The Imprint of Soil Biota Over Annual to Millennial Timescales

Organisms interact with soil to seek nutrition, water, or shelter during their entire life-cycle or parts thereof. In doing so, soil is altered chemically and physically. Here, we focus on biomechanical affects on soil (bioturbation), and link these to other soil traits to further understand pedogenesis.

In tropical, temperate, and Mediterranean climates, invertebrates are the dominant bioturbators. Such organisms produce surficial mounds with soil excavated from beneath. Open and infilled burrows (krotovinas) are characteristics of soil fabric and represent displaced soil. We quantify the proportion of displaced soil below the surface using blocks of resin-impregnated soil and regard this as an annual to decadal imprint of bioturbation. Additionally, biotic soil mounding buries former soil surfaces. We quantify soil burial over millennial timescales using Optically Stimulated Luminescence (OSL).

At our site in the temperate highlands of eastern Australia, biofabrics in residual soils on Triassic sandstone crests decrease with increasing soil depth to the saprolite. Additionally, the soil-saprolite boundary marks the junction of heavily bioturbated soil and partially bioturbated saprolite, thus soil production from saprolite is biogenic. OSL burial ages increase exponentially to the saprolite, which, combined with the fabric data, suggest biotic activity decreases below the surface and that mounding may have buried soil here for millennia.

Not only is burial, and thus mounding, a millennial-scale soil process, but it is also moderately rapid (>10^2 m/My)—this is an order of magnitude faster than both soil production from saprolite and landscape denudation at this site. As such, soil biota must be considered a strong and persistent pedogenic force that have the potential to account for vertical distributions of soil properties.