Left to right: Soybean, wheat, and rice plants

 

 

 

 

 

Isotope geochemists at Berkeley Lab are the first to establish the degree of Potassium (K) isotopic fractionation by plants. Isotopes are atoms of the same element that have a different number of neutrons. Scientists have just in the past decade begun studying for key nutrient elements (other than C, O, N and H which have been studied for decades) the degree to which plants may favor one isotope over another in a chemical or physical process–resulting in isotopic fractionation.

Knowledge about how plants take in nutrient elements and transport them internally for various functions is useful to understanding their biogeochemical cycling–or how an element or compound moves between its various living and nonliving forms and locations in the biosphere.

“Plants are major players in determining the fate and transport of different elements and compounds at the ecosystem level,” says John Neil Christensen, an isotope geochemist and staff scientist at Berkeley Lab. Christensen led a research team in using mass spectrometry to establish the degree of Potassium isotopic fractionation by three important food crops–soybean, rice, and wheat–and to compare the isotopic fractionation of Potassium and Calcium by these three plants. A recent paper describing their results is published here.

“It’s believed that as much as 40 to 70 percent of the dissolved Potassium in the world’s rivers comes from plant decay. Understanding the isotopic composition of Potassium could help track and quantify nutrient cycling in ecosystems, or lead to improvements in fertilizing food crops that depend on the element. Calcium is important to a plant’s cellular walls and membrane integrity against the uncontrolled, passive  transfer of ions.”

Understanding the isotopic composition of Potassium could help track and quantify nutrient cycling in ecosystems, or lead to improvements in fertilizing food crops that depend on the element.

For their study, soybean, rice, and wheat plants were grown hydroponically from seed and harvested before full maturity after 2, 4, 6, and 8 weeks. Harvested plants were divided broadly for soybeans into root, stem, and leaf samples, and for rice and wheat plants into samples of roots and leaves.  Aliquots were taken of the sample solutions before separation for K and Ca isotopic analyses.

The researchers conducted hydroponic experiments growing soybean (Glycine max), rice (Oryza sativa), and wheat (Triticum aestivum) under K and Ca replete conditions to establish the degree of K isotopic fractionation by plants, and compare the isotopic fractionation of Ca and K.

In the Isotope Lab at Berkeley Lab, Christensen’s team analyzed the K samples using a plasma-source mass spectrometer and the Ca samples using thermal ionization mass spectrometry. The researchers were able to measure differences in isotopic ratios between different parts of the plants and between the plants and the source K and Ca in the hydroponic solution.  

“Potassium is such an important nutrient for plants that it is often added to agricultural crops as fertilizer, so it could be useful to be able to isotopically distinguish among the Potassium within soil and plants, and the Potassium added in fertilizer to assess the efficiency of uptake and to fine tune fertilizer application,

They saw the greatest degree of K and Ca isotopic fractionation in the soybean plants, followed by wheat and then rice.  Comparing the amount of isotopic fractionation by plant part for each plant, the team found that while the pattern of root to leaf isotope fractionation by Potassium is similar to that of the isotopic fractionation by Calcium, the magnitude of the fractionation is about 2 times greater for K than for Ca–perhaps due to different methods of fractionation by plants.

“Potassium is such an important nutrient for plants that it is often added to agricultural crops as fertilizer, so it could be useful to be able to isotopically distinguish among the Potassium within soil and plants, and the Potassium added in fertilizer to assess the efficiency of uptake and to fine tune fertilizer application,” Christensen says.

“Isotopic fractionation provides another potential tool for understanding how crops respond and adjust to drought or salt stress,” he says. “If a plant is under drought stress, that condition could affect isotopic fractionation through changes in ion transport across cellular membranes. Different plants react differently to environmental stresses: Knowing how they will react to stress factors that affect their nutrient uptake could have a big impact on crop growth.”