They really need to update that paper with some better information. That is what you get when you chronically underfund the extension services and all the best and brightest bail to private industry to make more money. I had to pull up the source material the article because my bullsit meter went off.

https://sci-hub.se/https://doi.org/10.2134/agronj1996.00021962008800050025x

After reviewing the source material, you are 100% completely wrong :-)

First off the "transfer" of N from alfalfa to a grass was stated in the sourcing paper to be from mineralization of roots. Aka decomposing plant parts.

Perennial species often grow new roots and abandon older roots every year.

Alfalfa will abandon and regrow new roots after every cutting as the plant pulls carbohydrates from the roots to grow new stems and leaves.

The thing is that the grasses will do the exact same thing. Older roots die back and newer roots grow. So it's more of mutual swapping of N rather than a one-sided legume being leaky.