Unique Coevolution of Organic Matter and Selenium Enrichment in Cold Region Mollisols

Mollisols enriched with organic matter (OM) and selenium (Se) are a critical resource supporting global food security. Nonetheless, concerns arise as climate change and agricultural practices threaten Mollisol sustainability. Particularly, how the environmental conditions and biogeochemical processes constrain coevolution of OM and Se in Mollisols of cold regions is an unresolved issue. By combining pedogeochemical and multi-isotopic analyses with ultrahigh-resolution molecular and bioenergetic characterization of Mollisols in NE China, this research presents new evidence that unique cold region soil-forming environments are central to the coenrichment of OM and Se. While Mollisol OM originated from both C 3 (e.g., meadow) and C 4 (e.g., corn) plants, paleoclimate and redox shifts emerged as critical factors driving variations in the compositional and structural characteristics of OM, and substantial differences with the underlying parent material. The abundance of moderately energetic organic compounds (e.g., lipids and proteins) in Mollisols accounts for Se retention as organic-bound species. Under agricultural land use, the loss of OM and Se was accompanied by increasing proportions of microbial metabolites and fulvic acid-bound and hydrophilic Se species. By highlighting the dominant environmental drivers of changes in OM and Se chemistry in Mollisols, our research provides new knowledge that helps guide sustainable management of this valuable natural resource.