Non‐Abrupt Vegetation Changes due to Altered Nutrient Balance Make Complex Scale‐Dependent Warming and Cooling Effects

Land-atmosphere exchanges are mediated by biophysical properties (e.g., albedo change, evaporative cooling) and biogeochemical cycle (e.g., CO₂ fluxes), with both processes exerting global feedback as radiative forcing ( RF $$ RF $$ ). While most research on RF $$ RF $$ concentrated on the impact of abrupt vegetation changes, this study investigates the effects on non-abrupt changes due to altered nutrient levels (i.e., nitrogen [ N N ] and phosphorus [ P P ] deposition). We examined impacts of these changes by assessing RF $$ RF $$ , representing global effects, and linked it with surface temperature ( Ts $$ Ts $$ ), reflecting local influence. We hypothesized there are scale-dependent warming and cooling effects due to surface-atmosphere interactions. We explored this question using a 9-year dataset (2014-2023) from a large-scale nutrient manipulation experiment in a semi-arid savanna, Spain. Three co-located eddy-covariance sites are established: control, N N -added ( NT $$ NT $$ ), and N + P $$ N+P $$ -added ( NPT $$ NPT $$ ). The results indicate domination of changes in surface albedo over CO₂ fluxes, producing paradoxical effects: a net cooling at global scale ( RF $$ RF $$ differences are [mean ± SD]-0.46 ± 0.08 W m⁻² [global] m⁻² [surface] at NT $$ NT $$ and -0.39 ± 0.09 W m⁻² m⁻² at NPT $$ NPT $$ ) due to higher surface reflectivity, but localized warming at understory ( Ts $$ Ts $$ differences are 0.63°C ± 0.46°C at NT $$ NT $$ and 0.80°C ± 0.77°C at NPT $$ NPT $$ ) driven by shifts in energy partitioning. Furthermore, our findings indicate that N N -only addition has more canopy-level Ts $$ Ts $$ cooling than N + P $$ N+P $$ treatment, although Ts $$ Ts $$ increases at the understory. These contrasting responses reveal a layered and scale-dependent interplay of surface-atmosphere interactions. They highlight the critical role of nutrient stoichiometry in shaping climate feedbacks despite the vegetation changes are not abrupt, and emphasize that what cools the globe may still warm the land beneath our feet.