Fluorescent properties of soil humic acids of different formation conditions

Abstract

The purpose of the study. To identify the features of humic acids of soils of different formation conditions using quantitative indicators of fluorescence spectra.

Place and time of the event. The research was carried out on the territory of Siberia, where in the period 2000-2021, key areas with different natural conditions were identified in different regions.

Methodology. Humic acids (HA) are considered as an independent component of the system of humus substances (SHS) of soils, the properties of which depend on the ecological conditions of formation. Soil samples were taken during the period of the most stable state of the system of humus substances in a continuous column every 5–10 cm and (or) less within the visible boundaries of genetic horizons. The HA preparations were isolated with 0.1 n NaOH after preliminary soil decalcification followed by precipitation with 2 n HCl. Additional purification from ash elements was not carried out. Fluorescence spectra of humic acid solutions with the same concentration of the substance (0.01 % HA) at pH of 13 were recorded in the wavelength range of 300–650 nm on an Agilen Cary Eclipse Fluorescence Spectrophotometer at an excitation wavelength of 330 nm.

The main results. The average statistical quantitative indicators of the fluorescence spectra of humic acids of soils formed under different natural conditions were calculated: the position of the fluorescence maximum (λmax), the value of the first moment (M1), and the ratio of the integral fluorescence intensities in the areas of its maximum manifestation in the red and blue parts of the spectrum (coefficient α). The results showed that soils formed in different natural conditions have different ranges of values of all quantitative indicators of the fluorescent properties of humic acids. They differ significantly in tundra soils of plain and mountainous conditions, within the same mountainous country due to vertical zonality, within the same territory with ring zonality due to the change of humidification areas, and with other qualitative differences in the natural environment forming them. It is shown that M1 reflects the configuration features of humic acids due to the conditions of soil formation, while the coefficient α reflects the state of this soil component during the period of soil sampling depending on the local conditions of their functioning.

Conclusion. The analysis of quantitative parameters of the fluorescence spectra of humic acids of soils of different conditions of formation made it possible, using emission spectra at one excitation wavelength (330 nm), to identify the specifics of the fluorescent properties of humic acids, which manifests itself in the position of the fluorescence maximum on the spectral scale, the value of the first moment and the ratio of the values of integral intensities in the long-wave and short-wave spectral regions. The data showed that the fluorescence maximum as well as the value of the first moment in mountain-tundra soils lies within very narrow limits and is among the lowest values identified in our studies. Humic acids of plain tundra soils differ significantly from their mountain analogues in all indicators of fluorescent properties. In conditions of vertical zonation in the direction from the zone of tundra soils to the zone of steppe mountain areas, a shift of the fluorescence maximum towards an increase in the wavelength is observed, however, it does not go beyond the boundaries of the blue region of the spectrum. The maximum values of these indicators are characteristic of the fluorescence of GC soils of steppe areas of intermountain basins and steppe areas of plain conditions of the Trans-Urals and Altai Region Territory. The assessment of the significance of humic acid differences in the fluorescence properties of soils of hillocks and sports of plain tundra showed that the values of λ and M differ by 10 and 7 units, respectively, and that the differences between them are significant. Differences for other objects formed in different natural conditions at the local level in the absolute majority of cases turned out to be significant. The existence of a close correlation of the values of quantitative indicators of the fluorescent properties of humic acids with the ratio of the main structure–forming elements - H:C and with climatic parameters is shown, using the example of mountain soils of Tuva and pedogenically processed deposits of the archaeological monument Grotto Obi-Rahmat, which allows us to recommend the use of these easily and quickly obtaining characteristics as an indicator for assessing the state of soils and the natural environment.