I think you want to build up humus and humic compounds in the soil.
Adding organic matter is the way to do that.
Manures have their own nitrogen, so they do not cause nitrogen
draw-down in the soil when you mix them in. The only problems
to watch out for when using manure are too much ammonia in fresh
manure (not a problem in rabbit, alpaca, and llama manures), salt
buildup over successive years of adding manure, and, only recently,
contamination with aminopyralid or clopyralid broadleaf herbicides
spread on pastures (these herbicides pass through livestock undigested).
I have dug a foot of aged horse manure into a garden bed
and planted in it. No problem, the plants grew and produced great.
(I think this was before aminopyralid herbicides were for sale in the
US. Now, I would probably test the manure in a container first with some
peas or buckwheat. If those survive without looking like someone got too
close with the Roundup, it's good to go.)
Some thoughts from an expert on adding organic matter to soils:
http://humusandcarbon.blogspot.com/