Novel Insights Into the Role of Litter‐Soil Continuum for Soil and Water Conservation: A Mechanism‐Based Study of Mixed‐Species Eucalyptus Plantations

ABSTRACT Large‐scale eucalyptus ( Eucalyptus spp.) monocultures in southern subtropical China contribute to land degradation, characterized by severe soil erosion and hydrological imbalance. This study introduces the “litter–soil continuum” framework, treating the litter layer and mineral soil as a coupled system, to assess how converting monocultures to mixed‐species plantations enhances water retention and erosion resistance. We compared four mixed‐species systems: Eucalyptus mixed with Erythrophleum fordii , Dalbergia odorifera , Parashorea chinensis , and Castanopsis hystrix , with Eucalyptus monoculture under uniform site conditions. The study covered an area of 10 ha with 30 systematically sampled plots (100 m 2 each) established along an elevation gradient. Water conservation of the litter−soil continuum was evaluated using a coordinate‐based method, while soil aggregate stability was measured by the percentage of aggregate destruction (PAD) to assess erosion resistance. Mixed plantations, especially those with E. fordii and C. hystrix , showed significantly higher litter layer thickness and soil aggregate stability ( p < 0.05). Compared with monocultures, water‐holding capacity increased by 20% and 30%, and erosion resistance increased by 71% and 78%, respectively ( p < 0.05). Mantel tests and structural equation modeling also demonstrated that the semi‐decomposed litter layer serves as a critical interface linking litter and soil, playing a pivotal role in enhancing water storage capacity and aggregate stability. These findings provide mechanistic evidence that Eucalyptus plantations mixed with E. fordii and C. hystrix efficiently rehabilitate surface hydrological processes and enhance erosion resistance in degraded plantation ecosystems, suggesting that these species should be prioritized when establishing mixed‐species Eucalyptus plantations in erosion‐sensitive subtropical regions.