Soil potassium isotope composition during four million years of ecosystem development in Hawai'i

We combine spectroscopic and geochemical approaches to interpret the fate of potassium (K) during forest soil development along a 4-million-year chronosequence sampled from relatively undisturbed rainforests in Hawai'i. Potassium derived from weathering of lava is dominant in the youngest site (0.3 ky), but its contribution to the soil K budget declines as weathering progresses. Sites older than 0.3 ky are characterized by substantial K depletion (tau(K,Nb)similar to-1), with soil isotopic composition (K-41/K-39, delta K-41) varying from -1.91 +/- 0.08 parts per thousand to -0.09 +/- 0.08 parts per thousand, relative to the homogeneous basaltic substrate (-0.48 parts per thousand). Exchangeable and interlayered K show delta K-41 values ranging from-1.32 +/- 0.06 parts per thousand to 0.06 +/- 0.08 parts per thousand, higher than their corresponding bulk delta K-41 values. The delta K-41 patterns of soils and exchangeable components are vertically similar, implying similar environmental controls. The variability in K phase and isotope composition reflects the accumulative effect of different processes. Chemical weathering and plant cycling retain isotopically light K in soils, in particular for 20-150 ky sites. In contrast, atmospheric inputs of marine aerosols (0.14 parts per thousand) and mineral aerosols (-0.44 parts per thousand) add heavier K (than native basalts) and crust-like K (similar to basaltic delta K-41) in soils, respectively. In sites older than 150 ky, nearly complete depletion of basaltic K and reduced plant K imprints result in the dominance of atmospheric inputs in soils. In sum, this study emphasizes the sensitivity of delta K-41 to terrestrial biotic and abiotic K cycles. (C) 2022 Elsevier Ltd. All rights reserved.

Automated summary

We conducted a study to interpret the fate of potassium (K) during forest soil development in Hawai'i. We used spectroscopic and geochemical approaches and found that potassium derived from weathering of lava dominates in the early stages of soil formation, but its contribution declines over time. The isotopic composition of soil potassium reflects different processes, including chemical weathering, plant cycling, and atmospheric inputs.