Impact of altered nutrient balance on radiative forcing of a Mediterranean semi-arid savanna

Net radiative forcing (RF) of terrestrial ecosystems is controlled by changes in greenhouse gas fluxes (biogeochemical cycles) and albedo (biophysical properties). Semi-arid savannas, characterized by tree-grass coexistence, are highly sensitive to elevated nitrogen (N) deposition, which can alter biophysical and biogeochemical interactions on the land-atmosphere continuum. This study examines how altered N-to-phosphorus (P) ratios (simulated in a fertilization trial) affect RF at the top of the atmosphere (TOA) and surface temperature (Ts) at both the ecosystem and grass layer scales. We analyzed a long-term dataset (2014–2023) from three co-located eddy-covariance (EC) sites in a Mediterranean savanna in Spain: control (ES_LMa), N-added (ES_LM1 or NT; 16.9 ha), and N+P-added (ES_LM2 or NPT; 21.5 ha). Each site featured two enclosed-path EC systems at heights of 1.6 m and 15 m to capture grass and ecosystem-scale fluxes, respectively. Comparing between fertilized and control sites, we found net RF at TOA was dominated by change of albedo (± 98 %) over net ecosystem exchange (ΔNEE), with NT showing a stronger cooling effect (mean ± SD: -2.37 ± 1.52 Wm^-2) than NPT (-2.01 ± 1.82 Wm^-2). Interestingly, cooling effect that captured at TOA did not consistently correspond to Ts change (ΔTs) on the surface. At the ecosystem level, NT experienced cooler Ts (ΔTs = -0.41± 0.47 °C), whereas NPT had slightly warmer Ts (i.e., ΔTs = 0.03 ± 0.28 °C). At the grass layer, both fertilization treatments resulted in warming, with higher Ts observed for NPT (ΔTs = 0.80 ± 0.77 °C) than NT (ΔTs = 0.63 ± 0.46 °C). Surface conductance (Gs) patterns also diverged across scales, with NT showing the highest Gs at the ecosystem level, while NPT had the highest Gs at the grass layer. These findings emphasize differences in energy transfer processes across layers and highlight that N addition alone (without P) enhances tree canopy cooling capacity more effectively than combined N+P addition. Conversely, both treatments increased Ts at the grass layer, reshaping eco-physiological interactions in this water- and nutrient-limited ecosystem. Our results underscore the importance of nutrient stoichiometry in regulating biophysical and biogeochemical processes in semi-arid savannas, with implications for ecosystem management and climate modeling.