Whole-life evolution of soil aggregate structure and nutrient retention function in terraced citrus orchards

Extensive transformation of slopes into terraces, coupled with intensive agricultural practices, has markedly modified soil structure and nutrient balance in orchard ecosystems. Nevertheless, the evolution of soil aggregates and associated nutrient dynamics remains poorly understood. Soil samples were collected from citrus orchards of various ages (5-15-25-35-45 a) and soil depths (0-20 cm, 20-40 cm, 40-60 cm) to analyze the distribution and stoichiometry of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) within water-stable aggregates: macroaggregates (>0.25 mm), microaggregates (0.25-0.053 mm), and silt-clay particles (<0.053 mm). Results revealed that macroaggregates dominate, accounting for 79.64% of the entire soil. The stability of aggregates in the 0-20 cm layer first increased, then decreased, and increased again with orchard age, while a steady increase was noted in the 40-60 cm layer. As the planting years increased, the contents of SOC, TN, and TP in all size aggregates exhibited a general upward trend. Among them, the contents of SOC and TN in macroaggregates are significantly higher than those in microaggregates and silt-clay particles, whereas TP is more evenly distributed across different particle size aggregates. Stoichiometric analysis revealed that the C/N ratio remained relatively stable across all aggregate sizes, whereas the C/P and N/P ratios exhibited a decreasing trend over time, indicating a gradual weakening of phosphorus limitation in the soil. Under the while-life perspective, structural stability changes in soil aggregates can be attributed to soil erosion in young periods, frequent disturbances and fertilization during peak fruiting periods, and natural recovery in the declining periods. However, the nutrients stored within water-stable aggregates exhibit a lagged response to these structural changes. Optimizing terrace methods and human management practices in terraced orchards is crucial for synergistically enhancing both the structural stability of aggregates and the capacity for nutrient storage.