Redistribution of particulate organic matter during ultrasonic dispersion of highly weathered soils

Abstract

Summary The use of ultrasonic energy for the dispersion of aggregates in studies of soil organic matter (SOM) fractionation entails a risk of redistribution of particulate organic matter (POM) to smaller particle‐size fractions. As the mechanical strength of straw also decreases with increasing state of decomposition, it can be expected that not all POM will be redistributed to the same extent during such dispersion. Therefore, we studied the redistribution of POM during ultrasonic dispersion and fractionation as a function of (i) dispersion energy applied and (ii) its state of decomposition. Three soils were dispersed at different ultrasonic energies (750, 1500 and 2250 J g−1 soil) or with sodium carbonate and were fractionated by particle size. Fraction yields were compared with those obtained with a standard particle‐size analysis. Undecomposed or incubated (for 2, 4 or 6 months) 13C‐enriched wheat straw was added to the POM fraction (0.25–2 mm) of one of the soils before dispersion and fractionation. Dispersion with sodium carbonate resulted in the weakest dispersion and affected the chemical properties of the fractions obtained through its high pH and the introduction of carbonate. The mildest ultrasonic dispersion treatment (750 J g−1) did not result in adequate soil dispersion as too much clay was still recovered in the larger fractions. Ultrasonic dispersion at 1500 J g−1 soil obtained a nearly complete dispersion down to the clay level (0.002 mm), and it did not have a significant effect on the total amount of carbon and nitrogen in the POM fractions. The 2250 J g−1 treatment was too destructive for the POM fractions since it redistributed up to 31 and 37%, respectively, of the total amount of carbon and nitrogen in these POM fractions to smaller particle‐size fractions. The amount of 13C‐enriched wheat straw that was redistributed to smaller particle‐size fractions during ultrasonic dispersion at 1500 J g−1 increased with increasing incubation time of this straw. Straw particles incubated for 6 months were completely transferred to smaller particle‐size fractions. Therefore, ultrasonic dispersion resulted in fractionation of POM, leaving only the less decomposed particles in this fraction. The amounts of carbon and nitrogen transferred to the silt and clay fractions were, however, negligible compared with the total amounts of carbon and nitrogen in these fractions. It is concluded that ultrasonic dispersion seriously affects the amount and properties of POM fractions. However, it is still considered as an acceptable and appropriate method for the isolation and study of SOM associated with silt and clay fractions.

Response of microbial communities to long-term fertilization depends on their microhabitat

“…organic material still attached (Christensen, 1992;Amelung et al, 1998;Schmidt & K€ ogel-Knabner, 2002;Oorts et al, 2005), but when such a protocol was applied in this study, DNA and 16S rRNA genes detected in the soil suspensions indicated detachment of larger amounts of microbial cells from the soil particles. A reduction in the Microbial communities on soil particle-size fractions ultrasonic energy improved the recovery of particleattached DNA and 16S rRNA genes, but it also affected the efficiency by which silt and clay particles were separated.…”

Characterisation of organic matter from organo-mineral complexes in an Andosol from Reunion Island

“…They may have come from soil OM located on the surface of organo-mineral complexes and from free-OM, which was less protected. It has also been argued that ultrasonic treatment induced C redistribution in densimetric fractions [29,30]. However, Schmidt et al [31,32] carried out studies about C redistribution with ultrasonic treatments; they concluded that there was no evidence for OM separation and redistribution.…”

Separation of light and heavy organic matter fractions in soil — Testing for proper density cut-off and dispersion level

“…Therefore, in more developed soils, with a more complex particle-size distribution and higher in organic C (Oorts et al, 2005) we can expect much larger effects, potentially leading to wrong interpretations of the amount, role and reactivity of fractions. This becomes clear when considering the C and the N contents of O-LFs separated at various dispersion intensities in all the other test soils: their variability clearly indicates such differences in composition and mineral material redistribution.…”