Ontogenetic shifts in alfalfa root distribution and morphology drive soil water depletion and productivity decline in Loess Plateau pastures

Alfalfa (Medicago sativa) pasture sustainability in China’s Loess Plateau faces critical challenges from water scarcity and productivity decline in older stands. This two-year study (2020–2021) examined root distribution and morphology, biomass allocation, and soil water storage (SWS) dynamics across eight alfalfa stands aged 3–19 years (M3-M19), with annual cropland as control. Key findings revealed distinct ontogenetic shifts: in younger stands (<10 years, M10−), fine roots (Root length density = 0.18 cm cm⁻³) predominated in shallow layers (0–2 m), constituting 92.5 % of root systems, while coarse roots increased to 15.5 % in older stands (≥10 years, M10+). Notable vertical redistribution occurred in M10+, with fine root density increasing 62.5 % in deep soil layers (5–10 m) compared to M10−. Soil water deficit reached critical levels (740.1–822.2 mm in 0–10 m profile), with older stands exhibiting 57.5 % greater deep soil layer water depletion. Productivity declined sharply in M10+, showing 46.9 % lower aboveground biomass (peak yield at M6) despite stable belowground biomass. Multivariate analysis identified significant (p < 0.01) correlations between root traits (fine/coarse root ratio) and water depletion patterns, modulated by stand age and soil depth. These results demonstrate that stand rejuvenation before six years optimizes water-use efficiency and yield stability in this fragile ecosystem.