Use of near- And mid-infrared spectroscopy to distinguish carbon and nitrogen originating from char and forest-floor material in soils

Abstract

The presence of relatively inert organic materials such as char has to be considered in calibrations of soil C models or when calculating C-turnover times in soils. Rapid and cheap spectroscopic techniques such as near-infrared (NIRS) or mid-infrared spectroscopy (MIRS) may be useful for the determination of the contents of char-derived C in soils. To test the suitability of both spectroscopic techniques for this purpose, artificial mixtures of C-free soil, char (lignite, anthracite, charcoal, or a mixture of the three coals) and forest-floor Oa material were produced. The total C content of these mixtures (432 samples) ranged from 0.5% to 6% with a proportion of char-derived C amounting to 0%, 20%, 40%, 50%, 60%, or 80%. All samples were scanned in the visible and near-IR region (400-2500 nm). Cross-validation equations for total C and N, C and N derived from char (Cchar,Nchar) and Oa material were developed using the whole spectrum (first and second derivative) and a modified partial least-square regression method. Thirty- six samples were additionally scanned in the middle-IR and parts of the near-IR region (7000-400 cm-1 which is 1430-25,000 nm) in the diffuse-reflectance mode. All properties investigated were successfully predicted by NIRS as reflected by RSC values (ratio of standard deviation of the laboratory results to standard error of cross-validation) > 4.3 and modeling efficiencies (EF) ≥ 0.98. Near-infrared spectroscopy was also able to differentiate between the different coals. This was probably due to structural differences as suggested by wavelength assignment. Mid-IR spectroscopy in the diffuse-reflectance mode was also capable to successfully predict the parameters investigated. The EF values were > 0.9 for all constituents. Our results indicated that both spectroscopic techniques applied, NIRS and MIRS, are able to predict C and N derived from different sources in soil, if closed populations are considered. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.