Ca and Mg isotope systematics in Central European forest ecosystems in an era of retreating acidification

Martin Novák1, Juraj Farkaš1, Chris Holmden2, Pavel Krám1, Markéta Štěpánová1, Martin Šimeček1, Jan Čuřík1
1 Czech Geological Survey, Geologická 6, 152 00 Prague 5, Czech Republic
2 Saskatchewan Isotope Laboratory, Department of Geological Sciences, University of Saskatchewan, Saskatoon, Canada

O 8.1 in Trace element and metal biogeochemistry

14.07.2014, 11:15-11:35, H20

Two small upland catchments in the Czech Republic were selected for a Ca/Mg isotope study 25 years after the peak acid-rain period. The more polluted northern site, UDL, experienced severe spruce die-back. The less polluted western site, LYS, was also heavily acidified, but the signs of spruce decline were limited to canopy thinning and needle yellowing. Both sites are underlain by poorly buffering crystalline bedrock (orthogneiss and granite, respectively). In 1994, UDL received as much as 65 kg S and 26 kg N ha-1, due to proximity of obsolete coal-burning power plants. In the same year, LYS received 32 kg S and 10 kg N ha-1. In 2008, UDL still received as much as 38 kg S and 14 kg N ha-1. In the same year, LYS received 6 kg S and 8 kg N ha-1. Thus, in 2008, UDL remained to be more polluted than LYS at the beginning of the monitoring period (1994).

Ca isotope ratios (d44Ca) were measured in open-area deposition and spruce canopy throughfall, soil and soil solutes, bedrock and runoff. The more polluted site UDL exhibited a twice larger range of d44Ca ratios than the less polluted site LYS (3 vs. 1.5 ‰). Both sites had similar d44Ca of atmospheric input, close to -1.2 ‰, with little difference between open-area precipitation and throughfall. Ca of bedrock was isotopically lighter at LYS (-2.3 ‰) than at UDL (-1.8 ‰). At both sites, Ca of soil solutes was isotopically heavier than Ca of solid soil. At the more disturbed site UDL, d44Ca of soil water was -0.5 ‰, while d44Ca of deep soil was as low as -3.5 ‰. At LYS, Ca of soil solutes and deep soil was less contrasting, with d44Ca close to -1.3 ‰ and -2 ‰, respectively. Isotope composition of runoff Ca at UDL (-1.3 ‰) differed from that of soil solutes (-0.5 ‰). In contrast, runoff Ca at LYS was isotopically similar to that of soil solutes (both -1.3 ‰). At both sites, d44Ca of runoff was higher than d44Ca of bedrock, but the difference was larger at LYS. At UDL, we observed a distinct seasonality in d44Ca of the atmospheric input, with isotopically heavier Ca in summer, compared to winter. d44Ca of soil solutes exhibited the same seasonality, indicating a relatively low residence time of atmogenic Ca in the vertical soil profile. Overall, the more stressed ecosystem of UDL showed larger Ca isotope contrasts among catchment pools and fluxes than LYS.

Mg isotope ratios (d24Mg) were measured in various ecosystem compartments at LYS. Mg in 90-cm deep soil solutes was the isotopically heaviest reservoir (-0.5 ‰). Mg of atmospheric deposition was the isotopically lightest reservoir (-1.4 ‰). Mg in runoff was isotopically heavy (-0.7 ‰), similar to deep soil solutes, but different from both atmospheric input and shallow soil solutes. In the presentation, we will discuss mixing of isotopically contrasting Ca and Mg pools in the catchments, and the possible role of in-situ isotope fractionations in an era of easing pollution.

Acknowledgements: EC (SOIL TrEC; 244118) is thanked for funding.

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last modified 2014-06-19