Application of atomic force microscopy to the study of natural and model soil particles

Abstract

Summary The structure and surface chemistry of soil particles has extensive impact on many bulk scale properties and processes of soil systems and consequently the environments that they support. There are a number of physiochemical mechanisms that operate at the nanoscale which affect the soil's capability to maintain native vegetation and crops; this includes soil hydrophobicity and the soil's capacity to hold water and nutrients. The present study used atomic force microscopy in a novel approach to provide unique insight into the nanoscale properties of natural soil particles that control the physiochemical interaction of material within the soil column. There have been few atomic force microscopy studies of soil, perhaps a reflection of the heterogeneous nature of the system. The present study adopted an imaging and force measurement research strategy that accounted for the heterogeneity and used model systems to aid interpretation. The surface roughness of natural soil particles increased with depth in the soil column a consequence of the attachment of organic material within the crevices of the soil particles. The roughness root mean square calculated from ten 25 μm2 images for five different soil particles from a Netherlands soil was 53.0 nm, 68.0 nm, 92.2 nm and 106.4 nm for the respective soil depths of 0–10 cm, 10–20 cm, 20–30 cm and 30–40 cm. A novel analysis method of atomic force microscopy phase images based on phase angle distribution across a surface was used to interpret the nanoscale distribution of organic material attached to natural and model soil particles. Phase angle distributions obtained from phase images of model surfaces were found to be bimodal, indicating multiple layers of material, which changed with the concentration of adsorbed humic acid. Phase angle distributions obtained from phase images of natural soil particles indicated a trend of decreasing surface coverage with increasing depth in the soil column. This was consistent with previous macroscopic determination of the proportions of organic material chemically extracted from bulk samples of the soils from which specimen particles were drawn. Interaction forces were measured between atomic force microscopy cantilever tips (Si3N4) and natural soil and model surfaces. Adhesion forces at humic acid free specimen surfaces (Av. 20.0 nN), which are primarily hydrophilic and whose interactions are subject to a significant contribution from the capillary forces, were found to be larger than those of specimen surfaces with adsorbed humic acid (Av. 6.5 nN). This suggests that adsorbed humic acid increased surface hydrophobicity. The magnitude and distribution of adhesion forces between atomic force microscopy tips and the natural particle surfaces was affected by both local surface roughness and the presence of adsorbed organic material. The present study has correlated nanoscale measurements with established macroscale methods of soil study. Thus, the research demonstrates that atomic force microscopy is an important addition to soil s...

Prion Protein Interaction with Soil Humic Substances: Environmental Implications

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Organic matter identifies the nano-mechanical properties of native soil aggregates

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“…AFM adhesion data, which shows higher values for APW compared to CON samples (Fig. 3D), has been previously used to assess hydrophilicity states of several substrates with both silicon and silicon nitride tips: 25,27,28 sample hydrophilicity is expected to be associated with evident AFM pull-off adhesion values due to the formation of a water meniscus between the cantilever tip and the sample surface, 29 as schematically shown in Fig. S3A.…”

Biophysics of the Vadose Zone: From Reality to Model Systems and Back Again

“…Much smaller scale measurements of soil particle bonding are feasible with atomic force microscopy (AFM), which has been applied to measure the effects of humic acids (Cheng et al, 2008) and polysaccharides produced by ectomycorrhizal fungi (Gazze et al, 2013) on the roughness of soil particles at nanometer resolution, as well as the thermal behavior of soil organic materials (Schaumann and Mouvenchery, 2012). Great potential exists to study particle bonding caused by specific biological compounds in soil using AFM.…”