Water Availability Weakens the Forest Litter Carbon Sink

[Geoengineering News]

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025GB008731

Authors: X. L. Zhao, H. Y. Zhao, J. W. Chen, H. L. Chen, X. Y. Yu, W. Jia, G. Chen, T. T. Xu, Y. Z. Yao, X. L. Tang

First published: 31 January 2026

https://doi.org/10.1029/2025GB008731

Abstract

Litter carbon (C) release refers to the amount of litter C lost via microbial respiration or leaching, while the net litter C sink represents the C retained in soil following decomposition. Both are critical but understudied processes in forest C sequestration, particularly under variable water availability driven by climate and land-cover changes. Here, we modeled litter C decomposition rates (Kc) using 668 field observations with the Random Forest (RF) algorithm at 1 km resolution across Chinese forests. The RF model demonstrated good performance (R2 = 0.86), with predicted Kc exhibiting a clear latitudinal zonation. Using modeled Kc, we then estimated the spatial and temporal patterns of litter C release and net litter C sink. Litter C release increased toward lower latitudes, whereas net litter C sink peaked at mid-latitudes and declined toward both lower and higher latitudes. Over 2000–2018, litter C release increased (0.49 ± 0.07 g C m−2 yr−2), while the net litter C sink decreased (−0.10 ± 0.04 g C m−2 yr−2), driven primarily by changing water availability mediated by climate and canopy dynamics. Reduced water limitation in persistently arid zones enhanced decomposition, whereas increased water limitation in seasonally dry regions suppressed productivity. In humid regions, a reduction in water surplus led to asynchronous increases in litter input and decomposition. Our findings highlight the central role of water availability in regulating litter C fluxes and underscore the sensitivity of forest C cycling to climate variability, with important implications for projecting terrestrial carbon–climate feedback under future environmental change.

Plain Language Summary

Carbon stored in fallen leaves and branches cycles quickly through forest ecosystems, but how these fast-moving carbon pools respond to climate change is still poorly understood. In this study, we used field measurements and satellite data to predict how forest litter decomposes across China. We found that carbon released to the atmosphere through decomposition increased significantly at a rate of 0.49 g C m−2 yr−2, while the amount retained in soils through litter declined by 0.10 g C m−2 yr−2. These contrasting trends depended on how much water was available in different regions, and were further influenced by forest canopy cover and regional climate. Our findings help improve our understanding of how forests regulate carbon under climate change and support more accurate carbon–climate feedback predictions.

Key Points

The increasing rate of litter C release rose from 0.37 to 0.66 g C m−2 yr−2 as water availability transitioned from extreme to moderate

The decreasing rate of the net litter C sink intensified from −0.08 to −0.63 g C m−2 yr−2 under greater seasonal dryness or annual wetness

Litter C release and the net litter C sink showed opposite responses to changing water availability

Source: AGU