I have a copy of Ecology right in front of me. Page 20 - 21 Dianna is a
little all over the map as to ammonia uptakes by plants. She says depends
on the actual plants and their sensitivity. Then she says "plants rapidly
detoxify ammonia" and can enter by diffusion across membranes and can even
be stored in cell vacuoles. Next paragraph she says plants detoxify ammonia
by synthesizing with proteins. Again on the same page in a Q&A box she says
there is no definitive evidence that ammonia itself is actively taken up by
plants. So who really knows?