Soil extraction with pyrophosphate-dithionite mixture: a practical method to estimate organic carbon associated with metal cations and reactive mineral phases

Pyrophosphate (PP) extraction at pH 10 has been used to quantify organic carbon (OC) and metal cations in organo-metal complexes. However, the alkaline solution may solubilize some OC not complexed with metals. Recently, pyrophosphate-dithionite (PD) extraction at pH 7.5 was used to quantify OC bound to reactive mineral phases. Yet the sources of PD-extractable OC and metals in soils remain unclear. We thus examined the effects of pH and dithionite (a strong reductant) addition in PP extraction on the release of OC and metals presumably from organo-metal complexes, short-range-order minerals, and crystalline Fe oxide phases. We conducted three PP-based extractions, namely (i) PP at pH 10, (ii) PP at pH 7.5, and (iii) PD at pH 7.5 using 10 arable topsoils from 4 soil orders. The comparison of the two PP extractions showed more Al and Fe dissolution from reactive mineral phases at pH 7.5 and more OC solubilization at pH 10, suggesting alkali-induced OC solubilization at pH 10. More Al, Fe and OC were released by PD relative to PP at pH 7.5 for most of the samples. PD at pH 7.5 released almost 100% of Fe and 73–89% of Al relative to the sum of respective metals dissolved by sequential extraction (PP, acid oxalate, and then dithionite-citrate), indicating nearly complete removal of the metals from organo-metal complexes and reactive mineral phases. Based on the three extractions, we developed a conceptual model to assign the extractable OC to three pools (alkali-soluble, sorbed, and organo-metal complexes). While each C pool accounted for, on average, less than 10% of total C, 'sorbed OC' was positively correlated with Fe- and Al-bearing reactive mineral phases and 'complexed OC' was positively correlated with PP-extractable Ca. However, the model applicability needs to be tested further using a wider range of soils. Overall, our results suggest that PD at near-neutral pH appeared to fully dissolve metal cations and reactive mineral phases (with a caution on Al-bearing minerals) with less alkali-induced OC solubilization. This single extraction may thus serve as a practical method to estimate the OC associated with these metal and mineral phases.