Belowground and aboveground consequences of interactions between live plant species mixtures and dead organic substrate mixtures

In all terrestrial ecosystems, plant‐derived resources mainly enter the belowground subsystem in two ways – as dead plant parts (i.e. organic substrates) and as resources released from the rhizosphere of live plants. While the effects of identity of live plants or dead plant parts on aboveground and belowground properties have been well studied, much remains unknown about the effects of combinations of live plant species and dead plant‐derived substrates. In particular, no study to date has experimentally investigated the effects of simultaneously varying the composition or diversity of both live plants and dead plant parts. We performed a pot experiment consisting of a full factorial of 12 organic substrate addition treatments by eight live plant treatments, in which the composition and diversity of both substrates and live plants were varied. The 12 substrate treatments consisted of no substrates added, each of the four substrates represented singly (four treatments), each of the possible pairwise combinations of the four substrates (six treatments), and all four substrates added together. As expected, substrate and live plant identity both had important effects both above‐ and belowground. Mixtures of plant species always had effects that could be explained by the effects of the component plant species grown singly. Usually mixtures of substrates also had effects that could be predicted by the effects of component substrates added singly, but there were instances in which mixtures of certain substrates (notably mixtures containing Populus tremula leaf litter) had synergistic effects both belowground and aboveground, with especially strong effects on the growth of Pinus sylvestris seedlings. However, there was never any evidence of interactive effects between live plant species diversity or composition and added substrate diversity or composition, either aboveground or belowground. This means that whatever effects added substrates have on ecological properties operates independently of the influence of live plants, and vice versa. In total, this study shows that most resources entering the belowground subsystem have effects that are independent of the effects of other resources, though there are isolated instances in which strong (and potentially ecologically meaningful) synergistic interactions may occur for specific combinations of dead organic substrates.