Light penetration in soil and particulate minerals

Abstract

Knowledge of light penetration in soils is of particular interest for photolytic degradation of pesticides, for laser-induced fluorescence spectroscopy and remote sensing, and for understanding better the germination of seeds. To date little information has been available in the literature on this topic. In this paper light penetration in soils is determined successfully using diffuse reflectance and transmittance spectroscopy and the relatively simple Kubelka-Munk model. Using the latter model of light propagation in turbid media, the optical properties of kaolinite, montmorillonite, barium sulphate, goethite and 19 different soils were determined in the wavelength range 275-700 nm. In particular, the light absorption coefficient, k, and light scattering coefficient, s, were determined. The depth at which the light intensity at the surface is reduced by 99% (light penetration depth) and the depth of the sample contributing to the measured reflected radiation (information depth) could also be calculated from k and s. For kaolinite, the light penetration depth ranged between 10 and 200 mm for wavelengths between 275 and 700 nm, respectively; the information depth was between 5 and 80 mm. For soils, the penetration depth was in the range 17-110 mm at 275 nm and 120-300 mm at 700 nm, and the information depth was in the range 8-60 mm at 275 nm and 60-175 mm at 700 nm. Hence, the information depth is about half the penetration depth for media with reflectance smaller than 0.7 (e.g. soils). For dry soils, an empirical relationship was established between the light absorption coefficient and the amount of particlesize fractions. The effect of water content was also investigated: addition of water to kaolinite reduced its scattering coefficients, whereas the absorption coefficient was hardly affected. For the soils, addition of water had a more complex mode of action affecting both absorption and scattering coefficients. With the measured optical properties of dry minerals and soils it is possible to calculate light intensity profiles with depth and to quantify photochemical processes occurring in these media.

Volatile organic compounds (VOCs) in soils

“…Anyway, Atkinson and Arey (2003) give a very detailed overview on physical-chemical degradation of VOCs in the atmosphere where photolysis and spontaneous reactions with OH radicals (under the presence of light), NO 3 radicals (under the absence of light), O 3 (under the presence of light), and Cl atoms (at coastlines) are the main processes responsible for abiotic VOC degradation. Considering the fact that light can only reach habitats located in the soil surface (Woolley and Stoller 1977;Ciani et al 2005), photolysis and other lightdriven reactions (with OH radicals and O 3 ) only occur in a thin surface zone (Konstantinou et al 2001). In this zone, OH radicals and O 3 are produced by microorganisms or they diffuse from the atmosphere.…”

Physiological and taxonomic description of the novel autotrophic, metal oxidizing bacterium, Pseudogulbenkiania sp. strain 2002

“…To date, several microorganisms have been identified that are capable of anaerobic Fe (II) oxidation (Bruce et al 1999;Chaudhuri et al 2001;Edwards et al 2003;Finneran et al 2002;Hafenbradl et al 1996;Lack et al 2002a;Straub et al 1996;Straub et al 1999;Widdel et al 1993), however, few of these grow by the metabolism (Weber et al 2006a). Given that light penetration of soil and particulate matter is only between 8 to 200μm (Ciania et al 2005), light independent reactions such as nitrate-dependent Fe(II) oxidation are more likely to dominate bio-oxidative processes in saturated and subsurface soil/sedimentary environments.…”

The blurred line between photic and aphotic environments: a large Mexican cave with almost no dark zone

“…Light very rapidly declines in soil, with less than 99% of light remaining after one mm (Ciani et al, 2005). Light penetrates farther in leaf litter, and its decline follows the Beer-Lambert law (Facelli & Pickett, 1991): where RI is relative illuminance, b is transmittance constant of litter, and M is the amount of litter per g/m 2 .…”