Fire, invasion and herbicide Impacts on plant-soil feedbacks in the sagebrush steppe

Background/Question/Methods

Increased fire frequency coupled with plant species invasions in semi-arid ecosystems alter soil biochemical properties and processes that underpin ecosystem structure and functioning, thus threatening native plant communities. However, there is uncertainty about the magnitude of change, since soil recovery varies based on fire frequency, and local abiotic and biotic conditions. Post-fire management strategies -including herbicide application- further compound this uncertainty as they have the potential to directly and indirectly affect soil biochemical properties. We ask: 1) how do fire, plant community changes and herbicide application impact soil biochemical properties and processes, and 2) how do these changes feedback to native and invasive plant species? We have collected soils from areas in the sagebrush steppe that vary in local biotic and abiotic conditions, fire history and post-fire management, and measured soil biochemical properties (pH, soil organic matter (SOM), soil organic carbon (SOC), microbial community structure) and processes (N mineralization, enzymatic activity). We also conducted greenhouse experiments to evaluate plant-soil feedbacks and isolate impacts of changes in soil microbial communities on sagebrush (Artemisia tridentate ssp. Wyomingensis - native species) and cheatgrass (Bromus tectorum L. – invasive species) establishment.

Results/Conclusions

We have found that fire consistently reduced SOM contents and enzyme activity, while it increased soil N mineralization, but the magnitude of change varied by fire frequency. Furthermore, fire and associated changes in plant communities altered the structure of soil bacterial and fungal communities. These changes were exacerbated by herbicide addition, which also suppressed sagebrush biomass production. Cheatgrass invasion significantly changed the arbuscular mycorrhizal fungal (AMF) community structure and decreased AMF taxon richness. This resulted in a reduction in sagebrush root production, whereas soils inoculated with an AMF community associated with sagebrush reduced cheatgrass shoot production. Our results indicate that fire and invasion lead to significant and persistent changes in soil biochemical properties, and provide insight into feedback mechanisms that perpetuate species invasions and contribute to decreasing native plant performance upon fire, invasion and herbicide application.