This seems pretty straightforward in my eyes. Organic amendments are largely carbon in the first place. So if you add more C that is no surprise you are going to integrate more into the soil.
That is the e horizon which stands for eluvial. Spodosols form in acidic environments and this causes the iron and humus to move down the soil profile as a complex as rain moves through. So the black humus and red iron are removed which gives soil most of its color. What you are left with is the silt and sands that are the pale color you see.
Your lucky I left the greatness of Purdue's agronomy program out of the post. That came in a close third.
I just wanted to post two things. Spodosols are the best soil order and Spodosols are the best soil order. I do not give a damn about productivity, just look at that thing and tell me otherwise.