Mechanistic Modeling of Microtopographic Impacts on CO2 and CH4 Fluxes in an Alaskan Tundra Ecosystem Using the CLM‐Microbe Model

Yihui Wang,Margaret S Torn,Jitendra Kumar,Fengming Yuan,Liyuan He,Walter C Oechel,Fenghui Yuan,Robert Wagner,Melanie S Hahn,David A Lipson,Xiaofeng Xu,Peter E Thornton,Daniel M Ricciuto,Donatella Zona,Stan D Wullschleger,Baohua Gu

doi:10.1029/2019ms001771

Abstract

AbstractSpatial heterogeneities in soil hydrology have been confirmed as a key control on CO2 and CH4 fluxes in the Arctic tundra ecosystem. In this study, we applied a mechanistic ecosystem model, CLM‐Microbe, to examine the microtopographic impacts on CO2 and CH4 fluxes across seven landscape types in Utqiaġvik, Alaska: trough, low‐centered polygon (LCP) center, LCP transition, LCP rim, high‐centered polygon (HCP) center, HCP transition, and HCP rim. We first validated the CLM‐Microbe model against static‐chamber measured CO2 and CH4 fluxes in 2013 for three landscape types: trough, LCP center, and LCP rim. Model application showed that low‐elevation and thus wetter landscape types (i.e., trough, transitions, and LCP center) had larger CH4 emissions rates with greater seasonal variations than high‐elevation and drier landscape types (rims and HCP center). Sensitivity analysis indicated that substrate availability for methanogenesis (acetate, CO2 + H2) is the most important factor determining CH4 emission, and vegetation physiological properties largely affect the net ecosystem carbon exchange and ecosystem respiration in Arctic tundra ecosystems. Modeled CH4 emissions for different microtopographic features were upscaled to the eddy covariance (EC) domain with an area‐weighted approach before validation against EC‐measured CH4 fluxes. The model underestimated the EC‐measured CH4 flux by 20% and 25% at daily and hourly time steps, suggesting the importance of the time step in reporting CH4 flux. The strong microtopographic impacts on CO2 and CH4 fluxes call for a model‐data integration framework for better understanding and predicting carbon flux in the highly heterogeneous Arctic landscape.

Highlights

We aimed to address three questions: (1) How do different microtopographic types affect CO2 and CH4 fluxes in Arctic tundra ecosystems? (2) Which processes are more important in controlling CO2 and CH4 fluxes among the microtopographic types? (3) How do annual estimates of CO2 and CH4 fluxes differ under the microtopographic impacts in the Arctic? The field observational data from the U.S Department of Energy's Office of Science Generation Ecosystem Experiments (NGEE)‐Arctic project were integrated with the CLM‐Microbe model to understand microtopographic impacts on land surface CO2 and CH4 fluxes in an Arctic tundra landscape
The dynamics of CH4 was captured well by the CLM‐Microbe model for the trough, low‐centered polygon (LCP) center, and LCP rim, but those sites were simulated as a small CH4 sink in summer by the default CLM4.5 (Figures 2a–2c and Table 3)
net ecosystem exchange of CO2 (NEE) and ecosystem respiration (ER) had the similar patterns for the trough and LCP center but showed significant differences for LCP rim in the default CLM4.5 and CLM‐Microbe models (Figures 2d–2i)

Summary

Introduction

Spatial heterogeneity in land surface properties has been shown to be a key source of large variabilities and uncertainties in CO2 and CH4 fluxes in the Arctic (Bridgham et al, 2013; Davidson et al, 2016; Sturtevant & Oechel, 2013; Xu et al, 2014; Zona et al, 2011). Soil hydrological conditions affect vegetation growth and substrate availability, further influencing ecosystem C input and microbial community structure and altering the transport and production of CH4, root respiration, and microbial respiration (Davidson et al, 2016; von Fischer et al, 2010; Wagner et al, 2017). Accurate quantification of the strength of C sinks or sources requires explicit consideration of the microtopographic effects on C cycling in Arctic tundra ecosystems (Ebrahimi & Or, 2017)

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Results

Conclusion