OOS 30-1
Hydraulic redistribution of water, nutrient uptake, and carbon exudation by plant roots interact with soil properties to drive complex spatial and temporal hotspots of rhizosphere resource availability
Daily oscillation of soil water driven by plant transpiration generate spatially and temporally variable resource patterns in the rhizosphere. Nighttime hydraulic redistribution of water by plants (i.e., the movement of water from moist into dry soil layers through the root systems of non-transpiring plants) can enhance these daily oscillations by pushing water outward from the root during the night, potentially moving soluble carbon outward faster, and farther, than it would otherwise diffuse. In order to understand how physical transport processes driven by transpiration and hydraulic redistribution interact with root physiology (nutrient uptake and carbon exudation) and soil properties (cation exchange) to influence nitrogen and carbon concentrations, we created a rhizosphere-scale flow and transport model that integrates: 1) water flow into and out of plant roots, 2) solute transport in the soil (advection, diffusion and cation exchange), and 3) ion uptake and carbon exudation roots. We investigated the spatial and temporal distribution of soil carbon and nitrogen (nitrate and ammonium) gradients at 0-10 cm from the root undergoing cycles of transpiration and hydraulic redistribution, and assessed responses under different soil moisture levels, soil texture, root uptake kinetics, and root exudation rates.
Results/Conclusions
When hydraulic redistribution was incorporated into the model with daytime transpiration, we observed an increase in the spatial and temporal heterogeneity of rhizosphere nitrogen and carbon compared to the transpiration-only case. The redistributed water dilutes nitrogen concentrations near the root during the night, and day/night concentration oscillations, combined with soil cation exchange, create nitrogen hotspots farther from the root. Adding nighttime hydraulic redistribution also promoted greater transport of carbon exudates away from roots. Nonlinearities in root ion uptake (linked to Michaelis-Menten kinetics) caused asymmetry of the resource hotspots in space and time unexpected from the simple 24-hour sine wave flow of water into and out of roots. Increased spatial and temporal heterogeneity in rhizosphere resource availability caused by hydraulic redistribution presents a distinct stage for greater complexity of microbial interactions. We are currently exploring the parameter sensitivity of these responses and plan to couple in microbial processes to gain a better understanding of how resource heterogeneity influences carbon and nitrogen cycling controlled by microbes.