Hunting the Snark: Identifying the Organic Ice Nuclei in Soils

The contribution of soil organic matter as a potential source of atmospheric ice nuclei (IN) has long been postulated. Rather surprisingly, considering the abundance of IN active at warm temperatures in many soils, it remains unresolved. This research aimed to identify sources of high-temperature, organic IN in a range of Wyoming and Colorado soils. Methods used included physical, chemical and enzymatic tests combined with quantitative PCR to estimate the number of ice nucleation active bacteria. All soils contained 106 to >107 IN active at −10°C. Reductions in IN after heating or digestion with hydrogen peroxide suggested that IN active >−15°C were effectively all organic. Ice nuclei active >−7°C appear to be primarily a mixture of biological macromolecules. At colder temperatures there was a large pool of organic IN that were quite resistant to most physico-chemical challenges.