Close Encounters

Woody plants work with microbes to shape the soil they live in

The B-horizon in soil is created through interactions between plants and rhizosphere organisms.

There is no soil on the Moon. Instead, there’s regolith, loose deposits of rock that can be powder-sized. To get soil, you need organisms. Also, not all soil is the same. Frank Reith and colleagues have been examining how eucalypts create B-horizons in soil. The processes are a key step in converting life into geology.

Eucalypts. Image: Canva.

The B-horizon in soil is what you find below the A-horizon. I’ll admit this is not the most helpful comment by itself. When you scrape off the leaves on the surface (the O layer), you get to the topsoil, the A-horizon. The B-horizon lies beneath this level, as subsoil. This is a layer created by organic action, even though it may be low in organic material. The trees and their partners create clays and cretes that form the geology below the topsoil.

“The sheer magnitude of bioprecipitated clays and cretes across temperate Western Australia cannot be overemphasized, with an estimated 40 % of the area devoted to clay-depositing tree eucalypts and evidence from soil surveys and airborne radiometry of at least equally extensive areas of proteaceous-based systems associated with ferricretes, alcretes and coffee rock podzols,” write Reith and colleagues. “Here we find a truly impressive example of how bio-pedological forces have engineered outcomes of paramount significance to the diversity and sustainability of the bewildering complex of interacting ecosystems in soilscapes of this area.”

The team conclude that the soils form through the action of rhizosphere microbes, the microscopic organisms living next to the roots. The roots themselves provide solutes to these organisms to fuel their work in creating the soil. While the study was on eucalypts the authors find their results also shed light on how other plants work in the soil. “Comparisons of morphogenetic features of B horizons under eucalypts, tree banksias and other vegetation types show remarkably similar developmental trajectories involving pods of precipitation surrounding specialized fine rootlets and their orderly growth to form a continuous B horizon,” they write.

“Further acquisition of microbial information and contextual ecological data should provide not only a better understanding of how such biotic systems evolve, develop and operate but also a rational basis for taxonomic classification of soil–biota assemblages.”