Increasing Feldspar Tunneling by Fungi across a North Sweden Podzol Chronosequence

Abstract

Tunnels in feldspar grains, assumed to be created by fungal hyphae, were first discovered in a boreal podzol. In this paper, we further describe the phenomenon of mineral tunneling by determining the rate of feldspar tunneling across a north Sweden podzol chronosequence. The chronosequence is a result of ongoing land uplift, which started after the retreat of glaciers about 9000 years ago. The sequence comprises a series of soils that began developing on glacial tills 190–7800 years ago. Feldspar tunneling was concentrated in the uppermost 2 cm of the E horizon, and its frequency increased significantly with soil age. Although no tunnels were found in feldspar grains from the youngest soil (190 years), they were seen more frequently in soils aged 2000 years and older. This lag phase in tunnel formation of about 2000 years coincided with the disappearance of the easily weatherable potassium-(K) and calcium (Ca)-containing minerals biotite and hornblende and with the appearance of etch pits on feldspar grains. In the oldest soil (7800 y), about 25% of the feldspar grains in the upper 2 cm of the E horizon were tunneled. Within site variation in tunnel frequency was high, and we were able to exclude spatial variations in mineralogy and texture as a possible explanation. The shape of the tunnels, their depth distribution, and the fungal hyphae found inside them all offer support for the previous assumption that their formation is mediated by biological activity involving fungi. The results of this investigation also indicate that the bioavailability of Ca and K may be a factor in tunnel formation.