Potassium isotopic fractionation in a humid and an arid soil-plant system in Hawai'i

Plants play a critical role in the cycling of potassium (K) and the fractionation of its isotopes. However, little is known about K stable isotopic compositions in natural soil-plant systems and possible fractionation during intraplant transport and root-soil uptake of K. This study focuses on K isotopic fractionation within a humid and an arid soil-plant system sampled on the windward and leeward sides of Kohala Mountain, Hawai`i. We determined the K isotopic compositions of < 2-mm bulk soil, soil saturation extraction, and selected plant tissues by multi-collector inductively coupled plasma mass spectrometry and X-ray absorption spectroscopy. We studied soils and individual tissue samples such as roots, stems, barks, shoots (a sum of stems and fresh leaves), leaves (fresh and dead), seeds, and flowers of trees and grasses. The results demonstrated that: (i) tissue delta K-41 values ranged from 1.06 +/- 0.06 to 1.15 +/- 0.09 parts per thousand; (ii) within the same plant, stems (barks), dead leaves, and reproductive tissues (flowers and seeds) were isotopically lighter compared to fresh leaves, and to a lesser extent, roots; (iii) delta K-41 values of the humid soil (-0.54 +/- 0.07 to 0.49 +/- 0.06 parts per thousand) were lower than those of the arid soil (-0.24 +/- 0.07 to 0.14 +/- 0.06 parts per thousand); and (iv) soil bioavailable pool delta K-41 (saturation extracts) ranged from 0.63 +/- 0.08 to 0.34 +/- 0.08 parts per thousand and 0.48 +/- 0.08 to 0.54 +/- 0.10 parts per thousand in the humid and arid soils, respectively. From synchrotron-based analysis of K atoms, we identified two major K-bearing phases co-existing as ionic K+ and K-pectate association of different fractions. Based on K isotopic and synchrotron data, we conclude that two dominant processes are responsible for plant-mediated K isotopic fractionation, including (1) K redistribution during intra-plant circulation and (2) uptake at the root-soil interface. For intra-plant circulation of K, there is a high affinity of isotopically lighter K to organic complexes as K-pectate, and K-pectate is particularly enriched in roots and fresh leaves. For K uptake at root-soil interface, isotopically lighter K is preferentially taken by roots from soil bioavailable pools following a low-affinity (passive) transport path. Soil K budget in two sites reflects strong source mixing effects with limited plant imprints. This work provides exploratory data on the biogeochemical fractionation of K isotopes in the soil-plant system.

Automated summary

This study investigates potassium isotope fractionation within a soil-plant system in humid and arid environments on Kohala Mountain, Hawai‘i. The results show that within the same plant, different tissues have varying potassium isotopic compositions, and isotopic differences are observed between soils of different moisture levels. The study suggests that processes such as K redistribution during intra-plant circulation and K uptake at the root-soil interface contribute to potassium isotope fractionation.