These last twenty years have seen the development of an abundant literature on the influence of soil macrofauna on soil structure. Amongst these organisms, earthworms, termites and ants are considered to play a key role in regulating the physical, chemical and microbiological properties of soils. Due to these influential impacts, soil ecologists consider these soil macro-invertebrates as `soil engineers' and their diversity and abundance are nowadays considered as relevant bioindicators of soil quality by many scientists and policy makers. Despite this abundant literature, the soil engineering concept remains a `preach to the choir' and bioturbation only perceived as important for soil ecologists. We discussed in this article the main mechanisms by which soil engineers impact soil structure and proposed to classify soil engineers with respect to their capacity to produce biostructures and modify them. We underlined the lack of studies considering biostructure dynamics and presented recent techniques in this purpose. We discussed why soil engineering concept is mainly considered by soil ecologists and call for a better collaboration between soil ecologists and soil physicists. Finally, we summarized main challenges and questions that need to be answered to integrate soil engineers activities in soil structure studies. (C) 2014 Elsevier B.V. All rights reserved.

Why is the influence of soil macrofauna on soil structure only considered by soil ecologists

Earthworm burrow systems are generally described based on postulated behaviours associated with the three eco- logical types. In this study, we used X-ray tomography to obtain 3D information on the burrowing behaviour of six very common anecic (Aporrectodea nocturna and Lumbricus terrestris) and endogeic (Aporrectodea rosea, Allolobophora chlorotica, Aporrectodea caliginosa, Aporrectodea icterica) earthworm species, introduced into repacked soil cores for 6 weeks. A simple water infiltration test, the Beerkan method, was also used to assess some functional properties of these burrow systems. Endogeic worms make larger burrow sys- tems, which are more highly branched, less continuous and of smaller diameter, than those of anecic worms. Among the anecic species, L. terrestris burrow systems are shorter (9.2 vs 21.2 m) with a higher number (14.5 vs 23.5) of less branched burrows (12.2 vs 20.2 branches m �1 ), which are also wider (7.78 vs 5.16 mm) than those of A. nocturna .I n comparison, the burrow systems made by endogeic species appeared sim- ilar to each other. However, A. rosea burrows were short and narrow, whereas A. icterica had a longer burrow system (15.7 m), more intense bioturbation intensity (refilled macropores or soil lateral compaction around them) and thus a greater number of burrows. Regarding water infiltration, anecic burrow systems were far more efficient due to open burrows linking the top and bottom of the cores. For endogeic species, we observed a linear relationship between burrow length and the water infiltration rate (R 2 =0.49, p<0.01). Overall, the three main characteristics significantly influenc- ing water infiltration were burrow length, burrow number and bioturbation volume. This last characteristic highlighted the effect of burrow refilling by casts.

Morphological and functional characterisation of the burrow systems of six earthworm species (Lumbricidae)

Soil degradation is a worsening global phenomenon driven by socio-economic pressures, poor land management practices and climate change. A deterioration of soil structure at timescales ranging from seconds to centuries is implicated in most forms of soil degradation including the depletion of nutrients and organic matter, erosion and compaction. New soil-crop models that could account for soil structure dynamics at decadal to centennial timescales would provide insights into the relative importance of the various underlying physical (e.g. tillage, traffic compaction, swell/shrink and freeze/thaw) and biological (e.g. plant root growth, soil microbial and faunal activity) mechanisms, their impacts on soil hydrological processes and plant growth, as well as the relevant timescales of soil degradation and recovery. However, the development of such a model remains a challenge due to the enormous complexity of the interactions in the soil-plant system. In this paper, we focus on the impacts of biological processes on soil structure dynamics, especially the growth of plant roots and the activity of soil fauna and microorganisms. We first define what we mean by soil structure and then review current understanding of how these biological agents impact soil structure. We then develop a new framework for modelling soil structure dynamics, which is designed to be compatible with soil-crop models that operate at the soil profile scale and for long temporal scales (i.e. decades, centuries). We illustrate the modelling concept with a case study on the role of root growth and earthworm bioturbation in restoring the structure of a severely compacted soil.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/gcb.15289

A framework for modelling soil structure dynamics induced by biological activity

The representative elementary volume (REV) is related to the minimum size of a sample that is necessary for representative measurements of a property. The REV is frequently investigated in samples with characteristic sizes extending from millimeters to few centimeters. Most of the time, the obtained information in this kind of study does not reveal any important detail of the inner structure of a porous medium. Something that can only be attained using some high spatial resolution image analysis technique, as, for instance, the X-ray computed microtomography (μCT). In this research, 3D μCT images (60 μm of resolution) were used for investigating the REV for: soil macroporosity (MAP), number of macropores (NMAP), pore tortuosity (τ) and connectivity (Euler-Poincare characteristic, EPC) of a Rhodic Ferralsol under no-tillage (NT) and conventional (CT) tillage systems. Consecutive increasing volumes, centrally and randomly, with volumes ranging from ≈0.40 to ≈36,591 mm³, were selected inside the 3D images. Results showed that MAP and NMAP presented high variability for small investigated subvolumes and for the random selection scheme. The REV for MAP (under NT) was reached at volumes of ≈10,841 mm3 (centered scheme) and ≈17,216 mm3 (random scheme), respectively. For the CT, the REV for MAP occurred at volumes of ≈17,216 mm3 (random and centered schemes). The REV was not reached for NMAP, while the ratio between NMAP and sample subvolumes (here called specific NMAP, NMAPe) presented a REV of ≈17,216 mm3. The tortuosity and EPC were characterized by high variability for smaller subvolumes and random selection scheme. The REV for EPC was not found and for τ it was reached at volumes of ≈17,216 mm3 (both subvolume selection schemes and tillage systems). The results of our work present for the first time the REV definition for soil properties such as NMAPe, τ and EPC at the micrometer scale as well as the effect of tillage systems in the REV. The obtained results surprisingly demonstrated that the REV was not influenced by the tillage systems as well as it was the same for NMAPe, τ and EPC. In addition, the complexity of the soil porous system did not allow the REV determination for EPC, at least for the sample volume here analyzed.

X-ray microtomography analysis of representative elementary volume (REV) of soil morphological and geometrical properties

Biosolid application, wastewater irrigation, and plastic mulching technologies are major sources of plastic pollution in agroecosystems. Microplastics may interact with soil physicochemical propert...

Joachim Kiesel

Papers

Micro-scale dry bulk density variation around earthworm (Lumbricus terrestris L.) burrows based on X-ray computed tomography