Manufactured zeolite application to soil can rapidly increase pH and enhance inorganic carbon sequestration

Soils host enormous carbon stocks. Whilst considerable research has been dedicated towards soil organic carbon (C) dynamics, less attention has been paid to soil inorganic carbon (SIC). Here we investigated zeolite amendments for their potential to sequester SIC. The rationale underpinning the investigation is that zeolites – i.e., high-pH, alkaline-rich aluminosilicate minerals – could promote stabilisation of soil carbonate compounds. A four-month study was undertaken via an 80-container factorial laboratory incubation experiment comprising two soil types (a loamy Planosol and a clayey Vertisol), four manufactured zeolites, and a control (soil only), with four replicates for each treatment. An additional series of containers, containing crushed mafic (high magnesium) rock combined with the above treatments, was included as a benchmark inorganic carbon sequestration method. The zeolites increased soil pH by an average of 1.5 units (p < 0.05) at the trial’s conclusion, whereas the crushed mafics had much less impact on soil pH. Zeolite addition resulted in a 70 % increase (p < 0.05) in SIC concentrations compared with controls when averaged across all treatments. They were particularly effective in the Planosol, with each zeolite yielding significantly (p < 0.05) and substantially higher (up to 230 %) SIC concentrations relative to the controls. Mass balance calculations confirmed these increases cannot have been caused by the relatively minor IC contents of the zeolites. X-ray diffraction analysis confirmed the presence of sparingly-soluble carbonate species in some of the zeolite soil treatments. Mafic rock alone did not cause appreciably higher SIC concentrations than the controls. Moreover, no synergistic effects were observed when combining zeolites with mafics. It is likely that mafics require a longer timeframe to be effective. Overall, our study revealed that zeolites can potentially increase SIC stocks and buffer against soil acidification, although confirmation of the method at field scale is needed. Given previous research has reported that zeolites can achieve other benefits, including enhanced soil organic C sequestration and improved soil water and nutrient retention, these minerals could be developed into high-value and multi-benefit amendments to support agriculture, landscape restoration and climate change mitigation.