Strong scale-dependent relationships between fine-root function and soil properties uncovered with spatially coupled sampling

Substantial fine-root trait variation is found at fine spatial scales but rarely linked to edaphic variation. We assessed the spatial scales of variation in fine-root traits and adjacent soils using a spatially coupled, nested sampling scheme along a fertility gradient in a seasonally dry tropical forest tree, Handroanthus ochraceus. We examined relationships among fine-root traits and identified edaphic drivers of fine-root function. We collected fine-root samples at three scales: multiple samples within individual trees (separated by > 1 m), among trees in a site (3–60 m) and across three sites (15–60 km). We quantified physiological, symbiotic, morphological, chemical and architectural traits, and paired soil physical and chemical properties. Fine-root traits and soils often varied most at fine spatial scales. Root arbuscular mycorrhizal colonization and phosphomonoesterase activity were coordinated and driven by coarse-scale heterogeneity in bulk density, magnesium and phosphate. The trade-off between large diameter and high specific root length, respiration rate and nitrogen concentration was driven by fine-scale heterogeneity in ammonium. The role of base cations was notable, with nitrogen and phosphorus being less influential than expected. Intraspecific fine-root responses to edaphic properties can occur at multiple spatial scales simultaneously and be detected when variation in both is properly captured and spatially matched.