Positive feedback loop between earthworms, humus form and soil pH reinforces earthworm abundance in European forests

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Positive feedback loop between earthworms, humus form and soil pH reinforces earthworm abundance in European forests

1. In many terrestrial ecosystems earthworms operate at the interface between plants and soil. As ecosystem engineers, they affect key ecosystem functions such as decomposition, nutrient cycling and bioturbation. Their incidence and abundance depends on several soil properties, yet simultaneously they also impact soil properties themselves. The existence of a positive feedback loop in which earthworm activity maintains their own niche ‐ by promoting turnover rate in the forest floor, thereby increasing topsoil pH and creating suitable living conditions for themselves ‐ has been suggested before, yet lacks supporting evidence. 2. Here we assessed how tree species litter traits relate to such belowground interactions in forests across Europe. Specifically, we hypothesized a belowground feedback loop between burrowing earthworm biomass, humus form and pH, affected by litter quality. We tested this hypothesis by means of structural equation modelling. 3. Our results demonstrate that litter nutrient concentrations affect both burrowing and litter dwelling earthworm biomass, which in turn directly impact humus form and indirectly soil pH. At a continental scale, i.e. including all edaphic conditions, soil pH did not feed into earthworm biomass nor could we link leaf structural recalcitrance (e.g. lignin) or functional diversity to belowground interactions. 4. However, in forests where moisture is not limiting, soil acidity proved an important factor determining the context of belowground interactions. Therefore, we were able to confirm the hypothesized feedback loop for forest ecosystems with soil pH ≤ 5. In calcareous and/or periodically dry forests, other factors than soil chemistry and litter quality became determinant for earthworm biomass. 5. The activity of burrowing earthworms is pivotal in belowground ecosystem functioning of mesic forest soils, impacting litter accumulation and forest floor conditions aboveground, the pH and nutrient status belowground and ultimately their own living conditions. This highlights earthworm bioturbation as a key mechanism for understanding plant soil‐interactions in forests

The contribution of belowground processes to tree diversity effects on ecosystem functioning

Many studies found evidence for a positive relationship between biodiversity and ecosystem functioning in a large variety of systems. However, there is still debate about the mechanisms that drive these relationships. Insight in ecological linkages between above- and belowground components is critical to understand the functioning of terrestrial ecosystems. Recent observational and experimental research showed positive tree diversity effects on productivity and it has also been shown that tree diversity can improve the resistance of forests to pests. But whether the effects are driven by aboveground or belowground processes and through which mechanisms is largely unknown, especially at larger spatial scales. Moreover, it is known that the magnitude of effects and causal pathways might change along environmental gradients according to the ‘stress gradient hypothesis’. Changing climatic, biotic, and edaphic conditions and associated stress intensity at the continental scale, thus may distinctly affect biodiversity-functioning relationships and the driving mechanisms across forests. Furthermore, there is still much controversy regarding the potential role of diversity in reducing the vulnerability of forests to a changing climate. Here we explored the relative importance of above- and belowground mechanisms driving tree diversity effects on ecosystem functioning in European forests. The study was conducted across a network of permanent forest plots in Europe, established through the FunDivEUROPE project. The network was designed to test the effects of diversity on ecosystem functioning in mature European forests, spanning much of the continent’s bioclimatic gradient. To test whether the effect of tree diversity on ecosystem functioning is driven by above- or belowground processes, we modelled causal pathways using structural equation modelling. We explored the linkages for a set of ecosystem functions, for which diversity effects have been observed.status: publishe