Invasive Phragmites enhances decomposition of deep, stable soil organic matter
Understanding the processes that control soil organic matter (SOM) loss in wetlands through decomposition is of key importance, given the relevance of SOM for the health of ecosystems, and its role as a vast C pool and climate change buffer. Due to methodological constraints to measure SOM decomposition reliably in the field, most decomposition studies are done under laboratory conditions that cannot address the effects of the rhizosphere on SOM dynamics. This fundamentally limits our understanding on SOM processes, because the rhizosphere is known to play an important role in regulating nutrient cycling and organic matter decomposition. We have observed that an invasive lineage of the common reed, Phragmites australis, roots deeper than native vegetation, and therefore has the potential to dramatically alter SOM in natural ecosystems. To evaluate the effects of changing root distribution in the soil profile and potential changes of SOM decomposition with depth, we designed a mesocosm experiment to differentiate between surface and deep SOM mineralization by employing a novel stable isotope approach to determine if plant species with different root distribution (i.e., the deep-rooting Phragmites australis and the shallow-rooting Spartina patens) differ in their SOM decomposition profile.
We report that the root:shoot ratio of Spartina was 0.96 ± 0.39 while Phragmites’s was 2.55 ± 0.19, demonstrating clear differences in belowground allocation we predicted, with a significantly greater root fraction in deeper soils (>40cm in depth) in the invasive Phragmites mesocosms. Preliminary results from our stable isotope approach indicate that Phragmites communities mineralized 43 % more SOM from the deep soil layer than from the surface SOM, supporting our hypothesis that deep rooting by invasive plants enhances SOM mineralization. We also measured changes in biomass and C:N ratios of SOM decomposition bags inserted at different depths, to support our isotope-based method estimations. If Phragmites deep rooting favors the decomposition of deep buried SOM accumulated under native vegetation (i.e., previously dominated by Spartina), Phragmites invasion into native plant communities could fundamentally change SOM dynamics and lead to the loss of the carbon pool that was previously sequestered under the native vegetation.