Millimetre‐scale distribution of organic matter composition at intact biopore and crack surfaces

Abstract

Structured subsoil horizons are characterized by biopores and shrinkage cracks, which may serve as preferential flow paths. The surfaces of cracks and biopores may be coated by clay-organic material. The spatiallydistributed organic matter (OM) composition at such structural surfaces was studied at the millimetre scale using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy in the mid-infrared range (MIR). Intact biopores such as earthworm burrows and root channels, and crack surfaces of nine subsoil horizons were analysed. The samples were from arable and forest Luvisols, one Regosol, one Stagnosol and Cambisols developed from loess, till, mudstone and limestone. For better comparison between soils, the DRIFT signal intensities were corrected for the particle-size effects. The OM was characterized by the ratio between alkyl-(C-H) and carbonyl (C=O) functional groups (C-H/C=O), which represent an index of the potential wettability (PWI) of the OM. The PWI was larger for biopores than for crack surfaces and the soil matrix, indicating a smaller potential wettability of OM at biopore surfaces. The millimetre-scale spatial variability of OM was especially large for the surfaces of root channels. Samples from till-derived Luvisols had smaller PWI (with greater potential wettability than surfaces from loess-derived Luvisols) than other soil types. The mean PWI of the arable Luvisol crack surfaces was less than that of the forest Luvisol samples. The results suggest that the spatial distribution of OM properties at intact structural surfaces may be important for describing sorption and mass transfer processes during preferential flow.

Droplet infiltration dynamics and soil wettability related to soil organic matter of soil aggregate coatings and interiors

“…The summed signal intensities of C-H and C = O groups are shown in Table 4. The calculated PWI values ( Figure 2) were lower for uncoated intact aggregate surfaces as compared to the coated ones, which agree with the results of Leue et al (2013). Differences between clay-organic coated E(A) and uncoated F, as well as between clayey coated E(B) and uncoated F surfaces were statistically significant at the 99% confidence level.…”

“…The signal intensities of carboxylic C = O groups (which act hydrophilic) were measured in the regions between WN 1740 and 1698 cm -1 and between WN 1640 and 1600 cm -1 . The potential wettability index (PWI) (Leue et al, 2013) of the SOM was calculated as the ratio between the summed signal intensities of the C-H groups and the summed intensities of the C = O groups (Ellerbrock et al, 2005).…”

“…The potential wettability of SOM was characterized by the ratio between the FTIR signal intensities of hydrophobic aliphatic C-H groups and hydrophilic C = O groups (Ellerbrock et al, 2005). This ratio was proposed as a potential wettability index (PWI) of SOM (Leue et al, 2013). A higher value of PWI means that the SOM has relatively more C-H in comparison to C = O functional groups, and indicates decreased wettability of the surfaces.…”

“…Infrared spectroscopy in the diffuse reflectance mode (DRIFT) can be used to obtain infrared spectra from intact sample surfaces. DRIFT spectra were used to determine the ratio between hydrophobic and hydrophilic functional groups in the SOM (PWI) of intact aggregate coatings by Ellerbrock et al (2009) and Leue et al (2013). For the clay-organic coatings on aggregates from a Bt horizon of a loess-derived Luvisol, a nonlinear relationship between the PWI and soil wettability (expressed as WDPT) was recently found by Leue et al (2015).…”

Quantity of Hydrophobic Functional CH-Groups – Decisive for Soil Water Repellency Caused by Digestate Amendment

“…Soil organic matter is characterized by the amount of particulate organic matter, organo-mineral complexes (Leue et al, 2013), humic acids, the content of functional groups (CH and C=O) and their spatial conformation of organic molecules on the microscale (Morrison and Boyd, 1983). DRIFT ( The organic material of anaerobic digestates contains great amounts of amphiphilic components like long-chained fatty acids, which are transferred to the soil matrix after application (Täumer et al, 2005); it can flow into pore spaces or may surround mineral particles.…”