Soil organic matter (SOM) may be a significant source of atmospheric ice nucleating particles (INPs), especially of those active ????15?°C. However, due to both a lack of investigations and the complexity of the SOM itself, the identities of these INPs remain unknown. To more comprehensively characterize organic INPs we tested locally representative soils in Wyoming and Colorado for total organic INPs, INPs in the heat-labile fraction, ice nucleating (IN) bacteria, IN fungi, IN fulvic and humic acids, IN plant tissue, and ice nucleation by monolayers of aliphatic alcohols. All soils contained ?≈??106 to ?≈??5?×?107?INPs?g?1 dry soil active at ?10?°C. Removal of SOM with H2O2 removed ?≥??99?% of INPs active ????18?°C (the limit of testing), while heating of soil suspensions to 105?°C showed that labile INPs increasingly predominated ????12?°C and comprised ?≥??90?% of INPs active ????9?°C. Papain protease, which inactivates IN proteins produced by the fungus Mortierella alpina, common in the region's soils, lowered INPs active at ?≥???11?°C by ?≥??75?% in two arable soils and in sagebrush shrubland soil. By contrast, lysozyme, which digests bacterial cell walls, only reduced INPs active at ?≥???7.5 or ?≥???6?°C, depending on the soil. The known IN bacteria were not detected in any soil, using PCR for the ina gene that codes for the active protein. We directly isolated and photographed two INPs from soil, using repeated cycles of freeze testing and subdivision of droplets of dilute soil suspensions; they were complex and apparently organic entities. Ice nucleation activity was not affected by digestion of Proteinase K-susceptible proteins or the removal of entities composed of fulvic and humic acids, sterols, or aliphatic alcohol monolayers. Organic INPs active colder than ?10 to ?12?°C were resistant to all investigations other than heat, oxidation with H2O2, and, for some, digestion with papain. They may originate from decomposing plant material, microbial biomass, and/or the humin component of the SOM. In the case of the latter then they are most likely to be a carbohydrate. Reflecting the diversity of the SOM itself, soil INPs have a range of sources which occur with differing relative abundances.
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