COS 69-5 - Soil chemistry and rhizosphere microbial communities co-vary across an urbanization gradient

Abstract: Over the past two centuries, urbanization has altered physical, chemical, and biological aspects of ecosystems in ways that are just beginning to be understood. More than half the global human population resides in cities, and that percentage is expected to increase in the coming decades. Thus, understanding ecological processes in urban systems is necessary for informing land management practices that will promote ecosystem services of importance to humans. At the same time, urbanization gradients present a basic science challenge to untangle the effects of co-varying abiotic and biotic factors on ecological processes. Compared to macro-organismal studies, there are far fewer studies that examine patterns of microbial diversity (much less plant-associated microbial diversity) across urbanization gradients. Additionally, the causes and consequences of variation in microbial community composition across landscapes are poorly resolved.

I performed an observational study to characterize soil chemistry and microbial community composition across an urbanization gradient in St. Louis, Missouri. In each of 10 sites (three urban, five suburban, and two rural), I collected rhizosphere samples from five individuals of two common weedy plant species: Plantago lanceolata and Plantago rugelii. To characterize soil chemistry, I quantified trace and heavy metals with inductively coupled plasma mass spectrometry (ICP-MS) and quantified nutrients and exchangeable cations using inductively coupled plasma optical emission spectroscopy (ICP-OES). I characterized microbial communities in the same samples through sequencing of the bacterial 16S rRNA gene and fungal internal transcribed spacer (ITS) region, using a DNA metabarcoding approach.

Principal component analysis revealed soil chemistry differences between urban, suburban, and rural sites. Urban sites had higher concentrations of trace and heavy metals. Suburban sites had more cobalt, manganese, and phosphorus. Rural sites had more sulfur, calcium, magnesium, and silicon. Preliminary ordination analyses also revealed clustering of microbial communities by level of urbanization. In the next steps, I will assign taxonomy to the microbial communities of each sample and perform multivariate analyses on the soil chemistry and soil microbial data together to understand how these abiotic and biotic variables are related to each other.

This work will improve our understanding of spatial co-variation in abiotic and biotic soil conditions experienced by plants in human-altered landscapes. Such understanding is needed to develop strategies for managing soils to promote health of plants, animals, and people in an increasingly urbanized world.