Soil development constrains N availability through regulating N cycling in subtropical karst regions

Soil development profoundly shapes the physical, chemical, and biological properties of soils, yet its role in regulating nitrogen (N) cycling and shaping ecological succession remains underexplored. This gap is especially critical for large-scale ecological restoration, where similar climatic and geologic backgrounds can yield divergent outcomes. In this study, we investigated how varying degrees of soil development affect soil N availability and plant community succession after 60 years of natural recovery. Field surveys revealed that sites with moderately weathered soils transitioned to woodlands, while highly weathered soils remained as shrublands. Using foliar stoichiometry and a dual-¹⁵N isotope tracing approach, we found that plant communities in moderately weathered soils had largely overcome N limitation, reflected by higher N/P ratios, foliar N content, and δ¹⁵N values. In contrast, plant communities in highly weathered soils exhibited persistent N limitation, characterized by lower N/P ratios, foliar N content, and δ¹⁵N values. These differences were linked to inherent soil N transformation processes. In moderately weathered soils, both mineralization and autotrophic nitrification rates increased substantially, resulting in elevated ammonium (NH₄⁺) and nitrate (NO₃^-) concentrations. However, in highly weathered soils, although mineralization rates increased, much of the newly produced NH₄⁺ was immobilized by microbes, suppressing nitrification and limiting NO₃^- production. Our findings highlight that soil development degree is a critical determinant of plant succession trajectories through its regulation of inherent soil N cycling. Restoration goals and species selection strategies should be tailored to soil development status to optimize ecological restoration outcomes.