Plant-soil feedback has been highlighted as an important determinant of several major ecological processes including succession, coexistence, and the invasion of exotic species. However, most experimental evidence for plant soil feedback is taken from pot experiments in which monocultures of plant species condition the soil and then monocultures of the same and other species are grown on these conditioned soils. Such situations do not capture the complexity of real plant communities, in which heterospecifcs can share rooting space both when affecting and to responding to the soil, depending on the degree of intraspecific aggregation.           

Using a two-species annual herb system we performed a two-phase glasshouse experiment investigating how intraspecific aggregation affects plant-soil feedback. In the first phase species were grown in monoculture and in two-species mixtures at several levels of intraspecific aggregation. The soil  environments generated in the conditioning phase then formed the basis of the response phase. In this phase the same two species were grown, again in monocultures and in mixtures at several levels of aggregation, on all the different soil environments generated in the conditioning phase, and plant performance was measured.  

Results/Conclusions

We found that both plant species displayed a negative feedback when grown on soils in which they had previously occupied but that the feedback on one species Matricaria recutita only occurred when plants were grown on soil that had been Matricaria monocultures in the conditioning phase. Where plants were grown in mixtures in the conditioning phase, Solanum nigrum, dominated the community and the effect on the soil environment, regardless of the degree of aggregation. In the response phase there was no difference in competitive outcomes on these soils and those on which Solanum monocultures had been grown, i.e. Solanum performed poorly relative to Matricaria.

These results show that the strength of feedback is can be proportional to the abundance of a species and that subordinate species may not express feedback if their effects on the soil are diluted by those of more abundant species. The role of feedback in coexistence, succession and invasions should be reconsidered if this is true of field communities.