Dina in ‘t Zandt discusses her recent paper Local soil legacy effects in a multispecies grassland community are underlain by root foraging and soil nutrient availability. Find out more about their soil legacy experiments and how their results differed from the theory.
Biodiversity plays a key role in ecosystem structure and function, with plant community diversity an important basis for diversity on higher trophic levels. Natural grasslands are very diverse ecosystems, with up to 89 plant species per m2. Many of these occupy different niches in the community. Although this differentiation into niches is an important underlying mechanism of diversity, it does not explain the coexistence of ecologically similar species in species-rich communities. Why does the strongest competitor not exclude all other species? This question has captured the attention of ecologists for decades and still cannot be answered satisfactorily.
Over the past decades, evidence has accumulated that shows that interactions between plants and soil biota play a critical, structuring force in plant species co-existence processes. Soil biota (such as bacteria, fungi and nematodes) that plants accumulate in and around their roots during their lifetime have been found to play an important role in this. Especially important is the accumulation of species-specific antagonists that negatively affect their host but not other plant species. This helps prevent successful species from mono-dominating the community. This self-limitation of successful species is thought to be aided by soil biota communities that remain present in the soil as a ‘legacy’ once a plant dies. As a result of the presence of own, unique antagonists, plant species are hypothesised to be negatively affected by their own soil legacies, creating opportunities for other, subordinate plant species to survive and thus promoting species coexistence and diversity.
In an outdoor mesocosm experiment (Nijmegen Phytotron), we tested the negative effect of own soil legacy patches on the surrounding plant species in eight-species plant communities. Soil legacies were created in the greenhouse by growing plant species on a mixture of sterile soil and live soil from a species-rich grassland. Aboveground plant material was harvested and soils were filled into four cylinders in each mesocosm container. During the following two growing seasons, we determined root ingrowth into the soil legacy patches of the surrounding plant species using a novel sequencing based method, injected nutrient tracers in the middle of each soil patch and traced this back into the aboveground biomass of the surrounding plants.
Against expectations, we found that plant species did not place less roots into own soil legacy patches, nor did they take up less tracers from own legacies compared to other legacies. Root exploration of the soil patches was in many cases explained by differences in soil nutrient availability between the patches and indicated that many species selectively placed roots in soil with higher nutrient availability. This occurred even when differences in available nutrients between soils were small. Tracer uptake was also affected by soil nutrient availability as well as root exploration patterns.
Therefore, in complex, multi-species semi-field conditions, soil legacy effects do not match expectations based on theory and experiments in controlled conditions. This indicates that results from controlled experiments cannot easily be translated to longer-term, (semi)field situations. Among the many complicating factors that may modify or even overrule expected effects, we identified soil nutrient availability as a critical force that may, together with soil biota, shape plant species coexistence processes. To move forward in soil legacy research and close the gap between controlled and (semi)field experiments, it is imperative to understand the effects of soil legacies on soil nutrient cycling processes, such as decomposition and mineralisation, and the subsequent effects on plant performance. It will be challenging to understand the effects of soil legacies in multi-species plant communities, but this is a necessary step in advancing our understanding to disentangle mechanisms of plant species coexistence.
Dina in ‘t Zandt Department of Experimental Plant Ecology, Institute for Water and Wetland Research, Radboud University, Nijmegen, The Netherlands.
Read the full research article online: Local soil legacy effects in a multispecies grassland community are underlain by root foraging and soil nutrient availability