Comparison of Current and Long-Term Ca Weathering Rates in the White Mountains of New Hampshire.

Long-term soil weathering can be estimated by comparing concentrations of mobile elements in parent materials with those in soil profiles, referenced to immobile elements such as titanium.  These rates are much slower than current watershed denudation rates estimated from input-output budgets.  The difference might be attributable to contemporary influences such as acidic deposition or biological demand following forest harvesting.  

We compared the net hydrologic flux of calcium at the Hubbard Brook Experimental Forest (HBEF) with weathering rates calculated from soil profiles in 14 other stands in the White Mountain National Forest.  We estimated uncertainty in both methods.  Uncertainty in ecosystem weathering estimates are rarely reported but are essential to establishing confidence in the results.

The net hydrologic flux of Ca in the reference watershed of the HBEF was 7.1 kg/ha/yr, with a 95% confidence interval of 0.5 kg/ha/yr, based on a Monte Carlo sampling of uncertainty in the measurements.  Uncertainty in Ca inputs in precipitation was greater than that of Ca losses in runoff.  In contrast, the long-term weathering rate estimated from soil profiles averaged only 0.5 kg/ha/yr, with a range of -0.2 to 1.1 kg/ha/yr across the 14 sites.  This approach involves estimating the time for soil development and assuming that there is no loss or gain of material, that the C horizon represents the parent material, and that Ti is immobile during weathering.  Even with these uncertainties, current rates of Ca loss are undoubtedly higher that the average over 14,000 years of soil formation. Calcium loss was accelerated by 3.0 kg/ha/yr in the first 26 years after harvesting a watershed at the HBEF.  This difference could be due to biological demand for nutrients (including phosphorus in apatite, a Ca mineral) or to other changes associated with forest disturbance and regrowth, such as acidification.  Clearly, current rates of Ca loss are variable, affected by forest management, and high relative to the long-term average over past millennia.