Thanks for your reply Scott.

Similar rainfall in Southern, Western Australia past 3 years, but usually 16-17inch.

Our soils are ancient and naturally woodland so any carbon was sequestered in wood instead of soil carbon 🙁

We still work on 50N being left after grain legumes on average, with some in year 2 in some conditions. You dont see this from your peas? Our best zero bagged N results come from hairy vetch brown manure crops but any escapee vetch has become very difficult to control without extremely toxic chemistry.

Either way we grow more than we should from applied N plus legumes, so has to be some free living nitrogen fixers in our soil. But what to expect from them in an 8inch deep soil?