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Abstract. Soil aggregation is an important process in nearly all soil and land-use types across the globe. Aggregates develop over time through a series of abiotic and biotic processes and interactions, including plant growth and decay, microbial activity, plant and microbial exudation, bioturbation, and physicochemical stabilization processes, and are greatly influenced by soil management practices. Together, and through feedbacks with organic matter and primary soil particles, these processes form dynamic soil aggregates and pore spaces, which together constitute a soil’s structure and contribute to overall soil functioning. Yet, the concept of soil aggregates is hotly debated, leading to confusion about their function or even existence. We argue here that the opposition to the concept of soil aggregation likely stems from the fact that the methods for characterization of soil aggregates have largely been developed in the context of arable soils, where tillage promotes the formation of discrete soil aggregates that are easily visible in the topsoil. We propose that the widespread use of conceptual figures showing detached and isolated aggregates can be misleading and has contributed to the skepticism towards the validity or relevance of studies on soil aggregates. However, the fact that we do not always see distinct aggregates within soils in-situ does not mean that aggregates do not exist. Here, we illustrate how aggregates can form and dissipate within the context of undisturbed, intact soils, highlighting the point that aggregates do not necessarily need to have a distinct physical boundary and can exist seamlessly embedded in the soil. We hope our contribution helps to alleviate the debate on soil aggregates and supports the foundation of a shared understanding on the characterization and function of the ‘dual nature’ of soil structure.

Soil organic carbon (SOC) fashions soil structure, which is a key factor of soil fertility. Existing SOC content recommendations are based on SOC:clay ratio thresholds of >1:10. However, the corresponding SOC content might be considered hard to reach in clayey soils, whose structure degradation risk is assumed to be high. Here, we analysed the SOC content and soil structure quality of soils under similar cropping practices with clay contents ranging from 16% to 52%. Five undisturbed soil cores (5–10 cm layer) were collected from 96 fields at 58 farms in the Swiss Jura region. We assessed the soil structure quality visually using the CoreVESS method. Gravimetric air content and water content, and bulk density at −100 hPa were also measured, and the soil structure degradation index was calculated. We found that the relationship between SOC and clay content held over the clay content range, suggesting that reaching an acceptable SOC:clay ratio is not limited by large clay contents. This suggests that the 1:10 SOC:clay ratio may remain useful for clayey soils. In contrast to what was expected, it is not more challenging to reach this ratio in clayey soils even if it implies reaching very large SOC contents. SOC content explained the considered physical properties better than clay content. From a soil management point of view, these findings suggest that the soil texture determines a potential SOC content, while the SOC:clay ratio is determined by farming practices regardless of the clay content.

<p>Severe subsoil compaction can occur during construction due to heavy construction machinery or storage of excavation material. This has consequences on many soil functions such as water storage and purification capacity, water and nutrient uptake by plants, etc. The consequences of subsoil compaction are known but the means and time for recovery are not well documented. Subsoil recovery is sometimes even considered unreachable at human scale.</p><p>The aim of the ROCSUB project (Restoration Of Compacted SUBsoil) is to monitor the effects of subsoil compaction on the long term and evaluate the potential of two different restoration methods (mechanical and biological) and the time needed for recovery.</p><p>The experiment started in 2020 and takes place on a field in western Switzerland with loamy texture. The subsoil was severely compacted by a heavy pile of excavation material. Compaction occurred directly on the subsoil, after topsoil removal. Visible signs of compaction were detected up to 70 cm depth.</p><p>The experiment is designed along three mechanical axes (compacted, mechanically loosened, control) and three vegetal axes (permanent grass, crop, willow trees) with four replication of each combination, resulting in 36 plots. The mechanical loosening was performed with an excavator and willow trees were selected as the most promising bioengineering method for restoring the subsoil structure.</p><p>Following properties are monitored or sampled on a yearly basis: soil moisture via TDR probe, yield, plant biomass and physiology, soil structure properties including bulk density, air capacity, water holding capacity, air permeability. The soil structure evolution is also assessed via X-ray computed tomography.</p><p>Preliminary results show an improvement in plant biomass (grass and willow) after mechanical loosening treatment. We expect plants of the compacted plots to suffer most during extreme weather conditions (dry or wet). The mechanically loosened treatment is expected to recover drainage function rapidly while water holding capacity should take more time. The combination of mechanical loosening and willow tree is expected to recover most subsoil functions fastest.</p>

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<p>Soil structure degradation is considered a major threat to soil fertility in many regions, including the Swiss Jura. In order to investigate the extent of this degradation and the means to improve soil structure quality (SSQ) with different farming practices, a large scale project “Terres Vivantes” was launched in 2019 by the canton of Jura and Bern and is followed by a group of scientists.</p><p>90 farms, covering 3’000 ha of arable land with clay contents ranging from 16% to 60% are involved in the project. Two fields per farm were selected for closer investigation and monitoring. SSQ indicators included VESS and CoreVESS (visual evaluation on sample) scores, bulk density, water and air capacity at -100 hPa and soil organic carbon (SOC):clay ratio. Five VESS observations per field were made by the farmers via the VESS app for Smartphones/iPhones. Physical properties were analyzed on five undisturbed samples (150 cm<sup>3</sup>) per field at 5-10 cm depth. Texture, SOC, pH and CEC were determined on a composite sample. Earthworm abundance, biomass and diversity were measured after onion solution extraction and earthworm surface casts were collected and weighed. The farming practices of the past 5-10 years were documented and soil tillage intensity indicators were assessed (number of tillage and stubble operations, tillage depth, and STIR (soil tillage intensity rating)).</p><p>Our results show that the soils are carbon depleted as the SOC:clay ratio is in average below 0.10 threshold (0.08). VESS scores were in average Sq3, denoting a medium SSQ with a lack of aeration and of readily available water. Among a variety of farming practice descriptions, the temporary pasture duration and the number of tillage and stubble operations were significantly correlated to the following SSQ indicators: SOC:clay, bulk density and water content. Earthworm biomass was better correlated to the number of tillage and stubble operations than to the temporary pasture duration. These two farming practice descriptions also correspond to two of the three well-known pillars of conservation agriculture, namely maximum vegetal intensity and minimal mechanical soil disturbance.</p><p>In conclusion, the soils in the Jura region have medium SSQ and are carbon depleted. The effect of current farming practices can be observed on a series of biological and physical indicators and reveal conservation agriculture pillars as “best practices”. Future investigations from the project should reveal whether farmers will be able to adapt some farming practices and improve SSQ despite time and resource constraints.</p><p> </p>