P6 Controls for the Fate of Agrochemicals in Soils

Despite the undisputed importance of soils for the quality of ground- and surface water, we are still lacking the mechanistic understanding of the interacting biogeochemical processes that control the persistence of pesticides in soil. Project P6 therefore identifies and quantifies chemical, physical, and biological processes controlling pesticide turnover in soil to address three central research questions:

• Why do intrinsically biodegradable compounds (here pesticides) persist in soils under field conditions?
• Which physicochemical and microbial factors control sorption, biodegradation and leaching of pesticides (and their metabolites) and how are they interrelated?
• Which microbial and small-scale control mechanisms of pesticide turnover need to be considered in mechanistic and predictive pesticide fate models and what are the limits of model simplifications in terms of uncertainties?

In project P6, we combine field monitoring with targeted laboratory experiments and mechanistic modeling:

• to characterize the inventory of pesticides, other organic compounds as well as dissolved organic carbon and nitrogen species
• to quantify the biodegradation potential of the CAMPOS target compounds 2- methyl-4-chlorophenoxy acetic acid (MCPA), glyphosate and atrazine
• to identify physicochemical factors controlling the mobility and bioavailability of these target compounds
• to quantify concentration thresholds of pesticides triggering functional gene expression/ energy-limited growth of pesticide-degrading microorganisms
• to investigate how physiological responses of microorganisms (sensitivity to moisture and temperature, drought stress) affect biodegradation and leaching
• to assess how small-scale dynamics affect persistence and release of pesticides in soil
• to develop up-scaled expressions of pesticide turnover that integrate small-scale control mechanisms for implementation in vertical soil-crop models (Expert-N) in project P7

We have installed tension-controlled suction plate lysimeters at three representative soil monitoring stations in the Ammer catchment for intensive monitoring of pesticide leaching and stimulated field experiments. For detailed insights into control mechanisms of biodegradation, we have developed a gene-informed modeling approach and established qPCR protocols for quantifying gene transcripts of microbial degraders involved in the degradation of MCPA, glyphosate and atrazine.