Any environmental parameter can be valued economically if it can be measured perfectly. therefore, I believe that standardization of methodology is the basic criteria for economic valuation.
For Carbon storage if you mean CO2 sequestered there is a standard cost per unit weight that is fixed by world bank for carbon credits. For oxygen generation i dont think we have such formulla. However Raj is right if it can be measured we can value it. soil moisture retention capacity can have a value both positive and negative depending on wher it is ?
Can't we think of standardizing an economic valuation of oxygen (from producers) as this the only life sustaining gas for human beings? Lets roll the ball and initiate a discussion.