Sandy Soil Microaggregates: Rethinking Our Understanding of Hydraulic Function

Abstract

Core Ideas Sandy soil microaggregates are comprised of a solid sand core encrusted with fines. Thick films control a significant proportion of water retention in coarse sandy soil. Three water pools drain sequentially: capillary water, thick films, and adsorptive films. Film seepage rates should be considered when irrigating sandy soil. This study investigated the peculiar structure of microaggregates in coarse sandy soils that exhibit only external porosity and investigated their control on soil hydrology. The microstructure underpins a hydrologic existence that differs from finer textured soils where aggregates have internal porosity. Understanding the impact of these microaggregates on soil hydrology will permit improved agricultural irrigation management and estimates associated with ecosystem capacity and resiliency. Microstructure was investigated using a digital microscope, and aspects of the structure were quantified by sedimentation and computed microtomography. Sandy soil microaggregates were observed to be comprised of a solid sand‐grain core that is coated with fines, presumably cemented by organic media. This microstructure leads to three distinct water pools during drainage: capillary water, followed by thick films (1–20 μm) enveloping the outer surfaces of the crusted microaggregates, followed by adsorbed thin films (<1 μm). The characteristics of the thick films were investigated using an analytical model. These films may provide as much as 10 to 40% saturation in the range of plant‐available water. Using lubrication theory, it was predicted that thick film drainage follows a power law function with an exponent of 2. Thick films may also have a role in the geochemical evolution of soils and in ecosystem function because they provide contiguous water and gas phases at relatively high moisture contents. And, because the rough outer crust of these microaggregates can provide good niches for microbial activity, biofilm physics will dominate thick film processes, and consequently hydrologic, biologic, and geochemical functions for coarse sandy soils.

Explicit spatial modeling at the pore scale unravels the interplay of soil organic carbon storage and structure dynamics

“…This confirms a similar result of POM inducing the formation of large macroaggregates, independent of texture, found in an incubation experiment (Bucka et al, 2021). This implies that the POM‐induced aggregate formation can connect particles of all sizes and underlines that also sand grains can be found within aggregates and thus contribute to a larger size of aggregates (Bucka et al, 2021; Felde et al, 2020; Paradiś et al, 2017).…”

Clay content modulates differences in bacterial community structure in soil aggregates of different size

“…Aggregates with a coarse texture component were, hitherto, not at the center of scientific attention. One of the first studies looking at the functional role of sand grains inside microaggregates by Paradiś et al (2017) described sandy microaggregates with a solid sand grain nucleus that is surrounded by mineral fines. They conceptualized a model where water menisci forces draw fine soil constituents towards the solid surface of larger particles after multiple wetting-drying events.…”

Wet sieving versus dry crushing: Soil microaggregates reveal different physical structure, bacterial diversity and organic matter composition in a clay gradient

“…Aggregates with a coarse texture component were, hitherto, not at the centre of scientific attention. One of the first studies looking at the functional role of sand grains inside microaggregates by Paradiś, Brueck, Meisenheimer, Wanzek, and Dragila (2017) described sandy microaggregates with a solid sand grain nucleus that is surrounded by mineral fines. They conceptualized a model where water menisci forces draw fine soil constituents towards the solid surface of larger particles after multiple wetting‐drying events.…”