Contact Info:
Carmen Nezat
Department of Geological Sciences
University of Michigan
425 East University
Ann Arbor, MI 48109-1063
phone: (734) 763-9368
fax: (734) 763-4690
email: cnezat@umich.edu

Date Prepared: November 2002

Long-term weathering rates¹ can vary significantly within a watershed but are, nevertheless, frequently estimated from a small number of soil profiles within a watershed. Weathering rates are a function of parent material, topography, vegetation type, windthrows, animal burrowing, and other processes that differ across a watershed. Although many studies have observed systematic changes in weathering across soil chronosequences, watershed variability in weathering rates has not been investigated in detail, or in relation to elevation and/or vegetation type. This study examines 1) the variability of soil composition across the watershed, 2) the overall weathering rates and relative mineral weathering rates along an elevational gradient, and 3) the degree to which present-day cation release rates² differ from long-term weathering rates.

In addition, Ca/Sr and ⁸⁷Sr/⁸⁶Sr ratios of Hubbard Brook vegetation suggest that ectomycorrhizal trees (spruce and fir) obtain calcium directly from apatite grains, thus bypassing the bulk soil solution, whereas those without ectomycorrhizae (maple) acquire calcium from the soil exchange pool (Blum et al., 2002). Ectomycorrhizal fungi leave direct evidence of mineral dissolution in the form of microscopic elongate pores in grains and mineral etching.

During the summer of 1997, forty-seven, randomly located pits (0.5 m²) were excavated in Watershed 1 of Hubbard Brook Experimental Forest (Figure 1) by Chris Johnson and Tom Siccama and their assistants. Samples were collected by horizon from the Oie, Oa, E, Bh, Bs1, Bs2, and C horizons. To overcome the heterogeneity of the soils and obtain a representative estimate of soil properties over the entire watershed, W1 was divided into six zones (Zones A-F; Figure 1) and a composite soil sample was created for each horizon in each zone. Composite soil samples were sequentially digested and chemically analyzed.

In order to evaluate the role of ectomycorrhizae in mineral dissolution, we collected soil and root samples beneath spruce, fir, beech and maple trees at high, mid, and low-vegetation zones to look for presence (or absence) of micropore formation and mineral etching by ectomycorrhizal fungi. Root samples will be viewed by scanning electron microscopy.

The chemical composition of the soil parent material, or C horizon, is relatively homogeneous across the watershed. In contrast, the degree of weathering intensity varies with elevation and vegetation. Mineral weathering patterns also vary with landscape position. The average weathering rate at Hubbard Brook is similar to other watersheds across the U.S. with similar soil age and composition. However, the weathering rates within Watershed 1 vary by a factor of three over a change of 260 m elevation in the watershed and suggest the need for extensive, watershed-wide sampling to average out soil heterogeneity and obtain accurate estimates of long-term weathering rates. Lastly, present-day fluxes of base cations exceed present-day mineral weathering rates, indicating that the removal of base cations from the exchangeable pool is greater than the supply from mineral weathering.

¹ Long-term weathering rates of the Hubbard Brook soils are calculated as the sum of the base cations lost from a soil profile since deposition of the glacial till.
² Present-day cation release rates from soils, estimated from a watershed mass balance, are a measure of the current loss of base cations from soils due to both mineral weathering and removal from cation exchange pools and vegetation pools.

Blum J. D., Klaue A., Nezat C. A., Driscoll C. T., Johnson C. E., Siccama T. G., Eagar C., Fahey T. J., and Likens G. E. (2002) Mycorrhizal weathering of apatite as an important calcium source in base-poor forest ecosystems. Nature 417: 729-731.