Estimating leaf nitrogen concentration in ryegrass ('Lolium' spp.) pasture using the chlorophyll red-edge: theoretical modelling and experimental observations

Title
Estimating leaf nitrogen concentration in ryegrass ('Lolium' spp.) pasture using the chlorophyll red-edge: theoretical modelling and experimental observations
Publication Date
2002
Author(s)
Lamb, David
Steyn-Ross, M
Schaare, P
Hanna, M M
Silvester, W
Steyn-Ross, A
Type of document
Journal Article
Language
en
Entity Type
Publication
Publisher
Taylor & Francis
Place of publication
United Kingdom
DOI
10.1080/01431160110114529
UNE publication id
une:11708
Abstract
Chlorophyll red-edge descriptors have been used to estimate leaf nitrogen concentration in ryegrass ('Lolium' spp.) pasture. Two-layer model calculations have been used to predict the influence of chlorophyll content and Leaf Area Index (LAI) on the shape and location of the peaks observed in the derivative spectra of a ryegrass canopy. The complex structure of the resulting derivative spectra precluded extracting red-edge wavelengths by fitting inverted Gaussian curves to reflectance profiles. Fitting a combination of three sigmoid curves to the calculated reflectance spectra provided a better representation of subsequent derivative spectra. The derivative spectra in the vicinity of the chlorophyll red-edge is predicted to contain two peaks (705 and 725 nm), which on increasing the canopy LAI is generally found to shift to longer wavelengths. However, for a canopy containing leaves of low chlorophyll content and LAI>5, the wavelength of the first peak becomes insensitive to changes in LAI. The same phenomenon is predicted for high-chlorophyll leaves of LAI>10. The role of multiple scattering, primarily due to increased leaf transmittance at higher wavelengths, has also been verified. In subsequent experiments, the predicted shape of the derivative spectra was observed and the use of three sigmoid curves to better represent this shape verified. Changes in the descriptors used to describe the chlorophyll red-edge were observed to explain 60% and 65% of the variance of leaf nitrogen concentration and total leaf nitrogen content, respectively. The resulting regression equation was found to predict leaf nitrogen concentration, in the range of 2-5.5%, with a standard error of prediction (SEP) of 0.4%. The confounding influence of canopy biomass on the red-edge determination of leaf nitrogen concentration was found to be significantly less at higher canopy biomass, confirming both theoretical predictions and the potential of using the chlorophyll red-edge as a biomass-independent means of estimating leaf chlorophyll, and hence nitrogen, concentration in high-LAI ryegrass pastures.
Link
Citation
International Journal of Remote Sensing, 23(18), p. 3619-3648
ISSN
1366-5901
0143-1161
Start page
3619
End page
3648

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