Co-analysis of the calcium and sodium budgets of forested catchments increases the sensitivity of detecting temporal dynamics
Abstract
The retention of nutrients by forest ecosystems is thought to be affected by forest age and response to disturbances, such as acid deposition. Long-term mass balance studies at Hubbard Brook Experimental Forest suggest depletion of available Ca pools, in contrast to retrospective soil studies, at Hubbard Brook and elsewhere, which have not detected changes in soil exchangeable Ca pools. This scenario may be reconciled if (a) mineral weathering release of Ca has increased or (b) other, non-exchangeable, soil pools of Ca have been depleted. Here, we critically examined application of a Ca:Na ratio method in interpreting the long-term Ca dynamics. Co-analysis of the Ca and Na cycles utilizes differences in the biogeochemical behavior of Ca and Na to increase the sensitivity of detecting dynamics in the Ca cycle. Storage of Na in biomass and secondary minerals and on cation exchange sites was low enough so that net ecosystem Na loss was essentially equivalent to mineral weathering flux. Mineral chemistry and mass-balance considerations constrained the Ca:Na ratio of weathering products to a sufficiently narrow range that spatial and temporal changes in the net ecosystem Ca:Na ratio could be interpreted as changes in contribution of available Ca pools to ecosystem loss. Based on this indicator, depletion of available Ca pools was greater in the three experimentally manipulated watersheds with aggrading biomass compared to three reference watersheds with relatively mature forest conditions. Although accelerated loss of Ca in the first few years following disturbance has been documented by prior studies, this study suggests that excess Ca loss continues for at least three decades after treatment, with no trend toward conditions in the reference watershed. It is not likely that changes in mineral weathering flux, or in previously quantified Ca pools account for this sustained loss, suggesting that a previously unstudied Ca pool or release mechanism may be important in ecosystem response to disturbance.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.H22A..05B
- Keywords:
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- 1615 Biogeochemical processes (4805);
- 1655 Water cycles (1836);
- 1886 Weathering (1625)