Microbial carbon processing in a riparian soil: a microcosm experiment on responses to native vs. invasive root input

Invasive plant species can alter belowground biogeochemical processes, but the specific dynamics and mechanisms associated with these shifts remain unclear. We compared the effects of aqueous root extracts from the invasive Impatiens glandulifera and the native Urtica dioica on the carbon processing and functional stability of non-invaded riparian soil. In a 21-day laboratory incubation, we amended soil with two concentration levels of root extracts to simulate root lysis and exudation. We assessed microbial responses by measuring respiration rates (MicroResp™) and enzyme activities, while the stoichiometric properties of the root extracts were characterized by dissolved carbon (DC), nitrogen (DN), and total phenolics (TPC). We found that I. glandulifera extracts did not suppress microbial activity. Instead, plant origin of the root extract (U. dioica or I. glandulifera) determined the physiological efficiency of the community. The addition of native U. dioica extracts was able to support biomass accretion indicated by high substrate-induced respiration. Conversely, I. glandulifera extracts triggered a state of relatively higher basal respiration but low biomass formation resulting in lower physiological efficiency. By stoichiometric modelling we identified the dissolved carbon-to-nitrogen ratio as the primary driver of this inefficiency. Temporal dynamics were governed more by substrate concentration rather than plant origin. Low-dose treatments of both species exhibited “boom-and-bust” dynamics likely driven by rapid labile depletion and necromass recycling, whereas high-dose treatments exhibited a monotonic response. Furthermore, temporal spikes in TPC were linked to the possible enzymatic “unmasking” of reactive moieties as the labile fraction was selectively harvested. Overall, our findings indicate that changes in soil organic carbon cycling may be better explained by the stoichiometric quality of root input than by the biogeographic status of the plant itself.