Influence of organic stimulation on plant-microbe interactions in tree trenches exposed to urban runoff contaminants

Urban runoff transports diverse organic pollutants that threaten urban waters and soils. Blue-green infrastructures such as tree trenches may help to mitigate these impacts. Tree trenches are increasingly implemented in cities to manage urban runoff. While the hydraulic and physical retention functions of tree trenches are well studied, their potential to perform biological cleaning processes is less understood.

This study explores whether organic carbon amendments can stimulate the microbial transformation of organic pollutants in tree trench systems. We hypothesize that stimulation with low molecular weight organic carbon increases microbial activity and promotes co-metabolic degradation pathways in the tree rhizosphere. This would support active pollutant removal rather than passive retention.

To test this hypothesis, an outdoor mesocosm experiment was established that simulates a real tree trench in Leipzig, Germany. Linden trees (Tilia cordata) were planted in 1000 L containers filled with the volcanic substrate used in Leipzig, which has rapid permeability to ensure better infiltration. The systems received 60 L of water within two hours to simulate a rainfall event. The water contained a mix of fuel spills, fuel additives, and tire wear pollutants commonly found in urban runoff waters (naphthalene, methyl tert-butyl ether, and 1,3-diphenylguanidine). The common industrial by-products molasses and whey were applied as organic stimulants of microbial metabolism. The system’s response was investigated from a plant, geochemical, and soil microbial perspective.

Following the rainfall event, all tree trenches remained oxygen-depleted during incubation, which was evident from a consistently low redox potential of -40 mV in the percolating soil water. In the plant-available porewater of the linden trees, the redox potential further decreased to -60 mV over time across treatments, indicating microbial fueling through plant exudation. A minor increase in bulk and rhizosphere pH from 7.8 to 8.0 across 4 weeks in trenches amended with and without contaminants and/or organic stimulants indicated a well-buffering trench substrate and allowed comparison of biogeochemical data. An accompanying laboratory study confirmed the mineralization of 13C-labeled naphthalene and, furthermore, that organic stimulants enhanced this process. Overall, organic stimulants seemed to increase biological activity in the rhizosphere as indicated by changing nitrogen speciation and a decrease in dissolved organic carbon. Besides monitoring porewater geochemistry shifts, genes coding for key enzymes of degradation pathways specific to each contaminant were quantified. They were correlated with shifts in microbial community composition and activity by assessing the abundances of 16S rRNA genes and transcripts in the bulk and rhizosphere soil of the trench system. Together, these patterns demonstrate that stimulation with organic compounds can activate biological processes relevant for pollutant transformation, even under complex and heterogeneous tree trench conditions.

This work aimed to evaluate biological stimulation as a design principle for tree trenches in urban water management. By promoting active cleaning rather than passive retention, blue-green infrastructures could become more effective tools for sustainable water runoff treatment, thereby strengthening the role of nature-based solutions in sustainable urban water management.