Climate and plants regulate the spatial variation in soil multifunctionality across a climatic gradient

A patchy distribution of soil resources is a characteristic of most natural terrestrial biomes, potentially resulting in spatial variation in multiple soil functions (soil multifunctionality). However, less is known about how soil multifunctionality and its spatial variability respond to increasing dryness across extensive climatic gradients, making it difficult to predict changes in ecosystem functions under climate change scenarios. We surveyed 150 sites along a 1500 km climatic gradient in eastern Australia, from humid forests to arid shrublands, to explore the spatial variation in soil multifunctionality with increasing aridity. We assessed four functional groups (carbon stocks, organic matter decomposition, plant structure, soil stability) of multifunctionality and average (net) multifunctionality across four vegetation patch types (tree, shrub, grass and unvegetated open interspaces). We then used average dissimilarity across these four patches as our measure of spatial variability. Our results showed that 1) net soil multifunctionality remained unchanged as aridity increased, because increases in soil stability and plant structure compensated for reductions in carbon stocks and organic matter decomposition; 2) the response of soil multifunctionality to increasing aridity differed among vegetation patch types, with the greatest increases in plant structure and reductions in carbon stocks in the open, but with marginal changes beneath trees; 3) variation in soil multifunctionality increased with increasing aridity and was driven by changes in climate (aridity, rainfall seasonality), soil (pH, sand) and to a lesser extent, variation in plant size, with impacts varying with the target functional group. Our study provides empirical evidence that soils can sustain an average level of multifunctionality across the climatic gradient by regulating the trade-offs between nutrient cycling and soil stability. Furthermore, our results demonstrate that forecasted increases in aridity will increase the spatial variation in soil multifunctionality and enhance the dominance of biocrusts, which would be critical for stabilizing soils under drier global climates.