Just ADD LIME? Interactive Effects of Plants, Bacteria, and Amendments on Soil Arsenic Levels.

Unfortunately, historical mining activity has left deep landscape wounds. In the western United States, for example, there are more than 45,700 abandoned mine sites requiring soil remediation. Soil remediation efforts should include re’establishment of vegetation because this stabilizes soils through root growth, and transpirational dewatering of soils minimizes potential leaching losses of any metals.  We performed a greenhouse study to determine what combinations of soil amendments would lead to the best vegetative response, and potentially associated reductions in soil arsenic levels. Tailings were collected from Warm Springs, Montana, ~50 km downstream of Butte, the original source of contamination.  Pre-remediation soil metal levels were 2910mg Cu,  916 mg Pb,  1293 mg kg^-1 As. We planted Basin wildrye (Leymus cinereus), and after 12 weeks, we compared above- and belowground plant growth responses, foliar metal concentrations, soil metal concentrations, pot leachate metal concentrations and photosynthesis activity across the 13 treatments. Amendments included single or combinatorial additions of lime (“+lime”: 4.7% Ca(OH)₂); organic matter (“+OM”: 5%; Eko-compost, Missoula, MT); and oxidizing (“+ox”) strains of Agrobacterium tumefaciens.  All pots were fertilized with 0.13 g of KNO₃, watered with 600 ml water every 2 days, and exposed to 16 h day lengths.

Not surprisingly, the full combination of treatments (+lime, +OM, +ox) showed the best plant growth response.  Unexpectedly, the +OM +ox treatment yielded biomass gains that were not significantly lower than the full combination, implying a potential costs savings for phytoremediation efforts.  Photosynthetic rates were also greatest for the full combination of treatments (19.6 umol CO2 m’2 s’1), while +lime +OM rates were significantly lower (11.6 umol CO2 m’2 s’1), with +OM +ox showing intermediate rates.  Taken together, our study results suggest that cost-effective restoration approaches might be improved through a greater consideration of the microbial communities supported by these reestablishing vegetation communities, which could lead to more natural ecosystem successional trajectories.