An initial assessment of calcium and iron isotope systematics in forest ecosystems: clues to possible linkages?
Abstract
Calcium (Ca) and iron (Fe) are common major constituents of soils in watersheds and are essential nutrients for plants. Ca is potentially a limiting ecosystem nutrient due to its depletion from soil and biomass pools as a result of both anthropogenically-induced and natural leaching processes. In contrast, Fe is rarely limiting due to its typically greater abundance in soils as a result of the immobility of its oxidized weathering products. Although both Ca and Fe play critical roles in the biogeochemical dynamics of forest ecosystems, their differing chemical affinities provide little reason to expect similarities in biogeochemical behavior at the ecosystem level. However, initial assessments of the isotopic systematics of Ca at a watershed developed on granitoid glacial till in New Hampshire (USA) and of Fe at a watershed developed on marine sediments in northern California (USA) have revealed similar patterns of isotopic distribution and thus a potential linkage between their respective biogeochemical cycles. In each case, easily extractable Ca or Fe in the forest floor is isotopically heavier than residual Ca or Fe; in contrast, easily-extractable Ca or Fe in the deepest mineral soils is isotopically lighter than residual Ca or Fe. The development of these depth distributions of isotopic composition is consistent with either transport of relatively light, easily extractable Ca and Fe from deep mineral soils to shallow soils via plant root networks and/or soil water migration, or retention of relatively light Ca and Fe internally by plants with subsequent redistribution to and concentration in the shallow soils. An important role for the latter process is suggested at the New Hampshire site, where detailed analysis of red spruce tissues reveals that relatively light Ca is retained by foliage and bark, probably as a result of Ca-oxalate formation, and subsequently concentrated in the forest floor through litter deposition. We are currently determining the Fe isotope distribution in the soils at the New Hampshire site, the Ca isotope distribution in the soils and plants at the California site, and the Fe isotope distribution in the plants at both sites in order to understand potential linkages between the Ca and Fe biogeochemical cycles at these watersheds.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.H21D..06B
- Keywords:
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- 1030 Geochemical cycles (0330);
- 1040 Isotopic composition/chemistry;
- 1045 Low-temperature geochemistry