Technically not a paywall, but in the spirit of Rule 3, here’s a link to the full article without mandatory account creation.

I’ll add in the interest of fairness that:

• Campbell et al. is a workshop presentation and does not appear to be published in any sort of peer-reviewed scientific journal. However, its authors are researchers within the Canadian government’s agriculture department.
• Chapter 12 of Meena et al. goes into a lot of fantastic detail about green manuring with legumes, but the chapter by and large seems more concerned with comparing green manures to synthetic manures rather than green manures to animal manure. The book routinely points out issues with animal agriculture, but in Chapter 5, they do state: “[…] which is reasoned by the fact that animal manure is a very good substrate […].” (Chapter 5 goes into legumes as an energy crop, and it’s pretty neat!)
• The work by Forster is a master’s thesis and therefore did not go through the traditional peer-review process. Nonetheless, it’s extremely interesting, and of course it was still reviewed by faculty at the university.