Use of foliar Ca/Sr discrimination and 87Sr/86Sr ratios to determine soil Ca sources to sugar maple foliage in a northern hardwood forest
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Calcium/strontium and 87Sr/86Sr ratios in foliage can be used to determine the relative importance of different soil sources of Ca to vegetation, if the discrimination of Ca/Sr by the plant between nutrient sources and foliage is known. We compared these tracers in the foliage of sugar maple (Acer saccharum) to the exchange fraction and acid leaches of soil horizons at six study sites in the White Mountains of New Hampshire, USA. In a previous study, sugar maple was shown to discriminate for Ca compared to Sr in foliage formation by a factor of 1.14 ± 0.12. After accounting for the predicted 14% shift in Ca/Sr, foliar Ca/Sr and 87Sr/86Sr ratios closely match the values in the Oie horizon at each study site across a 3.6-fold variation in foliar Ca/Sr ratios. Newly weathered cations, for which the Ca/Sr and 87Sr/86Sr ratios are estimated from acid leaches of soils, can be ruled out as a major Ca source to current foliage. Within sites, the 87Sr/86Sr ratio of the soil exchange pool in the Oa horizon and in the 0–10 cm and 10–20 cm increments of the mineral soil are similar to the Oie horizon and sugar maple foliar values, suggesting a common source of Sr in all of the actively cycling pools, but providing no help in distinguishing among them as sources to foliage. The Ca/Sr ratio in the soil exchange pool, however, decreases significantly with depth, and based on this variation, the exchange pool below the forest floor can be excluded as a major Ca source to the current sugar maple foliage. This study confirms that internal recycling of Ca between litter, organic soil horizons and vegetation dominate annual uptake of Ca in northern hardwood ecosystems. Refinement of our understanding of Ca and Sr uptake and allocation in trees allows improvement in the use of Ca/Sr and 87Sr/86Sr ratios to trace Ca sources to plants.
KeywordsFoliage Soil Ca/Sr 87Sr/86Sr Sugar maple Acer saccharum
We thank A. Klaue, M. Johnson, K. Keller and C. Nezat for assistance in the laboratory and M. Vadeboncoeur and our student field crews for assistance with sample collection. Four anonymous reviewers are thanked for their helpful comments. We appreciate the opportunity provided by the USDA Forest Service Northeastern Research Station to conduct research in the White Mountain National Forest and in particular the cooperation of C. Costello at the Bartlett Experimental Forest. Support for this study was provided by National Science Foundation grant DEB 0235650, which contributes to the Hubbard Brook Ecosystem Study (http://www.hubbardbrook.org) and the Long-Term Ecological Research (LTER) program funded by the National Science Foundation.
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