Agriculture Reference
In-Depth Information
(N, P, K, S, Ca, and Mg), but also essential micronutrients such as Fe, Zn, Mn, Ni, Cu, and
Mo. Most of these micronutrients accumulate in the plant tissues for their metabolic needs,
but they never exceed 10 ppm. Higher levels of these metals (and other heavy metals) in
plant tissue might have phytotoxic effects, sometimes resulting in death (Winkelmann 2005).
Vegetation stress caused by heavy metal toxicity can show different symptoms, depending
on the heavy metal's type and concentration, and on the plant species' sensitivity. The
phototoxic effects typically cause physiological, morphological and ecological changes,
expressed in many different ways, from chlorosis (reduced production of chlorophyll and
hence reduced photosynthetic activity), leaf wilting, needle retention, branch density
reduction, defoliation, growth inhibition, flowering and fruiting changes, dwarf growth and
gigantism, to changes in plant species distribution, the establishment of adapted species,
and the dying-off of whole plants or communities (Winkelmann 2005). However, stress can
be induced in vegetation by a large number of other factors, including water deficiency,
poor soil drainage, poor soil aeration, soil salinity, weed competition, pest infestation,
nutrient deficiency, or nutrient poisoning (Levitt 1980; Lichtenthaler 1996).
A large number of studies have reported on spectra of metal-stressed vegetation being
clearly different from those of unstressed vegetation. These differences usually include
decreases in both NDVI and RVI with increasing plant stress (Davids & Tyler 2003;
Dunagan et al. 2007), signs of reduced biomass and a shift of the red edge position (REP)
(Kooistra et al. 2004; Dunagan et al. 2007) or red edge slope (Zhou et al. 2010), signs of
reduced photosynthetic activity due to chlorosis and decreased reflectance in the 700 nm to
2,500 nm wavelength region (Kooistra et al. 2003). Some heavy metals which are considered
contaminants are also essential micronutrients for all vegetation species, in particular Cu
and Zn. Therefore, in some cases, even positive effects in reflectance spectra can be observed
when one of the elements under investigation is clearly a micronutrient with limited
availability. Such effects include a red-shift of the red edge and decreased reflection in the
VIS wavelength region indicating increased photosynthetic activity (Horler et al. 1980). As
no vegetation stress symptoms and corresponding spectral characteristic could be related
specifically to heavy metal stress or any other contaminant stress, the sources causing the
observed stress need to be carefully identified and separated.
8. Limitations, obstacles and problems
8.1 Laboratory
A wide range of factors can affect soil reflectance spectra in both laboratory and field
domains. In the lab, different spectrometers, or even repeating a specific sample's
measurements in the same spectrometer may result in variations. Such variations might
include subtle or strong alterations in wavelength location, peak absorption shape or
radiometric intensity. In addition to the instrumentation itself, internal electronic noise can
affect the measurements and mechanical noise factors (e.g. homogeneity and purity of the
white reference panel, or subtle movement when holding the fiber optic) can strongly affect
their consistency. In soil samples, where very weak spectral features are monitored for
chemometric purposes, these noise factors can alter the robust use of a selected spectral
model for a wide range of spectrometers and users. Recently, Pimstein et al. (2011)
examined the variation stemming from the above sources using three ASD Fieldspec Pro
spectrometers and developed a standard protocol for laboratory spectral operation. They
also suggested using the same internal standard worldwide in order to correct the spectra of
