Contrasting Soil Carbon, Nutrients, and Isotopic Signatures Across Tropical Rainforest Restoration Strategies on Former Pastures

ABSTRACT Conversion of tropical rainforest to pasture reduces soil carbon (C), nitrogen (N), and phosphorus (P) contents. Ecological restoration supports soil nutrient recovery, but the effectiveness of restoration strategies may differ, informing management and choice. We compared soil nutrient concentrations in 0–5 cm and 5–20 cm layers of a 13‐year‐old restoration experiment in Los Tuxtlas, Mexico, under natural succession and mixed‐species plantings, with pasture and reference forest as controls, and also examined altitudinal (182–256 m) variation, C 4 ‐derived C, and natural abundance of 15 N. Plantings with animal‐dispersed species had 92% higher C concentration in the upper soil than pasture, likely due to high fine root biomass and decomposition. In addition, animal‐dispersed plantings showed a 122% higher NO 3 − :NH 4 + ratio in deeper soil than natural succession, possibly due to greater animal N deposition and enhanced N mineralization and nitrification processes. Other nutrient comparisons placed the three restoration treatments generally between forest and pasture; among restoration treatments, no other differences had p < 0.10. No evidence of nutrient runoff was detected, despite the slope (182–256 masl). C 4 ‐derived C in the upper soil was around 5 times higher in animal‐ and wind‐plantings than in forest, reflecting grass‐to‐forest conversion. In natural succession plots, 15 N natural abundance correlated negatively with Fabaceae basal area, reflecting the legacy effects of leguminous tree species. Overall, differences among restoration strategies were few and minor, indicating that nutrient recovery alone cannot guide the choice between natural succession and mixed‐species plantings; broader ecological and practical objectives should instead shape decisions.