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Vadose Zone Journal Abstract - Original Research

Sandy Soil Microaggregates: Rethinking Our Understanding of Hydraulic Function

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This article in VZJ

  1. Vol. 16 No. 9
     
    Received: May 02, 2017
    Accepted: July 30, 2017
    Published: September 7, 2017


    * Corresponding author(s): maria.dragila@oregonstate.edu
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doi:10.2136/vzj2017.05.0090
  1. Ashley Paradiśa,
  2. Christopher Brueckb,
  3. Douglas Meisenheimerb,
  4. Thomas Wanzeka and
  5. Maria Ines Dragila *a
  1. a Dep. of Crop and Soil Science, Oregon State Univ., Corvallis, OR 97331
    b Dep. of Chemical, Biological and Environmental Engineering, Oregon State Univ., Corvallis, OR 97331
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.

Abstract

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.

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