Biotic versus biogeochemical effects on plant community development during primary succession: The role of the soil microbial community

Background/Question/Methods

Soil microbial communities play a fundamental role in recycling nutrients and solubilizing minerals, processes that indirectly affect plant productivity by regulating soil nutrient availability. Soil microbes can also directly influence plants via positive (e.g., increased nutrient acquisition via mycorrhizal fungi) or negative (e.g., pathogen or herbivore effects) effects on plant seedling establishment and growth. For secondary succession systems it has been hypothesized that negative interactions between plants and microbes dominate in early succession, while positive interactions are more prevalent later in succession. To test this hypothesis for a primary succession, we collected soil samples from three distinct stages of a recently deglaciated chronosequence in the North Cascade Mountains. We tested the influence of soil biota on the growth of plant species common to each successional stage using a full factorial greenhouse experiment. First, we used sterilized and unsterilized soil treatments to examine the influence of soil biota on plant growth over and above the effects of variations in soil physical and chemical conditions. Next, we used a fertilization treatment to determine the extent of nutrient limitation. Finally, we successively grew two generations (G₁ and G₂) of plants in the same pots to assess the nature and strength of plant-microbe interactions. 

Results/Conclusions

We calculated response ratios  (RR_x= (Biomass_treatment-Biomass_control)/Biomass_control) in order to examine the influence of soil age, fertilization, and sterilization on biomass production. First, we found that the response of biomass production to fertilization was significantly reduced in the late succession soils (G₁ P<0.001; G₂ P=0.0526) reflecting greater nutrient availability with increasing soil development. Secondly, we observed that the influence of soil microbes on aboveground biomass production in the first generation of plants (G₁) differed by both soil origin and plant species, and generally the two early succession plants showed greater biomass production in sterile, early succession soil. At the same time, these early succession plants had a positive response to soil biota in late succession soil. By contrast, the late succession plant species showed a positive response to soil biota regardless of soil age. In the second generation (G₂), we found that plant-microbe feedbacks tended to be negative in late succession soils for all plant species. Taken together, our results suggest that the relative influence of microbial community composition on plant growth varies among plant species, and point to the potential importance of soil biota in influencing plant communities as ecosystems develop during primary succession.