Simulating rhizosphere structure alterations using finite element calculations

Berli, M., Ruiz, Siul Aljadi, Aravena, Jazmín E., Bolduc, E., Ghezzehei, T., Cook, D., Menon, Mahesh, Tyler, Scott W., Young, M., Nico, P and Regentova, E.E. (2011) Simulating rhizosphere structure alterations using finite element calculations. European Geosciences Union General Assembly 2011, Vienna, Austria. 03 - 08 Apr 2011.

Abstract

The rhizosphere is the thin layer of soil that surrounds the roots playing a critical role as an environmental interface that controls water, nutrient and solute transport from the soil to the biosphere. Despite its importance, relatively little is known about the processes of mechanical rhizosphere formation and alteration as well as its impact on rhizosphere hydraulic properties in highly structured soils as for example seed beds. In this study, we used synchrotron X-ray microtomography (XMT) and finite element calculations to explore rhizosphere alterations due to a radially expanding root and its influence on rhizosphere hydraulic properties. XMT images from beds of aggregates containing plant roots were used as templates for creating finite element meshes of structured soil surrounding the root. Rhizosphere deformation was then simulated by virtual root growth, i.e. radial expansion of a cylindrical body within a bed of elasto-plastic aggregates. For various load steps, water flow through the deformed rhizosphere to the “root” surface was calculated. Finally, XMT observed structure alterations around real roots were compared to alterations simulated by finite element calculations. Mechanical simulations show that “root” expansion within a bed of aggregates can increase inter-aggregate contact area, whereas interaggregate porosity decreases. Hydraulic simulations show an increase in partially saturated hydraulic conductivity of the rhizosphere with increasing interaggregate contact area; however, in this case saturated hydraulic conductivity decreases because of a decrease in inter-aggregate pore space. For a loose initial aggregate packing, as used in this study, inter-aggregate contact area controls partially saturated hydraulic conductivity of the rhizosphere. With increasing degree of rhizosphere compaction, however, the influence of inter-aggregate contact area on partially saturated hydraulic conductivity
decreased and the hydraulic conductivity of the aggregate matrix became increasingly important. In agreement with observations from XMT images, mechanical simulations also show that root-induced rhizosphere compaction occurs primarily within a shell around the root with a shell thickness of one to two root radii. Although mechanical and hydraulic simulations are limited to 2D at this point, they provide some quantitative insight of how plant roots
mechanically alter the surrounding soil and in particular its hydraulic conductivity.

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