Hi simy1,

The surrounding soil is extremely N-poor.

I assume it's a sandly soil and lack of organic matter to buffer
nutrient.

I must conclude that
decomposing wood absorbs nitrogen from the atmosphere.

Some bacteria will fix nitrogen from the air, provided there is carbon
available as energy. The carbon can get from exchange from legume, or
directly from soil. When the bacteria die and decompose, nitrogen will
release and available by plant. Some nitrogen will come from storm.
All this will return to air if there is nothing to hold it.

A lot of nutrient are keep in soil in life form, semi-decompose dead
body, either plant or animal. This act as nutrient bank that buffer
nutrient.

One thing I'm not quite clear, can fungus be group into plant, and
bacteria into animal?

Regards,
Wong