Microorganisms play a fundamental role in regulating the cycling of nutrients in soil through organo-mineral interactions. Our objective was to examine the composition and abundance of incipient weathering products extracted from minerals exposed to natural soils in a field experiment.
We deployed nylon mesh bags filled with granular basalt, granite, and quartz sand (250-53 micron) in surface soils of desert scrub, sub-humid conifer forest, and humid hardwood forest systems for three years. The retrieved samples were prepared for Fourier-transform ion cyclotron resonance mass spectrometry (FTICR-MS) using two solvents (water, chloroform) with different polarities to sequentially extract a representative fraction of organic matter associated with the granular materials. Water and dilute acid extracts were analyzed by Inductively Coupled Plasma- Optical Emission Spectrometry to quantify soluble weathering products from the bulk substrates. We also assessed incipient weathering features of the retrieved materials with a coupled scanning electron and helium ion microscopy approach.
Our results show that the composition of the organo-mineral compounds vary by rock type in the weathered soils of the humid forests, despite the randomized deployment of mesh bags in the same plots and topographic positions (Calhoun Critical Zone Observatory, SC; Phil Campbell Research Station, GA). Results from Principle Component Analysis (PCA) revealed that samples from the humid forest Phil Campbell site showed distinct organic compound clusters in basalt (CHO, CHON, CHONSP), granite (CHO, CHOS, CHOP), and quartz (CHOS, CHOP, CHONP, CHONS). Organic compounds from the Calhoun CZO grouped together for basalt (CHO, CHONSP) while compounds from quartz and granite overlapped, which indicated similarities in molecular composition. Conversely, granite and basalt were dominated by CHO at the subhumid forest site (Catalina Conifer Forest CZO, AZ) and the biogeochemical classes in the moisture-limited desert scrub site (Catalina Desert CZO, AZ) showed no separation by rock type. Our work suggests that the composition of the mineral matrix influences microbial activity under natural conditions and contributes to a broader understanding of biologically-mediated weathering across climate and topographic gradients.
Lybrand, Rebecca, Jason Austin, Rosalie Kae Chu, Odeta Qafoku, Tom Resch, Erin Rooney, Paul A Schroeder, Malak M Tfaily, Jason Toyoda, Vaithiyalingam Shutthanandan, Dragos George Zaharescu, Maya Bergmann, Benjamin Pierce, Rachel E Gallery (2019): Identifying the organic and inorganic products of incipient weathering in natural environments. American Geophysical Union Fall Meeting, San Francisco, CA, December 9-13, 2019.
This Paper/Book acknowledges NSF CZO grant support.