Iron(III) fate after complexation with soil organic matter in fine silt and clay fractions: An EXAFS spectroscopic approach

The role of soil organic matter (OM) on SO4 retention was investigated by comparing OM content, SO4 retention, and the distribution of Fe, Al and Si compounds in OM-poor (Grands-Jardins, PGJ) and OM-rich (Hermine, HER) Podzols from Québec, Canada. At both sites, four pedons were sampled by horizon; soil pH in H2O, organic C, phosphate-extractable SO4 and, sodium pyrophosphate, acid ammonium oxalate and dithionite-citrate-bicarbonate (DCB) extractable Fe, Al and Si were measured for each mineral horizon. The mineralogy of the clay (<2 µm) and fine silt (2–20 µm) fractions of selected horizons was determined by X-ray diffraction (XRD) and infrared spectroscopy (IR). Weighted mean organic C and pyrophosphate extractable Fe and Al contents were significantly higher in the HER than in the PGJ sola, while the PGJ soils were richer in amorphous inorganic Al. No trends were observed for inorganic Fe compounds. Chemical dissolution and IR allowed the identification of short-range ordered aluminosilicates, probably allophane, in the OM-poor and slightly acidic to neutral PGJ soils. These materials were absent from the OM-rich and acidic HER soils. Phosphate extractions showed that the weighted mean native SO4 content was five times higher in the PGJ than in the HER soil. Finally, native SO4 was strongly related to inorganic Fe, Al and Si (associated with allophane) at PGJ but only to inorganic Fe at HER. These results indicate that OM indirectly affects SO4 sorption through the influence organic substances exerts on the nature and distribution of pedogenic Fe, Al and Si compounds, such as allophane, in Podzolic profiles. Key words: Organic matter, sulfate, imogolite, allophane, silica, Podzol

Download Full-text

Multidecadal persistence of organic matter in soils: investigations at the submicrometer scale

10.5194/bg-2018-343 ◽

2018 ◽

Cited By ~ 1

Author(s):

Suzanne Lutfalla ◽

Pierre Barré ◽

Sylvain Bernard ◽

Corentin Le Guillou ◽

Julien Alléon ◽

...

Keyword(s):

Organic Matter ◽

Soil Organic Matter ◽

Coarse Silt ◽

Fine Silt ◽

Mineral Matrix ◽

Bare Fallow

Abstract. The mineral matrix, particularly clay-sized minerals, protects soil organic matter (SOM) from decomposition by microorganisms. Here we report the characterization of SOM and associated minerals over decades of biodegradation, in a French long-term bare fallow (LTBF) experiment started in 1928. The amounts of carbon (C) and nitrogen (N) declined with time for six fractions (sand, coarse silt, fine silt, coarse clays, intermediate clays and fine clays). The C : N ratios of SOM associated to silt fractions remained constant whereas they significantly decreased in clays, reaching very low values in intermediate and fine clays (C : N

Download Full-text

Which are important soil parameters influencing the spatial heterogeneity of <sup>14</sup>C in soil organic matter?

10.5194/bg-2016-11 ◽

2016 ◽

Cited By ~ 2

Author(s):

Stephan John ◽

Gerrit Angst ◽

Kristina Kirfel ◽

Sebastian Preusser ◽

Carsten W. Mueller ◽

...

Keyword(s):

Organic Matter ◽

Spatial Distribution ◽

Soil Organic Matter ◽

Microbial Biomass ◽

Organic Carbon ◽

Soil Texture ◽

Soil Parameters ◽

Soil Profiles ◽

Fine Silt ◽

Distribution Of Roots

Abstract. Radiocarbon (14C) analysis is an important tool that can provide information on the dynamics of organic matter in soils. Radiocarbon concentrations of soil organic matter (SOM) however, reflect the heterogeneous mixture of various organic compounds and are affected by different chemical, biological, and physical soil parameters. These parameters can vary strongly in soil profiles and thus affect the spatial distribution of the apparent 14C age of SOM considerably. The heterogeneity of SOM and its 14C signature may be even larger in subsoil horizons, which are thought to receive organic carbon inputs following preferential pathways. This will bias conclusions drawn from 14C analyses of individual soil profiles considerably. We thus investigated important soil parameters, which may influence the 14C distribution of SOM as well as the spatial heterogeneity of 14C distributions in soil profiles. The suspected strong heterogeneity and spatial variability, respectively of bulk SOM is confirmed by the variable 14C distribution in three 185 cm deep profiles in a Dystric Cambisol. The 14C contents are most variable in the C horizons because of large differences in the abundance of roots there. The distribution of root biomass and necromass and its organic carbon input is the most important factor affecting the 14C distribution of bulk SOM. The distance of the soil profiles to a beech did not influence the horizontal and vertical distribution of roots and 14C concentrations. Other parameters were found to be of minor importance including microbial biomass-derived carbon and soil texture. The microbial biomass however, may promote a faster turnover of SOM at hot spots resulting in lower 14C concentration there. Soil texture had no statistically significant influence on the spatial 14C distribution of bulk SOM. However, SOM in fine silt and clay sized particles (< 6.3 µm) yields slightly higher 14C concentrations than bulk SOM particularly at greater soil depth, which is in contrast to previous studies where silt and clay fractions contained older SOM stabilized by organo-mineral interaction. 14C contents of fine silt and clay correlate with the microbial biomass-derived carbon suggesting a considerable contribution of microbial-derived organic carbon. In conclusion, 14C analyses of bulk SOM mainly reflect the spatial distribution of roots, which is strongly variable even on a small spatial scale of few meters. This finding should be considered when using 14C analysis to determine SOM.

Download Full-text