized in ecosystems by several different processes, including (1) ion exchange (Larsson and Jarvis 1999), (2) adsorption onto iron and aluminum oxides (Nodvin et al 1986), (3) uptake by biota, including vegetation and microbes (Ohrui and Mitchell 1996;Lovett et al 2005;Bastviken et al 2009), and (5) conversion to organic forms of chlorine (Clutterbuck et al 1940;Myneni 2002;Bastviken et al 2007). …”