If you want to know whether - or when - the US will become a nitrogen exporter, read what the fertilizer company CEOs say during the latest round of quarterly earnings calls.
These guys should have formed pretty solid opinions by now about how the capacity expansions will affect long-term supply and demand, and how they're going to gain/keep market share and competitive advantage. But it can be a challenge to infer what those opinions might be.
I've summarized the pertinent parts of the debate here, with quotes from Agrium, CF Industries, KBR, LSB Industries, OCI, Potash Corp, and Yara.