EXPORT OF CHLORIDE AFTER CLEAR-CUTTING IN THE HUBBARD BROOK SANDBOX EXPERIMENT

Abstract

The objective of this study was to discern the source of higher than usual concentrations of chloride in drainage water collected from experimental forest plots after clear-cutting. When the sandbox experiments were initiated at the Hubbard Brook Experimental Forest Station three vegetation types were established: red pine, grass, and minimally vegetated (scattered lichens and bryophytes) as the "bare" control plot. After 15 years of growth the trees were cut down and above-ground biomass removed from the red pine sandbox. For several years prior to the cut, high concentrations (~ 75 μM) of dissolved Cl- in drainage waters occurred in November/December. This is attributed to the buildup of rainfall-derived Cl- due to evapotranspiration that depletes soil moisture to low levels resulting in a lack of drainage during this period. The excess Cl- is quickly flushed out by subsequent drainage over a few weeks and Cl- concentrations return to values characteristic of rainfall and throughfall. After the trees were removed in May, 1998, Cl- continued to be leached from the system. The concentration of Cl- peaked (175 μM) in Sept. 1998 and did not return back to base level concentration until Dec. 1999. The Cl- release pattern is distinctly different from that of dissolved NO3-, which peaked about one year later than Cl-. An excess (over that of the control sandbox) of 78 g Cl- was released in the 1.5 year period after clear-cut, showing that a large amount of leachable chloride is stored in the bulk soil/root/organic matter fraction. Lack of uptake by trees may be part of the reason for this chloride pulse. But an analysis of chloride content in roots and litter indicates that as much as 50% of the chloride leached from the sandbox may have come from the decaying roots and litter. Additional chloride may have been released from the soil organic matter by decomposition. The biochemical behavior of Cl- in systems such as this should be evaluated before assuming Cl- to be conservative for purposes of hydrological transport or soil weathering studies.