Amendments Improve Performance of Perennial Native Plants for Revegetation of Acidic Mine Tailings Over Time

In the US there are over 20,000 abandoned hardrock mines that may present risks to human health or the environment from long-term exposure to harmful substances. One site is the Formosa mine Superfund site located in Oregon, US. The site lacks plant cover due to phytotoxic tailings with high acidity, possible metal toxicity, insufficient nutrients, and poor soil structure which make it difficult to establish vegetation. We evaluated the effects of amending the tailings on tailing chemistry, and survival and growth of local native herbaceous species: Anaphalis maragitacea (ANMA), Carex inops (CAIN), Chamerion Angustifolium (CHAN), Elymus glaucus (ELGL), and Potentilla gracili (POGR).  The treatments were (1) withholding any amendments, (2) amending with lime, biosolids, and biochar, (3) amending with lime, biosolids, and K fertilizer, and (1), (2) or (3) plus inoculation with locally-sourced microbes (LSM). Amendments increased pH and reduced available heavy metals in the tailings, and improved plant survival and growth. The grass, ELGL, had greater survival and growth than the other species with the amendments. Plant biomass during the fifth year of growth was greater for ELGL with the amendments with the biochar treatment than the amendments with the K treatment. In contrast, the other species had greater biomass with the amendments with the K treatment. There were no LSM effects to date, though evaluation is continuing. Thus, soil amendments can improve soil chemistry and enhance native herbaceous survival and growth on mine tailings.

Impact/Purpose

Past mining activities in the US and in other nations, have led to many abandoned mine sites with acidic tailings, which can lead to heavy metal contamination of local ground and surface waters.  Because of the low pH, heavy metals, and lack of normal soil physical, chemical, and biological properties, plant cover can be sparse or nonexistent on the sites. Thus, the tailings must be amended with various substances for successful plant survival and growth. In a field study at an abandoned mine site, we evaluated the effects of a mixture of amendments (lime, biosolids, biochar or extra potasium fertilizer, and local microbial inoculum) on tailings pH and metal concentrations; and on corresponding survival and growth of a variety of native herbaceous plants growing in small plot.s There were three main tailing treatments: (1) To adjust tailing pH, Lime (CaCO3) was provided at ≈1% (dry weight basis). (2) To provide inorganic plant nutrients and soil carbon, municipal waste biosolids were added at ≈ 0.5% by weight.  Biochar produced from mixed conifer wood waste was added to the tailings at ≈ 2.5% by weight. (3) Lime and biosolids, but potassium fertilizer added at 0.2 % by weight to substitute for the biochar amendment. A locally-sourced microbial (LSM) inoculum solution was added to half the plots in May 2019, resulting in three additional treatments: (4) no amendments + LSM, (5) amendments with biochar + LSM, and (6) amendments with potassium +LSM.  We used perennial plant species which are native in the area.  Species which survived were Anaphalis maragitacea, Carex inops, Chamerion Angustifolium, Elymus glaucus, and Potentilla gracilis. We present results for plant growth following over three and one-half years of treatment to indicate multi-year responses to the amendment treatments. In 2023, four years post transplanting nearly all the plants in unamended plots that were only tilled had died, likely due to the low tailing pH and possibly due to high metal concentrations. In contrast, amending the tailings with lime, biosolids, and biochar or extra potassium allowed plants to survive and grow. There was a trend for higher growth of Elymus glaucus and lower growth of the other species with the amendments and biochar than with the amendments and potassium, though the high variability among plots made it difficult to determine statistical significance.  Field evaluation projects are necessary to determine the practical usefulness of amendments to enhance the reestablishment of vegetation on acidic mine tailings, after over four years of study, beneficial effects of the amendments: lime, biosolids, and biochar or potassium were clearly associated with changes in tailing chemistry conducive to plant survival and growth.  Future tailings and plant sampling and analysis are necessary to determine the cause of these changes, and additional amendment application may be required.

Citation

Bollman, M., D. Olszyk, M. Nash, M. Storm, K. Trippe, V. Manning, D. Watts, J. Novak, AND M. Johnson. Amendments Improve Performance of Perennial Native Plants for Revegetation of Acidic Mine Tailings Over Time. SETAC Europe, 34rd Annual Meeting, Seville, SPAIN, May 05 - 09, 2024.