Soil texture and depth differentially regulate total and available nutrient stoichiometry in Yilong Lake basin agroecosystem, China

The stoichiometric ratios of soil available nutrients serve as a pivotal indicator for assessing the sustainability of farmland and nutrient cycling efficiency within fragile ecosystems. Soil texture and depth are significant factors influencing nutrient availability, however, their regulatory mechanisms within highland lakeshore agricultural systems remain poorly understood. This study, conducted in the Yilong Lake watershed of the low-latitude plateau in southwest China, collected 216 soil samples. It innovatively determined both total organic carbon, nitrogen, and phosphorus (SOC, TN, TP) and available carbon, nitrogen, and phosphorus (DOC, AN, AP) simultaneously. Path analysis was employed to examine the direct and indirect associations between soil texture (Clay, Loam, Clay Loam, Sand) and depth (0–20 cm, 20–40 cm, 40–60 cm) on nutrient ratio measurements. Findings indicate: (1) Distinct nutrient limitation and leaching risk patterns emerged across soil textures: Loam exhibited strongest carbon and nitrogen sequestration capacity; Although Clay Loam exhibits reduced phosphorus availability due to iron and aluminium oxide immobilisation, it demonstrates pronounced nitrogen limitation rather than phosphorus limitation; Sand though rich in available phosphorus, had the lowest C:N, suggesting heightened carbon and nitrogen leaching risks. (2) Soil depth significantly altered nutrient ratios: topsoil (0–20 cm) showed marked nutrient enrichment, while subsoil (40–60 cm) exhibited elevated TN:TP, suggesting that nitrification may have intensified with increasing depth. (3) Path analysis revealed that texture directly regulates nutrient measurement ratios (DOC:AN) and indirectly governs C:N:P balance by influencing SOC, AP, and AN content; depth directly or indirectly affects measurement relationships through pathways such as TP. This study elucidates the key regulatory roles of soil texture and depth in nutrient ratios, particularly available nutrient balance, providing direct scientific evidence for implementing differentiated, precision nutrient management based on soil physical properties in Yilong Lake basin agroecosystems and other ecologically fragile areas.