The Effects of Whole-Watershed Calcium Addition on Preferential Uptake of Calcium vs. Strontium and the Isotopic Ratio of 87Sr/86Sr into Foliage at the Hubbard Brook Experimental Forest, New Hampshire.

Abstract

Calcium is an essential if not normally a limiting base cation nutrient in forest ecosystems. The increase in acid deposition (acid rain) in modern times has caused concern about long term leaching of the base cation exchange pool in soils, potentially causing serious damage across northern forests (Likens et al. 1996). Strontium, also an alkaline earth element, is present in trace amounts in rocks and soils. It is thought to be incorporated into plant tissues similarly to calcium because of its ionic size and charge (Ash-Dasch et al. 2006). Strontium forms four stable isotopes in nature 84Sr, 86Sr, 87Sr, and 88Sr (webelements.com). Of these isotopes of strontium, only 87Sr varies due to radioactive decay of naturally occurring 87Rb (Ash-Dasch et al. 2006). Because of this decay different terrestrial geochemical reservoirs develop unique ratios of 87Sr/86Sr over time. Because calcium and strontium are assumed to have similar geochemical behaviors, the ratio of calcium to strontium (Ca/Sr) and the ratio between the isotopes of 87Sr and 86Sr has commonly been used to trace the sources of calcium in forest ecosystems (Ash-Dasch 2006; Blum et a. 2000). Previous work in this field has established that the uptake rates of Ca and Sr vary between plant species (Ash-Dasch et al. 2006). Correctly interpreting Ca/Sr and 87Sr/86Sr data in order to trace calcium requires understanding the extent to which species discriminate between calcium and strontium during uptake. The discrimination factor (DF) between Ca and Sr can be estimated and applied to nutrient flow studies of calcium through different ecosystems. This study examines ten years of chemical data from several tree species in Ca-depleted forested watershed. Data came from a whole watershed calcium addition experiment in the Hubbard Brook Experimental-Forest, NH (Peters et al. 2004). The experiment was begun in October of 1999 and sample collection is ongoing. Results for data from 1999-2004 were published in Ash-Dash et al. 2006. Here, I will be updating the data set and calculating discrimination factors after including new data from 2004 through 2008. Following our calculations, a DF greater than 1 indicates a plant’s preference for calcium over strontium, and a DF less than one indicates a preference for strontium over calcium. Widespread calcium depletion has been documented across northeastern North America, including the watershed studied here. The depletion is due to increased acid deposition (Likens et a. 1998). The consequences of calcium depletion on northern forests are only partly understood. At the Hubbard Brook Experimental Forest unexpected declines in biomass accumulation have been observed since the late 1980’s, which may relate to increased acid deposition and subsequent leaching of calcium from the nutrient pool (Likens et al. 1996). While some research (focused on global sulfur and nitrogen cycles) on acid rain has indicated that the effects of acid rain on forest nutrient cycles would be limited, there is evidence that the effects of changes in calcium cation pools on forest health could be much stronger(Likens et al. 1996). Spruce (Picea spp.) trees are particularly vulnerable to calcium deficiencies in the form of increased freezing injuries in low Ca environments (DeHayes 1999). The vulnerability of forests to calcium depletion and the damage already done tothem by acid rain are reasons why being able to trace ecological sources of calcium could aid in long term forest preservation and help inform environmental policy. Adding calcium to an entire watershed (whole-watershed addition) allows for observations to be made about many aspects of calcium’s role in ecosystem health, and how this nutrient cycles through the ecosystem (Peters et a. 2004). The ratio of calcium to trace amounts strontium, and the isotopic ratio of 87Sr to86Sr in the source calcium added to the ecosystem can be used as a chemical marker to follow the added calcium as it flows through the ecosystem. In tracing that marker, observations can be made about relative uptake rates and absorption of calcium and strontium by various plant species. Because of divergent calcium chemistry between plants, characteristic preferences for calcium vs. strontium can then be applied to more reliably trace calcium sources over time in ecosystems.