EVIDENCE OF FUNGAL WEATHERING OF PRIMARY MINERALS IN A MARINE TERRACE CHRONOSEQUENCE, SANTA CRUZ, CALIFORNIA

It is well established that mycorrhizal fungal hyphae tunnel into primary silicate minerals for nutrient acquisition. The nutrients along with soil water are shared with symbiotic plants in exchange for carbon resources. The marine terrace chronosequence of this study is located north and west of Santa Cruz, California. The soils are developed in marine and beach sediments deposited on wave-cut bedrock terraces. Tectonic uplift in combination with sea level change has created a stair step sequence of terraces. We sampled 5 terrace soils ranging in age from 60 to 225 Ka. Fungal tunneling has been found in all 5 soils with field emission scanning-electron microscopy (FESEM). Qualitatively, fungal tunneling is more extensive in the older soils. In addition to fungal tunneling, two other types of fungal dissolution of feldspars have been observed in these soils: direct hyphal attachment and mineral traps. Fungal hyphae attached directly to the feldspar surface have been imaged by low voltage FESEM. The area of attachment is surrounded by a film of presumably extracelluar polysaccharides. The surfaces of these feldspars are highly etched. Curious structures, interpreted here as fungal mineral traps, have been found in Fe-rich soil nodules. The mineral trap consists of Fe-oxides and fungi in a pseudoparenchymatous conglutinate form, that completely surround a primary mineral grain. In many of these structures the mineral has dissolved creating a gap between it and the Fe-oxide coating. The mineral traps may be a mechanism for isolating primary minerals, presumably for nutrient extraction. Fungal tunneling of minerals have been reported to occur mainly in spodosols; a soil of some boreal and temperate forests. The chronosequence soils in this study are currently under a coastal prairie ecosystem with Mollisol soils in a Mediterranean (xeric) climate. The fungal tunneling in primary feldspar minerals of these Mollisol soils may indicate that tunneling is not limited to spodosols, or that these soils have tunneled minerals from a previous spodosol forming ecosystem.