Microbial communities are associated with indicators of soil surface condition across a continental gradient

Abstract

Soil microbes support multiple ecosystem functions in woody biomes and are regulated by both plant communities and soil properties. While most microbes reside in the uppermost soil layer, we have a poor understanding of how the condition of the soil surface affects soil microbes across different woody biomes. This knowledge is important because it could provide a cost-effective means of predicting the extent to which different soil surface types could be useful predictors of microbial communities in soil health monitoring programs. We collected data on soil bacterial and fungal communities, and 13 soil surface indicators and grazing intensity at 173 sites in eastern Australia along a climatic gradient (humid to arid) varying in tree cover. Our aim was to investigate potential relationships among the condition of the soil surface and soil microbes in forests and non-forests using linear regressions, correlations and structural equation modelling. We found that both bacterial and fungal richness were closely associated with measures of soil surface condition, particularly in forests. Greater microbial diversity was related to enhanced plant structure and richness, and lower litter characteristics in both ecosystems. In forests, these relationships were enhanced by higher temperatures or suppressed by greater tree richness. Bacterial and fungal communities responded differently to changes in soil surface attributes in forests, with winners (e.g., Proteobacteria) and losers (e.g., Cyanobacteria) among bacterial phyla, but consistent responses across fungal phyla. Our study provides empirical evidence of significant relationships among microbes and indicators of soil surface condition. This finding advances our understanding of plant-soil interactions, and indicates the possibility of using field-based soil surface attributes as potential cost-effective, practical, yet informative indicators to monitor changes in microbial richness and composition in forests and non-forests (open woodlands, shrublands).