Optimizing revegetation success at an Arizona copper mine

How can we optimize revegetation success in semi-arid environments? That’s the newest question that University of Arizona’s Center for Environmentally Sustainable Mining (CESM) is tackling.

Through a network of research cooperatives with copper mines across Arizona, CESM develops transferable technologies that help mines address their sustainability challenges. One of the outcomes of CESM’s work has been the identification of soil biological indicators to measure revegetation trajectory and success of copper mines’ tailings facilities, waste rock piles, and soils affected by smelter-generated arsenic.

Revegetation has a number of benefits. First and foremost, revegetation helps form and stabilize the soils in these areas, which is critical for two reasons: human health and ecosystem functioning. When plants take root, soil is stabilized and less likely to become airborne as dust. Without being bound in the ground, small soil particles can mobilize with wind and create fugitive dust emissions, which impact nearby communities in a number of ways. Revegetation also helps return these lands to self-sustaining ecosystems that can support a diversity of life: plants, mammals, birds, reptiles, and so forth. And, in the light of one of the hottest summers on record here in Arizona, establishing these ecosystems is crucial to battling climate change: revegetated soils become carbon “sinks” that sequester carbon dioxide from the atmosphere, where it would instead be contributing to the greenhouse effect.

Revegetation in the Southwest is heavily dictated by our region’s climate: intense, prolonged heat and sparse rainfall. Given the constraints of our climate, one of our current CESM projects works with an Arizona copper mine to evaluate which seeding season, seed dispersal method, and seeding amendments help vegetation most successfully (and sustainably) establish and grow. Specifically, we are curious to see if any of the following significantly influence revegetation success:

Season: Seeding during the summer monsoon vs. seeding during the winter rains
In evaluating seeding season, we are aiming to better understand if seeding conditions – eg, precipitation and ambient temperature – have a significant impact on vegetation establishment and soil development. As we experienced in Arizona this past summer, monsoon rains are not always reliable, which raises the question if hydroseeding during this time is advisable.

Dispersal method: Hydroseeding (slurry of mulch, fertilizer, and seeds) vs. seedball scattering
Dispersal method is another factor being investigated, especially for its impact on seed predation. In the hydroseeding technique, seeds are applied across the prescribed area in a slurry. While this slurry does provide the seeds with necessary germination and growth promoters (initial moisture, fertilizing nutrients, and organic matter), it does not protect the seeds from predation by small mammals or birds. Seedballs, a proposed alternative to hydroseeding or drill seeding, instead encapsulates seeds in a ‘nursery’ of nutrients, all held together by clay. Because of their clay content, these seedballs are relatively resilient to desiccation and predation but do require a heavy germinating rain to break apart. In our study, seedballs were scattered by hand, but they can be dispersed by airplanes to seed larger areas of land.

Amendment: Adding arbuscular mycorrhizae fungi (AMF) to the hydroseed mixture
The study will also evaluate if the addition of AMF significantly impacts vegetation cover and soil development. AMF are soil fungi that form beneficial relationships with plants and act as an ‘extension’ of the plants’ root system. This underground network of fungi helps plants access even more soil nutrients and moisture, which ultimately aids plant growth. These fungi also secrete a protein called glomalin (glow-mail-in), which has been nicknamed “soil glue” for its ability to form and stabilize soil aggregates. A commercially available AMF inoculate was selected for this study so that other mines could incorporate it into their revegetation plans if results from this study indicate a significant impact.

In 2019, we established a randomized, controlled study of these treatments in an area that had been previously contaminated with smelter-generated arsenic. Over the next 5 years, we will assess the impacts of these treatments on aboveground (vegetation cover) and belowground (pH, electrical conductivity, total nitrogen, DNA biomass, and glomalin concentration) indicators of revegetation progress.

This work is particularly novel because randomized, controlled studies at mines are typically not conducted. We are appreciative of our excellent partnership with the copper mine, as it allows us a rare, large-scale opportunity to tease apart the potential influences of these treatments on revegetation success. At the end of this study, our goal is to provide clear recommendations to the mine as to what seeding season, method, and amendment scenario best optimizes the potential success of their upcoming revegetation plans.