Rubber plant root properties induce contrasting soil aggregate stability through cohesive force and reduced land degradation risk in southern China

Abstract. In southern China, Hainan Island faces land degradation risks due to poor soil physical properties, such as a high proportion of microaggregates (< 0.25 mm), low soil organic matter (SOM) content, and frequent uneven rainfall. The cohesive force between soil particles, which is influenced by plant root properties and root-derived SOM, is essential for improving soil aggregate stability and mitigating land degradation. However, the mechanisms by which rubber root properties and root-derived SOM affect soil aggregate stability through cohesive forces in tropical regions remain unclear. This study compared rubber plants of varying ages to assess the effects of root properties and root-derived SOM on soil aggregate stability and cohesive forces. Older rubber plants (> 11-years-old) showed greater root diameters (RD) (0.81–0.91 mm), higher root length (RL) densities (1.83–2.70 cm/cm³), and increased proportions of fine (0.2–0.5 mm) and medium (0.5–1 mm) roots, leading to higher SOM due to lower lignin and higher cellulose contents. Older plants exhibited higher soil cohesion, with significant correlations among root characteristics, SOM, and cohesive force, whereas the random forest (RF) model identified aggregates (> 0.25 mm), root properties, SOM, and cohesive force as the key factors influencing mean weight diameter (MWD) and geometric mean diameter (GMD). Furthermore, partial least squares-path models (PLS-PM) showed that the RL density (RLD) directly influenced SOM (path coefficient 0.70) and root-free cohesive force (RFCF) (path coefficient 0.30), which in turn affected the MWD, with additional direct RLD effects on the SOM (path coefficient 0.45) and MWD (path coefficient 0.64) in the surface soil. Cohesive force in rubber plants of different ages increased macroaggregates (> 0.25 mm) and decreased microaggregates (< 0.25 mm), with topsoil average MWD following the order: CK (0.98 mm) < 5Y_RF (1.26 mm) < MF (1.31 mm) < 11Y_RF (1.36 mm) < 27Y_RF (1.48 mm) < 20Y_RF (1.51 mm). Rubber plant root properties enhance soil aggregate stability and reduce the land degradation risk in tropical regions.