Towards the development of field methods to characterize environmental controls on rhizosphere priming effects

Background/Question/Methods

The processes that control soil organic matter (SOM) decomposition have traditionally been modeled as a function of temperature and to a lesser extent soil moisture. Recent research suggests that inputs of labile carbon (C) – through their effects on microbial activity – can mediate SOM responses to environmental drivers. We sought to better understand how precipitation and temperature mediate the magnitude and biogeochemical consequences of rhizosphere priming effects on nitrogen (N) retention. We conducted two field experiments at the Boston-Area Climate Experiment (BACE) – an old-field ecosystem subjected to experimental changes in precipitation (-50% and +50% of ambient precipitation) and warming (from +1°C to +4°C above ambient) since 2008. In experiment one (hereafter the “rhizosphere simulator experiment”; RSE), we released root exudate mimics of varying C quality into soils for four weeks, and measured microbial biomass and nutrient transformation rates. In a second field experiment (hereafter the “ingrowth core experiment”; ICE), we quantified the response of root- and mycorrhizal-derived C to the experimental treatments over 5 months, and how changes in these inputs influence microbial biomass and nutrient transformations. We quantified C inputs by training roots and mycorrhizal fungi to colonize ingrowth cores containing “C4 soils”.

Results/Conclusions

In the RSE, we found that root exudates increased microbial respiration, with the greatest magnitude of effects occurring for exudates with a C:N of 50 (52% increase) and 100 (56% increase). This effect was not influenced by warming or altered precipitation (i.e. interactions with C quality: p > 0.05). Although net N mineralization rates were unchanged by the addition of exudates (across all treatments), enzymes involved in the breakdown of N-bearing SOM increased by 49% in plots warmed plots which received exudates. In the ICE, C inputs from roots and hyphae together were two orders of magnitude greater than those from hyphae alone, indicating that belowground C fluxes in these plots are dominated by root inputs. Notably, the magnitude of the root-derived C fluxes were positively correlated with microbial activities (e.g. net nitrogen mineralization and extractable P), indicating that exudates were likely fueling microbial release of nutrients from SOM. Collectively, our preliminary results provide some of the first field-based estimates of the extent to which changes in soil moisture and temperature can directly and indirectly alter the size of the SOM pool via rhizosphere priming effects and lay the groundwork for the development of more mechanistic models of SOM decomposition under climate change.