I read a cover crop study recently on radishes that showed they used more nitrogen than they sequestered, and this was after chopping tops and leaving them in the soil to decompose. Another cover crop study showed that a following crop of similar species or one that hosted similar microbiology gave poorer results. So cover crop selection can be important, and the ratio of root to shoot may be important. So always study your plant roots!
One of the reasons legumes perform so well as cover crops is because of the amount of biomass in the form of labile carbon they sequester in their roots. This increases CEC as they decompose, with some varieties able to feed the soil for years. One of the best performers I’ve seen in studies were faba bean. Legumes also work especially well in acidic soils because the roots raise the pH around them using the nitrogen they fix and create hot spots of microbial activity, and not just inside the nodules. The nodules themselves also don’t just house rhizobia, but a whole ecosystem. Interesting I’ve learned that the nodules themselves aren’t evidence of rhizobia colonization, and the best way to determine if they are nitrogen fixing is to look at the microbial biomass inside a nodule when sliced open. I also read recently that microbes use this nitrogen to create amino acids like alanine that when oxidized mineralize soil nutrients faster than the nitrogen and carbon alone would based on its C:N ratio. So microbial enzymes can supercharge plant available soil nutrients, with the study I saw showing a 40 times increase in rate of decomposition.
So you should get a better result if you can leave plant roots to feed the soil, and if you can’t otherwise find a good way to kill a cover crop (acidic vinegar spray perhaps?) without a herbicide (that’s how many no till farmers do it) then cutting plants and then laying down a mulch of compost, or biodegradable weed mat, cardboard or paper mulch with holes to plant larger crops into like the courgettes, may work. Flame weeding or using a cordless steam cleaner/mop on the leaf area are other options. Though matting itself isn’t without problems as they can attract fauna predators, which themselves have been found to aid labile carbon sequestration as the predators eat their prey and cycle those nutrients. I’ve also read that matting left on over time without amendment lowers labile soil carbon and this can increase fungal predators that need this carbon more than bacteria and archaea because of fungus’ higher C:N ratio being eukaryotes, and so fungi take that carbon from plants and subsequently cause disease, like powdery mildew. The same thing happens with nematodes, when there isn’t enough labile carbon for the fungi that prey on the nematodes, the nematodes attack plant roots.
Interesting observation he makes about the Red Russian Kale and predatory lady bugs, I’ll have to plant some in with aphid loving crops.