Potassium isotope fractionation of potassium-bearing mineral fertilizers and its implications for global potassium cycle

Abstract

Potassium (K) isotopes are increasingly recognized as promising tracers for evaluating the impact of anthropogenic activities on the biogeochemical K cycle, particularly given the widespread use of K fertilizers in agriculture to mitigate soil K depletion. Understanding K isotope behavior in soil is essential because soil acts both as a carrier of fertilizers and as a source of K from weathered rocks. In this study, we determined the K isotopic compositions of K-bearing mineral fertilizers (KMFs), which were produced from K-feldspar through artificial hydrothermal alteration, exhibiting physicochemical properties similar to those of soil. The δ⁴¹K values of bulk K in KMFs range from −0.755 ± 0.023 ‰ to −0.095 ± 0.020 ‰. The K isotopic compositions of 0.5 mol/L acid-soluble K are close to those of bulk K in KMFs, in the range of −0.751 ± 0.043 ‰ to −0.103 ± 0.035 ‰. The observation of isotopically heavier water-soluble K compared to bulk sample K confirms the preferential enrichment of heavy K isotopes in aqueous solutions, consistent with observations in weathered rocks. A combined mechanism involving K–O bond-driven and diffusion-mediated K isotope fractionation offers a plausible explanation for the discrepancy between theoretical models and experimental observations. The application of K-bearing fertilizers has the potential to impact δ⁴¹K values in rivers and seas due to interactions among soil, plants, and water. Our findings contribute to a more comprehensive understanding of K cycling in ecosystems.