Isn't the idea that you remove the nickel and then go back to farming "normal" crops? So this is a one off way to mine without destroying the environment. And maybe a way to remove excess metals from soil too. Not a continuous supply.

Edit, I guess you could use this technique in waste cleanup too, so maybe once a mine stops producing you plant these trees on the site for one last load. Or you bring waste from the mine (high nickle etc) to farms like this and "replenish" the sites

I wondered that myself. They'd need a succession plan. But high concentrations of certain metals are toxic to many other plants. So maybe the nickel harvesting plants are a first step to preparing the soil for another crop. Only this step allows the farmers to at least partially recover some of their investment in the cleaning step.

Another possibility would be to grind up olivine and spread that on the field. Olivine is a nesosilicate (orthosilicate) so it dissolves readily in weak acid conditions. And it's typically a few tenths of a percent nickel.

The dissolution of olivine would liberate magnesium ions, which could react with water and CO2, permanently sequestering the CO2 as carbonate.

The succession plan is reverting to stocks that can handle the elevated levels of nickel still remaining after the commercial viability of bioremediation has been exhausted.

Nickel is not really needed in any large amount for the growth of plants, I assume even these plants don't need it in more than a trace amount to grow.

The hydroponic fertilizer I use doesn't even list it on the label and I assume that the only way my plants get it is from impurities in the other minerals.

https://www.jacksnutrients.com/online-store/5-12-26-Part-A-p...

In the areas where they evolved, they've presumably been present for a very long time. Human action could possibly increase their nickel-concentrating abilities by some small multiplier, but that won't turn human timescales into geologic timescales.

Those trees would overwhelmingly be falling and decomposing in the same soil they grew in though, so the movement of minerals out of the soil would be minimized.

> 500 pounds of nickel citrate

That would be 115 pounds of nickel (plus some water and C02)