Spatial variability of the molecular composition of humic acids from subtropical forest soils

Abstract. Iron oxide minerals play an important role in stabilizing organic carbon (OC) and regulating the biogeochemical cycles of OC on the earth surface. To predict the fate of OC, it is essential to understand the amount, spatial variability, and characteristics of Fe-bound OC in natural soils. In this study, we investigated the concentrations and characteristics of Fe-bound OC in soils collected from 14 forests in the United States and determined the impact of ecogeographical variables and soil physicochemical properties on the association of OC and Fe minerals. On average, Fe-bound OC contributed 37.8 % of total OC (TOC) in forest soils. Atomic ratios of OC : Fe ranged from 0.56 to 17.7, with values of 1–10 for most samples, and the ratios indicate the importance of both sorptive and incorporative interactions. The fraction of Fe-bound OC in TOC (fFe-OC) was not related to the concentration of reactive Fe, which suggests that the importance of association with Fe in OC accumulation was not governed by the concentration of reactive Fe. Concentrations of Fe-bound OC and fFe-OC increased with latitude and reached peak values at a site with a mean annual temperature of 6.6 °C. Attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR) and near-edge X-ray absorption fine structure (NEXAFS) analyses revealed that Fe-bound OC was less aliphatic than non-Fe-bound OC. Fe-bound OC also was more enriched in 13C compared to the non-Fe-bound OC, but C ∕ N ratios did not differ substantially. In summary, 13C-enriched OC with less aliphatic carbon and more carboxylic carbon was associated with Fe minerals in the soils, with values of fFe-OC being controlled by both sorptive and incorporative associations between Fe and OC. Overall, this study demonstrates that Fe oxides play an important role in regulating the biogeochemical cycles of C in forest soils and uncovers the governing factors for the spatial variability and characteristics of Fe-bound OC.

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Statistical evaluation of the relationships between spatial variability in the organic carbon content in gray forest soils, soil density, concentrations of heavy metals, and topography

Eurasian Soil Science ◽

10.1134/s1064229313090044 ◽

2013 ◽

Vol 46 (11) ◽

pp. 1076-1087 ◽

Cited By ~ 14

Author(s):

P. A. Shary ◽

D. L. Pinskii

Keyword(s):

Heavy Metals ◽

Organic Carbon ◽

Spatial Variability ◽

Carbon Content ◽

Forest Soils ◽

Statistical Evaluation ◽

Organic Carbon Content ◽

Soil Density ◽

Gray Forest Soils

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Methods for the Assessment of Humic Substances Quality in Forest Soils

Soil and Water Research ◽

10.17221/6499-swr ◽

2013 ◽

Vol 1 (No. 1) ◽

pp. 3-9

Author(s):

Mládková Lenka ◽

Borůvka Marcela Rohošková and Luboš

Keyword(s):

Humic Acids ◽

Forest Soils ◽

Beech Forest ◽

Soil Extract ◽

Soil Samples ◽

Drift Spectroscopy ◽

Organic Matter Quality ◽

C And N ◽

Drift Spectra ◽

Humus Quality

This paper is focused on soil organic matter quality assessment in acid forest soils. Soil samples were collected in the Jizera Mountains region. Vegetation cover of sampling sites was formed by spruce or beech monocultures. Humus quality was assessed by the ratio of absorbances of pyrophosphate soil extract at the wavelengths of 400 and 600 nm (A400/A600). Humus fractionation was performed on selected soil samples. DRIFT spectra of individual fractions were measured. Higher pH and lower C and N contents were found in beech forest than in spruce forest. A400/A600 well correlates with C and N contents (r = 0.510*** and 0.615***, respectively). C and N content increases as to humus quality decreases. DRIFT spectra of fulvic acids turned out to be unsuitable for describing differences in humus quality. DRIFT spectra of humic acids and humin were hence more suitable. The difference between spruce and beech forest was found in 1514.cm–1 (C=C bounds of benzene rings) and 1550 cm–1 (N-H bounds in monosubstituted amides) bands intensities. Humic acids and humin coming from the O horizons of beech forest are relatively enriched by nitrogen functional groups. Values of humic acids aromaticity index did not differ between beech and spruce forests. DRIFT spectroscopy was shown as a possible method for detailed humus quality studying.

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Molecular size distribution and hydrophilic and hydrophobic properties of humic acids isolated from forest soil

Soil and Water Research ◽

10.17221/2113-swr ◽

2008 ◽

Vol 2 (No. 2) ◽

pp. 45-53 ◽

Cited By ~ 8

Author(s):

B. Debska ◽

M. Drag ◽

M. Banach-Szott

Keyword(s):

Humic Acids ◽

Forest Soils ◽

Tree Species ◽

Plant Material ◽

Molecular Size ◽

European Beech ◽

Plant Litter ◽

Hydrophobic Properties ◽

Degree Of Maturity ◽

Molecular Size Distribution

The aim of the present paper was to determine hydrophilic and hydrophobic properties and the degree of polydispersity of humic acids depending on their degree of maturity and the properties of the plant material participating in the process of humification. The study involved humic acids isolated from samples taken from organic and mineral horizons of forest soils. The samples were taken under the tree stands of: pine with an admixture of hardbeam, European beech, elm, fir, spruce, and thuja. It was demonstrated that the properties of humic acids of the organic horizon and mineral horizons are determined by the kind of plant litter, mainly by the tree species. The humification process is connected with an increase of the proportion of humic acids of lower molecular size as compared to the proportion of molecules greater in size, and with an increase of hydrophilic fractions and a decrease of hydrophobic fractions. Based on the correlation relationships, it was shown that the degree of polydispersity of molecules of humic acids is related to their hydrophilic and hydrophobic properties. Humic acids with a larger proportion of high-molecular fractions demonstrate also a higher proportion of hydrophobic fractions.

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