Natural Potassium (K) Isotope Fractionation during Corn Growth and Quantification of K Fertilizer Recovery Efficiency Using Stable K Isotope Labeling

An improved understanding of the potassium (K) cycle in soil-plant systems is scientifically and economically significant, but the conventional research based on K concentration measurements has several known limitations. The recent advent of high-precision stable K isotope analysis (reported as delta K-41 values can facilitate the use of both stable K isotope labeling and mass-dependent isotopic fractionation in studying the K nutrient cycle, including K fertilizer utilization, and plant-soil interactions. As a proof of concept, we conducted a pot study to quantify the uptake of K fertilizer by corn. Three groups of treatment (50, 100, 200 mg K kg(-1) soil) were conducted using soils premixed with different amounts of K-41-labeled fertilizer. A control group used the same soil without fertilizer treatment. Aboveground shoots and soils were sampled and analyzed after similar to 6 weeks. The control group showed preferential uptake of light K isotopes by corn with an estimated mass-dependent fractionation of similar to-0.37 parts per thousand (+/- 0.23 parts per thousand) in K-41/K-39 between the shoot and soil. In fertilized experiments using an enriched K-41 tracer, delta K-41 data unambiguously quantifies fertilizer-derived K in corn shoots, yielding apparent fertilizer recovery efficiency of 59-81%. In comparison, the K concentration-based method underestimated fertilizer utilization at low K treatment and overestimated fertilizer utilization at high K treatment because it cannot distinguish different K sources whose relative contributions to the bioavailable K pool in the soil can vary in response to plant-soil interactions. Our study demonstrates the potential of stable K isotopes in improving the understanding of the K cycle in soil-plant systems.