Evaluating the impact of elevated CO2 on the hydrological cycle and carbon budgets in the loess plateau

Loess Plateau (LP), China Understanding the impact of elevated CO₂ (eCO2) concentrations on water-carbon interactions is critical for informing climate mitigation strategies in semi-arid regions susceptible to hydrological stress. Although CO₂ fertilization effects on terrestrial processes are well documented, existing models often oversimplify land cover heterogeneity and neglect region-specific feedbacks, limiting their applicability to complex landscapes like the LP. This study addresses these limitations by employing the Community Land Model (CLM5), which represents diverse land types, including forests, grasslands, and croplands (such as corn and wheat), and is validated against flux tower and remote sensing data. This work reveals that eCO2 nearly doubles gross primary productivity by 2.78 gC/m²/yr, while suppressing evapotranspiration by 1.42 mm/yr and increasing baseflow by 1.09 mm/yr. Despite vegetation carbon gains (∼19.16 gC/m²/yr), soil organic carbon exhibited a continued decline at 6.01 gC/m²/yr, suggesting slower soil carbon recovery. Water use efficiency (WUE) improved disproportionately, with forests exhibiting a 50 % increase due to deep-rooted systems and stomatal optimization, and croplands showing only a 10 % improvement, likely constrained by irrigation practices. Spatial heterogeneity emerged as a dominant control, with the marginal gain in WUE per unit increase in CO₂ (WUE/CO₂ ratio) declined by 14 % over two decades, highlighting water constraints in semi-arid ecosystems. By integrating land cover-specific responses, this study advances the homogenized CO₂ representations in land surface models and offers actionable insights for targeting afforestation in ecological restoration zones. These findings highlight the necessity of resolving spatial heterogeneity in Earth system models to enhance climate adaptation strategies and optimize water-carbon synergies under escalating CO₂ scenarios. • Elevated CO₂ boosts GPP, reduces ET, with responses modulated by land cover. • Despite vegetation carbon gains, SOC declines persist due to high respiration. • WUE/CO₂ ratio drops, highlighting water constraints on diminishing returns.