# Global Evidence of the Unimodal Response of Ecosystem Respiration to Soil Moisture

## TL;DR

This study shows that ecosystem respiration peaks at a specific soil moisture level and then declines, which could improve predictions of carbon emissions under climate change.

## Contribution

The paper provides global evidence of a unimodal soil moisture response in ecosystem respiration and demonstrates its adaptation to water availability.

## Key findings

- 106 out of 135 sites showed a unimodal ER-soil moisture relationship with an optimal soil moisture level.
- Drier sites had lower optimal soil moisture for respiration, indicating adaptation to local water conditions.
- A field experiment confirmed that long-term water deficit shifts the optimal soil moisture level downward.

## Abstract

Ecosystem respiration (ER) is the largest flux of carbon from land to the atmosphere and is strongly influenced by soil moisture. Nevertheless, the response of ER to soil moisture remains poorly understood. Here, the ER‐soil moisture response curves at 135 sites from the global FLUXNET are analyzed. In contrast to the typically assumed monotonic increase in respiratory carbon emissions with increasing soil moisture derived from land surface models, the study shows that 106 sites exhibit a unimodal soil moisture response with a peak in ER at an apparent optimum soil moisture (SMER opt), implying a prevalent inhibitory effect of soil moisture excess on ER. Among the 12 biotic and abiotic variables examined, the SMER opt is mostly related to local water availability, with drier sites showing lower SMER opt than wetter sites, possibly reflecting water adaptation of ER. This adaptation is further supported by a field experiment that exclusively manipulates water and holds other factors constant, showing a downward shift in SMER opt after long‐term water deficit. These results reveal the large variation of SMER opt and the water adaptation of ER, providing novel insights for understanding and forecasting carbon‐climate feedbacks.

Using global eddy covariance observations and a field experiment, this paper provides compelling evidence for the widespread unimodal relationship between soil moisture and ecosystem respiration and its water adaptation. Such knowledge facilitates understanding of soil moisture in affecting respiratory carbon emissions and has the potential to improve the predictive power of global carbon cycling in the context of climate change.