Substantial role of check dams in sediment trapping and carbon sequestration on the Chinese Loess Plateau

The global carbon cycle is significantly influenced by the transfer of terrestrial organic carbon (OC), yet the processes governing this lateral transfer remain a subject of considerable debate. While OC deposition in aquatic ecosystems is relatively well-understood, the fate of OC in dry depositional environments is less clear. A significant proportion of eroded OC (34-82%) is deposited in terrestrial systems, highlighting their importance as carbon sinks. However, the role of these sinks, particularly dry environments like alluvial fans, floodplains, and check dams, is understudied compared to aquatic systems such as reservoirs and lakes. Reservoirs and lakes often act as net carbon sources due to high emission rates, while the OC dynamics in dry environments may differ significantly. Check dams, widely used for erosion control, particularly in arid and semi-arid regions, are crucial in this context. The Chinese Loess Plateau, characterized by severe soil erosion and a high density of check dams, provides a unique setting to investigate the role of these structures in OC burial and sequestration. The Yellow River, originating from this plateau, has experienced a substantial reduction in sediment load in recent decades, largely attributed to the construction of these check dams. This study aims to quantify the sediment retention and OC burial in check dams on the Loess Plateau using remote sensing and extensive field surveys to determine their contribution to the terrestrial carbon budget.

Existing literature emphasizes the importance of understanding carbon sequestration in both aquatic and terrestrial environments. Studies have shown that a substantial portion of eroded organic carbon is deposited in terrestrial systems, making them important carbon sinks. However, research has primarily focused on aquatic environments like reservoirs and lakes, with limited understanding of the role of dry depositional environments such as check dams. While the hydrological, ecological, and geomorphological effects of check dams have been explored, their contribution to carbon sequestration remains poorly understood. Existing studies on the Chinese Loess Plateau highlight the massive sediment reduction in the Yellow River due to check dam construction, but the associated carbon sequestration potential remains largely unquantified. This study addresses this gap by focusing specifically on the carbon sequestration capacity of check dams in this region.

This research employed a multi-faceted approach combining remote sensing techniques and extensive field surveys. The spatial distribution of check dams on the Chinese Loess Plateau was mapped using an object-oriented classification method applied to multi-source remote sensing data, including high-resolution Google Earth imagery. A total of 50,226 active check dams were identified. The area of silted land behind each dam was determined, and an empirical formula linking silted area to sediment volume was developed using unmanned aerial vehicle (UAV) photogrammetry and virtual dam construction. Sediment mass was estimated using measured bulk density data from 60 deep sediment profiles. To assess organic carbon (OC) dynamics, 2121 samples were collected from 86 deep drillings or profiles across the Loess Plateau. OC content was measured using the K2Cr2O7-H2SO4 oxidation method, and total nitrogen was determined using the Kjeldahl method. Radiocarbon (Δ14C) and stable carbon isotope (δ13C) analyses were performed to determine the age and source of the buried OC. A binary mixing model was used to quantify petrogenic and biospheric OC contributions. The OC burial efficiency was estimated based on the regression slope of OC content with depth in the sediment cores, reflecting the decomposition rate of OC. The relationship between OC content and fine particles was also investigated to understand the mechanisms of OC preservation.

The study revealed that the 50,226 active check dams on the Chinese Loess Plateau have intercepted a staggering 10.2 ± 0.6 Pg of eroded sediment between 1970 and 2020. This represents approximately 46% of the total sediment load carried by the Yellow River during this period, underscoring their significant role in sediment reduction. Analysis of 86 deep sediment cores showed that these dams have buried 21.6 ± 9.9 Tg of organic carbon over the past 50 years. This translates to an area-normalized OC burial rate of 468 ± 204 g C m−2 yr−1, substantially higher than that reported for global lakes and reservoirs (approximately one order of magnitude higher). The OC burial efficiency in check dams was estimated to be around 80%, significantly greater than other depositional environments (reservoirs ~44%, lakes ~43%, colluvial/alluvial sediments ~18%, and marine sediments ~24%). Isotopic analysis indicated that a significant portion (29.9%) of the buried OC is petrogenic in origin, highlighting the geological impact of check dams. Furthermore, a strong positive correlation (P < 0.01, n = 540) was found between OC content and fine particles, suggesting that adsorption by fine particles contributes to OC preservation. There was no significant difference in δ13C and C/N ratios between erosion and deposition areas, suggesting low OC decomposition rates within the check dams.

The findings strongly support the hypothesis that check dams represent a significant, yet previously underestimated, terrestrial carbon sink. The exceptionally high organic carbon burial rate and efficiency, exceeding those of other depositional environments, are attributed to a combination of factors. The dry and anoxic conditions in check dams inhibit microbial activity, reducing OC decomposition. The relatively low OC content in the sediment, coupled with its high recalcitrance (largely due to the petrogenic OC fraction and pre-aged biospheric OC), contributes to enhanced preservation. The results emphasize the importance of considering check dams in regional and global carbon budgets. The findings also have implications for future erosion control strategies and sustainable development goals, particularly in arid and semi-arid regions worldwide.

This study provides the first comprehensive quantification of sediment retention and organic carbon burial by check dams on the Chinese Loess Plateau. The results demonstrate that these structures act as significant terrestrial carbon sinks, surpassing the carbon sequestration capacity of other depositional environments. The high OC burial efficiency and the role in trapping petrogenic OC further emphasize their importance. Future research should investigate the long-term stability of this carbon sink under changing climate conditions and explore the potential for scaling up check dam construction strategies in other regions with similar environmental conditions to mitigate soil erosion, enhance carbon sequestration and improve food security.

While this study provides valuable insights into the role of check dams in sediment trapping and carbon sequestration, several limitations exist. The sampling strategy might not perfectly represent the entire spatial variability of OC content across the entire Loess Plateau. Further research using more extensive sampling could help to refine estimates. Long-term monitoring is necessary to better understand the temporal dynamics of carbon storage within check dams and assess the long-term stability of the stored carbon under changing environmental conditions. The study focuses on a specific region and it is important to investigate the generalizability of findings to other geographical areas with different climatic and geological settings.