There are two stable isotopes of nitrogen: 14N and 15N. All nitrogen compounds contain both isotopes, but because of isotopic fractionation they are incorporated into compounds in differing ratios depending on the nature of the reactions that produce the compounds. For example, as nitrogen compounds are passed up the food chain, the lighter isotopes are excreted in urine and the heavier isotopes are retained. Nitrogen in animal waste is hydrolyzed to ammonia and then converted to nitrate. During this process more of the heavy isotope is concentrated in the resulting nitrates. When various sources of nitrogen compounds are mixed together in surface runoff or in a body of water, the ratio of light to heavy nitrogen isotopes in the water can be used to estimate the relative contributions of the various sources.