The estimation of root water uptake and water flow in plants is crucial to quantify transpiration and hence the water exchange between land surface and atmosphere. In particular the soil water extraction by plant roots which provides the water supply of plants is a highly dynamic and non-linear process interacting with soil transport processes that are mainly determined by the natural soil variability at different scales. To better consider this root-soil interaction we extended and further developed a finite element tree hydrodynamics model based on the one-dimensional (1D) porous media equation. This is achieved by including in addition to the explicit three- dimensional (3D) architectural representation of the tree crown a corresponding 3D characterisation of the root system. This 1D xylem water flow model was then coupled to a soil water flow model derived also from the 1D porous media equation. We apply the new model to conduct sensitivity analysis of root water uptake and transpiration dynamics and compare the results to simulation results obtained by using a 3D model of soil water flow and root water uptake. Based on data from lysimeter experiments with young European beech trees (Fagus silvatica L.) is shown, that the model is able to correctly describe transpiration and soil water flow. In conclusion, compared to a fully 3D model the 1D porous media approach provides a computationally efficient alternative, able to reproduce the main mechanisms of plant hydro-dynamics including root water uptake from soil.

In a risk assessment study the environmental fate of the herbicide glyphosate was studied with the specific background of the presence of genetically modified (GM) plants. Aim was to simulate the environmental behaviour of glyphosate in sandy field soil lysimeters after multiple herbicide applications and under the presence of GM soybean and to test and enhance model reliability in the simulation of the herbicide fate including biodegradation in the soil and herbicide translocation in GM plants. The modelling of the herbicide behaviour in the present study was based on the pesticide transport model LEACHP and the model PLANTX to simulate the pesticide uptake by plants. Both models were implemented in the modular modelling system EXPERT-N. Glyphosate transport measurements and the mathematical modelling results indicate that due to the high sorption of glyphosate to the soil matrix and the high microbial capacities for glyphosate degradation in the lysimeter soil, leaching risk can be considered to be low. We confirmed that the introduction of more adequate conceptual descriptions of microbial response to pesticide and nutrient additions can contribute to a reduction in the uncertainty of pesticide degradation modelling. Moreover, the consideration of uncertainty in sorption, dispersivity and degradation parameters revealed a considerable variability in model output. The observed accumulation of glyphosate in roots and nodules was reproduced by the simulation results. Under the restriction that the prevailing model assumptions are valid, the simulation results indicate that glyphosate may accumulate also in beans of trangenic soybean.

Column experiments are often used to study transport properties of sediments to predict the fate of contaminants in the subsurface. However, water flow in column experiments is rarely monitored over the entire length of experiments, despite the known impact of biogeochemical processes on pore structures. To study if flow path changes occur over time, water flow and solute transport were investigated in a homogeneous, nearly saturated column by conducting three tracer experiments over 7 mo. Water flow and transport parameters were determined from tracer breakthrough curves using different model approaches. The initial homogeneous transport was adequately described by a simple advection‐dispersion model. However, after some months, the porous medium changed from being uniform to non‐uniform. The observed bimodal breakthrough curves were simulated with a simple multi‐flow advection‐dispersion and a dual‐permeability model. Both models supported the development of non‐uniform water flow and solute transport due to flow paths changes. Those changes were attributed to clogging of small pores near the column inlet due to microbial growth and calcite precipitation leading to a heterogeneous infiltration front. The results highlight the need to determine transport properties continuously over the course of column experiments to reliably predict parameters for groundwater flow and understand contaminant transport into aquifers.

Please cite this article as: Braunschweig, J., Klier, C., Schröder, C., Händel, M., Bosch, J., Totsche, K.U., Meckenstock, R.U., Citrate influences microbial Fe hydroxide reduction via a dissolution-disaggregation mechanism, Geochimica et Cosmochimica Acta (2014), doi: http://dx.doi.org/10.1016/j.gca. 2014.05.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Modelling of abiotic dissolution kinetics revealed that colloid stabilization was most pronounced at citrate:Fe ratios of 0.1 -0.5, whereas higher ratios led to enhanced dissolution of both colloidal and larger aggregated fractions. Mathematical simulation of the microbial reduction kinetics under consideration of partial dissolution and colloid stabilization showed that the bioaccessibility increases in the order large aggregates < stable colloids < Fe-citrate.These findings indicate that much of the organic acid driven mobilization of Fe oxy(hydr)oxides is most likely due to colloid formation and stabilization rather than solubilisation.