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

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.

The immediate soil surface is a critical interface between the above- and below-ground compartments of the terrestrial system in terms of hydrological processes, crop emergence, yield and yield quality. This zone also supports distinct but hitherto barely-considered microbial communities, which are likely to have profound influences upon soil:water interactions, particularly in the context of climate change impacts upon the energy imparted to the surface via rain and/or irrigation drop impacts. We propose to elucidate fundamental relationships between the constitution of the microbial community at the immediate soil surface pertinent to UK crop production systems, and the biophysical behaviour in this zone with respect to soil structural dynamics, hydrophobicity, water infiltration and penetrative resistance. We will quantify such phenomena in a coherent manner using microcosm-based experimental approaches involving both prescribed direct manipulation of the surface biota, and via adopting soils subjected to contrasting tillage management practices. We will characterise the microbial community phenotype, which is the functionally-relevant construct in these terms, and relate this to the hydrological and soil structural responses of soil surfaces subjected to highly-controlled simulated rainfall, whose energy profiles will be co-quantified. Application of X-ray computed tomography to non-destructively visualise and quantify the 3-D structure of soil surfaces prior to rainfall impact, immediately thereafter and following subsequent drying will enable the study of the temporal dynamics of the soil porous architecture in these contexts at unprecedented resolution. Resultant data will test the basic hypotheses that soil microbial communities govern hydrological and structural responses at the soil surface. The findings will inform appropriate strategies for management practices to optimise the biophysical interactions in this crucial soil zone.

By establishing the extent to which the soil surface biota affect key hydrological and structural properties of managed soils, we will be in an informed position to establish the extent to which such phenomena may be managed, and potentially by what means. Just as importantly, we will be informed as to where and when this may not practicable. Such information is of certain interest and relevance to land managers. We will produce briefing material accordingly, which will be made freely available via online-accessible material, and promulgated through a range of professional trade associations and societies (publication in trade journals) and to a wider audience through the production of briefing notes and press releases. The availability of such information will be advertised via our institutional networks, such as the opt-in National Soil Resources Institute E-zine system.

Project outputs will be disseminated to commercial growers via regular agronomy forum meetings and annual grower days which the project team regularly participate in. These occur as a matter of routine and hence there is no additional costs incurred in this mode of promulgation. Such a route also foreshortens the impact pathway and leads to direct communication with commercial growers.

The project outputs will also feed directly into the next phase of state-of-the-art soil information risk management systems, such as the Soil-for-life system currently being developed by Cranfield University and Produce World through a KTP-Partnership co-funded by BBSRC/NERC/DEFRA.

And of course, scientific outputs will be primarily delivered via the conventional (and highly appropriate) form of peer-reviewed papers in international journals, and conference attendance. All other open-source material will also be available to this community.