Bismuth shotshell has been used as a non-toxic alternative to lead but the environmental fate of bismuth has not been studied adequately. In an upland hardwood forest in south-eastern Ontario, Canada, soil beneath mature sugar maple, red maple and bitternut hickory was experimentally manipulated from 1999 to 2003 with bismuth pellets and ammonium sulfate, or left untreated to serve as a control. Although soil bismuth was below detection (0.1 μg g−1 dry mass) across treatments, sugar maple and hickory receiving bismuth pellets and acidification had elevated bismuth in sapwood compared to control trees. This finding suggests that soil acidification can mobilize metals from bismuth pellets into the forest biogeochemical cycle within a 4-year period.
The study site is an upland mixed hardwood stand at the Queen's University Biological Station (QUBS) approximately 50 km north of Kingston, Ontario, Canada. Trees were located within a 4 ha area on Ordovician carbonate rocks near the Precambrian Canadian Shield (William, 1982). Soils consist of well-drained sandy material deposited as outwash on gently sloping rocky terrain and with low fertility (Gillespie and Miller, 1968). During the early 1980s, annual wet deposition of sulfate (30–35 kg ha−1) exceeded nitrate (15–20 kg ha−1) but sulfate deposition decreased during 1996–2000 (25–30 kg ha−1), while that of nitrate increased to 20–25 kg ha−1 (Environment Canada, 2002). Mean annual precipitation is 964 mm with maximum daily temperatures in July (24.8 °C) and minimum daily temperatures of −12.6 °C in January (Environment Canada, 2002).
Sugar maple (Acer saccharum Marsh.) was the most abundant tree species in our study area. Our stand also included bitternut hickory (Carya cordiformis (Wangenh.) K. Koch) and red maple (Acer rubrum L.). In November 1999, eight mature trees (dbh=22–57 cm; age=37–71 year) with straight boles and no visible signs of bark damage were assigned randomly to one of four treatments: bismuth pellets, acidifying agent (ammonium sulfate [(NH4)2SO4] applied annually as an acid rain simulation), bismuth pellets plus acidifying agent, or control (no bismuth pellets or acidifying agent). Treatments were applied to 5 m×5 m plots centered on individual trees with the following species by treatment groupings: untreated (sugar maple, red maple), bismuth (sugar maple, red maple), acidified (two sugar maple), bismuth and acidified (sugar maple, hickory). Target trees were at least 10 m apart to minimize nutrient acquisition of target trees from soil of neighboring trees that may have received a treatment.
Bismuth pellets were applied with a seed spreader on December 12, 1999 at 50 pellets m−2 (equivalent to 200 g of bismuth per tree), within the range of lead pellets for upland forests used for recreational hunting in southern Ontario (Holdner et al., 2004). The first acidification treatment was also applied on December 12, 1999 and applied in subsequent years between leaf fall and the first snowfall. Hutchinson et al. (1998) applied annual doses of ammonium sulfate between 250 and 1000 kg ha−1 year−1 for 2 years at two sugar maple forests on coarse textured acidic tills in central Ontario. They found a reduction of about 0.5 in soil pH at one site but no reduction at the second site. Thus, given the potential that even 1000 kg ha−1 year−1 of ammonium sulfate may not acidify the soils at QUBS, we chose to increase our dose to 2000 kg ha−1 year−1 (=5 kg tree−1 year−1). This is equivalent to 424 kg N ha−1 year−1 and 484 kg S ha−1 year−1 and therefore substantially higher than current annual nitrogen and sulfur loads for the region. Soil pH from non-acidified plots increased an average of 0.7 during the experimental period (1999–2003), perhaps in response to decreased sulfate deposition (Environment Canada, 2002) and the ameliorating effect of forest canopy interception to acidic rainfall deposition (Carlson et al., 2003). By comparison, soil pH of the acidified plots decreased an average of 1.0 after 4 years of acidification. The decrease in soil pH measured at QUBS is similar to that found in forest soils receiving acidic atmospheric deposition over the past 50 years in Germany (Wesselink et al., 1995) and southern Sweden (Falkengren-Grerup et al., 1987).
Soil samples were collected after leaf fall between late October and early December, the first collection made immediately before treatment application on December 12, 1999. Leaf litter was cleared and a 7-cm diameter stainless-steel corer used to extract the top 5-cm of soil. Soil was not collected within 0.5 m of the tree trunk to avoid the zone of stemflow leaching. Three soil samples were collected per plot and pooled into one composite sample. Soils were air-dried to constant weight and passed through a stainless-steel 1-mm sieve.