Estrogens: Properties, behaviors, and fate in dairy manure-amended soils

In 2012 there were 63% fewer dairies in the United States than there were in 1997 as a result of conglomeration of the dairy industry into concentrated animal feeding operations at the expense of smaller farms. Today, 60% of all milk produced in the United States comes from 5% of the nation's dairies (operations with >= 500 cows). Concentrated animal feeding operations are touted as economically efficient agricultural business models, hailed for their increased milk yields. Yet, with an average daily manure production of over 27 000 kg for a 500-head dairy farm, manure storage and disposal are serious management and environmental concerns. A common economical mode of manure disposal is application to nearby agricultural fields. However, a major concern with land application of dairy manure is the fate of manure-borne hormones, compounds considered chemicals of emerging concern, and the potential threat these hormones pose to humans and the environment. The fate of these chemicals in the soil environment is complicated by multiple edaphic variables including pH, mineralogy, organic matter, microbial activity, and redox status. Estrogens are sorbed by soil organic matter and transformed to nonbioactive, highly soluble conjugated forms or to metabolites that exhibit yet additional properties distinct from their parent compounds. However, deconjugation frequently occurs, regenerating endocrine-disrupting free estrogen compounds. It is challenging to fully understand the behavior and predict the fate of estrogenic compounds from dairy manure in soils because of variable and complex interactions with soil factors, as well as possible interactions among the different chemicals of emerging concern. This review focuses on the behavior of naturally occurring estrogen hormones present in dairy manure in the soil environment. Heightened understanding of the fate of these compounds in soil will enhance our ability to reduce their potential risks.