The eight-day hearing in Ebbw Vale is examining the request to take over 250 hectares (600 acres) of common land.
Backers, including Blaenau Gwent council, have offered around 320 hectares (800 acres) replacement land.
The inquiry has heard conflicting accounts from experts over claims the circuit would damage habitats and plant life by building on the common land.
Natural Resources Wales said it was "difficult to conclude" that the replacement land was of equal value, but it was not opposing the development overall.
But an ecology report presented by the developers said there would be a "net gain" in biodiversity because the plans included work to enhance other habitats nearby - which the Gwent wildlife trust argued against.
The Planning Inspectorate for Wales will make a recommendation after the inquiry in Ebbw vale, before Welsh government ministers have the final say on whether the plans can go ahead.

Ronald Chigunwe worked for Wessex Heartbeat, which supports the cardiac centre at Southampton General Hospital.
The 40-year-old, of Breadels Field, Basingstoke, pleaded guilty to four offences of fraud and money laundering.
However, he denied four other charges of money laundering. The Crown Prosecution Service will now decide whether he should face trial.
A decision is due within the next 14 days.
The fraud was uncovered when a new chief executive took over at the charity and became suspicious after asking Chigunwe for financial information.
The chief executive's wife - an accounts expert - was asked to look at the records and discovered the fraud.

The origin of the giant 'A' at Playhatch Roundabout on the A4155 Reading to Henley road is not yet known.
The roundabout is near Sonning-on-Thames, where a red letterbox 'appeared' on a bridge last year.
A BBC Radio Berkshire reporter saw the 'pin' and posted an image on a BBC Facebook page.
"I was just driving to Sonning to go play touch rugby on Monday night and the sign caught my eye as I went round Playhatch Roundabout," said reporter Catherine Bolsover.
"I asked around on Twitter but no-one knew where it had come from or who had made it.
"One suggestion was that it's linked to whoever put a postbox on Sonning Bridge."
In response to her Facebook photograph, one person commented: "Noticed this yesterday nearly drove on to it!"
Anita Trif, a waitress at The Crown pub nearby, said no-one knew where the sign had come from.
"I first saw it on Friday or Saturday," she added.
"We have all seen it but have no idea where it's come from. We have all been wondering about it."
A public inquiry into plans to build a £325m motor racing circuit in the south Wales valleys has started.
[NEXT_CONCEPT]
A former financial controller has admitted stealing more than £440,000 from a hospital charity.
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A sign resembling a large, red Google Maps pin has been spotted on an Oxfordshire roundabout.