Rubber Plantation Age Affects Soil Aggregate Stability, Organic Carbon, and Nitrogen Distribution in Hainan Island, China

Soil aggregate stability is influenced by the soil organic carbon (SOC) and total nitrogen (TN) content. However, the mechanisms by which the age of rubber plantations (RPs) affects soil aggregate stability and aggregate-associated C and N contents remain inadequately understood. Soil samples were collected from four RPs (10-, 20-, 30-, and 40-year-old) at depths of 0–20 cm and 20–40 cm, respectively. The water-stable aggregates (WSAs) were divided into five size fractions of > 2, 1–2, 0.5–1, 0.25–0.5, and < 0.25 mm by a standard wet-sieving technique, and their C and N concentrations were ascertained. The soil was dominated by the < 0.25 mm WSAs fraction, the proportion of which decreased with plantation age, contrary to that of the > 2 mm WSAs. Aggregate distribution was significantly affected by particle size, plantation age, and particle size interactions. Mean weight diameter (MWD) and geometric mean diameter (GMD) were directly proportional to plantation age but inversely to soil depth. The WSA0.25, MWD, and GMD were the highest in 40- but the lowest in the 10-year-old RPs. The aggregate-associated C and N concentrations positively correlated with the plantation age; the C and N stock profile of the soil at a depth of 0–40 cm in 30-year-old RPs was the highest. The pH, soil porosity, and C/N ratio positive correlated with both aggregate stability and distribution. The stability and associated with C and N stocks of the aggregates, and soil pH enhanced, but soil bulk density reduced with plantation age. This trend suggested that mature RPs play an essential ecological role in preventing soil degradation and maintaining soil C and N sinks during long-term rubber cultivation.