Avoid copper-based bio/fungicides, as they'll probably kill earthworms.
*Any* fungicide applied to the wood may inhibit fungi in the surrounding
soil, though weatherproof fungicide treatments may be water-insoluble so
might not spread far into the soil.
One way to minimise contact with moist soil (which minimises leaching),
while improving soil moisture retention would be to use a linseed
lacquer on the inner surfaces (or all surfaces?). Linseed lacquer will
harden into a thin but tough bioplastic layer and is a time-honoured way
of protecting woods. :)
--
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