NUTRIENT HOTSPOTS AND HOT MOMENTS IN THE RHIZOSPHERE GENERATED BY OSCILLATORY WATER FLOW AND SOIL CATION EXCHANGE

Plant transpiration is often considered an inevitable cost of photosynthetic carbon uptake, but transpiration also facilitates solute transport in the plant and through rhizosphere soil surrounding roots. Further, the diel nature of transpiration (oscillation of water flow between day and night) is thought to play an important role in controlling both plant and microbial access to soil nutrients. We hypothesized that when combined with competitive soil cation exchange, oscillatory plant-driven water flow can increase nutrient availability in the rhziosphere by promoting dis-equilibrium between dissolved and sorbed phases. We reasoned that when transpiration-driven transport of solutes stops at night, solutes that accumulated near the root during the day (e.g. low-demand cations such as Ca²⁺, Mg²⁺ and Na⁺) diffuse away from the root into the rhizosphere soil where they competitively exchange with cations that are more weakly adsorbed to soil (e.g., NH₄⁺ and K⁺). Desorption of these cations and their release into the soil solution increases their availability for biological uptake. To test this idea, we developed a single-root model that included plant-driven water flow, solute transport, competitive soil cation exchange, and root uptake of nutrients. As hypothesized, our model generated time-varying non-monotonic concentration gradients for NH₄⁺ and K⁺ (i.e., ‘hotspot’ peaks at ca. 0.5 cm from the root that intensified during the night). Plant uptake of NH₄⁺ and K⁺ and hotspot intensity were positively related, and both increased as the soil cation exchange capacity increased. In addition, reductions in diel oscillatory water flow driven by nighttime transpiration decreased hotspot generation, while enhancements in diel oscillatory water flow driven by nighttime hydraulic redistribution intensified hotspot formation. Our study elucidates a novel mechanism where transpiration-driven flow and solute uptake rates interact with soil cation exchange to generate nutrient hotspots and hot moments in the rhizosphere. By adjusting their diel patterns of water use plants can potentially influence spatial and temporal heterogeneity of nutrient distributions, microbial activity and, ultimately, nutrient cycling.