QUANTIFYING CARBON FLUXES AND ISOTOPIC SIGNATURE CHANGES ACROSS GLOBAL TERRESTRIAL ECOSYSTEMS

Description

<p>This thesis is a collection of three research
articles to quantify carbon fluxes and isotopic signature changes across global
terrestrial ecosystems. Chapter 2, the first article of this thesis, focuses on
the importance of an under-estimated methane soil sink for contemporary and
future methane budgets in the pan-Arctic region. Methane emissions from
organic-rich soils in the Arctic have been extensively studied due to their
potential to increase the atmospheric methane burden as permafrost thaws.
However, this methane source might have been overestimated without considering
high affinity methanotrophs (HAM, methane oxidizing bacteria) recently identified
in Arctic mineral soils. From this study, we find that HAM dynamics double the
upland methane sink (~5.5 TgCH₄yr^-1) north of 50°N in
simulations from 2000 to 2016 by integrating the dynamics of HAM and
methanogens into a biogeochemistry model that includes permafrost soil organic
carbon (SOC) dynamics. The increase is equivalent to at least half of the
difference in net methane emissions estimated between process-based models and
observation-based inversions, and the revised estimates better match site-level
and regional observations. The new model projects double wetland methane
emissions between 2017-2100 due to more accessible permafrost carbon. However,
most of the increase in wetland emissions is offset by a concordant increase in
the upland sink, leading to only an 18% increase in net methane emission (from
29 to 35 TgCH₄yr^-1). The projected net methane emissions
may decrease further due to different physiological responses between HAM and
methanogens in response to increasing temperature. This article was published
in <i>Nature Climate Change</i> in March
2020.</p>

<p>In Chapter 3, the second article of this
thesis, I develop and validate the first biogeochemistry model to simulate
carbon isotopic signatures (δ¹³C)
of methane emitted from global wetlands, and examined the importance of the wetland
carbon isotope map for studying the global methane cycle. I incorporated a carbon isotope-enabled module into an
extant biogeochemistry model to mechanistically simulate the spatial and
temporal variability of global wetland δ¹³C-CH₄. The new
model explicitly considers isotopic fractionation during methane production,
oxidation, and transport processes. I estimate a mean global wetland δ¹³C-CH₄ of
-60.78‰ with its seasonal and inter-annual variability. I find that the new
model matches field chamber observations 35% better in terms of root mean
square estimates compared to an empirical static wetland δ¹³C-CH₄ map.
The model also reasonably reproduces the regional heterogeneity of wetland δ¹³C-CH₄ in
Alaska, consistent with vertical profiles of δ¹³C-CH₄
from NOAA aircraft measurements. Furthermore, I show that the latitudinal
gradient of atmospheric δ¹³C-CH₄ simulated by a chemical
transport model using the new wetland δ¹³C-CH₄ map
reproduces the observed latitudinal gradient based on NOAA/INSTAAR global
flask-air measurements. I believe this study is the first process-based
biogeochemistry model to map the global distribution of wetland δ¹³C-CH₄,
which will significantly help atmospheric chemistry transport models partition
global methane emissions. This article is in preparation for submission
to <i>Nature Geoscience</i>.</p>

<p>Chapter 4 of this thesis, the third
article, investigates the importance of leaf carbon allocation for seasonal
leaf carbon isotopic signature changes and water use efficiency in temperate
forests. Temperate deciduous trees remobilize stored carbon early in the
growing season to produce new leaves and xylem vessels. The use of remobilized
carbon for building leaf tissue dampens the link between environmental stomatal
response and inferred intrinsic water use efficiency (iWUE) using leaf carbon
isotopic signatures (δ¹³C). So far, few studies consider carbon
allocation processes in interpreting leaf δ¹³C signals. To
understand effects of carbon allocation on δ¹³C and iWUE estimates,
we analyzed and modeled the seasonal leaf δ¹³C of four temperate
deciduous species (<i>Acer saccharum, Liriodendron tulipifera, Sassafras
albidum, </i>and <i>Quercus alba</i>)
and compared the iWUE estimates from different methods, species, and drought
conditions. At the start of the growing season, leaf δ¹³C values
were more enriched, due to remobilized carbon during leaf-out. The bias towards
enriched leaf δ¹³C values explains the higher iWUE from leaf
isotopic methods compared with iWUE from leaf gas exchange measurements. I
further showed that the discrepancy of iWUE estimates between methods may be
species-specific and drought sensitive. The use of δ¹³C of plant
tissues as a proxy for stomatal response to
environmental processes, through iWUE, is complicated due to carbon
allocation and care must be taken when interpreting estimates to avoid proxy
bias. This
article is in review for publication in <i>New
Phytologist</i>.</p>

<p> </p>

Links & Downloads

1. 10.25394/pgs.12730481.v1

Tags

Environmental Science

Climate Change Processes

Ecological Impacts of Climate Change

Arctic permafrost soils

Methane oxidizing bacteria

Wetlands

Global methane cycle

Leaf carbon allocation

Water use efficiency

Additional Fields

Identifer	oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/12730481
Date	29 July 2020
Creators	Youmi Oh (9179345)
Source Sets	Purdue University
Detected Language	English
Type	Text, Thesis
Rights	CC BY 4.0
Relation	https://figshare.com/articles/thesis/QUANTIFYING_CARBON_FLUXES_AND_ISOTOPIC_SIGNATURE_CHANGES_ACROSS_GLOBAL_TERRESTRIAL_ECOSYSTEMS/12730481