I got this information from Squirrely Cakes:
First of all, are there molasses in this spice cake recipe? If so, there is already baking soda in the recipe because molasses are acidic and the baking soda releases the carbon dioxide gas.
Is there enough baking soda to also work with the buttermilk, that is difficult to say. Maybe, but you will have to experiment.
Cupcake_Kisses stated that the buttermilk substitution ( when regular milk is called for) causes too high a level of acidity and that is correct. But you want baking soda in a recipe where you are using acidic ingredients like buttermilk, sour cream, regular cocoa, coffee, yogurt etc. The baking soda is needed where you have acidic ingredients in your recipe. It doesn't make the cake more acidic. So you wouldn't want to decrease the amount of baking soda added.
When we talk about Dutch Processed cocoa, it is alkalized. So that is why you shouldn't really substitute Dutch Processed cocoa for regular cocoa when it is called for, where there is baking soda in a recipe. But this will depend on what other ingredients are in the recipe and if baking soda action is working on these other ingredients and therefore doesn't have much affect when you substitute the Dutch Processed Cocoa. For example a recipe that also contains sour cream and coffee - two other acidic ingredients may be fine if you use the Dutch Processed cocoa in place of regular cocoa called for. Regular cocoa is acidic.
Think about it in terms of how people drink baking soda mixed in water, to neutralize too much acid in their stomach, this is one of the functions of baking soda.
The buttermilk is the acidic ingredient, so it needs baking soda to make the finished goods leaven properly. If the recipe has some baking soda in it already, the substituting of the buttermilk for regular milk called for, may work just fine.
I will use buttermilk as a substitute when there is already baking soda in a recipe. But a lot depends on the other ingredients in the recipe. Is there enough soda to work with the acidity of the buttermilk or is there only a small amount to work with ingredients such as coffee or regular cocoa? Those are things to consider and experiment with.
Baking soda is 4 times the strength of baking powder - which also contains some baking soda. So you cannot just substitute one for the other. And too much soda will cause changes in leavening action, a funny smell to a cake and sometimes a funny taste.
The taste isn't adversely affected from the buttermilk, in fact in many recipes it enhances taste, in small quantities anyway. Where it does affect taste is in something like buttermilk pancakes, where it is a main ingredient. It will give a recipe a distinct taste but in a recipe containing other flavourings, you will not notice it.
You are usually best off using the milk called for in the recipe but in this case, I think it may well be fine if you leave the baking soda the way it is.
The issue can be that the acid ingredients if not used in conjunction with some baking soda, will have an affect on the way the gasses are producing the leavening action. So you may not get as good results with the rise and texture of your baked good.