In the seeds, the nitrogen is converted into amino acids, the building blocks of protein. When the beans or peas are removed, most of the fixed nitrogen is harvested as well. If the rest of the plant is removed, there is likely no net gain of nitrogen. Removing the plants and the roots could result in a loss of nitrogen. To add the most nitrogen to your soil, green manures should be incorporated when they start to flower.
Legumes break down quickly, and the nitrogen is released into the soil as nitrates.
Nitrates are readily available for plants, but are also susceptible to leaching. So, even if a green manure is successfully established, nodulated and incorporated, the nitrogen may still be lost. If a green manure is turned under in late summer or early fall, and the soil stays bare throughout from the fall rainy season through the next spring rain, much of the nitrogen may be leached. There are many ways to retain the nitrogen in the soil. The nitrogen can be taken up by living plants such as catch crops.