Plants can cause changes in the structure of soil microbial communities which can in turn affect seedling performance nearby, forming plant-soil feedback (PSF). It is typically assumed that the strength of PSF is fixed. However, soil cultivation by plants is often a gradual process, and PSF strength may change over time as plants continue to modify its surrounding.
We took a two-step approach to studying how soil microbial communities might vary with cultivation length and how these temporal changes might feed back to affect plant performance. First, we used a series of aerial photos at the Bodega Dunes in California to estimate the age of individuals of the four dominant plants, including two introduced species (Ammophila arenaria and Carpobrotus edulis) and two native species (Lupinus arboreus and Baccharis pilularis). For each species, we collected soil from 30 individuals of different ages and characterized its fungal and bacterial communities using Illumina Miseq. Second, we analyzed corresponding changes in the effects of soil microbes on seedlings in a greenhouse experiment. We grew seedlings of the four species individually in pots with soil collected from different individuals, representing different plant species, plant age, and sterilization treatment (live and sterile soil).
Results/Conclusions
In the field, plant species differed in the composition of soil fungal and bacterial communities. Moreover, microbial communities became progressively different from communities in bare sand with longer cultivation. Microbial functional groups changed differently through time depending on the species identity of the plants. After three months in the greenhouse, we quantified PSF strength for each seedling replicate by comparing seedling biomass when grown in live vs. sterile soil. We found that L. arboreus grew better in live than sterile soil (positive PSF), while the other three species grew worse (negative PSF). In the two introduced species, PSF became more negative with longer cultivation, especially when grown in soil cultivated by a conspecific individual.
Together, these results suggest that PSF is more dynamic than usually assumed, with both soil microbial communities and their effects on plant performance changing with plant age. When PSF is dynamic, the timing of disturbance and species arrival can play a major role in the structuring of plant communities.