Examining the Incipient Stages of Mineral Weathering in Semiarid and Humid Landscapes.

Soil microorganisms play a crucial role in soil structure, nutrient cycling, and plant growth in the soil environment, yet biological contributions to mineral weathering are underrepresented in field systems. The objective of this research was to examine the responses of biological constituents (e.g., fungal hyphae) to topography and mineral assemblage in dry to humid landscapes. The in-soil mesh bag field experiment was deployed in divergent summit and convergent footslope positions across two Critical Zone Observatories (CZOs): desert and mixed conifer ecosystems in the Catalina CZO (Arizona) and the wet, humid hardwood forests in the Calhoun CZO (South Carolina). Ground rock materials (basalt, granite, quartz sand; 53-250 µm) were sealed into nylon mesh bags and subsequently buried in surface soils during 2015. The bags were retrieved after one year and analyzed using helium ion microscopy and scanning electron microscopy. Preliminary results confirm the presence of microbe-mineral interfaces in the form of organic coatings and fungal hyphae. Biological specimens observed in the mesh bags exhibited contrasting surface features based on mineral type and mesh size including fungal hyphae that varied in length, diameter, and the presence/absence of adhered exudates, mineral particles, and bacterial communities. The results of this work suggest that hyphae may preferentially attach to and transform minerals that contain calcium, magnesium, and/or potassium in both CZO locations. Conversely, the most prevalent organic coatings were detected in downslope, water-gathering positions of the Calhoun CZO that represents the humid, wet climate end member in the study. The continuation of this work requires quantitative assessments of soil geochemistry, microbial inputs to bulk ground rock samples, and the integration of microbial community profiling to identify the dominant communities driving mineral weathering in field systems. Outcomes of this research include a stronger understanding of biological-driven weathering on toposequences established along a climate gradient-- an imperative task for advancing global carbon and nutrient cycling models where biological contributors to mineral weathering require improved representation.