Adsorption-desorption behavior of dissolved organic carbon by soil clay fractions of varying mineralogy

Abstract

Soil clay minerals significantly influence the accumulation and stabilization of organic carbon (OC). However, the effect of interactions among phyllosilicate clay minerals, native OC and sesquioxides (Fe/Al oxides) on the adsorption-desorption of dissolved organic carbon (DOC) under different background electrolyte types and concentration is poorly understood. A set of batch adsorption-desorption experiments were conducted using pedogenic clays extracted from soils dominated by kaolinite-illite (Kaol-Ill), smectite (Smec) and allophane (Allo). The clay samples were sequentially treated to remove native OC and sesquioxides, and tested for adsorption-desorption of DOC under various solution conditions. All the experiments were conducted at pH 7 using water extractable fraction of OC from wheat residues. DOC adsorption increased with increasing background electrolyte concentration, and the presence of Ca 2+ significantly enhanced the uptake in comparison to Na + due to a possible cationic bridging effect. Under all electrolyte conditions, the maximum DOC adsorption capacity (Q max ) (mg g −1 ) of the soil clay fractions (SCF) maintained the order: Allo N Smec N Kaol-Ill. A similar order was also observed when the adsorption capacities were normalized to the specific surface area (SSA) of the SCFs (mg m −2 ). DOC adsorption showed a positive relationship with SSA, and sesquioxides and allophanic minerals provided the largest contributions to the SSA in the SCF. Removal of sesquioxides from the SCF resulted in a decrease in SSA and thus DOC adsorption, whereas removal of native OC increased the SSA and subsequent DOC adsorption. Because this study used pedogenic SCFs which represented soils formed in different environments instead of processed clays from geological deposits, it provided realistic information about the interaction of DOC with SCF in relation to their native OC and sesquioxide contents. It also revealed the importance of Ca 2+ in enhancing the carbon adsorption capacities of these SCFs.

Game Changer in Soil Science. Functional role of clay minerals in soil

“…Because of its greater surface area, especially compared with those of the more crystalline oxides such as goethite and hematite ( e.g ., Churchman and Lowe , ), ferrihydrite has a greater affinity for the uptake of adsorbates. Allophane, which has a similarly high surface area to ferrihydrite, also has a strong affinity for adsorbates, as noted by Singh et al (). Singh et al () also found that, even after removal of iron oxides, the allophanic soil showed a greater uptake of DOC than either kaolinite + illite or smectitic soils before oxides had been removed.…”

“…Allophane, which has a similarly high surface area to ferrihydrite, also has a strong affinity for adsorbates, as noted by Singh et al (). Singh et al () also found that, even after removal of iron oxides, the allophanic soil showed a greater uptake of DOC than either kaolinite + illite or smectitic soils before oxides had been removed.…”

“…In particular, dissolved organic carbon (DOC) is strongly adsorbed by oxides associated with phyllosilicate clays in soils. Singh et al () found that removal of oxides from soil clay fractions decreased the uptake of DOC by soils with kaolinite and illite (together), with smectite, and with allophane as dominant phyllosilicates. While the soils with kaolinite + illite and smectite had mainly goethite and hematite as oxides, the allophanic soil had mainly ferrihydrite, and the diminishing effect of oxide removal was much greater in the allophanic soil than in the other two soils examined ( Singh et al., ).…”

Minerals in the rhizosphere: overlooked mediators of soil nitrogen availability to plants and microbes

“…Microbes capable of oxidative enzyme production may have a disproportionate impact on MAOM destabilization. Microbial synthesis of lignindegrading and other oxidative enzymes serves to release N from organic matter (Craine et al 2007;Shukla and Varma 2011;Rinkes et al 2016); more specifically, oxidative attack may be an important pathway for fungi to acquire N in macromolecular structures, like protein-polyphenol complexes (Bending and Read 1996). Indeed, peroxidase enzymes have been characterized as a ''proximate control for C and N mineralization '' (Tian et al 2010).…”

“…Finally, changes in pH or soil solution composition can cause organic matter to be mobilized off mineral surfaces, or alternatively to be sorbed onto them (Avena and Koopal 1998;Rashad et al 2010;Singh et al 2016). Such variability points to MAOM's potential to serve as an active N source and sink.…”