The biogeochemistry of calcium at Hubbard Brook

G. E. Likens, C. T. Driscoll, D. C. Buso, T. G. Siccama, C. E. Johnson, G. M. Lovett, T. J. Fahey, W. A. Reiners, D. F. Ryan, C. W. Martin, S. W. Bailey

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A synthesis of the biogeochemistry of Ca was done during 1963-1992 in reference and human-manipulated forest ecosystems of the Hubbard Brook Experimental Forest (HBEF), NH. Results showed that there has been a marked decline in concentration and input of Ca in bulk precipitation, an overall decline in concentration and output of Ca in stream water, and marked depletion of Ca in soils of the HBEF since 1963. The decline in streamwater Ca was related strongly to a decline in SO4/2- + NO3/- in stream water during the period. The soil depletion of Ca was the result of leaching due to inputs of acid rain during the past 50 yr or so, to decreasing atmospheric inputs of Ca, and to changing amounts of net storage of Ca in biomass. As a result of the depletion of Ca, forest ecosystems at HBEF are much more sensitive to continuing inputs of strong acids in atmospheric deposition than expected based on long-term patterns of sulfur biogeochemistry. The Ca concentration and input in bulk precipitation ranged from a low of 1.0 μmol/l and 15 mol/ha-yr in 1986-87 to a high of 8.0 μmol/l and 77 mol/ha-yr in 1964-65, with a long-term mean of 2.74 μmol/l during 1963-92. Average total atmospheric deposition was 61 and 29 mol/ha-yr in 1964-69 and 1987-92, respectively. Dry deposition is difficult to measure, but was estimated to be about 20% of total input in atmospheric deposition. Streamwater concentration reached a low of 21 μmol/l in 1991-92 and a high of 41 μmol/l in 1969-70, but outputs of Ca were lowest in 1964-65 (121 mol/ha-yr) and peaked in 1973-74 (475 mol/ha-yr). Gross outputs of Ca in stream water were positively and significantly related to streamflow, but the slope of this relation changed with time as Ca was depleted from the soil, and as the inputs of sulfate declined in both atmospheric deposition and stream water. Gross outputs of Ca in stream water consistently exceeded inputs in bulk precipitation. No seasonal pattern was observed for either bulk precipitation or stream water concentrations of Ca. Net soil release varied from 390 to 230 mol/ha-yr during 1964-69 and 1987-92, respectively. Of this amount, weathering release of Ca, based on plagioclase composition of the soil, was estimated at about 50 mol/ha-yr. Net biomass storage of Ca decreased from 202 to 54 mol/ha-yr, and throughfall plus stemflow decreased from 220 to 110 mol/ha-yr in 1964-69 and 1987-92, respectively. These ecosystem response patterns were related to acidification and to decreases in net biomass accretion during the study. Calcium return to soil by fine root turnover was about 270 mol/ha-yr, with 190 mol/ha-yr returning to the forest floor and 80 mol/ha-yr to the mineral soil. A lower content of Ca was observed with increasing elevation for most of the components of the watershed-ecosystems at HBEF. Possibly as a result, mortality of sugar maple increased significantly during 1982 to 1992 at high elevations of the HBEF. Interactions between biotic and abiotic control mechanisms were evident through elevational differences in soil cation exchange capacity (the exchangeable Ca concentration in soils was significantly and directly related to the organic matter content of the soils), in soil/till depth, and in soil water and in streamwater concentrations at the HBEF, all of which tended to decrease with elevation. The exchangeable pool of Ca in the soil is about 6500 mol/ha, and its turnover time is quite rapid, about 3 yr. Nevertheless, the exchangeable pools of Ca at HBEF have been depleted markedly during the past 50 years or so, >21,125 mol/ha during 1940-1995. The annual gross uptake of trees is about 26-30% of the exchangeable pool in the soil. Some 7 to 8 times more Ca is cycled through trees than is lost in stream water each year, and resorption of Ca by trees is negligible at HBEF. Of the current inputs to the available nutrient compartment of the forest ecosystem, some 50% was provided by net soil release, 24% by leaching from the canopy, 20% by root exudates and 6% by atmospheric deposition. Clear cutting released large amounts of Ca to stream water, primarily because increased nitrification in the soil generated increased acidity and NO3/-, a mobile anion in drainage water; even larger amounts of Ca can be lost from the ecosystem in harvested timber products. The magnitude of Ca loss due to whole-tree harvest and acid rain leaching is comparable for forests similar to the HBEF, but losses from harvest must be superimposed on losses due to acid rain.

Original languageEnglish (US)
Pages (from-to)89-173
Number of pages85
Issue number2
StatePublished - 1998


  • Calcium biogeochemistry
  • Forest disturbance
  • Forest ecosystem
  • Landscape patterns
  • Soil chemistry
  • Stream chemistry
  • Weathering
  • Wet and dry deposition

ASJC Scopus subject areas

  • Environmental Chemistry
  • Water Science and Technology
  • Earth-Surface Processes


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