Wildfire-Derived Pyrogenic Carbon Modulates Riverine Organic Matter and Biofilm Enzyme Activities in an In Situ Flume Experiment.

Stefan Helmut Doerr,Gabriel Sigmund,Lukas Thuile Bistarelli,Caroline Poyntner,Matthew V. Talluto,Cristina Santín,Gabriel Singer

doi:10.1021/acsestwater.1c00185

Stefan Helmut Doerr, Gabriel Sigmund + Show 5 more

Open Access

https://doi.org/10.1021/acsestwater.1c00185

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Abstract

Wildfires produce large amounts of pyrogenic carbon (PyC), including charcoal, known for its chemical recalcitrance and sorption affinity for organic molecules. Wildfire-derived PyC can be transported to fluvial networks. Here it may alter the dissolved organic matter (DOM) concentration and composition as well as microbial biofilm functioning. Effects of PyC on carbon cycling in freshwater ecosystems remain poorly investigated. Employing in-stream flumes with a control versus treatment design (PyC pulse addition), we present evidence that field-aged PyC inputs to rivers can increase the dissolved organic carbon (DOC) concentration and alter the DOM composition. DOM fluorescence components were not affected by PyC. The in-stream DOM composition was altered due to leaching of pyrogenic DOM from PyC and possibly concurrent sorption of riverine DOM to PyC. Decreased DOM aromaticity indicated by a lower SUVA245 (−0.31 unit) and a higher pH (0.25 unit) was associated with changes in enzymatic activities in benthic biofilms, including a lower recalcitrance index (β-glucosidase/phenol oxidase), suggesting preferential usage of recalcitrant over readily available DOM by biofilms. The deposition of particulate PyC onto biofilms may further modulate the impacts of PyC due to direct contact with the biofilm matrix. This study highlights the importance of PyC for in-stream biogeochemical organic matter cycling in fire-affected watersheds.

Highlights

Vegetation fires annually burn ∼4% of Earth’s vegetated land surface, forming approximately 256 Tg of pyrogenic carbon (PyC).[1]
We hypothesized that PyC would affect (i) in-stream dissolved organic matter (DOM) composition and dissolved organic carbon (DOC) concentration due to sorption of riverine DOM and leaching of pyrogenic DOM leading to a net increase in DOC concentration and (ii) microbial functions, measured via enzymatic activities by altering substrate composition and pH
O2 content and conductivity were not strongly affected by PyC addition. This is in good agreement with the well-documented alkalinity of biochar, an engineered PyC primarily used in agricultural applications

Summary

INTRODUCTION

Vegetation fires annually burn ∼4% of Earth’s vegetated land surface, forming approximately 256 Tg of pyrogenic carbon (PyC).[1]. The less aromatic, more labile fraction of PyC can be a relevant component of in-stream carbon turnover.[21,22] Changes in DOC quantity and DOM composition induced by PyC may alter microbial functioning, on the basis of observations in non-fire-affected aquatic systems.[12,23,24] For example, Freixa and colleagues[24] showed that shifts in DOM sources (i.e., allochthonous to autochthonous) along the river continuum were accompanied by a change in extracellular enzymatic activities. In addition to pyrogenic DOM, PyC particles can affect DOM composition and its bioaccessibility by interacting with riverine DOM via selective adsorption This process has previously been observed for other carbonaceous materials, including carbon nanomaterials,[25,26] graphite, and biochar.[26]. We hypothesized that PyC would affect (i) in-stream DOM composition and DOC concentration due to sorption of riverine DOM and leaching of pyrogenic DOM leading to a net increase in DOC concentration and (ii) microbial functions, measured via enzymatic activities by altering substrate composition and pH

METHODS

RESULTS AND DISCUSSION

Addition of Pyrogenic Carbon Changes the DOM

IMPLICATIONS

■ ACKNOWLEDGMENTS

■ REFERENCES