Mid-Infrared Soil Spectral Changes Due to Cultivation, C Mineralization, and Short-Term Substrate Utilization.

Mid infrared diffuse reflectance spectroscopy is a valuable technique for the study of the C quantity and quality of soils. We analyzed soils from two sites (Hoytville, OH, and Akron, CO), under different managements (native, vs. different levels of cultivation). Each soil was also analyzed before and after a 707 d incubation in order to determine how long–term management and C mineralization affects the soil spectral properties and chemistry. Multivariate analysis shows that the largest spectral differences occurred between sites, with the Hoytville soils having more organic absorption bands (amides, aliphatic, carboxylic and phenolic), while the Akron soils had more quartz and clay absorbance. This is consistent with the higher sand content and lower soil C of the Akron site. Within each site, the long-term tillage management affected the spectra, showing that SOM does have an effect within the larger influence of soil mineral composition. The native soils, which lost more C upon incubation, also absorbed more than the cultivated soils at the 2930-2870 cm^-1 CH band, and several bands between 1680 and 1450 cm^-1 that mark the presence of several organic functional groups. The 707 d day incubation affected the spectral properties of the soils, but the effect was different between sites. This suggests that the SOM dynamics during incubation may follow different patterns depending on the initial soil mineralogical, organic, and microbiological composition. An ancillary experiment was carried out in which soil samples were incubated for up to 42 days with the addition of corn or wheat residue. In the native Akron soils, the decomposition of the wheat and the corn resulted in different spectral dynamics.  The wheat residue caused an initial change at 3 weeks that reverted to the unamended soil spectral properties. The corn residue caused more unidirectional spectral changes, and the 42 d spectra were different from the earlier sampling times. Corn addition and incubation resulted in increased absorbance at 1348 cm^-1, suggesting that this band may indicate recalcitrant SOM. The spectral data is supported by carbon loss during incubation, which shows that more of the corn C was retained in the soil than the wheat C.