Destabilization of carbon in tropical peatlands by enhanced weathering

The Paris Agreement aims to limit global temperature rise to well below 2°C, ideally under 1.5°C. Current commitments are insufficient to meet this target, highlighting the need for atmospheric CO₂ removal (CDR) techniques. Enhanced weathering (EW) is a promising CDR method that accelerates natural CO₂ uptake by weathering through rock powder dispersion. Tropical regions, particularly tropical peatlands, are considered prime candidates for EW due to their warm, humid climates which enhance weathering rates. Model studies project significant CO₂ uptake potential in these areas, but the effects of EW on peatland carbon stocks remain poorly understood. Tropical peatlands function as vital carbon sinks, offering coastal protection against erosion and flooding, and providing habitat for biodiversity. However, large-scale disturbances, primarily from agricultural expansion (e.g., palm oil), have transformed many peatlands from carbon sinks into significant sources. This study focuses on evaluating the potential impact of EW on CO₂ emissions from tropical peatlands, considering the effects on peat soils, rivers, and coastal waters. The research investigates how EW-induced pH changes affect soil carbon mobilization and CO₂ emissions across the land-ocean continuum. Sumatra, a region with extensive peatlands, serves as the case study area. By utilizing data from Southeast Asian rivers and coastal waters, this study develops and validates a carbon dynamics model to quantify the net effect of EW on the carbon cycle in tropical peatlands.

Existing literature supports the potential of enhanced weathering (EW) as a CO2 removal strategy. Studies highlight the accelerated CO2 uptake through increased silicate weathering when rock powder is dispersed over land surfaces, leading to the formation of bicarbonate and its subsequent transport to the ocean. Tropical regions are favored due to the accelerated weathering rates under warm and humid conditions. However, studies investigating the potential of EW in tropical peatlands are limited. Research indicates high potential CO2 uptake (20-200 MgC km⁻² year⁻¹), but this does not account for the potential destabilization of peat carbon due to pH changes. Furthermore, the literature underscores the importance of peatlands as significant carbon sinks and their role in coastal protection and biodiversity. Concerns about the sustainability of expanding agriculture in these regions are also highlighted. The existing literature lacks a comprehensive assessment of EW's impact on the carbon cycle in tropical peatlands, considering both the terrestrial and marine components. The current study aims to fill this knowledge gap.

This study uses a combined approach of observational data and modelling to estimate the response of CO₂ emissions to EW application in tropical peatlands of Sumatra. First, the impact of EW-induced pH increase on soil carbon mobilization was assessed. This involved estimating the response of soil CO₂ emissions and dissolved organic carbon (DOC) leaching to changes in pH based on existing studies linking peat soil pH and soil carbon mobilization. Two cases were considered: (1) EW application solely on tropical peat soils, and (2) EW application across Sumatra, accounting for peat coverage. The study then developed a river box model to investigate the fate of leached DOC and dissolved inorganic carbon (DIC) along the land-ocean continuum. The model incorporated pH and O₂-dependent decomposition processes, atmospheric gas exchange, and inorganic carbon dynamics. It was validated against data from Southeast Asian rivers. DIC and DOC leaching rates were varied in the model to assess the impact of enhanced carbon leaching. Finally, simple mixing calculations were applied to trace the fate of carbon transported to the ocean and estimate CO₂ emissions from estuaries and coastal oceans using data on estuarine and coastal salinities to derive the ratio of river to ocean water. The study used data from measurement campaigns in Southeast Asian rivers and the coastal ocean of Sumatra to parameterize and validate the models. A case study focusing on Sumatra's peatlands was performed, considering the significant peat coverage in the region. The study integrates the emissions from peat soils, rivers, estuaries, and coastal oceans to assess the net impact of EW on the region's carbon balance.

The study's key findings indicate that EW application in tropical peatlands can destabilize carbon reservoirs and lead to significant CO₂ emissions. A moderate basalt application of 1 kg m⁻² year⁻¹ could increase soil CO₂ emissions by 12–81 gC m⁻² year⁻¹, largely due to increased peat decomposition (12-81 gC m⁻² year⁻¹), potentially exceeding the CO₂ uptake by EW. Focusing on Sumatra's peat soils (15.6% peat coverage), the carbon mobilization is estimated at 2–13 gC m⁻² year⁻¹ as CO₂ and 2–17 gC m⁻² year⁻¹ as DOC leaching, partially offsetting the CO₂ captured by EW (25–50 gC m⁻² year⁻¹). In-river processes further amplify emissions. A 40 gC m⁻² year⁻¹ increase in DIC leaching, consistent with EW estimates, raises river CO₂ emissions by 11–13% (32% for rivers with high peat coverage). The additional increase in DOC leaching from enhanced soil carbon mobilization significantly increases river emissions, particularly in rivers with low peat coverage. The oceanic carbon export increases substantially, but CO₂ emissions from estuaries and coastal oceans also increase significantly, potentially offsetting the CO₂ uptake from EW by more than 150%. Overall, land-based emissions from Sumatra’s soils and rivers could reduce the net EW CO₂ uptake by 18–60%, with coastal emissions potentially completely offsetting the EW CO₂ uptake.

The findings challenge the assumption that EW universally enhances CO₂ removal. In tropical peatlands, the increased soil acidity resulting from EW can have unintended consequences, enhancing carbon mobilization and re-emission. While EW might create a carbon sink on land, the significant increase in coastal CO₂ emissions could negate the benefits, emphasizing the importance of considering the entire carbon cycle. The study highlights the substantial uncertainties associated with predicting the response of complex ecosystems to large-scale interventions. The results strongly suggest that tropical peatlands should be excluded from EW projects, and that soil type must be carefully considered when implementing EW strategies. This necessitates further research into the soil-specific responses to EW.

This study demonstrates the potential for enhanced weathering (EW) to destabilize carbon stocks in tropical peatlands, leading to substantial CO₂ emissions from soils, rivers, and coastal waters, potentially exceeding the CO₂ uptake by EW. The findings underscore the critical importance of considering soil type and the entire land-ocean carbon cycle when assessing the efficacy and environmental impacts of EW. Further research is crucial to fully understand the complex interactions between EW and diverse soil types, particularly in sensitive ecosystems like tropical peatlands. This research emphasizes a precautionary approach to geoengineering strategies and the need for site-specific assessments before implementation.

The study's estimates are based on a case study for Sumatra, and the extrapolations to other tropical peatlands may have limitations. The models used, particularly the simplified mixing calculations for coastal emissions, introduce uncertainties. The lack of data on tropical peat soils necessitates the use of data from temperate peat soils, which may not fully capture the complexities of tropical peatland ecosystems. The precise response of DOC leaching rates to EW is also uncertain due to limited data, especially for tropical peat soils.