Soil surface matters: Microbial controls upon hydraulic behaviour at the soil surface

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Soils are fundamental to terrestrial function, and underpin human civilisation by virtue of the range of goods and ecosystem services they provide, including food production, water storage and filtration, and supporting biodiversity. The soil surface is a crucially important zone since it is the first point of contact between rainfall or irrigation water and the underlying soil. The nature of the soil surface, for example in terms of porosity, structural resilience or hydrophobicity, affects the extent to which water enters the soil matrix (and thus is stored and available to plants) or runs off (potentially leading to soil erosion, pollution of waterways and increased flooding). Some commercially significant soils are prone to aggregate breakdown at the surface under rainfall/irrigation, leading to the formation of seals and caps which can impair crop emergence, affecting both yield and quality and significantly reduce infiltration. Whilst there has been extensive research into the physico-chemical processes involved in this zone, there has been remarkably little consideration of the role that micro-organisms play in affecting key processes occurring at the soil surface in arable and horticultural systems, particularly in the UK context. Soil organisms are known to create, destroy and modify soil structure and water-repellency by a number of mechanisms. However it is only recently that is has been revealed that the biology of the extreme soil surface is rather different to that which occurs in deeper soil zones, and hence the contribution of the soil biota to governing soil:water interactions at the surface needs to be better understood.We propose to quantify the extent to which different types of soil micro-organisms affect the interactions between rainfall, and the structural and hydrological properties of the immediate soil surface. We will achieve this by experimentally manipulating which organisms dominate the microbial community at the soil surface, applying simulated rainfall, and measuring soil structure, the extent to which water infiltrates the soil, how much the soil repels water, and the physical strength of developed surfaces. We will include soils which have been managed by farmers to promote re-aggregation and structural integrity in ways which lead to different microbial communities being present. We will also study how the intensity and kinetic energy of rainfall affects these phenomena, which is important to understand since climate change will affect both storm as well as the energies of individual raindrops impacting the soil surface. Climate change is predicted to increase rainfall intensity, but currently we have no knowledge about how soil management will affect such biological and physical interactions. The results of this research will tell us how the soil microbial community affects the performance of the soil surface, and provide the fundamental knowledge necessary for designing ways to manage soil systems more effectively and sustainably.