10 REASONS NOT TO TILL PART 1. SOIL STRUCTURE AND MICRO-ORGANISMS
Based on Hartmann & Six. 2023. Soil structure and microbiome functions in agroecosystems. Nat. Rev. Earth Env.
Soils consist of mineral particles and organic constituents packed in aggregates of different sizes and organised as three-dimensional porous structures. These habitats are colonised by microorganisms in which they perform different functions (nutrient recycling, nitrification, denitrification, plant growth promotion, pest control, etc., …).
The basic components of soil are the mineral soil particles which are classified according to their size into clay (<2 µm), silt (2-63 µm) and sand (63-2000 µm). These soil particles bind carbonates, metal oxides (Fe, Al), organic matter (particulate, humified, carbonised), biotic components and organic macromolecules (mucilages, polysaccharides, proteins, lipids and agglomerates such as extracellular polymeric substances) in organo-mineral complexes to form silt-sized aggregates smaller than 50 µm or microaggregates of 50-250 µm size; the forces holding particles together in these aggregates are stronger than the forces between adjacent particles. Silt-sized aggregates and microaggregates are organised as larger, less stable macroaggregates (>250 µm) that are held together mainly by root complexes and fungal hyphae. Macroaggregates form first and function as a nucleus for the formation of microaggregates in the centre of the macroaggregate.
Decomposing plant and fungal debris or root-derived particulate organic matter is progressively embedded with clay particles and microbial products to form the core of stable micro-aggregates and silt-sized aggregates. Once the organic matter in the core is depleted, microbial activity and metabolite production stops, leading to a loss of stability of the aggregate and eventually to the decomposition of the aggregate.
Therefore, primary particles organised as soil aggregates separated by voids are a relevant microhabitat for soil microorganisms. Most soil microorganisms live as interconnected assemblages associated with these particles, so that soil structure ultimately determines its resources through oxygen diffusion, water flow, organic matter accessibility and nutrient availability. Understanding how agricultural management influences the link between soil structure and the soil microbiome is therefore essential to anticipate changes in soil microbial communities and their ecosystem services under different agricultural management regimes.
Photo: Conceptual view of a model pore network. Arrows: O2. In yellow: anaerobic bacteria. In blue aerobic bacteria. Image: Borer et al. 2018. Spatial organization of bacterial populations inresponse to oxygen and carbon counter-gradients in pore networks. Nat. Com. 9:769