Do plants absorb nitrate at all?

[mrzap]

or are water changes the only thing that removes nitrate?

 

[rich311k]

Plants will remove nitrates from the water. You would need a lot of plants and a low bioload to replace water changes. If you have a lot of plants and high light you will need to change water anyway to reset the ferts from your dosing schedule.

It is possible to run a low bio load tank with many plants almost maintenace free. If you search around you will find threads here about it and articles on the internet about doing it.