Calcium concentrations as a proxy for inorganic carbon content in salt marsh soils

• Calcium concentrations can be used as a proxy for inorganic carbon concentrations. • pXRF calcium correlates strongly with inorganic carbon from loss-on-ignition and coulometry. • The pXRF method offers practical advantages: rapid analysis, minimal training, and is non-destructive. Tidal salt marshes are habitat for shelled organisms such as crabs, mollusks, and foraminifera. As these organisms die, their shells are buried in the accumulating soil which should be a reservoir of inorganic carbon (IC), primarily present as calcium carbonate. Soil IC, however, is an understudied component of salt marsh soils, limiting our understanding of coastal carbon budgets and their potential climate feedback. The lack of knowledge of soil IC can be hindered by methodological constraints. Conventional measurement techniques such as loss-on-ignition (LOI), coulometry, and the Scheibler method are destructive, time-consuming, or often require costly, specialized equipment and training. To address these challenges, we tested a rapid, non-destructive method using a portable X-ray fluorescence (pXRF) instrument that measured calcium as a proxy for the IC content of salt marsh soils. We analyzed 419 soil samples from 11 salt marshes along four coastlines of Eastern Canada. To isolate the calcium associated with carbonates, we normalized calcium concentrations with aluminum to account for calcium from other lithogenic minerals like plagioclase. We compared pXRF measurements of calcium to IC values determined by sequential LOI and coulometry. Our analysis revealed a strong and significant linear correlation between the normalized calcium from pXRF and the IC from LOI and coulometry. This strong correlation is particularly notable as it was achieved in low-carbonate soils (mean IC: 2.6 ± 1.5%), demonstrating the method’s reliability even at low concentrations. Our results validate pXRF as a cost-effective, rapid, and robust technique for quantifying IC in salt marsh soils. Its non-destructive nature is a significant advantage, preserving sample integrity for subsequent biogeochemical analyses and helping to build more complete coastal carbon budgets.