Limitations of pollutant transformations in topsoils

Why appear biodegradable pollutants much more recalcitrant in soils than in the lab?

Environmental chemicals such as pesticides persist in soils at low, but relevant concentrations, even though they are fully biodegradable under laboratory conditions. My doctoral study will shed light on the complex interplay of biological and physiochemical processes that control pollutant transformations in soil. The major goal is to identify processes and boundary conditions governing the build-up of pollutant residues pools in agricultural soils. The central hypothesis is that soil structure and associated small-scale spatial dynamics of biophysical processes limit the biodegradation of pollutants. Based on that it will be addressed, what the consequences of soil heterogeneity at small-scale for pollutant fate at field scale are and how small-scale information can be simplified to be applicable in hydrological models.

This PhD thesis builds on a biophysical model (PECCAD) which simulates degradation cou-pled to carbon turnover in soil (Pagel et al. 2014 - Micro-scale modeling of pesticide degradation coupled to carbon turnover in the detritusphere: model description and sensitivity analysis, Biogeochemistry 117.1 (2014): 185-204). The biophysical model reflects that pesticide degradation depends strongly on carbon turnover, degradation is controlled by activity of the microbial community and it is regulated by substrate availability through sorption and transport processes. Currently techniques from dynamical systems theory (stability and bifurcation analysis) are applied to the biophysical model in order to identify parameter values and initial conditions possibly leading to a change in system behaviour. Starting from an equilibrium state of a system, stability analysis reveals whether small changes in parameters or initial conditions will lead to similar behaviour. If this is not the case, the system undergoes a bifurcation.The dynamical system point of view shall particularly provide insight into the role of the physiological state of microbial populations.

In a second step, PECCAD will be extended to consider the role of soil structure and spatial processes on the degradation of organics. The model dimension will be extended to 2D or 3D so that the model can be tested against planted microcosm systems. Sensitivity studies will elucidate the impact of small-scale processes on pedon-scale degradation and transport of organic pollutants.