# Widespread Higher Soil Respiration Rates at Nighttime Than Daytime Across Global Forest Ecosystems

## TL;DR

Soil respiration rates are often higher at night than during the day in forests, which could affect predictions of carbon emissions and climate change.

## Contribution

Discovery of widespread higher nighttime soil respiration rates in forests and their implications for climate models.

## Key findings

- Nighttime soil respiration rates are higher than daytime rates across global forest ecosystems.
- Using daytime temperature relationships underestimates nighttime soil respiration by up to 28.7%.
- Current climate models likely underestimate nighttime root respiration dynamics.

## Abstract

Soil respiration (R
s) is the second largest terrestrial carbon flux and therefore its temporal dynamics exert a significant influence on the soil carbon budget. While the seasonal and annual dynamics of R
s and its temperature sensitivity have been well documented, the diel R
s dynamics remain poorly understood. Earth system models (ESMs) typically assume a constant temperature response of R
s over the diel cycle, thereby predicting lower R
s at night than during the warmer daytime. Here, by analyzing extensive in situ R
s datasets from 36 global forest sites, we reveal an unexpectedly widespread pattern of higher nighttime than daytime R
s, which is likely driven by the hourly temporal lag between recent photosynthetic assimilation and R
s associated with the transportation of recent photosynthates to the roots. Moreover, applying daytime R
s‐temperature relationships systematically underestimates nighttime R
s by 2.5% to 28.7% across 31 sites, due to the significant diel difference in the temperature response of R
s. However, ESMs predict lower R
s at night than during the day, largely resulting from the significant underestimation of nighttime root respiration dynamics. Our findings demonstrate significant diel R
s patterns across forest ecosystems, suggesting that daytime and nighttime R
s may respond distinctly to future climatic changes. Incorporating these diel dynamics is essential for improving predictions of terrestrial carbon‐climate feedbacks under global warming.

Soil respiration is the second largest carbon flux between terrestrial ecosystems and the atmosphere, yet its diel dynamics remain poorly understood. By synthesizing continuous, in situ soil respiration measurements from forests worldwide, we reveal widespread higher soil respiration rates at night than during the day. The diel difference in the temperature response of soil respiration leads to up to a 28.7% underestimation of nighttime soil respiration across sites when using daytime‐based relationships. Our findings suggest that daytime and nighttime soil respiration in forests can experience distinct changes under climate warming, with significant impacts on the future trajectory of soil carbon emissions.