What does this biomass measurement mean and how to translate it to volume
50 g m-2 d-1 translates to "50 grams per square meter per day": the negative signs are negative exponents (check a math textbook). This is equivalent to writing

((50 g)/m2)/day

You cannot transform this into volume, doing so would lose the meaning: if you are talking about biomass growth of 50 grams per square meter per day, you are saying that there is 50 grams of new biological material produced within a square meter of land (say, a patch of grassland or forest). Talking about it in terms of volume doesn't make sense because the "height" coordinate doesn't produce additional biomass, since the source of energy is the sun.