Soil heterogeneity and species composition interactively affect the seed germination of three typical forage species

Soil heterogeneity has been shown to affect seed germination of plant species in monocultures, and plant species' interactions in a community may also impact seed germination. However, whether and how both factors interact to modulate seed germination is still unclear. To investigate these interactive effects, a controlled experiment was conducted in which seven species compositions derived from all the possible combinations of three forage grasses (Festuca elata, Bromus inermis, and Elymus breviaristatus) were factorially crossed with four levels of soil heterogeneity. This heterogeneity gradient was developed by applying resource-rich and resource-poor substrates in an alternating pattern in pots, creating fine, small, medium, and large patch sizes. The results showed that patch size and species composition interactively affected seed germination percentage. Specifically, the small patch size supported a higher seed germination percentage in the mixture with three species compared with other patch sizes (i.e., fine, medium, and large patch sizes). Moreover, this pattern was only found in the resource-rich substrate, not in the resource-poor substrate. A combination of abiotic (e.g., soil moisture and temperature) and biotic effects (e.g., allelopathic suppression) may have caused these varying patterns. Designs such as ours that vary soil heterogeneity in three dimensions should be combined with additional measurements of the underlying mechanisms to further elucidate the factors governing seed germination.