Organic Matter Stocks and the Interactions of Humic Substances with Metals in Araucaria Moist Forest Soil with Humic and Histic Horizons

Abstract

Soils with humic and histic horizons in tropical and subtropical ecosystems play an important role in determining the atmospheric C stock and its stabilization, climate regulation, water holding capacity, and environmental filtering, due to the different functions of soil organic matter (SOM). However, the processes and mechanisms that regulate SOM dynamics in these soils are not clear. The objectives of this study were: i) determine the C and N stocks and ii) investigate the SOM chemical fractions and their interactions with Fe and Al ions in soils with humic and histic horizons of a toposequence under Araucaria moist forest in southern Brazil. The soils sampled were classified as Humic Hapludox (top -not hydromorphic), Fluvaquentic Humaquepts (lower third -hydromorphic), and Typic Haplosaprists (floodplain -hydromorphic). The C and N contents were determined in bulk soil samples and SOM chemical fractions; in these fractions, Fe and Al co-extracted contents were also determined. The chemical composition of humin and humic acid fractions was investigated by FTIR spectroscopy. The C content in the toposequence increased from the top to the lowest position. The differences observed in SOM content and SOM chemical composition were defined by the differences in soil water regime. The amount of C stored in the subsurface horizons is about 70 % of total organic C. The carbohydrate-like structures in the humin fraction were protected from solubilization through interaction with iron oxides, which may represent an important mechanism for labile organic compound preservation in these soils. The soluble humic substances showed the highest Fe and Al contents, and their compartments have different affinities for Fe and Al.

Aggregate Stability in Soil with Humic and Histic Horizons in a Toposequence under Araucaria Forest

“…In fact, the Feo/Fed ratio in FH and TS ranged from 0.46 to 1.0. The same ratio in OX showed very low values (Hanke and Dick, 2017), confirming the higher proportion of lower crystallinity oxides in C-rich soils. However, this mechanistic contribution may be less important than the protection conferred by the aliphatic zone, which was confirmed by the CCA.…”

“…This result suggests that the C-O groups that are part of carboydrate-like structures (primary and secondary alcohols), which are more polar than other C-O groups, interact with the polar sites of mineral surfaces. This hypothesis is supported by the higher iron extracted by ammonium oxalate and iron extracted by dithionite-citrate-bicarbonate (Feo/Fed) ratio of FH and TS in comparison to OX (Hanke and Dick, 2017). In these two C-rich soils, the low crystallinity Fe oxide content is higher than in OX and, therefore, they have a higher density of monocoordinated hydroxyls.…”

A comparison of soil texture measurements using mid-infrared spectroscopy (MIRS) and laser diffraction analysis (LDA) in diverse soils

“…Reeves et al 32 observed that with ashing of soils to remove OC, the region 3000-2800 cm −1 did not alter the spectra of clay, indicating that no spectral over-lap with OC and clay occurred in this region, as observed here. The peak at 1113/1115 cm −1 has previously been identified as relating to Si-O stretching in kaolinite and montmorillonite and amorphous silica 29,30,34 , and more specifically the sorption of organic matter 30 , humic [35][36][37] and fulvic 38 acids on to the surfaces of these clays. This suggests that the intensity of the peak at 1115 cm −1 in the clay model coefficients in the soils with OC was enhanced by the OC associated with the clay, and with this peak no longer so predominant in the coefficients in soils with OC removed, the prediction improved.…”

Assessment of Aqueous Extraction Methods on Extractable Organic Matter and Hydrophobic/Hydrophilic Fractions of Virgin Forest Soils

“…Two major types of DOM can be identified as nonhumic and humic substances. Nonhumic substances are the compounds of well-described classes such as amino acids, lipids, proteins, waxes, and carbohydrates [ 12 ]. Humic substances are supramolecular (a complex of molecules), rich in oxygen-containing functional groups, commonly COOH, phenolic and enolic OH, alcoholic OH, and C=O of quinones [ 13 ].…”