Influence of the root–soil complex on soil infiltration stages and their temporal changes in Cunninghamia lanceolata plantations

• High fine root density inhibits soil infiltration before the steady infiltration stage. • Soil organic matter promoted infiltration in the slow stage. • Plugging effect of fine root was weaken in the 15-years-old plantation. As the intensive monoculture plantations continue to expend, their water demand of these plantations is concurrently rising. Investigating the soil infiltration processes in plantations is essential for developing informed water resource management and appropriate ecological management practices. Field experiments were carried out to examine variations within soil infiltration among different-aged Cunninghamia lanceolata plantations (5-, 8-, 11-, and 15-year-old) using an improved surface double-ring infiltration infiltrometer. Results showed that soil infiltration rates improved as the forest matures. During the rapid infiltration phases, including initial infiltration rate , early-stage infiltration rate and mid-stage infiltration rate , the rates varied between 3374 to 10196 mm·h −1 . In the slow infiltration phases, including late-stage infiltration rate, steady infiltration rate , the rates ranged from 1022 to 3195 mm·h −1 . Soil structural indicators (clay and silt contents) were the primary physical factors responsible for differences in entire infiltration process across different plantation ages: increased clay content and decreased silt content enhanced soil infiltration. Furthermore, the effects of biochemical factors — soil organic matter and fine root density (FRD) — on infiltration rate exhibited significant stage specificity. Before the stable infiltration stage, FRD was the main factor inhibiting infiltration, and this inhibiting effect was evident in initial infiltration rate, early-stage infiltration rate, mid-stage infiltration rate, and late-stage infiltration rate. In contrast, soil organic matter showed a promoting effect on infiltration during the slow infiltration phase, with a particularly prominent enhancement on steady infiltration rate. The inhibiting effect of FRD decreased in the 15-year-old C. lanceolata plantation and notably increased in its infiltration rate. Consequently, to enhance soil infiltration, the rotation period for C. lanceolata plantations should ideally be extended beyond 15 years. These findings offer scientific foundations for promoting sustainable development and water conservation in subtropical plantations.