Phytoremediation Efficacy of Salix discolor and S. eriocephela on Adjacent Acidic Clay and Shale Overburden on a Former Mine Site: Growth, Soil, and Foliage Traits

Plants regularly experience suboptimal environments, but this can be particularly acute on highly-disturbed mine sites. Two North American willows—Salix discolor Muhl. (DIS) and S. eriocephala Michx. (ERI)—were established in common-garden field tests on two adjacent coal mine spoil sites: one with high clay content, the other with shale overburden. The high clay content site had 44% less productivity, a pH of 3.6, 42% clay content, high water holding capacity at saturation (64%), and high soil electrical conductivity (EC) of 3.9 mS cm−1. The adjacent shale overburden site had a pH of 6.8, and after removing 56.5% stone content, a high sand content (67.2%), low water holding capacity at saturation (23%), and an EC of 0.9 mS cm−1. The acidic clay soil had significantly greater Na (20×), Ca (2×), Mg (4.4×), S (10×), C (12×) and N (2×) than the shale overburden. Foliar concentrations from the acidic clay site had significantly greater Mg (1.5×), Mn (3.3×), Fe (5.6×), Al (4.6×), and S (2×) than the shale overburden, indicating that these elements are more soluble under acidic conditions. There was no overall species difference in growth: however, survival was greater for ERI than DIS on both sites, thus overall biomass yield was greater for ERI than DIS. Foliar concentrations of ERI were significantly greater than those of DIS for N (1.3×), Ca (1.5×), Mg (1.2×), Fe (2×), Al (1.5×), and S (1.5×). There were no significant negative relationships between metal concentrations and growth or biomass yield. Both willows showed large variation among genotypes within each species in foliar concentrations, and some clones of DIS and ERI had up to 16× the Fe and Al uptake on the acidic site versus the adjacent overburden. Genetic selection among species and genotypes may be useful for reclamation activities aimed at reducing specific metal concentrations on abandoned mine sites. Results show that, despite having a greater water holding capacity, the greater acidity of the clay site resulted in greater metal mobility—in particular Na—and thus a greater EC. It appears that the decline in productivity was not due to toxicity effects from the increased mobility of metals, but rather to low pH and moisture stress from very high soil Na/EC.