I am not a chemist, but in ...

I am not a chemist, but in general thiosulphate (a strong reducing agent) will react with an oxidizing agent.  Nitrate is not generally an oxidizing agent, and it can be expected that the thiosulphate could react with the cation associated with the nitrate (e.g. iron Fe).  The resultant products will tend to be the reduced form of the cation, together with a sulphate or sulphite.  Nitrate at greater than say 20 mg/l in drinking water is harmful to human beings - especially babies and toddlers.   thiosulphate being a strong reducing agent will be reactive to compounds that can be reduced, which may not be good for the environment.  Under some conditions (usually requiring heat) the nitrate itself can be reduced to nitrogen dioxide which is a noxious gas and hence should be in a well ventilated area.