Plant-soil feedbacks: The benefit of long-term, field-based community level study in uncovering their role in California grasslands

Background/Question/Methods

Plant-soil feedbacks (PSFs) can be a factor in plant invasions and may negatively impact native restoration success. PSFs occur when a plant changes soil properties that then directly influence plant growth, influencing community assemblage. Short-term greenhouse studies show that invasive species often develops PSFs that negatively affect natives. However, the relative significance of PSFs in more realistic long-term field settings needs more study. In California grasslands, Eurasian grasses are known to alter nitrogen cycling, deep soil organic matter, and microbial community composition, which negatively impact native grasses in greenhouse experiments. To uncover whether these changes may be behind restoration failures of unknown cause and to determine the relative importance of PSFs in a more natural setting, we investigated a) whether exotic grasses and/or native grasses condition soil properties differently, b) whether those differences result in plant-soil feedbacks that negatively impact the other on a community level in field conditions. From a larger grassland experiment in Davis, CA, 90-cm deep soil cores (0-15, 15-30, 30-60, 60-90 cm) were taken from 16 plots dominated by either exotic or native grasses for the last 10 years to compare soil properties. Denuded plots were divided into subplots, seeded with either native mix, exotic mix, or native + exotic mix. Plant performance variables encompassing all life stages were measured for two growing seasons.

Results/Conclusions

Net mineralization and nitrification rates in the top 15-cm of soil were lower in exotic-conditioned soil; but soil organic matter, %C, %N, and water holding capacity did not differ at any depth. The microbial community composition (bacterial and fungal) as a whole, as well as specifically the nitrifying bacterial community, significantly differed between native and exotic soils at all depths. Native grasses performed better in exotic-conditioned soil than their home soil in seed head production, height, and community-level cover by 20%. These results suggest that invasive-driven PSFs observed in the greenhouse are weakened in the field and that pathogen accumulation may drive native performance, highlighting the need for more long-term studies. Additionally, future studies should also include community level analysis as species-level analyses may obscure overall effects.