Soil aggregates as massively concurrent evolutionary incubators

Soil aggregation, a key component of soil structure, has mostly been examined from the perspective of soil management and the mediation of ecosystem processes such as soil carbon storage. However, soil aggregation is also a major factor to consider in terms of the fine-scale organization of the soil microbiome. For example, the physico-chemical conditions inside of aggregates usually differ from the conditions prevalent in the bulk soil and aggregates therefore increase the spatial heterogeneity of the soil. In addition, aggregates can provide a refuge for microbes against predation since their interior is not accessible to many predators. Soil aggregates are thus clearly important for microbial community ecology in soils (for example, Vos et al., 2013; Rillig et al., 2016) and for microbially driven biogeochemistry, and soil microbial ecologists are increasingly appreciating these aspects of soil aggregation. Soil aggregates have, however, so far been neglected when it comes to evolutionary considerations (Crawford et al., 2005) and we here propose that the process of soil aggregation should be considered as an important driver of evolution in the soil microbial community

Water repellence and soil aggregate dynamics in a loamy grassland soil as affected by texture

De Gryze, S.; Jassogne, L.; Bossuyt, H.; Six, J.; Merckx, R

Factors controlling aggregation in a minimum and a conventionally tilled undulating field

A

Pore structure changes during decomposition of fresh residue: X-ray tomography analyses

The morphology of soil pores is a crucial factor in the understanding of the ecology of soil microorganisms at a small scale. Xray computer tomography (CT) enables to visualize the soil pore space in three dimensions. We aimed at exploring possible changes in porosity and pore morphology during the incubation of soil to which fresh residue was added. These changes were compared to changes in porosity and pore morphology in field aggregates. A set of CT images is presented from newly formed, incubated and field aggregates from both a sandy loam and a silt loam soil. These aggregates are about 6-8 mm diameter. After incubation of the artificial aggregates, we observed the formation of micro-cracks. At the same time, the total void porosity > 27 mu m increased from 3.5% to 7%. The pore size distribution indicated that this extra porosity was present in the 27-67 mu m range. Furthermore, it was found that this new pore space was (partly) located near decomposing residue. To quantify the observed changes in pore morphology, we explored the use of the mass fractal dimension and variograms. After correction for differences in porosity, we found that incubation decreased the mass fractal dimension to the level of field aggregates in the sandy loam soil, while no differences were observed in the silt loam soil. In contrast, variogram parameters of the different treatments did not change consistently with the observations in both soils. In conclusion, the data suggest that microbial activity changes the morphology of the pore structure. However, the observed changes in the pore architecture were not sufficient to explain the large differences in aggregate stability among the analyzed samples. (c) 2005 Elsevier B.V. All rights reserved.status: publishe

Measuring and understanding carbon storage in afforested soils by physical fractionation

Includes bibliographical references (page 1987).Forested ecosystems have been identified as potential C sinks. However, the accuracy of measurement and understanding of the underlying mechanisms for soil organic C (SOC) storage in forested ecosystems needs to be improved. The objective of this study was to use aggregate and soil organic matter (SOM) fractionation techniques to identify SOC pools that preferentially stabilize SOC in the long term and elucidate SOC sequestration mechanisms in forested soils. At two sites (Wildlife area, Ohio and Kemptville, Ontario) representing two different soils (Hapludalf and Hapludoll), we sampled soils under agriculture, afforestation, and forest and separated them into aggregates. Different size classes of intra-aggregate particulate organic matter (iPOM) fractions were isolated by density flotation, dispersion, and sieving. At both sites, aggregation and whole SOC content were greater in the forested than in the agricultural ecosystems. The greater aggregation in forested ecosystems resulted in greater iPOM C concentrations, especially the iPOM C fractions associated with microaggregates (53–250 μm) and microaggregates occluded within macroaggregates (mM) (250–2000 μm). The sum of C in these fractions (microaggregate protected C) was 468 ± 29, 696 ± 171, 673 ± 70 g C m−2 in the agricultural, afforested, and forested soils at Kemptville, respectively. The difference in the microaggregate protected C between the agricultural and the afforested soils accounted, on average, for 20% of the difference in whole SOC stocks between the soils. We conclude, SOC is stabilized for a relatively longer term within microaggregates formed in afforested and forest systems. Therefore, we suggest a new fractionation scheme to isolate this microaggregate associated SOC for assessing the impact of land use, land management, and climate change on C storage