Three to six times more microbial biomass carbon and nitrogen depending on soil type.
These results provide evidence that carbon (C) inputs from frequent cover cropping are the primary driver of changes in the soil food web and soil health in high-input, tillage-intensive organic vegetable production systems.
Fresh is best.
Cover cropping frequency is the main driver of soil microbial changes during six years of organic vegetable production
Researchers have used nanoparticles to create a a fertilizer that releases nutrients over a week, giving crops more time to absorb them (ACS Nano 2017, DOI: 10.1021/acsnano.6b07781).
They attached urea molecules to nanoparticles of hydroxyapatite, a naturally occurring form of calcium phosphate found in bone meal. Hydroxyapatite is nontoxic and a good source of phosphorous, which plants also need.
In water, the urea-hydroxyapatite combination released nitrogen for about a week, compared with a few minutes for urea by itself. In field trials on rice in Sri Lanka, crop yields increased by 10%, even though the nanofertilizer delivered only half the amount of urea compared with traditional fertilizer.
Slow-release nitrogen fertilizer could increase crop yields | Chemical & Engineering News http://cen.acs.org/articles/95/web/2017/02/Slow-release-nitrogen-fertilizer-increase.html
They should call it UreaCa! Geddit?
Alternately you could just use fresh plant litter or cover crop residues that leach nitrogen over two weeks and also feed soil microbes carbon. Or faba bean that will release it over three years and build soil carbon so eventually you don’t need to add any.
 Carbon and Nitrogen Release from Legume Crop Residues for Three Subsequent Crops
Abstract | Digital Library https://dl.sciencesocieties.org/publications/sssaj/abstracts/79/6/1650
 Formation of soil organic matter via biochemical and physical pathways of litter mass loss : Nature Geoscience : Nature Research http://www.nature.com/ngeo/journal/v8/n10/full/ngeo2520.html
The priming effect, i.e. the increase in soil organic matter (SOM) decomposition rate after fresh organic matter input to soil, is often supposed to result from a global increase in microbial activity and competition due to the higher availability of energy released from the decomposition of fresh organic matter.
However a new study suggests that:
The chemical structure of added compounds on the priming effect is much larger than the effect of energy-content.
Different substrates resulted in different priming effect but appeared to stimulate the growth of similar bacterial groups. This suggests that the added compounds stimulate different enzyme systems within similar bacterial taxa.
Priming of soil organic matter: Chemical structure of added compounds is more important than the energy content