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Mogens Humlekrog Greve

The Effects of Moisture Conditions—From Wet to Hyper dry—On Visible Near-Infrared Spectra of Danish Reference Soils

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Changes in soil water content are known to affect soil reflectance. Even
though it was suggested some time ago that the phenomenon of increased
forward scattering due to the presence of water in the soil is related to water
film thickness and matric potential, there has been no detailed investigation
of this in any studies. The effects of moisture conditions on visible nearinfrared
(vis-NIR) spectra of four representative soils in Denmark have been
assessed as a function of both water film thickness (expressed as the number
of molecular layers) and matric potential. Complete water retention curves,
from wet (pF 0.3, pF = log(|j|), where j is the matric potential in cm) to
hyper dry end (oven-dried and freeze-dried soil), were obtained by initial
wetting followed by successive draining and drying of soil samples, performing
NIR measurements at each step. Soil reflectance was found to decrease
systematically, yet not proportionally, with decreasing matric potential and
increasing molecular layers. The changes in molecular layers were best captured
by the soil reflectance of clay-rich soils. Here the largest increase in
reflectance occurred between pF 3 and 4, caused by the shift from capillary
to adsorptive surface forces. In support of this, the smallest changes in reflectance were seen in the sandiest soil. Freeze drying the soil highest in organic C increased reflectance, possibly due to an alteration in organic matter during freezing. The different reflectance behavior of soil with a higher organic
C content may be linked to differences in the amount, but also the quality
(higher hydrophobicity) of the organic matter. However, this needs to be confirmed in further studies.
TidsskriftSoil Science Society of America Journal
Sider (fra-til)422-433
StatusUdgivet - 2014

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