Physical carbon‐sequestration mechanisms under special consideration of soil wettability

Abstract

The protective impact of aggregation on microbial degradation through separation has been described frequently, especially for biotically formed aggregates. However, to date little information exists on the effects of organic‐matter (OM) quantity and OM quality on physical protection, i.e., reduced degradability by microorganisms caused by physical factors. In the present paper, we hypothesize that soil wettability, which is significantly influenced by OM, may act as a key factor for OM stabilization as it controls the microbial accessibility for water, nutrients, and oxygen in three‐phase systems like soil. Based on this hypothesis, the first objective is to evaluate new findings on the organization of organo‐mineral complexes at the nanoscale as one of the processes creating water‐repellent coatings on mineral surfaces. The second objective is to quantify the degree of alteration of coated surfaces with regard to water repellence. We introduce a recently developed trial that combines FTIR spectra with contact‐angle data as the link between chemical composition of OM and the physical wetting behavior of soil particles. In addition to characterizing the wetting properties of OM coatings, we discuss the implications of water‐repellent surfaces for different physical protection mechanisms of OM. For typical minerals, the OM loading on mineral surfaces is patchy, whereas OM forms nanoscaled micro‐aggregates together with metal oxides and hydroxides and with layered clay minerals. Such small aggregates may efficiently stabilize OM against microbial decomposition. However, despite the patchy structure of OM coating, we observed a relation between the chemical composition of OM and wettability. A higher hydrophobicity of the OM appears to stabilize the organic C in soil, either caused by a specific reduced biodegradability of OM or indirectly caused by increased aggregate stability. In partly saturated nonaggregated soil, the specific distribution of the pore water appears to further affect the mineralization of OM as a function of wettability. We conclude that the wettability of OM, quantified by the contact angle, links the chemical structure of OM with a bundle of physical soil properties and that reduced wettability results in the stabilization of OM in soils.

Evidence of a union between organic carbon and water content in soil

“…Hydrophilic tendencies maintain a water film within aggregate complexes and thus contribute to transmission of water and promote increased wetting. The pore space in soil is also modified by the SOC composition; the hydrophobicity of some elements of SOC act to selectively block soil pores (Schulten and Schnitzer 1997;Bachmann et al 2008). From the orientation of carbon molecules, small changes in SOC within aggregates can alter the flow of water between and within molecules.…”

“…However, it is not as obvious to understand the persistent long-term relationship among SOC, texture, and time-averaged SWC, which would include feedback between the variables. The SOC is also associated with changes in hydrophilic versus hydrophobic attractions to water on the surface of aggregates (Bachmann et al 2008). Increased hydrophobicity from SOC within aggregates is proposed to regulate water entry into pores (Schulten and Schnitzer 1997;Bachmann et al 2008).…”

“…The SOC is also associated with changes in hydrophilic versus hydrophobic attractions to water on the surface of aggregates (Bachmann et al 2008). Increased hydrophobicity from SOC within aggregates is proposed to regulate water entry into pores (Schulten and Schnitzer 1997;Bachmann et al 2008). The distribution of water within aggregates may be a link in the "regulatory gate hypothesis", whereby a first step in mineralization of aggregates depends on oxidation and enzyme reactions (Kemmitt et al 2008).…”

CO<sub>2</sub> efflux from soils with seasonal water repellency

“…SWR has also been reported to reduce soil microbial activity and it has been considered as one of the factors protecting SOM from microbial decomposition by separation of the microorganisms from their food and water sources (Piccolo and Mbagwu, 1999;Piccolo et al, 1999;Bachmann et al, 2008). Goebel et al (2007) demonstrated that SWR affects the distribution and continuity of the liquid phase in the soil matrix and therefore restricts the accessibility of SOM and the availability of water, O 2 and nutrients to the microorganisms.…”

Occurrence and spatial pattern of water repellency in a beech forest subsoil

“…However, research of the last decades revealed also some negative effects associated with the amount and composition of SOM like soil water repellency (WR), which may lead to adverse effects with respect to soil hydraulic functions. Bachmann et al (2008) showed that the availability and distribution of water in the soil matrix depends on soil particle wettability. WR occurs in soils of different texture, land use and under a variety of climatic conditions (DeBano, 1981;Doerr et al, 2000;Woche et al, 2005).…”