Modeling the Role of Root Exudation in Critical Zone Nutrient Dynamics

Abstract

The multifaceted interface of plant roots, microbes, water, and soil can be considered a critical zone within the Critical Zone as it is host to many important dynamically linked processes, including the promotion of nutrient cycling through absorption and rhizodeposition, interaction and feedbacks with microorganisms and fungi, root‐facilitated hydraulic redistribution, and soil carbon dynamics. Such important processes in the Critical Zone have not been fully characterized and modeled in an ecohydrologic framework linking above‐ground natural and/or anthropogenic processes to below‐ground biogeochemical cycling. Specifically, 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 the systematic modeling of root exudation in an interconnected ecohydrologic framework. REWT incorporates an explicit dynamic root exudation transport model, nutrient absorption, and coupled microbial processes within the framework of a validated ecohydrologic model. Model simulations demonstrate the influence of root exudation of glucose, a polysaccharide that serves as fuel for microbes, and flavonoids, which can act as a biological nitrification inhibitor on microbial processes linked to soil carbon and nitrogen cycling. To demonstrate the capabilities of this theoretical framework, we parameterize REWT for corn and soybean crops in the Midwestern United States, and simulations indicate that rates of nitrification and respiration were substantially altered compared to model simulations in which root exudation was not explicitly included. This work demonstrates the importance of systematically incorporating root exudates into hydrobiogeochemical models and can serve to inform experimental design for active root zone processes.

Progresses of weathered bedrock ecohydrology in the Earth’s critical zone

“…距地表60 m深的基岩垂直裂隙中仍有牧豆树根系生长 [82] 。基岩裂缝中生长的根系可能比通过深 层土壤生长的根系遇到的机械阻力小得多，为植物根系生长提供相对开放的空间，同时因深层人 为和土壤动物扰动较小，根系寿命可能更长，深根吸水率更高；即便是植物的根系在下扎过程中 碰到岩石裂隙等障碍物时，也会逐渐降低生长速率，开始生长侧根 [83,84] 。量化研究发现，植物根 系可穿透到风化基岩中>4 m的深度，在干燥季节，从2.9 m厚的风化基岩吸取394 mm水，大约是 从0.35 m厚的土壤中可吸水的10倍 [81] ，基岩风化层作为储水器，提供了生长季节树木至少70％的 水 [76] 。然而，先前研究也指出，仅在土壤水分吸收部分枯竭而减少后，基岩风化层水分才可能 成为重要的植物有效水源 [4,45,81,86] ，且从基岩风化层中提取的水是植物可利用的最后一种来源 [87][88][89][90] ，该部分水分仍对植被分布、持续生长、年度蒸腾和植被生产力起到至关重要的作用 [4,[91][92][93][94] 。 此外，在岩石风化过程中，根的穿插生长可以增加基岩孔隙度和破碎度，将惰性岩石转化为可为 植物和微生物提供水分和营养物质的非土壤基质，其基岩风化层裂隙中的粘粒组成、矿物类型、 可溶性有机碳、可矿化性C、N与上伏土壤层相似，同时还含有大量的Ca、Mg、P、K等营养元 素，在相对贫瘠地区植物深根能插入基岩风化层中充分利用其养分和水分 [18,91,95,96] 。例如，对我 国喀斯特地区研究也发现，基岩化学特征能够解释55%的植被生产力差异性 [50] 。因此，基岩风化 层水分对植物可用水及部分元素补给意义重大，但其在生态水文循环过程中的调节功能、影响程 度和作用机理仍需进一步研究和量化。…”

Roots induce hydraulic redistribution to promote nutrient uptake and nutrient cycling in nutrient-rich but dry near-surface layers

“…Moreover, recent modeling advances have accounted for the effect of HR on biogeochemical cycles (Roque-Malo et al, 2020;Quijano et al, 2013). Our study adds a novel framework for integrating short-distance processes (rhizodeposition-facilitated rhizosphere hydrodynamics) with long-distance feedback (HR).…”

Reflections on Earth surface research

“…Carbon, water and nutrient cycling appear particularly intense at some interfaces and the interaction between transport and biological activity at these interfaces is a fascinating problem down to the nanometer scale. The export of nutrients or pollutants such as nitrate and arsenic from a system may be controlled by intense activity at local chemical and redox interfaces, and the position and activity of these interfaces themselves can shift in response to seasonal, climatic and anthropogenic forcing 8 . The nested spatial and time scales of such interaction need to be better understood to permit broader predictive modeling of how such systems will respond to changing climates.…”

“…of Geography, University of California Santa Barbara, Santa Barbara, CA, USA 6 Dept. of Earth Science, University of California Santa Barbara, Santa Barbara, CA, USA 7 National Oceanography Centre, Southampton, UK 8 School of Earth and Environmental Sciences, Seoul National University, Seoul, Korea…”