Bedrock weathering is the dominant proton sink in strongly acidic subtropical critical zones

Strongly acidic soils in subtropical and tropical regions often represent a transitional stage where soil acidity buffering shifts from cation exchange to aluminum hydroxide dissolution, yet neutral-to-alkaline surface waters are commonly observed. How these acidic regoliths and underlying bedrock collectively regulate water chemistry—from rainwater to groundwater—remains poorly understood in such settings. To address this, we investigated a representative, strongly acidic critical zone in subtropical China, comprising highly weathered regolith overlying sandstone bedrock, and quantified its proton consumption processes. Our results show that while rainwater, soil water, and regolith are strongly acidic, streamwater and groundwater are neutral to slightly alkaline. Soil water at 20–200 cm depth was more acidic than rainwater. The strongly acidic Quaternary red clay functioned as a net proton source, not a sink, due to active proton-producing processes (e.g., nitrogen transformation, plant cation uptake, acid deposition, and Al hydrolysis) and limited buffering capacity (scarcity of exchangeable base cations and weatherable minerals). In contrast, weathering of the bedrock sandstone—which contains calcite, albite, and microcline—emerged as the dominant proton consumption mechanism. From 2020 to 2023, the estimated mean proton consumption rate at the groundwater–bedrock interface was 583.5 ± 166.7 mol ha −1 yr −1 , corresponding to a bedrock weathering rate of 149.1 ± 40.8 kg ha −1 yr −1 (equivalent to 6.7 ± 1.8 μm yr −1 ). These findings indicate that bedrock weathering provides a plausible mechanism explaining the widespread occurrence of neutral-to-alkaline groundwater and surface water in strongly acidic subtropical regions. By neutralizing acidic soil water and rainwater, this deep weathering process effectively prevents the acidification of groundwater and surface water, underscoring the critical—and previously underappreciated—role of deeply buried bedrock in critical zone biogeochemistry.