Steroid dissipation and formation in the course of farmyard manure composting

Steroids are used as biomarkers for tracing faecal material in the environment, but their dissipation behaviour in the course of composting has remained unclear. To assess their stability, we tracked the fate of Δ5-sterols, 5α-stanols, 5β-stanols, epi-5β-stanols, stanones and bile acids after 0, 7, 14, 28, 56, 112 and 168 days and after 0 and 168 days of composting cattle and horse farmyard manure, respectively. After composting, extractable steroid content decreased by 93.8–99.9% for cattle manure and by 54.0–100% for horse manure relative to the initial amount. The loss was with 98.8–99.9%, particularly pronounced for bile acids in the cattle manure compost. Our findings challenge the assumption that bile acids are generally more resistant towards degradation than other steroids. Contrary to the bile acids, the Δ5-sterol content did not decrease constantly, but showed a temporary increase, pointing to a delayed release from straw (β-sitosterol, stigmasterol and cholesterol) or to temporary production by fungi (cholesterol). Similarly, there was no continued loss of cholesterol transformation products (5α-cholestanol, cholestanone and epicoprostanol); they either increased temporarily or showed delayed degradation. These changes in steroid patterns complicate the identification of compost as faecal matter by way of commonly used steroid ratios and stress the importance of additional bile acid analysis in geo-archaeological research. Nevertheless, slower dissipation and smaller relative loss of epi-5β-stanols vs. 5β-stanols increased the value of r = epi-5β-stigmastanol/5β-stigmastanol + epicoprostanol/coprostanol during composting and allowed differentiation between fresh farmyard manure, and immature and mature compost.