Plant-based controls on soil structural dynamics: elucidating the interactive roles of the genotype, phenotype and soil microbial community

The production of sufficient good quality crops from intensive production systems can be successful only if the soil has an appropriate structure. This structure is created by interactions between soil mineral particles and organic matter. Organisms in soil, in particular the roots of plants, play an important role in this process. Without good soil structure the soil cannot supply adequate water to plants, root systems cannot develop, nutrients are lost and infiltration of rain fall into soil is reduced resulting in floods and erosion. There has been a general loss of soil structure under intensive crop production, resulting from excessive tillage, often by larger and heavier machinery, the loss of soil organic matter and extreme weather events.

It is an urgent necessity to improve and maintain good soil structure, while sustaining production. There are limited options for doing this, and it is likely that success will require a variety of approaches used across a broad front. Reducing tillage operations, residue management to increase organic matter content, and growth of appropriate cover-crops are possible strategies. The latter can be effective because plants 'engineer' soil structure, both directly through the mechanical action of roots, and indirectly by promoting the activity of other organisms in soil. However, cover crops are generally only in the soil for relatively short periods and their likely effectiveness thus limited.

Little attention has been given to the possibility of using main crop varieties which have a particular capacity to engineer good soil structure. This would be better than use of cover crops because soil structure would be promoted, along with water and nutrient-use efficiency and soil biodiversity, throughout the period of crop growth.

We know that there is considerable variation in the sizes and architecture of root systems within crop species, but we do not know whether there is corresponding variation of the capacity of the plant to engineer soil structure, and the dynamics of soil structure in interaction with these root systems. That is the question to be addressed in this project. We propose to study the a wide range of wheat plants known to have very different root properties and to examine their ability to penetrate compacted soil and to promote the development of soil aggregation through interactions with soil microbes that are known also to play a 'bioengineering' role.

We will grow such plants in controlled experimental systems in which the soil structure is degraded in different ways. We will visualise the 3-dimensional distribution of roots, their ability to penetrate compacted soil, and and how they promote the development of a sound soil structure using X-ray computed tomography. This will be done at the scale of the whole root system, but also at fine scale (thousandths of a mm) in the immediate vicinity of the roots (the rhizosphere). To do this we shall adapt and develop mathematical methods to analyse complex spatial variability, and use these to model how the root modifies the local variation of soil structure. These methods will characterize the properties of root systems, and their immediate surroundings. We have a detailed characterisation of the genetic background of the plants that are used. Methods of genetic analysis can be used to show the extent to which properties of an organism depend on particular elements in that organism's genome, which is essential for showing how those properties can be targeted for selection and breeding of new varieties with enhanced properties. We shall use these methods, but the properties we shall examine will not be confined to the plants themselves, but will include measures of how the plant engineers improved structure in the surrounding soil. We shall therefore show how wheat plants can be bred to improve the structure of soils in which they are grown and the sustainability of the production system.

Healthy soils have good structure - a diverse and well-connected pore network. This arises from aggregation of soil constituents dependent on biological activity; plant roots modify soil mechanically and promote microbial activity and these effects interact. We will examine a range of 94 wheat lines, known to have extreme variation in root phenotypic architecture, which constitute a quantitative trait loci mapping population. We will measure the effects of all these lines on soil structural genesis and dynamics, and will treat both the soil physical structure and microbial phenotype as part of an 'extended trait space' along with the root architectural phenotype. We will screen the lines in microcosms with experimentally-destructured soils to allow high-throughput. This will show where there are major genetic loci regulating the extended trait space of soil and microbial communities manifest by the wheat lines, and associated soil structural genesis. A subset of lines with a range of responses will then be grown in experimental soil columns at larger scales, and for the plant life cycle, to characterize their effects in more detail, and to elucidate factors that affect soil structural conditioning, e.g. root biomass, root architecture, extra-cellular polysaccharide, and operationally active components of the microflora, particularly fungi. Root and soil architecture will be determined in situ and in 3D by X-ray computed tomography. Soil structural measures at core scale will be related to the overall root architecture and microbial properties. Novel methods for multiscale analysis will also be used to examine the pore to core scales. These will make use of wavelet transforms and non-stationary linear mixed models to characterize the modification of soil structure in the vicinity of roots. We will then examine whether there are synergies with respect to effects on soil structure when lines with contrasting but complementary properties are grown in combination.

The principal impacts we wish to attain with this project are:

(i) a practitioner-level awareness of the concepts and findings arising from this research, particularly in relation to the potential that maincrops could contribute to soil health through enhanced structure;

(ii) a wider understanding of the concepts and potential of such approaches by policy communities;

(iii) utilisation of the data and knowledge acquired by the environmental sciences academic community.

These will be achieved by the establishment of a project related website and periodic production of briefings which will be posted on the site and distributed via a group of 'Promulgation Partners' who have agreed to use their respective communications networks to disseminate the instigation of the project and the key findings as they are realised. This will include material on their websites and online portals, newsletters and annual member meetings and trade conferences. Targeted dialogue will be carried out with various policy interest groups and organisations.

Project members will also give series of talks and seminars to growers and various fora which the investigators are regularly contribute to, which will include two attendances to the key UK cereal-producer event, Cereals. A range of other practitioner and public engagement events will be made.