What Regulates Net Carbon Uptake in Coastal Ecosystems?

Abstract

Coastal ecosystems are hotspots of biological activity and carbon storage, accounting for a disproportionately high level of carbon burial relative to their land area. However, they are undergoing rapid land-use change due to increasing population pressure, with about 1 billion people living within 10 km of the coast globally in 2018 (Cosby et al. 2024). When coastal wetlands are converted to croplands, they can switch from CO₂ sinks to sources owing to factors like increased decomposition of soil carbon by microbes (Tan et al. 2020). Moreover, coastal regions are vulnerable to threats from climate change such as flooding, saltwater intrusion, and erosion. These ecosystems have been understudied in comparison to their social and ecological significance, increasing the relevance of the recent work by Wei et al. (2025).

In Wei et al. (2025), the authors studied interannual and spatial variations in net carbon uptake using eddy covariance technology for more than 10 years in native tidal salt marsh, non-tidal reed marsh, and cotton-dominated cropland. On average, all three sites were annual net C sinks during the study period, indicating their importance for long-term C sequestration. In order to understand the biological factors controlling the uptake, they applied a novel approach that distinguished physiology (i.e., rates of photosynthesis and respiration) from phenology (i.e., timing and duration of the growing season) (Fu et al. 2019). Interannual variations in C uptake in cropland were primarily regulated by the length of the growing season, which was in turn limited by precipitation. In the reedy non-tidal marsh, the maximum C uptake rate was the dominant indicator, and this rate was reduced during wet summers with periodic flooding. In contrast, the annual C uptake in the tidal wetland was controlled by the maximum C loss rate, which increased in years with warmer spring conditions (Wei et al. 2025). Despite the close proximity of the three sites, the vulnerability of C uptake was regulated by different climate conditions with disparate underlying biological mechanisms. Clearly, vegetation and land management need to be accounted for when upscaling C storage rates from individual sites.

The authors addressed concerns that their site-level results might be idiosyncratic or non-representative by analyzing publicly available carbon flux data from similar global ecosystems (Pastorello et al. 2020). This global analysis validated their results and demonstrated that the method of determining the biological indicators was robust across numerous sites. Their new insights into biological regulation can be used to improve global carbon cycle models and can be further improved by applying sensitivity analyses to assess the relative importance of seasonality, photosynthesis, and respiration rates across other vegetation types.

Nevertheless, questions remain regarding the vulnerability of carbon sinks in coastal ecosystems. First, tidal wetlands were poorly represented in the global database, despite being important interfaces between land and oceans with substantial value as “Blue Carbon” reservoirs (Tan et al. 2020). Tidal wetlands exist at the upper range of inundation and may be forested or dominated by shrubs, grass, or sedge; they fix CO₂ and have large organic C stocks, but tend to emit less methane than freshwater wetlands because they are more saline (Adame et al. 2024). Recognition of the potential for tidal wetlands to sequester C is increasing, along with management efforts to restore or mitigate damage. Second, climate changes are hitting coastal areas hard; warming temperatures and increased intensity of precipitation are interacting with land-use change, leading to increased rates of soil salinization, subsidence, erosion, and carbon degradation. It is critical to enhance protections, restoration efforts, and monitoring of low-lying coastal regions to maintain these valuable carbon sinks and reduce their vulnerability in the future.

Elise Pendall: conceptualization, writing – original draft, writing – review and editing.

The author declares no conflicts of interest.

This article is a Invited Commentary on Wie et al., https://doi.org/10.1111/gcb.70029.