Calcium and strontium isotope dynamics in three polluted forest ecosystems of the Czech Republic, Central Europe

Calcium and strontium isotope ratios were used to identify Ca sources for bulk soil, soil solutions and runoff in the stressed forest ecosystems of Central Europe. All three study sites are underlain by Ca-poor crystalline bedrock (orthogneiss, leucogranite and serpentinite, respectively), but differ in historical rates of acid deposition. UDL, the spruce die-back affected site at the Czech-Polish border underlain by orthogneiss, received six times more acidifying compounds from the atmosphere than LYS and PLB, two paired catchments near the Czech-German border. The paired catchments near the Czech-German border, whose forests were only mildly damaged by industrial pollution, differ in their acid buffering capacity (extremely low for LYS leucogranite, extremely high for PLB serpentinite). At the spruce die-back affected catchment UDL, 19 years of monthly hydrochemical monitoring revealed six times higher atmospheric deposition and runoff fluxes of Ca, compared to the paired catchments LYS and PLB. Across the three sites, the mean delta Ca-44 values increased in the order: spruce bark (-0.84 parts per thousand) < spruce xylem (-0.31 parts per thousand) < spruce fine roots (0.0 parts per thousand) < bedrock (0.08 parts per thousand) < soil (0.11 parts per thousand) < spruce needles (0.31 parts per thousand) < open-area precipitation (0.68 parts per thousand) < spruce throughfall (0.71 parts per thousand) < runoff (0.74 parts per thousand) < soil water (1.11 parts per thousand). At all three sites, Ca from atmospheric deposition was isotopically similar to Ca in runoff, indicating export of recent atmospheric Ca. All three catchments had isotopically lighter Ca in silicate bedrock and exported isotopically heavier Ca via runoff. Trees mostly represented the isotopically lightest Ca pool, whereas soil water collected by lysimeters contained isotopically heavy Ca. The Sr-87/Sr-86 isotope ratios were nearly uniform at the serpentinite site PLB (0.710). The( 87)Sr/(86) Sr ratios across the two felsic sites (UDL and LYS) increased in the order: open-area precipitation (0.710) < spruce canopy throughfall (0.715) < spruce xylem (0.723) < spruce needles (0.725) < soil water (0.734) < runoff (0.740) < soil (0.862) < bedrock (0.881). Collectively, the delta Ca-44 and Sr-87/Sr-86 isotope systematics indicated that organic Ca cycling, along with atmospheric input of isotopically heavy Ca, largely contributed to the high-delta Ca-44 values of runoff. The studied silicate bedrock had significantly lower delta Ca-44 values than the reported worldwide average. While bulk soil Ca was likely affected by bedrock Ca even after the period of acid rain, the delta Ca-44 difference between soil and soil water indicated a major role of recycled organic Ca in supplying nutrition to the trees. Future Ca-Sr isotope research in headwater catchments should include isotope analysis of sequentially leached Ca fractions from weathered minerals in soil to better assess the geogenic Ca inputs to runoff. In catchments currently recovering from pollution, Ca-Sr isotope fingerprinting of dust emitted from coal-fired power plants will be needed. Isotope analysis of local coal can indicate to what extent Ca-Sr isotope composition of past deposition contributes to the isotope signatures of present-day runoff.