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There are two possible sources: either active volcanoes, none of which have been found yet on Mars, or microbes.
The articale then says...
So far the PFS has observed a depletion of carbon dioxide and an enrichment of water vapour over some of the large extinct volcanoes on Mars.
That statement contradicts this statement.
"There are no known ways for ammonia to be present in the Martian atmosphere that do not involve life," the Nasa scientist said.