Rhizosphere Processes and Root Traits Determining the Acquisition of Soil Potassium

Abstract

Plants acquire K ions from the soil solution, and this small and dynamic pool needs to be quickly replenished via desorption of surface-adsorbed K from clay minerals and organic matter, by release of interlayer K from micaceous clay minerals and micas, or structural K from feldspars. Because of these chemical interactions with soil solid phases, solution K concentration is kept low and its mobility is restricted. In response, plants have evolved efficient strategies of root foraging. Root traits related to root system architecture (root angle and branching), root length and growth, together with root hairs and mycorrhiza-related traits help to determine the capacity of plants to cope with the poor mobility of soil K. Rooting depth is also important, given the potentially significant contribution of subsoil K in many soils. Root-induced depletion of K shifts the exchange equilibria, enhancing desorption of K, as well as the release of nonexchangeable, interlayer K from minerals in the rhizosphere. Both these pools can be bioavailable if plant roots can take up significant amounts of K at low concentrations in the soil solution (in the micromolar range). In addition, roots can significantly acidify their environment or release large amounts of organic compounds (exudates). These two processes ultimately promote the dissolution of micas and feldspars in the rhizosphere, contributing to the mining strategy evolved by plants. There are thus several root or rhizosphere-related traits P. Hinsinger (*) Eco&Sols, University of Montpellier, CIRAD, INRAE, Institut Agro, IRD, Montpellier, France e-mail: [email protected] M. J. Bell School of Agriculture and Food Science, The University of Queensland, Brisbane, Australia e-mail: [email protected] J. L. Kovar USDA-ARS, National Laboratory for Agriculture and the Environment, Ames, IA, USA e-mail: [email protected] P. J. White Ecological Sciences, The James Hutton Institute, Dundee, UK King Saud University, Riyadh, Saudi Arabia e-mail: [email protected] © The Author(s) 2021 T. S. Murrell et al. (eds.), Improving Potassium Recommendations for Agricultural Crops, https://doi.org/10.1007/978-3-030-59197-7_4 99 (morphological, physiological, or biochemical) that determine the acquisition of K by crop species and genotypes. 4.1 Soil Properties and Processes Determining the Acquisition of Potassium by Plants A number of soil characteristics determine K mobility, availability, and bioavailability to plants. These properties, together with the actual distribution of the various pools of K in the soil profile and horizons, ultimately determine the most desirable root and rhizosphere-related traits to search for in order to improve K acquisition efficiency in crops. 4.1.1 Potassium Mobility: Mass Flow Versus Diffusion in the Rhizosphere Potassium is present in the soil solution as K ions, which experience rather strong interactions (adsorption/desorption) with the many soil constituents contributing to cation exchange capacity, notably clay minerals and organic matter (Sparks and Huang 1985; Sparks 1987; Chap. 7). The consequences of such interactions are twofold. First, they buffer the concentration of K in the soil solution to values that commonly range from one to several hundred micromoles per dm (Asher and Ozanne 1967; Hinsinger 2006), i.e., concentrations that are significantly greater than those of phosphate, but less than those of nitrate. Second, they limit K mobility in the soil. Thus, compared with nitrate, K leaching occurs in significant amounts only in fertilized, light-textured soils. In addition, while mass flow can contribute significantly to the transport of nitrate toward the root surface as a consequence of transpiration-driven water uptake and corresponding solute movement, its contribution to the supply of K and phosphate ions is small (Barber 1995). Hence most K is transported to the root surface by diffusion, as a consequence of the concentration gradients that develop in the rhizosphere (Tinker and Nye 2000; Jungk 2001, 2002). Barber (1995) estimated that diffusion contributed about 80% of the K delivered to maize (Zea mays L.) roots in a Chalmers silt loam (Mollisol) soil (Table 4.1). Mobility is used here to describe the ability of K ions to move in soils, either vertically through leaching or laterally, through mass flow and diffusion (e.g., Hinsinger 2004). 100 P. Hinsinger et al.