There is little to argue about with your statements, RickCorey. Of course, sawdust mixed into the soil has a "thousand" times greater effect. Also a "thousand" times more temporary (and the much much smaller effect of the top mulch a "thousand" times longer lasting) . Due to the fantastic surface area of sawdust compared to wood chips, the "interface" between sawdust mulch and soil is a "thousand" times greater, and is in fact a "thousand" times more blurred. Such an interface is thus more of a transition zone, rather than a line of demarcation, as with wood chips. Such a zone has more ramifications than the simple (and logical) physical aspects of your interface scenario.
If soil microorganisms weren't so darn efficient compared to plant roots in gobbling up nutrients, the admittedly small physical reach of nitrogen sucking microorganisms would have little effect, long or short term. But because of the "thousand" times effect (I can't say if it's really a thousand), natural sources of adding nitrogen to the soil(that occurs with other mulches) is essentially void, resulting in an increasing nitrogen deficiency as plants struggle to remove the remaining soluble nitrates. Remember too, that a portion of these nitrates move down the soil column with the drainage of excess moisture, and out of the reach of roots. The soil becomes increasingly nitrogen deficient, though not in the way you might expect.
As for your other tangential statements(which I generally agree with), If you could explain the relevance to the subject at hand, I'd be happy to engage..... Or, perhaps they were just comments.....
---- But any weed would love to root in sawdust, IF it can reach any N with its roots before it starves.
---- But I've read many times that wood CHIPS cause N deficit only if mixed INTO soil (and seen it happen myself). A top-dressing of lots of wood CHIPS doesn't cause N deficit. Or so I've read in multiple places.
---- C [carbon]-hungry soil
---- I guess someone would have to do a study to be sure that fungi with long mycelia or hyphae can't digest cellulose at one end of the hyphae, and steal N with the other end, then transport energy compounds down and N up. My belief is that active internal transport like that is very "expensive" for a fungus to perform, so not very much of it happens.
I'm not aware of any internal "pumps" that require direct energy input to work. The mechanism(s) are there or not, and work as needed on their own. Indeed hyphae do transport nitrogen (and other compounds) regularly, but for your stated purpose, I have no idea.
