Mark,

This (soil OM change) is an interesting question that has come up many times over the years.  A number of no-tillers claim big increases in 5 or 10 years.  Yet when we examine the situation more closely, usually it is better explained by sampling error, lab changes, etc.  Realize that by 'error' I'm not saying that you did anything wrong, and that you might well have attempted to sample methodically in the same areas and to the same depth as previously.  However, very small spatial differences (a few feet, sometimes mere inches) can result in large variations in values.  

So how many fields or tested areas are we talking about, for which you have data?  How many cores went into each sample? Do you have some markings on the probe or other procedure to ensure cores are taken to the same depth each time?  Since the early samples were in the '80s, I will assume no GPS or other marking system was used. 

My main point is that it takes lots of sampling and lots of smoothing or averaging of results to know what you've got.  (Additional testing may confirm what you have now, but what if the original values were low?)  When a person calculates just how much N, S, P, etc go into a 1% change in OM on an acre, it becomes apparent just how difficult it is to change.  And a review of the science literature further emphasizes how difficult it is to increase soil OM. 

I like your idea of soil as big composter.  You are ahead of the curve in using cover crops.  Good for you.