Title: Hot Spots and Hot Moments in the Critical Zone: Identification of and Incorporation into Reactive Transport Models
Biogeochemical processes are often spatially discrete (hot spots) and temporally isolated (hot moments) due to variability in controlling factors like hydrologic fluxes, lithological characteristics, bio-geomorphic features, and external forcing. Although these hot spots and hot moments (HSHMs) account for a high percentage of carbon, nitrogen and nutrient cycling within the Critical Zone, the ability to identify and incorporate them into reactive transport models remains a significant challenge. This chapter provides an overview of the hot spots hot moments (HSHMs) concepts, where past work has largely focused on carbon and nitrogen dynamics within riverine systems. This work is summarized in the context of process-based and data-driven modeling approaches, including a brief description of recent research that casts a wider net to incorporate Hg, Fe and other Critical Zone elements, and focuses on interdisciplinary approaches and concepts. The broader goal of this chapter is to provide an overview of the gaps in our current understanding of HSHMs, and the opportunities therein, while specifically focusing on the underlying parameters and processes leading to their prognostic and diagnostic representation in reactive transport models.
Bhavna Arora, Martin A.(
, Biogeochemistry of the Critical Zone. Advances in Critical Zone Science)
Wymore, A.S.
(Ed.)
The Critical Zone encompasses the biosphere and its heterogeneities, with an extremely high differentiation of properties and processes within each compartment from bedrock to canopy, and across terrestrial and aquatic interfaces. Given this complexity, a comprehensive areal characterization of the critical zone environment at multiple temporal resolutions is needed but not always possible, and failing which the ecosystem fluxes, exchange rates and biogeochemical functioning may be under- or over-predicted. The hot spots hot moments (HSHMs) concept provides an opportunity to identify the dominant controls on carbon, nutrients, water and energy exchanges. Hot spots are regions or sites that show disproportionately high reaction rates relative to surrounding area, while hot moments are defined as times that show disproportionately high reaction rates relative to longer intervening time periods (McClain et al. 2003).
Jensen, Alexandria; Ford, William; Fox, James; Husic, Admin(
, Transactions of the ASABE)
Abstract. Water quality models serve as an economically feasible alternative to quantify fluxes of nutrient pollution and to simulate effective mitigation strategies; however, their applicability is often questioned due to broad uncertainties in model structure and parameterization, leading to uncertain outputs. We argue that reduction of uncertainty is partially achieved by integrating stable isotope data streams within the water quality model architecture. This article outlines the use of stable isotopes as a response variable within water quality models to improve the model boundary conditions associated with nutrient source provenance, constrain model parameterization, and elucidate shortcomings in the model structure. To assist researchers in future modeling efforts, we provide an overview of stable isotope theory; review isotopic signatures and applications for relevant carbon, nitrogen, and phosphorus pools; identify biotic and abiotic processes that impact isotope transfer between pools; review existing models that have incorporated stable isotope signatures; and highlight recommendations based on synthesis of existing knowledge. Broadly, we find existing applications that use isotopes have high efficacy for reducing water quality model uncertainty. We make recommendations toward the future use of sediment stable isotope signatures, given their integrative capacity and practical analytical process. We also detail a method to incorporate stablemore »isotopes into multi-objective modeling frameworks. Finally, we encourage watershed modelers to work closely with isotope geochemists to ensure proper integration of stable isotopes into in-stream nutrient fate and transport routines in water quality models. Keywords: Isotopes, Nutrients, Uncertainty analysis, Water quality modeling, Watershed.« less
Abstract. Watersheds are the fundamental Earth surface functioning units that connect the land to aquatic systems. Many watershed-scale models represent hydrological processes but not biogeochemical reactive transport processes. This has limited our capability to understand and predict solute export, water chemistry and quality, and Earth system response to changing climate and anthropogenic conditions. Here we present a recently developed BioRT-Flux-PIHM (BioRT hereafter) v1.0, a watershed-scale biogeochemical reactive transport model. The model augments the previously developed RT-Flux-PIHM that integrates land-surface interactions, surface hydrology, and abiotic geochemical reactions. It enables the simulation of (1) shallow and deep-water partitioning to represent surface runoff, shallow soil water, and deeper groundwater and of (2) biotic processes including plant uptake, soil respiration, and nutrient transformation. The reactive transport part of the code has been verified against the widely used reactive transport code CrunchTope. BioRT-Flux-PIHM v1.0 has recently been applied in multiple watersheds under diverse climate, vegetation, and geological conditions. This paper briefly introduces the governing equations and model structure with a focus on new aspects of the model. It also showcases one hydrology example that simulates shallow and deep-water interactions and two biogeochemical examples relevant to nitrate and dissolved organic carbon (DOC). These examples are illustrated in twomore »simulation modes of complexity. One is the spatially lumped mode (i.e., two land cells connected by one river segment) that focuses on processes and average behavior of a watershed. Another is the spatially distributed mode (i.e., hundreds of cells) that includes details of topography, land cover, and soil properties. Whereas the spatially lumped mode represents averaged properties and processes and temporal variations, the spatially distributed mode can be used to understand the impacts of spatial structure and identify hot spots of biogeochemical reactions. The model can be used to mechanistically understand coupled hydrological and biogeochemical processes under gradients of climate, vegetation, geology, and land use conditions.« less
The study of Critical Zone (CZ) biogeochemistry in Intensively Managed Landscapes is a study of transitions. Large-scale anthropogenic inputs in the form of agricultural practices have induced significant shifts in the transport and transformation of water, carbon, and nutrients across the landscape. Disentangling the present-day complexity of physical, biological, and hydrologic CZ processes in intensively managed landscapes requires us to first understand the interplay between underlying natural processes which have occurred over geologic time scales from the overpowering, comparatively abrupt onset of intensive agricultural practices that have dominated the landscape in recent centuries. Modeling provides a unique advantage to extricate such complex processes. Advancements in recent years have improved our ability to elucidate (1) the coevolution of soil organic carbon storage, movement, and decomposition under climate and land cover changes, (2) the impacts of intensive agricultural management practices on age-nutrient dynamics and their consequential modification of rates and landscape fluxes, and (3) the integral regulatory role of vegetation and root exudation on CZ biogeochemical processes. In this chapter, we will review several recent models developed in the Intensively Managed Landscape Critical Zone Observatory in Illinois, USA that advance our understanding of critical transitions in biogeochemical dynamics due to intensive managementmore »and discuss future challenges.« less
O’Connell, Christine S.; Anthony, Tyler L.; Mayes, Melanie A.; Pérez, Tibisay; Sihi, Debjani; Silver, Whendee L.(
, Frontiers in Forests and Global Change)
Soil nitrous oxide (N 2 O) emissions are an important driver of climate change and are a major mechanism of labile nitrogen (N) loss from terrestrial ecosystems. Evidence increasingly suggests that locations on the landscape that experience biogeochemical fluxes disproportionate to the surrounding matrix (hot spots) and time periods that show disproportionately high fluxes relative to the background (hot moments) strongly influence landscape-scale soil N 2 O emissions. However, substantial uncertainties remain regarding how to measure and model where and when these extreme soil N 2 O fluxes occur. High-frequency datasets of soil N 2 O fluxes are newly possible due to advancements in field-ready instrumentation that uses cavity ring-down spectroscopy (CRDS). Here, we outline the opportunities and challenges that are provided by the deployment of this field-based instrumentation and the collection of high-frequency soil N 2 O flux datasets. While there are substantial challenges associated with automated CRDS systems, there are also opportunities to utilize these near-continuous data to constrain our understanding of dynamics of the terrestrial N cycle across space and time. Finally, we propose future research directions exploring the influence of hot moments of N 2 O emissions on the N cycle, particularly considering the gaps surroundingmore »how global change forces are likely to alter N dynamics in the future.« less
Arora B., Briggs, Zarnetske, J., Stegen, J., Gomez-Velez, J. D., Dwivedi, D., and Steefel, C.. Hot Spots and Hot Moments in the Critical Zone: Identification of and Incorporation into Reactive Transport Models. Retrieved from https://par.nsf.gov/biblio/10278844. Biogeochemistry of the Critical Zone .
Arora B., Briggs, Zarnetske, J., Stegen, J., Gomez-Velez, J. D., Dwivedi, D., & Steefel, C.. Hot Spots and Hot Moments in the Critical Zone: Identification of and Incorporation into Reactive Transport Models. Biogeochemistry of the Critical Zone, (). Retrieved from https://par.nsf.gov/biblio/10278844.
Arora B., Briggs, Zarnetske, J., Stegen, J., Gomez-Velez, J. D., Dwivedi, D., and Steefel, C..
"Hot Spots and Hot Moments in the Critical Zone: Identification of and Incorporation into Reactive Transport Models". Biogeochemistry of the Critical Zone (). Country unknown/Code not available. https://par.nsf.gov/biblio/10278844.
@article{osti_10278844,
place = {Country unknown/Code not available},
title = {Hot Spots and Hot Moments in the Critical Zone: Identification of and Incorporation into Reactive Transport Models},
url = {https://par.nsf.gov/biblio/10278844},
abstractNote = {Biogeochemical processes are often spatially discrete (hot spots) and temporally isolated (hot moments) due to variability in controlling factors like hydrologic fluxes, lithological characteristics, bio-geomorphic features, and external forcing. Although these hot spots and hot moments (HSHMs) account for a high percentage of carbon, nitrogen and nutrient cycling within the Critical Zone, the ability to identify and incorporate them into reactive transport models remains a significant challenge. This chapter provides an overview of the hot spots hot moments (HSHMs) concepts, where past work has largely focused on carbon and nitrogen dynamics within riverine systems. This work is summarized in the context of process-based and data-driven modeling approaches, including a brief description of recent research that casts a wider net to incorporate Hg, Fe and other Critical Zone elements, and focuses on interdisciplinary approaches and concepts. The broader goal of this chapter is to provide an overview of the gaps in our current understanding of HSHMs, and the opportunities therein, while specifically focusing on the underlying parameters and processes leading to their prognostic and diagnostic representation in reactive transport models.},
journal = {Biogeochemistry of the Critical Zone},
author = {Arora B., Briggs and Zarnetske, J. and Stegen, J. and Gomez-Velez, J. D. and Dwivedi, D. and Steefel, C.},
editor = {Wymore, A. and Yang, W. and Silver, W. and McDowell, B. and Chorover, J.}
}
