Abrupt land-use changes from forest to arable land, natural disturbances (forest fires) and tillage practices modify soil structure affecting soil functions and soil resilience. Use of heavy machinery may lead to soil compaction affecting soil functions like water regulation and retention, habitat provision and therefore ability of soil to provide ecosystem services such as primary production. Compaction and reduced plant growth can lead to increased runoff of nutrients and carbon, and reduced drought tolerance. Compaction may cause problems for soil organisms and their function (Meurer et al. 2020). For example, their biological activity may decrease affecting the decomposition of soil organic matter, maintenance of soil structure (exopolysaccharides, glomalin, fungal hyphae). The emerging issue of microplastics in European soils is conceptually also a physical contaminant and affects soil aggregation and pore-size distribution (Wang et al. 2023, Han et al. 2024). However, the impact is likely to fluctuate based on the textural composition of the soil, as well as the size, shape, and aging characteristics of the microplastics particles (Lehmann et al. 2021, Wang et al. 2022). The change in management or caused by natural disturbances may lead to new structural state in soil or the change may be short-lived and there will be a reversion to the pre-disturbance state. The consequences of these changes in land management or changes resulting from natural disturbances, and the rates of these changes may differ depending on climate, soil and vegetation cover, management, and disturbance history. Therefore, more information is needed on specific management practices in different climatic environments and soil types that consider the land-use and functioning of the soil for provision of as many as possible ecosystem services. The improvement of soil structural quality can be assessed by physical-structural-hydrological parameters (aggregate stability, MWD, pF-curves, bulk density) linked to soil microbiology (at least, Microbial carbon, Basal soil respiration, Enzyme activity). There is a gap that should be explored between soil physical-structural and biological builders of soil structure. A particular challenge is that in many cases soil in poor condition is not responsive to management practices as it should.

Soil operations affect the soil structure, but with optimal timing the destabilizing effect can be reduced. For example, soil wetness and inherent soil properties contribute to soil structural vulnerability and their interaction is complicated depending also on the management practices (Hu et al. 2023). Minimum tillage has been considered the best approach from numerous biological points of views such as symbiotic fungi and arthropods, although this might not necessarily be the case with increasing number of weeds and reduction in yield levels. We need information on soil specific management options in different climatic conditions and land-use systems to improve the functionality of soil structure.

Soil aggregates are considered for hot spots for biological activity and biogeochemical processes and are of high importance defining soil structure and pore space. However, the efficacy of aggregate research in elucidating functioning of soil structure has come under scrutiny. Sampling aggregates has required disrupting the surrounding soil environment, raising concerns that aggregates may partially result from the sampling procedure, thus potentially compromising their representativeness (Young et al. 2001, Garland et al. 2023). Furthermore, non-destructive imaging techniques have failed to detect aggregates in undisturbed soils or in deeper soil layers (Garland et al. 2023). Recently, Garland et al (Garland et al. 2023) concluded that aggregates can be separate units but taking into account the processes contributing to the formation and turnover of aggregates, they don’t need to have distinct physical boundaries. In fact, tillage-produced aggregates are often loosely packed and form inter-fragment spaces whereas natural aggregates are more likely to be seamlessly embedded in the surrounding soil matrix (Or et al. 2021). Yudina and Kuzyakov (Yudina and Kuzyakov 2023) stated that they “consider the pores and the interfaces as the arena of the physico-chemical and biological processes, but aggregates as the result of these processes“. Consequently, aggregates ensoul the concept of stable pedogenic features (soil memory) and allow to realize a thermodynamic view on the soil structure. This further highlights the importance of understanding aggregation and developing methods to study aggregates in their functional surrounding. From a biological perspective the pore network is highly pertinent as it is the habitable space for microbial species.

The cementing agents that enhance aggregate formation are well-known and natural aggregates are formed as a result of biological activity resulting in stabilization by biopolymers, and mineral particle enmeshing by hyphae and roots. Small and fine roots produce optimal conditions to form and to stabilize aggregates due to the polysaccharides produced by the microorganisms (Hallett et al. 2022). Furthermore, the roots maintain the aggregates mechanically separated. However, tillage produces soil fragments similar to biologically formed aggregates, but the stability of the fragments against mechanical disturbance and wetting is lower (Or et al. 2021). More information is needed how these differently formed aggregates impact the functioning of arable and natural soils and what is the relative importance of these different types of aggregates in preserving soil organic carbon stocks in different soil types and in soils under different land-use and management. Small sized aggregates seem to improve soil hydrological properties like water retention capacity and infiltration, so the estimation of this fraction or derived indexes or ratios, which relate the percentage of micro to macroaggregates, can give an interesting information about the condition and degradation of Mediterranean soils.

Machado et al. 2018

With changes in weather events and in annual timing of them, there is a transition in timing of the soil management practices at both forest soils, agricultural soils and in the urban areas. When the soil is too moist, for example for the lack of frost period due to milder winters, certain machinery cannot be used without causing dramatic effects to the soil structure. Thus, the proper winter in Northern Europe with frost period protects soils from damage and allows use of heavy machinery (in forests). In addition, frost and the freeze thaw cycles are known to improve structure in arable soils by maintaining a good distribution of aggregate size. Unfortunately, currently climate change appears as milder temperature and increased precipitation in winter period, leading to greater leaching of organic material from the soils. Increased occurrence of heavy rain is possible also in more Southern regions, and thereby the concern of the loss of soil organic matter and soil structural changes is global. Abnormal weather events make trees susceptible to forest diseases, and in turn, loss of trees alter soil stability. In addition, the possibility for increased leaching is not restricted only to organic matter but may concern also particulate material (suspended solids) as well as nutrients essential for e.g. forest ecosystems in the long run. (Machado et al., 2018b)

On forest land there is a growing interest among landowners towards sc. continuous cover forestry, where one avoids clear-cuts and site preparations. If continuous cover forestry practices get more common and grow in area that results a significant change by reducing the need for soil preparation and for maintenance ditching on drained peatlands. Different harvesting practices may also have a variable effect on the forest soil structure and nutrient amounts remaining in the site after cuttings. If cutting covers all tree compartments (whole-tree harvesting), this increases the loss of organic matter and nutrients compared to that remaining in the soil in stem-only harvesting. The distribution of logging residue piles on the site may also affect soil structure (physical properties) and nutrition (organic matter, chemical properties), i.e. if the logging residues are located only on restricted parts in the harvested area due to modern harvesting techniques. In addition to physical soil management, human induced land use include change in plant species, particularly in agriculture but to certain extent also in forest systems. The narrowing of plant species selection has further extended to genetic diversity via the use of breeding of plant material often to maximize productivity. Plant breeding has changed root exudates, root microbes, soil chemistry via microbes, lack of arbuscular mycorrhiza, glomalins and other extracellular polymeric substances (EPS) thus affecting the soil structure.

We need information, not just on agricultural soils, but on the physico-chemical processes, all the biological processes and interactions, from larger plants and animals to fungal hyphae and tiny microbes. How soil organisms interact with each other and with the abiotic environment affects soil structure. The effect of soil invertebrates has been neglected for crop production. The biotic part maintains the structure, how is it affected by climate change and changes in the soil habitat? How do soil animals and microbes respond to extreme events? In addition, the role of microbes in the presence and absence of OM may be different and should be understood.

Biodiversity crisis and the change in biodiversity can have an effect on the extracellular polymeric substances produced by microbes and thus have a strong effect on soil structure. In forests, the forest stand age and management can affect the resilience of soil to draught and heat waves, and on how the soil responds to the drought. In addition, the different management practices bring along forest floor vegetation changes mediating the effects of drought on soil. One example are the forest fires in Portugal which are a major threat affecting soil structural, soil biota, soil physico-chemical with also off-site effects (flooding, ash deposition in damns, …,). Besides that, forest management practices affect soil structural properties (timber extraction, land preparation by terraces, and so on), forest fires themselves modify environment, being a major threat.

Last knowledge gap is associated to the recovery of soil after disturbances, which is tightly linked to soil structure. We do not know how long it takes for soil to recover and also how do we measure soil recovery. The anthropogenic effects have a major role in shaping soil structure, but we do not have a complete and soil- and climate-specific understanding on their direct impacts on soil structure and how to retain sustainability of soil after disturbance. The potentially important role of plants in restoration needs also more soil and management specific understanding. Furthermore, as the functioning of soil results from an interplay of soil structure and activity of soil organisms, recovery of the vast areas of deteriorated soils on earth is a challenge.