Quantitative constraints on plant K uptake from fertilizers using a novel stable K isotope labeling technique

Abstract:

Potassium (K) is a macronutrient essential for critical metabolic functions during plant growth, such as photosynthesis, enzyme activation, osmotic regulation, and pH homeostasis. K fertilizers are commonly used in agriculture to ensure sufficient bioavailable K in soils that can sustain healthy plant growth. An optimized use of K fertilizers is highly desirable for economic and environmental reasons, but conventional soil tests based on K concentrations are often inadequate in accurately predicting crop response to the amount of fertilizers applied, because tracing of a particular K source is complicated by the presence of different K pools in soils (i.e., soluble, exchangeable, non-exchangeable, and minerals) and possible K mass transfer among them. The recent advent of high-precision stable K isotope analysis (⁴¹K/³⁹K or δ⁴¹K) enables the use of a stable K isotope labeling technique, providing a novel quantitative method to study the K cycle in soils that can facilitate K biogeochemical models.

As a proof-of-concept, we conducted a pot study that grew corn in Plano silt loam soil in a greenhouse to quantify plant uptake of K specifically from fertilizers. Soil was pre-mixed thoroughly with different amounts (50, 100, 200 mg K/kg soil, in triplicate) of a ⁴¹K-labeled fertilizer (δ⁴¹K ≈ 5.5‰). The control used soil without fertilizer addition. Adequate levels of N and P fertilizer were used throughout to avoid deficiencies of those elements. Above-ground plants were harvested after ~6 weeks. For the control group, δ⁴¹K values of plants were lower than those of Bray-extracted K from remaining soils, indicating preferential uptake of light K isotopes by plants with an isotopic fractionation factor of ca. −0.2‰. In contrast, δ⁴¹K values of plants from the fertilizer treatments were significantly higher than values of Bray-extracted K from remaining soils. These results cannot be explained by K isotope fractionation during K uptake by plants but reflect preferential uptake of K from fertilizers, indicating that K transfer between soluble and exchangeable pools in soils may be much slower than traditionally believed. Moreover, calculations based on our K isotope results indicated that conventional concentration-based soil tests could underestimate the true fertilizer utilization efficiency by 10 to 30%.