Depth Profiles of C:N:P Stoichiometry and Land-Use Nutrient Contrasts in Basalt-Derived Soils of Northern Chengmai, Hainan Island

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Abstract
Soil C:N:P stoichiometry reflects biological inputs, land management, and mineral weathering, but how these controls vary with depth remains unclear in tropical basaltic landscapes. We analyzed 500 soil profiles sampled at 0–20, 80–100, and 180–200 cm in northern Chengmai County, Hainan Island, to test directional predictions about weathering–carbon associations, surface enrichment, land-use contrasts, and nutrient covariation. Mean Chemical Index of Alteration (CIA) was high at all depths (92.8–93.7), while molar SiO2/Al2O3 decreased from 14.0 to 10.5. Surface organic carbon (Corg) was positively associated with CIA, Al2O3, and TFe2O3. From surface to deep soil, Corg, total nitrogen (TN), and total phosphorus (TP) decreased by 68.4%, 64.4%, and 36.3%, respectively. Within-profile enrichment factors were greater for Corg and TN than TP (Holm-adjusted p < 0.001), accompanying larger declines in C:P and N:P than C:N. Fruit orchards, dryland, and paddy fields had higher mean surface TP and lower C:P and N:P than grassland and forest land. Corg differences among land uses disappeared at 180–200 cm. Corg–TP and TN–TP associations weakened with depth, whereas TP remained positively associated with Fe- and Al-rich components. These results support unequal biological and mineral influences on nutrient depth patterns.
Funding Information
  • National Nonprofit Institute Research (AS2023P01)
  • National Nonprofit Institute Research (AS2025P03)
  • Science and Technology Project of the Jiangsu Bureau of Geology (2024KJ07)
  • Geological Survey Program of China (DD20190305)

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