Do microbes matter?: Soil biotic versus biogeochemical effects on plant community development during primary succession

Background/Question/Methods

Microbial communities play a fundamental role in soils by 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 or negative effects on plant seedling establishment and growth. During primary succession (e.g., following glacial retreat), soil biota undergo changes in both function and community composition, but if and how these changes influence plant community development remains largely unknown. It has been hypothesized that negative interactions between plants and microbes (e.g., pathogen or herbivore effects) may dominate in early succession, while positive interactions (e.g., via mycorrhizal mutualisms with plants) may be more prevalent later in succession. To test this hypothesis, we collected soil samples from three progressive successional stages of a recently deglaciated, developing chronosequence in the North Cascade Mountains where we have already documented changes in soil microbial community composition through time. Using soil collected from those same sites, we tested the influence of soil biota on the growth of plant species common to each successional stage in a full factorial greenhouse experiment. 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, and we used a fertilization treatment to quantify the role of different microbial communities when nutrients are plentiful. 

Results/Conclusions

In the unfertilized treatments, all plant species performed significantly better in late succession soil, likely reflecting higher nutrient availability in those soils (P=0.014). When nutrient limitation was overcome by fertilizer additions, plant growth differences that were previously observed across successional stages disappeared (P=0.21). In fertilized soils, we also observed that the influence of soil microbes on aboveground biomass production differed by both soil origin and plant species, and the two early successional plants showed greater biomass production in sterile, early succession soil. At the same time, these early succession plants showed greater biomass production in unsterilized late succession soil. By contrast, the late succession plant species showed greater biomass production in unsterilized soil regardless of soil origin. Taken together, our results suggest that the relative influence of microbial community composition on plant growth varies among plant species, and point to the importance of soil biota in influencing plant communities as ecosystems develop during primary succession.