Plain English Summary

Root exudation is the highly complex and variable process by whichroots release carbon and other nutrients into the surrounding soil to inuence microbial activity andbiogeochemical conditions to their benet. It is a process common to all types of plants and trees and hasimportant implications for nutrient cycling and soil health. Here we develop a model, REWT (RootExudation in Watershedscale Transport), which couples exudation with moisture transport and microbialtransformation of nutrients in agricultural and natural soils in an effort to more accurately capturedynamic biogeochemical processes in the Critical Zone. We present REWT simulations for corn and soybeancrops in the Midwestern United States. These demonstrate that explicitly incorporating root exudation intohydrobiogeochemical models can result in substantial increases or decreases of microbial biomass, rateof nitrication, and total nitrate leached by the soil system, in this case on the order of ±4%, up to 100%decrease, and up to 16% decrease, respectively. REWT simulations are not only promising for future workrelated to plant breeding and modeling of biogeochemical cycling in natural and agroecosystems; they alsoshed light on the need for more empirical work related to rhizosphere processes, particularly themeasurement of root exudation rates.

Abstract

The multifaceted interface of plant roots, microbes, water, and soil can be considered a criticalzone within the Critical Zone as it is host to many important dynamically linked processes, including thepromotion of nutrient cycling through absorption and rhizodeposition, interaction and feedbacks withmicroorganisms and fungi, rootfacilitated hydraulic redistribution, and soil carbon dynamics. Suchimportant processes in the Critical Zone have not been fully characterized and modeled in an ecohydrologicframework linking aboveground natural and/or anthropogenic processes to belowground biogeochemicalcycling. Specically, the relation between root exudates and nutrient cycling remains an open challenge.Here we present the model REWT (Root Exudation in Watershedscale Transport) to demonstrate thesystematic modeling of root exudation in an interconnected ecohydrologic framework. REWT incorporatesan explicit dynamic root exudation transport model, nutrient absorption, and coupled microbialprocesses within the framework of a validated ecohydrologic model. Model simulations demonstrate theinuence of root exudation of glucose, a polysaccharide that serves as fuel for microbes, and avonoids,which can act as a biological nitrication inhibitor on microbial processes linked to soil carbon and nitrogencycling. To demonstrate the capabilities of this theoretical framework, we parameterize REWT for corn andsoybean crops in the Midwestern United States, and simulations indicate that rates of nitrication andrespiration were substantially altered compared to model simulations in which root exudation was notexplicitly included. This work demonstrates the importance of systematically incorporating root exudatesinto hydrobiogeochemical models and can serve to inform experimental design for active root zone processes.

Citation

Roque-Malo, S., Woo, D.K., and P. Kumar (2020): Modeling the Role of Root Exudation in Critical Zone Nutrient Dynamics. Water Resources Research. DOI: 10.1029/2019WR026606