Root inputs from exotic annual species in desert soils increase soil moisture but reduce plant growth: Implications for plant-soil feedbacks and restoration

Background/Question/Methods

Invasions by exotic annual species are increasingly impacting Southern California deserts, where they can alter ecosystem processes and plant community composition. With climate change the Southwestern US is expected to experience increasingly variable rainfall, and larger rainfall events may facilitate invasion because exotic species often benefit over native species under high resource conditions. Exotic annual species also have dense shallow root systems that could create positive feedbacks to further invasion by increasing soil organic matter and soil water holding capacity. Alternatively, fine root inputs could create negative feedbacks to exotic plant growth if they stimulate soil microbial nutrient immobilization. We evaluated the dual influences of rainfall frequency and fine root inputs on species performance in a greenhouse experiment. Common native and exotic desert species were grown in factorial combinations of three treatments: individually versus in competition, with and without additions of fine roots from exotic species in the soil, and with frequent versus infrequent watering (holding total rainfall constant).

Results/Conclusions

Mean soil moisture increased with larger infrequent watering events, and also with root addition. Plant growth increased with larger watering events, but declined with root addition, and exotic species growth declined more than native species growth with root additions. Mechanistically, root addition lowered inorganic nitrogen (N) availability, and microbial N immobilization increased with soil moisture content. Together these results show that increased fine root production can promote negative feedbacks to growth of exotic species via microbial N immobilization, especially under conditions of high soil moisture. Further, our results suggest that carbon additions are a potentially effective strategy for suppressing growth of problematic desert invaders.