Roots under water stress induce K release from phlogopite, bio-transforming to vermiculite

Abstract

Drought is an unavoidable abiotic stress that restricts the growth rate of many crops in arid regions. Potassium (K) is an essential nutrient and the most abundant cation in the plant, which is critical to decreasing the adverse effects of drought stress. K is supplied naturally by mica and other K-bearing primary minerals in the soil. This paper aimed to investigate the ability of barley roots (without interaction with soil organisms) to release K from phlogopite, to utilize the released K, and to transform this mineral to other minerals under drought stress conditions. A greenhouse pot experiment involving no soil organism was conducted in a completely randomized design with a factorial arrangement and three replications. The growth medium was a mixture of quartz sand (as filler) and three levels of phlogopite as a K-bearing mineral. During the five-month growth period, the test plant was supplied with either a complete nutrient solution or a K-free one. Three levels of drought stress were applied including no drought stress, moderate drought stress, and severe drought stress. At the end of the growth period, both shoot and root of the plant were separated and, their total K content was extracted and then determined by a flame photometer. Besides, the mineralogy of the clay fraction of the samples taken from the rhizosphere was studied using the XRD technique. The results indicated that drought stress reduced the K uptake of the plant. However, the application of K as either phlogopite mineral or K containing nutrient solution could somehow ameliorate the negative effects of drought stress. In K-free treatments, the amount of K released from phlogopite mineral was higher than in the treatments which received a complete nutrient solution. The higher level of added phlogopite did not necessarily supply more available K for plant, indicating that the application of a low amount of phlogopite was sufficient to supply the level of K needed. Results also stated that phlogopite was partly biotransformed to vermiculite due to the uptake of K by the plant roots. The rate of phlogopite transformation to vermiculite was found to be inversely correlated with the amount of phlogopite added. It was also negatively correlated with the level of drought stress applied. In general, it appears that water stress dramatically influences the K release rate from mineral and its biological weathering rate. Besides, the negative effects of drought stress could be managed by adding K-bearing minerals to growth media.