Abstract
Tree root exudation (TRE) of water soluble organic carbon (WSOC) is an important but under-assessed component of net primary production, and is thought to strongly influence rhizosphere biogeochemistry. Riparian systems in particular are often viewed as biogeochemical hot spots fueled partially by root exudate WSOC. However, TRE rates have not been previously reported for these systems. The δ13C signatures of exudates may provide important insights into plant physiology and inform isotope-based methods to identify sources of soil CO2 fluxes, but this information is also generally lacking. In the present study, root exudate WSOC was collected in situ to assess both net exudation rates and exudate δ13C values in a temperate riparian forest. Net TRE rates were found to be most strongly related to a combination of tree species, root characteristics and net ecosystem exchange (Adj. R2 = 0.73; p < 0.001). In contrast, exudate δ13C values were correlated to time-lagged vapor pressure deficit (Adj. R2 = 0.21; p < 0.05) and air temperature (Adj. R2 = 0.43; p < 0.05), suggesting a rapid transfer of photosynthate from the canopy to the rhizosphere. Extrapolation of mean net TRE rates (13 µmol C g root−1 day−1) from a root mass basis to the entire sampling area suggests that TRE may account for as much as 3% of net annual C uptake and represents an important input of organic matter to riparian soils. Our findings of predictable TRE rates and exudate δ13C values in the present study suggest that future studies examining δ13C values of different plant components, soil organic matter and respired soil CO2 would benefit by accounting for the impact of root exudates.
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Acknowledgements
We thank Dr. Rich Phillips and his lab group at Indiana University for guidance in the early phases of this research. Amber Bellamy, Jordan Myers and Amy Weber of Ohio State University’s (OSU) Aquatic Biogeochemistry Laboratory are acknowledged for their assistance with lab and/or field work. We also thank Mike Zianni of the OSU Plant Microbe Genomics Facility for root DNA sequencing. Drs. Peter Curtis and Brian Lower at OSU and the Evans Lab group at W.K. Kellogg Biological Station provided valuable feedback on this study and earlier versions of the manuscript. This work was partially supported by National Science Foundation awards DEB-0234533, EAR-0403949 and OCE-0961860 to J.E.B. and funding from OSU. Partial financial support was also provided by the U.S. Geological Survey. Data supporting the conclusions in the present study are displayed in the tables, figures and supporting information of this submission. The authors declare no real or perceived conflicts of interest. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
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Gougherty, S.W., Bauer, J.E. & Pohlman, J.W. Exudation rates and δ13C signatures of tree root soluble organic carbon in a riparian forest. Biogeochemistry 137, 235–252 (2018). https://doi.org/10.1007/s10533-017-0415-9
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DOI: https://doi.org/10.1007/s10533-017-0415-9