Mikkeller has been releasing a new round of Spontaneous Series beers in 2013. This list is pretty long and diverse, and continues to follow that trend. For the offering you see above, beetroot (the earthy tasting taproot) is added to the lambic and fermented in oak.
Lambic, The Rundown.
A lambic is a spontaneously fermented beer. What’s that? The beer is exposed to wild yeasts and bacteria. In this case (most likely) these wild yeasts live in the oak of the barrels they are aged in. The result is a tart/sour tasting beer.