(Draft answer till I get around to find some figures)
There's a few ways I see to go about this: Running a Haber Bosch process or similiar with a non-fossile source for energy and H2, or finding other N sources. There's principally two routes, N fixing plants in crop rotation + organic fertilizer and reusing N-rich wastes and wastewater. The key difference beeing that the first is basically happening within one or a few farms, while the second version involves municipalities.
Nitrogen as hard limit
Plants need nitrogen to build protein, typically 1kg N for 6,25 kg protein. The amount of nitrogen available to the plant is a hard upper limit on the amount of protein we can get. While it's not enough to look at the nutrient balance of a field, the balance puts limits on the achievable yields.
Nitrogen is the nutrient that is most strongly fossile fuel based, since production of N-fertilizers requires natural gas. So in this answer I will look only at nitrogen.
However, another important nutrient is phosphorus and here the world could face a shortage in the coming decades. Many of the ways to recycle nitrogen also recycle phosphorus.
Non-fossil Haber Bosch:
Haber Bosch (HB) needs power to drive turbocompressors and Methane for heat and hydrogen. For the latter, one could use Biogas. HB yields a bit less than one mole NH3 per mole CH4. One ha of corn yields about 47t/y, that amounts to roundabout 9400Nm³ Biogas at 52% methane. So we have ~220kmol methane, a slightly smaller amount of ammonia that amounts to maybe 2500kg/a of nitrogen fixed. This actually a lot. This approach is scalable if there's a gas grid in place: Biogas plants can feed the (upgraded) Biogas into the grid, where it is transported to the HB plant.
There are electrically powered approaches to HB that I have not yet investigated.
Crop rotation + Organic fertilizers:
needs a side by side comparision of conventional/intensive planting and extensive planting without. Even when fertilizers are used, the farmer wil have to follow some crop rotation to replenish topsoil, protect the ground from winter erosion etc. This means we cannot look at just plant yields with different fertilizer applications, but we must look at complete crop rotations at the farm level.
According to wikipedia, fixing 30-40kg N/ha is possible. Typical fertilizer application is in the range 100-170kg N/ha, for high yield crops.
This meta study states that on average organic farming practices (which in this context mainly means no fossils based nitrogen fertilizer) yield about 20% less than conventional agriculure - at the field level. The yield gap is assumed to be larger at the farm level, when nutrient limits come into play stronger. The authors themselves stress that whole crop rotations have to be looked at. Also there's a huge span, in some cases organic practices outperformed conventional agriculture. So far I found no farm level multi year comparison between organic and conventional agriculture.
While the question may still be open wether organic agriculture can feed the world (and this will depend on meat consumption, biofuels etc.), organic farming is scalable with one caveat: Manure is not always available in the quantities neccessary. Solutions that utilize waste and or wastewater will beimportant in the years to come.
With activated sludge treatment, about 90% of the nitrogen content of the wastewater is sequestered in the sewage sludge, along with some phosphates and other interesting nutrients. Sludge from municipal wastewater is often contaminated with heavy metals and not usable for land application. These contaminants usually come from industrial wastewater. Banning untreated industrial wastewater from municipal sewer lines could help.
Most Wastewater treatment plants are designed for elemination of the remaining N (turning nitrate into athmospheric N), not for fixing nitrogen in a usable form. But apparently there are promising tests with MAP (Magnesium amoonium phosphate, can be used as a fertilizer) precipitation or duckweed for municipal wastewater. To my knowledge, technologies established for high strength wastewater (like ANA strip or Dekastrip) cannot be used economically for municipal wastewater.
Using municipal wastes and wastewater
Source separated organic wastes can be composted (or fermented) and applied to land. This depends heavily on the behaviour of the households doing the separation. I've seen compost from SSO that looked pretty good, and I've seen some with lots of visible plastic contaminants.
I think one key to close nutrient cycles between city and countryside is cleaner waste, meaning better separation, separation between industrial and household wastewaters (and maybe between gray and black and yellow wastewaters).
There is no simple comprehensive plan for a future agriculture without fossile fule based fertilizer. The trick will be to employ closed nutrient cycles at many scales - on the farm, betweeen farm and food processing, between city and countryside. Closed loops are only possilbe with good waste and wastewater management and are aided by source separation.
Where these loops are insufficient, green fertilization with legumes etc. and non-fossile HB are needed to supplant fertilizer.