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Aims Over the past decades, vast croplands have been converted into forestland on the Chinese Loess Plateau (CLP). Although a few studies have investigated the effects of afforestation on soil inorganic carbon (SIC), there has been little information on the changes in the two fractions of SIC, pedogenic inorganic carbon (PIC) and lithogenic inorganic carbon (LIC), along a deep profile following afforestation. Methods We selected forestland ( Robinia pseudoacacia ; 31 years) and adjacent cropland (control), and investigated the stocks of SIC, PIC and LIC, at 0-300 cm under the two lands on the CLP. Results The SIC stock significantly decreased by 60.2 Mg ha −1 at 0-80 cm but increased by 57.8 Mg ha −1 at 80-220 cm, and no significant difference in total SIC stock at 0-300 cm was observed between the two lands. Afforestation significantly reduced the LIC stock in each layer; afforestation decreased the PIC stock at 0-80 cm but elevated it at 80-300 cm. In the 80-220 cm layer of forestland, the LIC stock decreased by 41.1 Mg ha −1 while the PIC stock increased by 98.9 Mg ha −1 . Conclusions Afforestation on cropland induces the loss of SIC in upper layers. However, it promotes the accumulation of PIC in deep layers, which causes an increase of SIC in these layers. Because the net increased PIC in deep layers compensates for the loss of SIC in the upper layers, afforestation on cropland does not alter the total SIC storage but redistributes the SIC along the profile on the CLP.

In arid and semiarid areas, the effects of afforestation on soil organic carbon (SOC) have received considerable attention. In these areas, in fact, soil inorganic carbon (SIC), rather than SOC, is the dominant form of carbon, with a reservoir approximately 2-10 times larger than that of SOC. A subtle fluctuation of SIC pool can strongly alter the regional carbon budget. However, few studies have focused on the variations in SIC, or have used stable soil carbon isotopes to analyze the reason for SIC variations following afforestation in degraded semiarid lands. In the Mu Us Desert, northwest China, we selected a shifting sand land (SL) and three nearby forestlands ( ) with ages of 8 (P-8), 20 (P-20) and 30 (P-30) years, and measured SIC, SOC, soil organic and inorganic δ C values (δ C-SOC and δ C-SIC) and other soil properties. The results showed that SIC stock at 0-100 cm in SL was 34.2 Mg ha , and it increased significantly to 42.5, 49.2, and 68.3 Mg ha in P-8, P-20, and P-30 lands, respectively. Both δ C-SIC and δ C-SOC within the 0-100 cm soil layer in the three forestlands were more negative than those in SL, and gradually decreased with plantation age. Afforestation elevated soil fine particles only at a depth of 0-40 cm. The entire dataset (260 soil samples) exhibited a negative correlation between δ C-SIC and SIC content ( = 0.71, < 0.01), whereas it showed positive correlation between SOC content and SIC content ( = 0.52, < 0.01) and between δ C-SOC and δ C-SIC ( = 0.63, < 0.01). However, no correlation was observed between SIC content and soil fine particles. The results indicated that afforestation on shifting SL has a high potential to sequester SIC in degraded semiarid regions. The contribution of soil fine particle deposition by canopy to SIC sequestration is limited. The SIC sequestration following afforestation is very probably caused by pedogenic carbonate formation, which is closely related to SOC accumulation. Our findings suggest that SIC plays an important role in the carbon cycle in semiarid areas and that overlooking this carbon pool may substantially lead to underestimating carbon sequestration capacity following vegetation rehabilitation.

• The evolutionary and ecological story of coccolithophores poses questions about their heterotrophy, surviving darkness after the end-Cretaceous asteroid impact as well as survival in the deep ocean twilight zone. Uptake of dissolved organic carbon might be an alternative nutritional strategy for supply of energy and carbon molecules. • Using long-term batch culture experiments, we examined coccolithophore growth and maintenance on organic compounds in darkness. Radiolabelled experiments were performed to study the uptake kinetics. Pulse–chase experiments were used to examine the uptake into unassimilated, exchangeable pools vs assimilated, nonexchangeable pools. • We found that coccolithophores were able to survive and maintain their metabolism for up to 30 d in darkness, accomplishing about one cell division. The concentration dependence for uptake was similar to the concentration dependence for growth in Cruciplacolithus neohelis, suggesting that it was taking up carbon compounds and immediately incorporating them into biomass. We recorded net incorporation of radioactivity into the particulate inorganic fraction. • We conclude that osmotrophy provides nutritional flexibility and supports long-term survival in light intensities well below threshold for photosynthesis. The incorporation of dissolved organic matter into particulate inorganic carbon, raises fundamental questions about the role of the alkalinity pump and the alkalinity balance in the sea.

Macroalgal ecosystems remain underrepresented in blue carbon frameworks, largely due to uncertainties surrounding the fate of macroalgal carbon during decomposition. Laboratory experiments on Sargassum horneri, Codium fragile, and Ulva australis revealed concurrent increases in total alkalinity and dissolved inorganic carbon, indicating substantial bicarbonate (HCO3−) production via previously unrecognized mechanisms in oxygenated environments. This pattern was corroborated by field data from Korean coastal macroalgal habitats and Yellow Sea macroalgal blooms. Contrary to the conventional view that macroalgal decomposition primarily releases CO2, over half of the inorganic carbon is released as HCO3− in oxygenated waters. This bicarbonate surge—potentially driven by sulfate reduction in anoxic macroalgal aggregates or intracellular HCO3− release—not only raises seawater alkalinity but also enhances carbon sequestration potential and mitigates ocean acidification. These findings highlight the critical role of macroalgal habitats in the global carbon cycle and call for their inclusion in blue carbon strategies.

During 2016–2017, the Indian Ocean experienced a pronounced increase in dissolved inorganic carbon (∼0.39 PgC/yr), approximately four times greater than the annual mean air–sea CO2 flux. Using a reconstructed data product and a state‐of‐the‐art ocean biogeochemical model, we attribute this anomaly to an enhanced Southern Ocean inflow and a weakened Indonesian Throughflow associated with an El Niño event accompanied by a positive Indian Ocean Dipole (IOD), and followed by a negative IOD during the El Niño‐to‐La Niña transition. The resulting carbon accumulation leads to a decline in aragonite saturation and a shoaling of the aragonite saturation horizon in the southeastern Indian Ocean. This subsurface acidification may pose risks to deep‐water calcifying organisms. Our findings demonstrate that ocean carbon storage and acidification are strongly modulated by circulation‐driven transport processes, highlighting the need for improved subsurface observations and model capabilities to better capture the interior carbon response to climate variability.

PurposeBasalt weathering has the potential to absorb and sequester CO2 as inorganic carbon, while its weathering byproducts, montmorillonite and kaolinite, have the capacity to stabilize organic carbon. Nonetheless, the practical viability of basalt weathering in achieving the stabilization of inorganic carbon and its impact on organic carbon dynamics in the soil priming effect (PE) remains unclear.MethodsAn incubation experiment was conducted by adding 13C-glucose with or without basalt, montmorillonite, or kaolinite to a Luvisol soil planted with peach (Prunus persica (L.) Batsch) for more than 20 years. CO2 emission and its 13C value were continuously measured to calculate the PE and soil net carbon balance.ResultsAfter a 28-day incubation, basalt resulted in an increase in soil pH from 5.32 to 7.17 and showed a 143.7% and 168.6% increase in dissolved organic carbon (DOC) and soil inorganic carbon (SIC), respectively. Subsequently, basalt induced the highest cumulative PE among all treatments, with the activities of soil β-glucosidase (S-β-GC), soil leucine amino peptidase (S-LAP), and soil catalase (S-CAT) being the highest. Furthermore, kaolinite significantly decreased emissions of CO2-C, glucose mineralization, and cumulative PE (P < 0.05). It is worth noting that all treatments significantly enhanced the net soil net carbon balance, with the most significant improvement observed in the kaolinite treatment.ConclusionsThe weathering process of basalt can significantly promote the stabilization of SIC in PE, whereas kaolinite exhibits the most pronounced impact on the stabilization of soil organic carbon (SOC), resulting in the greatest increase in soil net carbon balance.

Soil organic carbon (SOC) pool has been extensively studied in the carbon (C) cycling of terrestrial ecosystems. In dryland regions, however, soil inorganic carbon (SIC) has received increasing attention due to the high accumulation of SIC in arid soils contributed by its high temperature, low soil moisture, less vegetation, high salinity, and poor microbial activities. SIC storage in dryland soils is a complex process comprising multiple interactions of several factors such as climate, land use types, farm management practices, irrigation, inherent soil properties, soil biotic factors, etc. In addition, soil C studies in deeper layers of drylands have opened-up several study aspects on SIC storage. This review explains the mechanisms of SIC formation in dryland soils and critically discusses the SIC content in arid and semi-arid soils as compared to SOC. It also addresses the complex relationship between SIC and SOC in dryland soils. This review gives an overview of how climate change and anthropogenic management of soil might affect the SIC storage in dryland soils. Dryland soils could be an efficient sink in C sequestration through the formation of secondary carbonates. The review highlights the importance of an in-depth understanding of the C cycle in arid soils and emphasizes that SIC dynamics must be looked into broader perspective vis-à-vis C sequestration and climate change mitigation.

Inorganic carbon acquisition is essential to algal growth, while the limitations of dissolved inorganic carbon (DIC) on phytoplankton are still less known in lakes. Sediment is an active hot spot for microbial metabolism, driving the migration and transformation of elements in shallow lakes, which may control the DIC availability to influence algal spatiotemporal dynamics. Hence, we investigated the spatiotemporal changes of phytoplankton, DIC and sediment respiration rates in a eutrophic shallow freshwater lake under non-bloom conditions. There was a widespread deficiency of DIC in the lake, except the estuary. Sediment respiration was positively associated with changes in DIC concentrations, indicating that carbon metabolic activity of sedimentary microorganisms was an important inorganic carbon source for water columns. The availability of DIC in water columns regulated by sediment microbial respiration influenced the algal biomass, composition and productivity. The synergistic effects of seasonal temperature changes and sediment microbial respiration influenced the vertical distribution and migration of phytoplankton. Our results emphasized that carbon metabolic intensity of sediment microorganisms might play a key role in dynamics of phytoplankton, further impacting the spatiotemporal pattern and formation of algal bloom in eutrophic shallow freshwater lakes.

Soil inorganic carbon (SIC) is critical for carbon sequestration, infiltration, and climate modeling, yet quantifying its precise spatial distribution at continental scales remains challengings. We introduce a high‐resolution (30 m) CONUS SIC map using machine learning (ML) models trained on the ISRIC World Soil Information Service (WoSIS) database. Integrating point‐based soil data with high resolution climate and land surface variables enhances predictions and reduces uncertainty in unsampled areas over previous polygon‐based inventories. Our SIC Random Forest Regression (RFR) yielded an Root Mean Square Error (RMSE) of 15 kg/m2, and the multi‐class classifier had an accuracy of 0.56. Our model estimates that CONUS soils store 77 ± 1.8 Pg of SIC in the top 1 m, a significant increase in SIC storage over prior inventories. We distinguish two SIC storage types: lithogenic carbonates persisting in humid regions and pedogenic carbonates, which characteristically form in arid soils.

Widespread shrub encroachment in global drylands may increase plant biomass and change soil organic carbon stocks of grassland ecosystems. However, the response of soil inorganic carbon (SIC), which is a major component of dryland carbon pools, to this vegetation shift remains unknown. We conducted a systematic field survey in 75 pairs of shrub‐encroached grassland (SEG) and control plots at 25 sites in the grasslands of the Inner Mongolia Plateau to evaluate how shrub encroachment affects SIC density (SICD) in these ecosystems. We found that shrub encroachment significantly reduced SICD in the upper 100 cm (3.85 vs. 4.74 kg C m−2, p < .05), especially in the subsurface soil (20–50 cm layer). The magnitude of SICD changes was related to the change in soil pH, shrub patch size and initial SICD, reflecting that the reduction in SICD might be attributed to the shrub encroachment‐related soil acidification. Our results also revealed that the lost SIC was mainly released into the atmosphere rather than redistributed into deeper soil layers. Synthesis. We provide the first evidence for the soil acidification‐induced SIC loss caused by shrub encroachment. Our findings highlight the non‐negligible role of SIC dynamics in the C budget of SEG ecosystems and the need to consider these dynamics in terrestrial C cycle research. We provide the first evidence for the soil acidification‐induced soil inorganic carbon (SIC) loss caused by shrub encroachment. Our findings highlight the non‐negligible role of SIC dynamics in the C budget of shrub‐encroached grassland ecosystems and the need to consider these dynamics in terrestrial C cycle research.