Okay, the more I read the methods of their study the more "What the ever living fuck are these idiots doing?"

It's a greenhouse study of a plugs in pots extracted from an established field. Okay, that works except ... It's an irregular mix of species including a legume. Sort of tossed the entire study into question. They discussed this issue a bit at the end and blew it off.

The source of fertilizer is mostly manure slurry. Okay odd choice given that manure slurry is known to vary dramatically in the balance of nutrient content and microbial composition. This massively effects its availability and uptake ratio into plants. Specifically important is a ratio of ammonium, nitrite and nitrate.

But there's the a kicker they added in a heavy isotope of N (15N) in ammonia sulfate form as a marker into the manure. That's how they attempted to tell the difference between existing soil N and N from the shit. Are you fucking kidding me....

What the study actually showed was plants under environmental stress uptake a heavier isotope of N at a slower rate. Their results have nothing to fucking do with where the plants extracted the nitrogen from (the soil or the manure). That's not what they tested... Dumbasses.