Stable isotopes as integrators of soil biogeochemistry and tree growth across contrasting hardwood forest soils

Abstract

Abstract Background & aims Forest productivity and resilience are influenced by soil conditions; however, the mechanisms linking soil properties, isotopic signatures, and tree growth remain incompletely understood. This study evaluated whether soil carbon (δ 13 C) and nitrogen (δ 15 N) stable isotopes were related to differences in hardwood growth across contrasting plantation environments. Methods We examined the growth, soil physicochemical properties (texture and nutrient availability), and stable-isotopic composition of Juglans nigra (black walnut) and Quercus rubra (northern red oak) across three Midwestern plantations using isotope-ratio mass spectrometry of A-horizon soil and Random Forests to identify soil factors linked to growth and isotopic signatures. Results Tree growth varied across species and sites and was driven by soil texture, nutrient availability, and dual isotope signatures (δ 13 C and δ 15 N) along with soil physicochemical properties (organic carbon, pH, cation exchange capacity (CEC), extractable nutrient concentrations). Plantations with higher silt content (Indiana) generally supported higher growth than those with coarser soil textures (Michigan). Higher soil δ 13 C values were reflected in lower growth in J. nigra . Q. rubra growth was more strongly associated with soil phosphorus and CEC. δ 15 N corresponded with growth in both species and reflected shifts in nitrogen availability, microbial processing, and nutrient requirements. Random Forest identified soil texture, base cations, and pH as primary drivers of isotopic responses. Conclusions These results indicate that stable isotopes capture physiological responses and soil–tree feedbacks and highlight the value of δ 13 C and δ 15 N for monitoring soil–tree interactions and guiding site selection and sustainable management across heterogeneous forest landscapes.