Origin and chemical composition of DOM fractions from topsoil and subsoil layers of a silty and a sandy forest soil using FT-IR and fluorescence spectroscopy

The different environmental conditions (temperature, oxygen and water availabilities), microbial composition, availability of fresh organic inputs and textural and mineralogical properties of soil layers with the depth result differences between the origin, composition, C/N ratio and stability of the dissolved organic matter (DOM) of topsoils and subsoils.

This research examines the content and chemical composition of the DOM of topsoil and subsoil layers of a silty Luvisol and a sandy Arenosol. Both soils are derived from oak forests from Hungary. The soils were collected as composite samples (10 random subsamples within a 20 m × 20 m area) from the 0–20 and 30–50 cm layers.

The DOM was extracted with ultrapure water for 12 h at room temperature with a tumbling shaker. The sample was centrifuged for 35 min (1400 × g) and the supernatant was decanted and passed through a 250 µm-sieve. The fraction that passed through the sieve was filtrated through a 0.45 μm membrane filter to obtain the DOM samples. The filtered samples were acidified to pH 2 with HCl, passed through a solid phase extraction cartridge using a styrene divinyl benzene polymer sorbent (Agilent Mega Bond Elut PPL), eluted with methanol and dried.

The dried DOM samples were analyzed with a Bruker Vertex 70 FT-IR spectrometer. For each sample a spectral range of 4000–400 cm^–1, a resolution of 4 cm^–1, 128 scans, and three replicates were recorded. Relative absorbances were calculated for six peaks (2920, 2850, 1730, 1640, 1515, or 1420 cm^–1) representing characteristic organic matter compounds.

The dried DOM samples were dissolved in 0.05 M NaHCO₃ solution in order to determine the C and N content and the fluorescence and UV-VIS-NIR spectroscopical properties. The total organic carbon and nitrogen content of the DOM samples were analysed using a TOC/TN analyser (Shimadzu TOC-L). The chemical composition of the DOM samples was determined using fluorescence (Shimadzu RF6000) and UV-VIS-NIR (Shimadzu UV3600) spectrometry. Excitation-emission matrices were obtained by measuring fluorescence intensity excitation wavelengths ranging from 230–450 nm and emission wavelengths ranging from 260–600 nm with 2 nm increments. Fluorescence, humification and biological indices were determined from the fluorescence spectra in order to determine the sources, structural complexity and humification degree of the DOM samples. Synchronous fluorescence spectra were recorded with a fixed wavelength difference (Δλ=18) to separate SOM components with different molecular weights. Specific UV absorption (SUVA₂₅₄ and SUVA₂₈₀, L mg^-1 m^-1) was calculated by dividing the absorption at 254 and 280 nm by the DOC concentration.

The study aimed to assess the differences between the sources and the structural and chemical variability of the DOM samples from varying soil depths with different textural properties.

This work was supported by the Development and Innovation Fund of Hungary [grant No. NKFIH 142936] and the Eötvös Loránd Research Network [grant No. SA41/2021].