I don't remember the exact words used in the pesukim to describe shemittah, but I do remember that it uses a very similar lashon to shemirat Shabbat. As in, it is forbidden to work the land because of a sabbatical year.
Since they both seem to use similar language, my question is as follows:
On Shabbat it is forbidden to perform a melakhah on a Biblical level (i.e. an "av" melakhah). However, if it is performed with a shinui then it's only rabbinically prohibited. Do the same rules apply to shemittah (i.e. does prohibited labor during the seventh year become only rabbinically prohibited when done in an unusual manner)?