Hydraulic effects of root-soil interactions

Management strategies to stabilize and improve the physical and hydraulic properties of the soil are a major challenge for agriculture, soil and hydrological engineering facing increasing soil degradation. It is widely recognized that plants play a significant role among the various factors influencing soil structural porosity. Particularly roots are supposed to be a core element in plant related effects on soil properties. However, root effects on soil porosity and hydraulic properties have received less attention compared to chemical and biological rhizosphere processes. Still there is a lack of appropriate experimental methods to study root-pore interactions and modelling approaches for their quantitative description. Therefore soil water modelling still reduce plants to a sink term extracting water out of the system, while root influences on the soil pore system itself are neglected. Based on an interdisciplinary approach integrating root research, soil physics, hydraulics and agronomy, the present project has three main objectives: (i) to provide a modelling framework describing hydraulic property evolution under the influence of an external driving force such as plant roots, (ii) to clarify the strength, form and hierarchical structure of root effects on the soil pore system in a conceptual model, and (iii) to exemplify the role of roots as biological component for the management of hydraulic properties under field conditions for the agro-environmental measure of cover cropping. The project proposes the use of column experiments to extract root influences on water flow properties as an experimental method. Changes in soil pore parameters will be described using a pore evolution model and causal pathways between plant roots and soil porosity will be analysed. Results obtained under controlled conditions will be compared to field observations and modifications in the root-pore relations by environmental factors will be studied. For both, column and field experiments, two cover crop species (mustard and rye) with different root characteristics will be used as model plants compared to bare soil. Finally the impact of changes in the pore system, induced by the roots, on the soil water dynamics will be assessed in order to derive their relevance for soil and water management purposes in agriculture and environmental engineering. Qualitatively plants are known as driving forces for soil formation and a variety of soil processes. The dynamic quantitative view on soil hydraulic properties presented in our project could be a substantial advance not only for water flow simulation. It could open new ways for a model based design and optimization of management measures oriented directly on the physical state of the soil, with hydraulic properties being a major concern. Applying the quantitative framework we propose for this general purpose, we expect to be able to clarify the root systems potential role as (biological) component for such management strategies.

Plant roots increase the volume of both large water transmission pores as well as fine water storage pores. Although the importance of roots for soil physical quality (aggregates, pores) is recognized, still there is uncertainty (i) which pore classes are influenced, (ii) if there are different effects depending on plant species, and (iii) which is the impact under agricultural field conditions. We determined the soil pore size distribution in soil columns under laboratory conditions as well as in field trials in order to determine the influence of roots compared to an unplanted control. Roots increased the volume of pores with large (functionally related to water transmission) as well as fine radius (functionally related to water storage). Plant species with thicker root axes (e.g. legumes) showed a stronger influence on large pores, while fine rooted species (e.g. mustard) mainly increased fine pores. All plant species stabilized the pore system against natural settlement. A loose soil (e.g. after tillage) can only conserve its structural porosity if it is stabilized by plant roots. In a conceptual model we suggested that roots rearrange the fabric of solid soil particles thereby leading to a more structured arrangement of the pore voids (heterogenization). Thick roots can directly shift soil particles, while fine roots growing into existing pores can lead to indirect reorganization of soil structure via local drying processes. The first behavior results in enhanced macropore volume, while the latter is expressed in higher fine pore volume. Using a mathematical diffusion-type model we tried to describe the observed dynamics of pore size distribution. We could properly reproduce the shift towards fine pores, while the increase in macroporosity could not be reproduced. Model application thus showed that biological root effects cannot be fully captured as an exclusively physical process. When considering a prolonged period (crop rotation) under field conditions, we could show a structural effect of additional plant rooting by inclusion of cover crops in the rotation, which however was less pronounced compared to mechanical influences (tillage) and natural wetting-drying cycles. From this we concluded that permanent soil coverage by actively rooting plants is required for persistent root effect on soil structure. The results therefore underline the importance of plant coverage (e.g. via cover crops) on agriculturally used field soil as an alternative to prolonged fallow periods in order to conserve the physical quality of soil (structure).