Partitioning net ecosystem exchange of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; on the pedon scale in the Lena River Delta, Siberia

Tim Eckhardt,Gillian Simpson,Christian Knoblauch,Evgeny Abakumov,Eva-Maria Pfeiffer,David Holl,Lars Kutzbach

doi:10.5194/bg-16-1543-2019

Partitioning net ecosystem exchange of CO&lt;sub&gt;2&lt;/sub&gt; on the pedon scale in the Lena River Delta, Siberia

Tim Eckhardt, Gillian Simpson + Show 5 more

Open Access

https://doi.org/10.5194/bg-16-1543-2019

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Abstract

Abstract. Arctic tundra ecosystems are currently facing amplified rates of climate warming. Since these ecosystems store significant amounts of soil organic carbon, which can be mineralized to carbon dioxide (CO2) and methane (CH4), rising temperatures may cause increasing greenhouse gas fluxes to the atmosphere. To understand how net the ecosystem exchange (NEE) of CO2 will respond to changing climatic and environmental conditions, it is necessary to understand the individual responses of the processes contributing to NEE. Therefore, this study aimed to partition NEE at the soil–plant–atmosphere interface in an arctic tundra ecosystem and to identify the main environmental drivers of these fluxes. NEE was partitioned into gross primary productivity (GPP) and ecosystem respiration (Reco) and further into autotrophic (RA) and heterotrophic respiration (RH). The study examined CO2 flux data collected during the growing season in 2015 using closed-chamber measurements in a polygonal tundra landscape in the Lena River Delta, northeastern Siberia. To capture the influence of soil hydrology on CO2 fluxes, measurements were conducted at a water-saturated polygon center and a well-drained polygon rim. These chamber-measured fluxes were used to model NEE, GPP, Reco, RH, RA, and net primary production (NPP) at the pedon scale (1–10 m) and to determine cumulative growing season fluxes. Here, the response of in situ measured RA and RH fluxes from permafrost-affected soils of the polygonal tundra to hydrological conditions have been examined. Although changes in the water table depth at the polygon center sites did not affect CO2 fluxes from RH, rising water tables were linked to reduced CO2 fluxes from RA. Furthermore, this work found the polygonal tundra in the Lena River Delta to be a net sink for atmospheric CO2 during the growing season. The NEE at the wet, depressed polygon center was more than twice that at the drier polygon rim. These differences between the two sites were caused by higher GPP fluxes due to a higher vascular plant density and lower Reco fluxes due to oxygen limitation under water-saturated conditions at the polygon center in comparison to the rim. Hence, soil hydrological conditions were one of the key drivers for the different CO2 fluxes across this highly heterogeneous tundra landscape.

Highlights

An estimated 1000 Pg of organic carbon (OC) are stored in the upper 3 m of northern permafrost-affected soils (Hugelius et al, 2014)
As changes in soil temperature and moisture can significantly alter the individual fluxes contributing to net the ecosystem exchange (NEE), this study aims to improve the current understanding of CO2 flux dynamics in permafrost-affected ecosystems by (i) partitioning NEE into individual flux components at the pedon scale of the polygonal tundra and (ii) gaining insights into the response of these individual fluxes to different environmental parameters
The daily averaged photosynthetically active radiation (PAR) values showed a strong seasonality with decreasing daily mean values towards the end of the season, there was a period at the end of July with rather low daily averaged PAR values

Summary

Introduction

An estimated 1000 Pg (petagrams) of organic carbon (OC) are stored in the upper 3 m of northern permafrost-affected soils (Hugelius et al, 2014). Carbon has been sequestered in permafrost-affected soils and sediments due to cold conditions and poor drainage, resulting in water saturation and slow organic matter decomposition. Arctic ecosystems are facing amplified warming (AMAP, 2017; Taylor et al, 2013), which will lead to the longer and deeper thawing of permafrost-affected soils (Romanovsky et al, 2010). Higher temperatures increase the assimilation of CO2 by tundra vegetation due to a prolonged growing period and increased nutrient availability in the deeper layers of thawed soils (e.g., Beermann et al, 2017; Elmendorf et al, 2012; Salmon et al, 2016; Parmentier et al, 2011)

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