Seasonal decoupling of water supply and CO2 fluxes of urban greening shrubs is driven by leaf area index.

Urbanization exacerbates water scarcity, necessitating the adoption of deficit irrigation (DI) strategies to balance water conservation and the carbon sink function of greening plants. This study quantified the seasonal responses of CO2 fluxes (net ecosystem exchange, NEE; gross ecosystem productivity, GEP) in three common urban shrub species (Euonymus japonicus, Berberis thunbergii, Ligustrum × vicaryi) under four irrigation treatments (sufficient irrigation, SI; deficit irrigation, DI; natural rainfall, NR; and drought, DT) during spring (April 17-June 17) and summer (June 18-August 27) in Beijing, China. Our results revealed distinct seasonal dynamics of CO2 fluxes: In late spring, sufficient irrigation (SI) enhanced shrub carbon sequestration capacity to the maximum extent, with gross ecosystem productivity (GEP) ranging from 23.34 to 40.25 μmol/m2/s. During this period, shrub CO2 fluxes exhibit a more rapid response to variations in water availability. Deficit irrigation (DI) induced only negligible differences in NEE and GEP compared with sufficient irrigation (SI), except in Ligustrum × vicaryi, and summer shrub CO2 fluxes show insensitivity to irrigation levels (p > 0.05), and demonstrated nonlinear saturation responses when soil water content exceeded 0.35-0.40 m3/m3. Leaf area index (LAI) emerged as a key driver, explaining 55% and 38% of such variability in spring and summer, respectively. Additionally, air temperature and soil moisture regulated the seasonal differences in CO2 fluxes. These findings emphasize that irrigation in spring plays a critical role in enhancing the carbon sink capacity of shrubs, while summer irrigation can be reduced by 50% without significantly compromising carbon sink function. Optimizing seasonal irrigation strategies based on these quantitative thresholds can reconcile urban water conservation with carbon sequestration objectives in water-stressed regions.