If composted organic matter is spread like mulch atop the ground on
lawns or around ornamentals and allowed to remain there its nitrogen
content and C/N are not especially important. Even if the C/N is
still high soil animals will continue the job of decomposition much
as happens on the forest floor. Eventually their excrement will be
transported into the soil by earthworms. By that time the C/N will
equal that of other soil humus and no disruption will occur to the
soil's process.

Growing vegetables is much more demanding than growing most
perennial ornamentals or lawns. Excuse me, flower gardeners, but
I've observed that even most flowers will thrive if only slight
improvements are made in their soil. The same is true for most
herbs. Difficulties with ornamentals or herbs are usually caused by
attempting to grow a species that is not particularly well-adapted
to the site or climate. Fertilized with sacked steer manure or
mulched with average-to-poor compost, most ornamentals will grow
adequately.

But vegetables are delicate, pampered critters that must grow as
rapidly as they can grow if they are to be succulent, tasty, and
yield heavily. Most of them demand very high levels of available
nutrients as well as soft, friable soil containing reasonable levels
of organic matter. So it is extremely important that a vegetable
gardener understand the inevitable disruption occurring when organic
matter that has a C/N is much above 12:1 is tilled into soil.

Organic matter that has been in soil for a while has been altered
into a much studied substance, humus. We know for example that humus