Exudation rates and δ13C signatures of tree root soluble organic carbon in a riparian forest

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.