Taxability on transfer of fund from nro account to nre a/c

Agricultural land in some specified area, is capital asset, and when sold, capital gain arises, which is taxable.

So, any registered CA's certificate will be required for the transfer of the amount to NRE acc.

Being US citizen, check any tax liability in USA, with any expert there; but once any tax paid in India, you will get compensation as per DTAA between two countries.

For more details refer: https://www.hdfcbank.com/htdocs/common/pdf/FEMA_Declaration.pdf