Weather types and soil moisture modulate surface energy partitioning in a subtropical CAM pineapple field

Surface energy partitioning governs canopy thermal conditions and water use by determining how available energy is dissipated as latent versus sensible heat. In tropical and subtropical croplands, compound heat-drought events increase atmospheric evaporative demand, yet crassulacean acid metabolism (CAM) crops may respond differently from C₃/C₄ systems because daytime stomatal regulation constrains transpiration. Here we quantify subdaily energy partitioning in a subtropical CAM pineapple field in southern China using Bowen-ratio energy balance observations. Net radiation (R_n ), soil heat flux (G), and vertical gradients of temperature and vapor pressure were used to estimate sensible (H) and latent (LE) heat fluxes and to compute available energy (A = R_n - G). To diagnose the coupling relationship between atmospheric demand and water supply, hourly data were classified into four weather types (WT4) using median thresholds of R_n and vapor pressure deficit (VPD): LR_n-LVPD (low R_n , low VPD), LR_n-HVPD (low R_n , high VPD), HR_n-LVPD (high R_n , low VPD), and HR_n-HVPD (high R_n , high VPD). Soil water content (SWC) at a depth of 20 cm was further classified into three soil moisture states (SWC3) using percentiles (Dry, Normal, and Wet). Scenario medians and valid-hour counts were used to summarize LE/A, H/A, and G/Rn, complemented by Bowen ratio (β = H/LE) as an integrative indicator of sensible versus latent heat partitioning. The results showed that energy partitioning was dominated by compensation between LE/A and H/A, while G/R_n remained small. LE/A was consistently lower under HR_n-HVPD than under LR_n-LVPD across soil-moisture states. Under LR_n-LVPD, LE/A was 0.47 in Dry and Normal and increased to 0.56 in Wet; under HR_n-HVPD it ranged from 0.26 (Dry) to 0.37 (Wet). Moisture effects were demand dependent: within HR_n-HVPD, LE/A increased by 0.11 from Dry to Wet, whereas the LR_n-LVPD Dry-to-Wet increase was 0.09. β responses reinforced this interaction. Under Dry conditions, β increased with VPD and remained high at high VPD, whereas under wet soils β was lower and tended to level off. Under HR_n-HVPD, β declined steeply as SWC increased over 0.23-0.27 m³ m^-3, then approached a plateau near 1.7; under non-HR_n-HVPD conditions β showed weaker dependence and plateaued near 2.0. Composite daytime patterns further showed sustained H dominance under HR_n-HVPD-Dry, while HR_n-HVPD-Wet shifted partitioning toward higher LE. These results demonstrate that atmospheric demand sets a strong constraint on daytime evaporative cooling in this CAM pineapple system, and that soil water supply enhances latent heat dissipation most effectively under high-demand conditions, thereby informing model parameterization and targeted water management.