Sweet pea, Lathyrus odoratus, is a member of the legume family and can fix nitrogen in the soil. Nitrogen fixation occurs when specialized strains of bacteria (rhizobia) infect the roots of legumes (beans, peas, clover, vetch, alfalfa, lupins, sweet clover, etc.). Nodulation occurs when the legume roots form a growth, called a nodule, around the bacteria. Within the nodules, the bacteria convert atmospheric nitrogen into usable plant compounds (such as ammonia and nitrate). After the plant flowers, the nitrogen moves from the roots into the seeds. Eventually the legume dies and releases the stored nitrogen as it decomposes. About half of the nitrogen will be released within a year after incorporation into the soil. The rest of the nitrogen will be released over a period of a few years.

so yes you can dig them in.

Also you can collect the seeds for next years planting as I do -- Nitrogen what's that ? lol

Many thanks for answering my question . As i said i just couldnt find the answer.