Effects of mixed plant root systems on soil water retention in the Loess Plateau

Plant roots play a critical role in improving soil water retention by regulating soil structure and pore characteristics. Although the effects of monoculture root systems on soil hydrological properties have received considerable attention, natural succession drives vegetation from monoculture to more structurally stable mixed-species patterns, resulting in more complex root systems. However, the mechanisms by which mixed root systems affect soil water retention remain poorly understood. This study examined fibrous and tap root systems of herbaceous plants under three mixing ratios: 1:3 (tap root dominant), 2:2 (equal proportions of fibrous and tap roots), and 3:1 (fibrous root dominant), with bare land as a control. The effects of root systems on soil initial water content (IWC) and saturated water content (SWC) under different mixing patterns were analyzed. The results showed that mixed root systems significantly improved soil water retention, with the 2:2 mixing ratio exhibiting the most pronounced effects. Under this ratio, IWC and SWC increased by 58.4% and 39.6%, respectively, compared to bare land. With increasing root density, IWC and SWC first increased and then decreased, with a critical root length density of 10.4 cm/cm³. Within the mixed system, fibrous roots (IWC: 12.2%; SWC: 16.0%) had stronger effects on soil water retention than tap roots (IWC: 1.5%; SWC: 11.7%). Root systems primarily influence soil water retention indirectly by regulating soil organic matter rather than through direct effects. This study provides theoretical support for understanding infiltration mechanisms of vegetation in the Loess Plateau region.