Applying fingerprint Fourier transformed infrared spectroscopy and chemometrics to assess soil ecosystem disturbance and recovery

Abstract

The assessment and monitoring of soil disturbance and its effect on soil quality (i.e., ability to support a range of ecosystem services) has been hindered due to the shortcomings of many traditional analytical techniques, including high cost, long-term time investment and difficulties with data interpretation. Consequently, there is a critical need to develop rapid and repeatable approaches for quantifying changes in soil quality that land managers may use to assess the condition and trend of natural and managed ecosystems. Here we report on a rapid, high throughput approach using fingerprint Fourier transformed infrared (FTIR) spectroscopy and chemometric modeling. Fingerprint FTIR incorporates all information embedded within the FTIR spectrum, thus producing a biogeochemical or ecological “fingerprint” of the soil. This methodology was applied in a highly disturbed forest ecosystem over a 19-year sampling period to detect, via spectral analysis, changes in dynamic soil properties (e.g., soil organic matter and reactive mineralogy) that can indicate changes in soil quality. Two chemometric statistical techniques (i.e., hierarchical clustering analysis [HCA] and discriminate analysis of principal components [DAPC]) were evaluated for interpreting and quantifying similarities/dissimilarities between samples utilizing the entire FTIR spectra from each sample. We found that both statistical approaches provided a means for clearly discriminating between degraded soils, soils in recovery, and reference soils. DAPC analysis provided additional information on the spectral wavenumbers most important in differentiating between soil samples across our disturbance time series. Wavenumbers relating to stretching of phenolic (C-O), aromatic (C=C, C-C), carboxylic acids (C=O), and to a lesser extent aliphatic (C-H) bonds were important in discriminating between soil samples. These results confirmed our visual interpretation of FTIR spectra, where an increase in aliphatic (fats and lipids) and decrease in phenolic/aromatic (lignin) absorption occurred following intensive site disturbance, followed by a return to predisturbance absorption values 17 years later. Results from this study illustrate the potential of fingerprint FTIR and chemometrics as an efficient technique for quantifying changes in soil quality that may be used in the monitoring and assessment of soil landscape change. Additional research across a range of soil ecosystems and under different types of ecosystem disturbance is needed to further validate this approach and to demonstrate its wide applicability as a land management tool.