390-3 Diffuse Reflectance Spectroscopy for Total Carbon Analysis of Hawaiian Soils.



Wednesday, October 19, 2011: 8:40 AM
Henry Gonzalez Convention Center, Room 209, Concourse Level

Meryl McDowell1, Greg Bruland1, Jonathan Deenik1, Sabine Grunwald2 and Nichola M. Knox2, (1)University of Hawaii, Honolulu, HI
(2)University of Florida, Gainesville, FL
Accurate assessment of total carbon (Ct) content is important for fertility and nutrient management of soils, as well as for carbon sequestration studies.  The non-destructive analysis of soils by diffuse reflectance spectroscopy (DRS) is a potential supplement or alternative to the traditional time-consuming and costly combustion method of Ct analysis, especially in spatial or temporal studies where sample numbers are large.  We investigated the use of the visible to near-infrared (VNIR) and mid-infrared (MIR) spectra of soils coupled with multivariate chemometric analysis to determine their Ct content.  Our specific focus was on Hawaiian soils of agricultural importance.   

This study benefits from the remarkably diverse soils of Hawaii.  Our sample set included 216 soil samples from 145 pedons from the main Hawaiian Islands archived at the National Soil Survey Center in Lincoln, NE, along with 89 newly-collected samples from Kauai, Oahu, Molokai, and Maui.  In total, over 100 series from 10 of the 12 soil orders were represented.  The Ct values of these samples ranged from < 1% – 55%. 

We evaluated two methods in our effort to develop a prediction model for soils of unknown carbon content given only their spectral signature.  For each method, the calibration set was a random 70% subset of the full data set, and the validation set consisted of the remaining 30% of the data set.  Using Partial Least Squares Regression (PLSR), we found R2 values of 0.91/0.94 (calibration/validation) for the VNIR spectra and 0.95/0.94 for the MIR spectra.  With a preliminary trial of random forests ensemble tree regression, we achieved slightly lower R2 values for the VNIR spectra (~0.85) but comparable R2 values for the MIR spectra (~0.96).  These results indicate that VNIR and MIR spectroscopy provide a viable alternative to combustion techniques for Ct analysis on Hawaiian agricultural soils.

See more from this Division: S11 Soils & Environmental Quality
See more from this Session: Symposium--Changes In Soil Carbon Due to Climate and Human Activities