Wet sieving versus dry crushing: Soil microaggregates reveal different physical structure, bacterial diversity and organic matter composition in a clay gradient

Abstract

Soil microaggregates contain particles of different sizes, which may affect their potential to store organic carbon (OC). A variety of methods can be used to isolate microaggregates from the larger soil structures, among which wet sieving approaches are widely employed. We developed a novel dry crushing method that isolates microaggregates along failure planes due to mechanical stresses rather than hydraulic pressures and compared the mechanical stability, OC contents and microbial community composition between dry‐crushed and wet‐sieved samples with contrasting clay contents. Dry‐crushed samples exhibited a higher stability and bacterial diversity compared to wet‐sieved samples. As a result, the dry‐crushed microaggregates had different size distributions when analysed dry and after wetting. In the dry state, dry‐crushed microaggregates were larger and contained more sand‐sized primary particles within the aggregate structures. The wetting of dry‐crushed aggregates caused a disintegration of larger microaggregates and sand‐sized primary particles into smaller microaggregates that contained finer particles. In the soils with lower clay contents, the diameter of dry‐crushed microaggregates was 40 μm larger due to more sand‐sized primary particles remaining within the aggregates. Depending on how much volume in microaggregates is occupied by large primary particles, the OC concentration increased in the soil with higher clay content. Wet‐sieved size fractions also showed a similar pattern of OC distribution, whereas more primary particles were observed outside of aggregates. Wet sieving approaches disperse the soil into OC‐rich aggregates and might be preferable if OC dynamics are investigated. Differences in bacterial community composition in dependence on clay content were more pronounced in dry‐crushed microaggregates. If intact aggregate architectures are of interest for the isolation of soil structural units, the presented dry crushing method might provide an advantageous alternative that also better preserves bacterial diversity.

Experiences of soil physical measurements with laser diffractometer and their application possibilities in soil water management research

“…In the current phase of our correlation analysis, we did not perform any filtering on the database before the calculations, so we did not remove outliers. Consistent with the literature, the presence of organic matter was found to have a high stabilising (Tisdall-Oades 1982;Totsche et al 2018;Felde et al 2021), while exchangeable sodium a destabilising (Rengasamy-Marchuk 2011;Bennett-Marchuk-Marchuk 2016;Almajmaie et al 2017) effect on the microaggregates. The literature experience was supported by the not very clear negative effect of water soluble salt content, the effect of salt depends on a lot of other soil chemical properties (Sumner-Naidu 1998;Rengasamy-Marchuk 2011;Voelkner-Holthousen-Horn 2015).…”

Assessing the source and delivery processes of organic carbon within a mixed land use catchment using a combined n-alkane and carbon loss modelling approach

“…Uncertainty could be further reduced by using different methods to isolate the finer fraction within the soil samples. Under field conditions, various mechanisms cause soil aggregates to break apart creating finer particle fractions; disintegration of aggregates is a complicated mixture of mechanical (raindrop impact, field traffic/tillage, roots, earthworms) and hydraulic stresses (Felde et al 2021). Therefore, using different methods to isolate the finer fraction within the soil samples could highlight any differences in biomarker distribution due to breaking down aggregates using methods such as dry crushing (along more "natural planes of mechanical weakness" i.e., those likely to fail in the field (Felde et al 2021)) compared to wet/dry sieving and/or sample grinding.…”

Accounting for soil architecture and microbial dynamics in microscale models: Current practices in soil science and the path ahead

“…Although we are still far from having a handle on the sub-resolution porosity, the approaches that have been explored so far at least confirm the importance of "hidden" smaller pores in understanding soil processes and thereby encourage future research efforts. Recent advances in imaging techniques should bring better knowledge of pore arrangement at the infra-micron scale (Felde et al, 2020;Schlüter et al, 2019) and address their potential role in shaping soil microbial activity.…”