Aboveground-belowground linkages across vegetation degradation gradients differ among native eucalypt communities

Native vegetation degradation impacts soil communities and their functions. However, these impacts are often studied by comparing soil biotic attributes across qualitatively defined, discrete degradation levels within a single plant community at a specific location. Direct quantification of the relationships between vegetation and soil attributes across continuous degradation gradients and at larger scales is rare but holds greater potential to reveal robust patterns in aboveground-belowground linkages that may apply across different plant communities. We investigated how native vegetation attributes relate to soil communities and their functions across a degradation gradient within three native temperate eucalypt woodland and forest communities that differed in soil nutrient availabilities. Across remnant patches of native vegetation in the Sydney Basin bioregion, we established plots representing different levels of decline in their vegetation quality (i.e., increased exotics and canopy changes) compared to relevant reference communities. In those plots, we assessed soil community groups (microbes and fauna), carbon (C) and nutrient cycling (litter decomposition, enzyme activity, and phosphate and nitrate accumulation rates), soil pH, texture and vegetation attributes (composition, structure, and function). Our unique study design revealed that the relationships between vegetation degradation and soil biota across the food web (i.e., AM fungi, Fungi:bacteria ratio, Gram-positive bacteria, total nematodes) were highly dependent on the plant community. However, the degradation impacts on soil functions (i.e., total enzyme activity, and phosphate availability) were mostly consistent, suggesting their potential as belowground indicators of ecosystem degradation, with a notable positive association observed in phosphate availability rates. Additionally, the effects of vegetation degradation on soil biota and their functions appeared stronger in the nutrient-poor plant communities, suggesting greater vulnerability of their belowground components. Our findings call for caution when generalizing belowground responses to degradation and for further research on how nutrient availability mediates the impacts of degradation on aboveground-belowground linkages.