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

## TL;DR

This study shows that changes in nutrient levels, like nitrogen and phosphorus, can cause complex warming and cooling effects at different scales in semi-arid ecosystems.

## Contribution

The study reveals scale-dependent climate feedbacks from non-abrupt vegetation changes due to nutrient imbalances, emphasizing the role of nutrient stoichiometry.

## Key findings

- Surface albedo changes dominate CO2 fluxes, causing global cooling but local warming in semi-arid savannas.
- Nitrogen-only addition leads to more canopy cooling than nitrogen plus phosphorus treatment.
- Nutrient stoichiometry significantly influences climate feedbacks despite non-abrupt vegetation changes.

## Abstract

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). While most research on 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] and phosphorus [P] deposition). We examined impacts of these changes by assessing RF, representing global effects, and linked it with surface temperature (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‐added (NT), and N+P‐added (NPT). The results indicate domination of changes in surface albedo over CO2 fluxes, producing paradoxical effects: a net cooling at global scale (RF differences are [mean ± SD]—0.46 ± 0.08 W m−2 [global] m−2 [surface] at NT and −0.39 ± 0.09 W m−2 m−2 at NPT) due to higher surface reflectivity, but localized warming at understory (Ts differences are 0.63°C ± 0.46°C at NT and 0.80°C ± 0.77°C at NPT) driven by shifts in energy partitioning. Furthermore, our findings indicate that N‐only addition has more canopy‐level Ts cooling than N+P treatment, although 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.

Semi‐arid ecosystems are particularly vulnerable to elevated N deposition. However, there is still a critical gap in understanding the impact of N:P imbalance by considering biophysical changes and biogeochemical cycle. This study presents the complex scale‐dependent warming and cooling effects due to altered nutrient balance on the Mediterranean semi‐arid ecosystem. The arrows represent the values being higher or lower at the nutrient addition than the control sites, and asterisk marks indicate the differences are significant across whole seasons. 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 feedback despite the vegetation changes not being abrupt, and emphasize that what cools the globe may still warm the land beneath our feet.