Green manure like Chinese milk vetch can help rice grow better and use nitrogen more efficiently compared to just using chemical fertilizer. By using a special labeled nitrogen compound, researchers found that when Chinese milk vetch was added with urea, rice plants grew bigger, took up more nitrogen, and used urea more effectively. When less urea was used, more nitrogen from Chinese milk vetch was taken up by the rice. This method changed how urea was used, with more being taken up by the rice and less remaining in the soil compared to using urea alone. Substituting some chemical fertilizer with green manure can help rice plants grow better by providing nitrogen for them to use efficiently.