2.2. Aggregate stability

Soils with high organic matter content tend to have larger, stronger, and more stable aggregates that resist compaction, whereas the opposite is true for soils with less organic matter. An improvement in soil aggregate stability has several consequences for an agroecosystem, including reduced risk of soil compaction and erosion [30]. The quality of soil structure greatly depends on the soil organic carbon (SOC) content [31], especially on the fraction of labile SOC (also called the "particulate organic matter" because of this fraction cycles relatively quickly in the soil). Labile organic matter also plays an important role in maintaining soil structure and providing soil nutrients [32].

Aggregate stability is a keystone factor in questions of soil physical fertility and can be enhanced by means of an appropriate management of organic amendments, which can maintain an appropriate soil structure. This agronomic procedure could improve pore space suitable for gas exchange, water retention, root growth, and microbial activity [9]. Aggregate stability at the soil surface is affected mainly by exposure to rainfall (drop impact and runoff). A bare soil (e.g., a soil from which crop residues have been exported or incorporated into the soil by plowing) is in direct contact with raindrops, which facilitates a breakdown of soil aggregates, increasing soil erodibility. Aggregate degradation can lead to surface sealing and crust formation, which reduces the water infiltration rate and increases the risk of soil erosion and the loss of valuable topsoil [33]. High silt content, together with low organic matter content, results in soils that are more prone to aggregate breakdown and surface crusting [29, 34]. Organic matter applied on the topsoil protects to the erosion and favors the aggregation of mineral particles.

#### 2.3. Soil compaction

Soil compaction is a form of physical degradation in which soil biological activity and soil productivity for agricultural and forest cropping are reduced, resulting in environmental consequences. Compaction is a process of densification and distortion in which total and airfilled porosity and permeability are reduced, strength is increased, soil structure are partly destroyed, and many changes are induced in the soil fabric and in various characteristics [35].

Generally, four indicators quantify soil compaction: total porosity, pore size distribution, bulk density, and penetration resistance. Given that root growth is impeded by soil compaction, these indicators are probably negatively correlated with root growth and rooting depth [29]. Even more, these properties are closely related to water movement, water availability for plants, and soil gas exchange.
