Abstract
The investigation of iron oxides in soil using spectral reflectance is very common. Their spectral signal is significant across the visible–near infrared (VIS–NIR) spectral range (400–1000 nm). However, this range overlaps with other soil chromophores, such as those for water and soil organic matter (SOM). This study aimed to investigate the effect of different SOM species on red soil from Israel, which is rich in hematite iron oxide, under air-dried conditions. We constructed datasets of artificially mixed soil and organic matter (OM) with different percentages of added compost from two sources (referred to as A2 and A5). Eighty subsamples of mixed soil–OM were prepared for each of the OM (compost) types. To investigate the effect of OM on the strong iron-oxide absorbance at 880 nm, we generated two indices: CRDC, the absorbance spectral depth change at 880 nm after continuous removal, and NRIR, the normalized red index ratio using 880 and 780 nm wavelengths. The different OM types influenced the soil reflectance differently. At low %SOM, up to 1.5%, the OM types behaved more similarly, but as the OM content increased, their effect on the iron-oxide signal was greater, enhancing the significant differences between the two OM sources. Moreover, as the SOM content increased, the iron-oxide signal decreased until it was completely masked out from the reflectance spectrum. The masking point was observed at different SOM contents: 4% for A5 and 8% for A2. A mechanism that explains the indirect chromophore activity of SOM in the visible region, which is related to the iron-oxide spectral features, was provided. We also compared the use of synthetic linear-mixing practices (soil–OM) to the authentic mixed samples. The synthetic mixture could not imitate the authentic soil reflectance status, especially across the overlapping spectral position of the iron oxides and OM, and hence may hinder real conditions.
Highlights
Qualitative and quantitative detection of iron oxides using reflectance spectroscopy is an important task in soil mapping practice [1,2,3,4,5,6]
As the visible–near infrared (VIS–NIR) spectral region has some limitations in predicting Soil organic matter (SOM), as already noted, we strongly suggest that the entire spectral VIS–NIR–SWIR region be used for such predictions to obtain more spectral information as examined here
The overlapping spectral responses of organic matter (OM) and iron oxides in the VIS–NIR spectral region should be taken into account when both iron-oxide and OM contents in the soil are to be spectrally estimated due to this eventual obliteration ("saturation") effect of OM on the iron-oxide spectral signal
Summary
Qualitative and quantitative detection of iron oxides using reflectance spectroscopy is an important task in soil mapping practice [1,2,3,4,5,6]. Soil iron oxide content reflects the duration and intensity of pedogenic processes [7,8]. They appear as either a standalone crystallized mineral (e.g., hematite (Fe2O3) and goethite (FeOOH)), or a pseudo amorphous entity (e.g., ferryhydrates) that coats the soil particle surface and colors the soil, even if their content is low [5]. It is important to study the impact of different types of organic matter (OM) on the spectral signal of iron oxides. Complex as well, can cluster several soil properties based on their spectral responses and can reduce the dimension of uncertainty in the soil system using multivariate algorithms [10]
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