Modeling of mineral nutrient uptake of spruce tree roots as affected by the ion dynamics in the rhizosphere: Upscaling of model results to field plot scale

- A multi-ion rhizosphere model is used in a Monte Carlo upscaling approach.
- The variability of rhizospheric ion concentration changes is modeled.
- Rhizospheric ion concentration changes are compared with measurements.
- The variability of actual nutrient uptake rates of a spruce plot is determined.

The effects of acid soil conditions on mineral nutrition and growth of forest trees are discussed controversially. It is hypothesized that approaches are needed which determine the root nutrient uptake rates as affected by root-induced processes in the rhizosphere. A multi-ion rhizosphere model (MIM) has been developed which calculates the reactive dynamics of all major ions (H+, Al3+, Mn2+, Fe3+, Ca2+, Mg2+, K+, Na+, NO3−, SO42− and Cl−) in the rhizosphere of forest tree roots growing in acid soils. MIM calculates fine-scaled ion concentration gradients extending from the unrooted bulk soil (Bulk) to the root surface (RS) and the temporal dynamics of the average concentrations in rhizospheric sub-volumes termed as soil-root-interface (SRI), inner rhizosphere (Rh) and outer rhizosphere (oRh) of all ions (Mi) involved. SRI, Rh and oRh are defined as cylindrical soil volumes around the root which have distances to the root surface of 0.5 mm, 2.0 mm and 8-12 mm, respectively. The SRI-to-Bulk, Rh-to-Bulk and Rh-to-oRh ion concentration ratios (VMi-SRI, VMi-Rh, RMi) and the actual rates of root nutrient uptake (UMi) and H+ or OH− root excretion (EH/OH) are determined. The model is used in a Monte Carlo upscaling-procedure to calculate the UMi- and EH/OH-rates of non-mycorrhizal long roots of spruce trees growing on a long-term monitoring plot in Solling, Germany. The objectives of this study are (i) to show the plot-specific heterogeneity of modeled VMi-Rh- and VMi-SRI-values of H+, base cations (Ca2+, Mg2+, K+; Mb-cations), NO3− and SO42−, to present a comparison with rhizospheric measurement data and, to model the UMi and EH/OH rates and (ii) to present the impact of major influencing processes. The VMi-Rh-data comprise a range of about 0.5 up to 3.0 and more depending on the ion considered. In an equivalence-testing the modeled RMi-ratios of Ca2+, SO42−, Fe3+ and Na+ agree with corresponding ratios (RˆMi) of measured concentrations in Rh and oRh if extreme RˆMi-values are neglected. Means of modeled UMi-rates are 0.27, 0.126, 0.09, 1.09 and 0.12 mmol m−2 d−1 for Ca2+, Mg2+, K+, NO3− and SO42−, respectively. The UMb-rates are determined by root uptake capacities (UMb-max), height of water fluxes, Mb concentrations in bulk and rhizosphere soil, amounts of desorbed exchangeable Mb cations and EH/OH-rates. In most calculations OH− root excretions (EOH) have been calculated. Low UMb-rates have been calculated at low water fluxes and low bulk soil solution concentrations even at high UMb-max-values and are associated with EOH-rates. Based on the UMi-rates an assessment of the contribution of long roots on the total annual nutrient uptake of the spruce stand is given. It is concluded that the measured proceeding reduction of Mb-solution concentrations and the prospective NO3− saturation in the bulk soil of the spruce plot will lead to extreme low Mb/NO3 root uptake ratios.