1. Introduction
Stream restoration can potentially increase floodplain carbon stocks by
enhancing deposition of organic matter and sequestration of soil organic
carbon. Within the global carbon cycle, however, potential magnitudes of
organic carbon stocks in the freshwater hydrosphere are not yet well
constrained and the uncertainty is particularly substantial for carbon
stocks in river corridors (i.e., active channels and floodplains; Battin
et al., 2009; Aufdenkampe et al., 2011; Hilton and West, 2020). This
uncertainty partly reflects the limited number of field-based
quantifications of river corridor carbon stocks (Sutfin et al., 2016;
Hinshaw and Wohl, 2021) and partly reflects the substantial spatial
variability that can be present in these stocks, with limited portions
of a river corridor accounting disproportionately for total carbon stock
in the river network or the entire watershed (Wohl et al., 2012; Wohl
and Knox, 2022). Despite these uncertainties, there is a growing need to
quantitatively estimate and predict organic carbon stocks in river
corridors in connection with the potential for carbon sequestration as
one of the goals of stream restoration (e.g., Yan et al., 2022).
Here, we examine whether there are detectable differences in floodplain
carbon stock in categories of impacted or degraded floodplains,
treatment floodplains that have recently undergone restoration, and
reference floodplains that display relatively greater floodplain
function and connectivity. This study design conceptually models carbon
storage potential as a result of restoration at short and longer
timescales, where treated floodplains represent short timescales and
reference floodplains represent longer timescales. Before-after
restoration studies are necessary to directly attribute changes in
carbon stocks to restoration activities. We recommend this strategy in
future as more restoration projects emphasize hydrologically
reconnecting the channel and floodplain, but these studies will be most
effective when they include a timespan of a decade or more from before
to after treatment. We were not able to access the restoration sites in
this study before treatment.
We first review existing knowledge of organic carbon stocks in river
corridors and how human activities have altered these stocks. We then
discuss how stream restoration might enhance carbon stocks and describe
the design of this study, in which our objective is to determine whether
there are differences in carbon stocks in the three categories of
floodplains at diverse sites in the western United States.
Organic carbon stocks in river corridors
Carbon stock refers to the mass of carbon stored in a carbon pool such
as soil or living vegetation. Carbon sequestration refers to the ability
to capture and store carbon; sequestration can maintain or increase
carbon stocks. Within river corridors, floodplains typically contain
much greater carbon stock than active channels. Within floodplains,
carbon stocks occur as living biomass (i.e., vegetation and aquatic
organisms), dissolved carbon in surface and ground water, dead biomass
including large wood in the floodplain, and soil organic carbon (Sutfin
et al., 2016; Wohl et al., 2017). Floodplain soil organic carbon (SOC)
typically forms the largest stock within a river corridor unless the
river corridor lacks a floodplain (Scott and Wohl, 2018b). Published
values for floodplain SOC range from 1.4 Mg C/ha at a site in South
Carolina, USA to 7735 Mg C/ha on a floodplain in northwestern Montana,
USA (Sutfin et al., 2016). We seek to quantitatively estimate existing
and potential floodplain SOC stocks in connection with floodplain
restoration. Although riparian vegetation growth increases organic
carbon stocks within aboveground biomass (e.g., Hanberry et al., 2015),
we do not account for living biomass (plants and vegetation) in this
study. Rather, we focus on SOC as an integrative representation of
above- and below-ground carbon stocks at longer timescales.
Although typically the largest carbon stock in floodplains, SOC is
highly spatially variable (Samaritani et al., 2011; Sutfin et al., 2016;
Wohl et al., 2017). In this context, we use soil to refer to all mineral
sediment and particulate organic matter in floodplain alluvium.
Floodplain SOC stocks reflect complex interactions among climate;
geology; soil moisture, texture, and residence time; biomass; and
organic matter supply (Hinshaw and Wohl 2021). Optimal conditions for
large floodplain SOC stocks are wide, wet, relatively stable valley
bottoms with long sediment residence times, cooler climates, and high
organic matter inputs (Sutfin et al., 2016; Hinshaw and Wohl 2021;
Sutfin et al., 2021). Residence times of floodplain sediment and
associated SOC depend on fluvial erosion rates and vary from years to
decades in small floodplains or locations close to the active channel(s)
to thousands of years in larger floodplains and at locations farther
from the channel (Wohl, 2015; Sutfin and Wohl, 2019). Residence time of
floodplain SOC also reflects rates of mineralization through microbial
processing that releases CO2 to the atmosphere and
dissolved organic carbon to downstream transport and to groundwater
(Bouillon et al., 2009; Handique, 2015).
Spatial and temporal variations in soil moisture, organic matter inputs,
and soil residence time can create significant lateral and longitudinal
variations in floodplain SOC in large floodplains (Lininger et al.,
2018) and longitudinal variations in smaller mountain streams (Scott and
Wohl, 2018a; Sutfin and Wohl, 2019). Consequently, quantitative
estimates of floodplain SOC stock may be most accurate and appropriately
applied at the reach scale, where a reach is a length of river corridor
with consistent channel and valley geometry that is at least several
times as long as average channel width. Spatial variations in floodplain
SOC also complicate attempts to use space-for-time substitutions in
which sites from different rivers or different portions of a river are
used to understand potential temporal changes following restoration, for
example. This is the approach we use in this study, but we acknowledge
the limitations and uncertainties inherent in this approach.
Human alterations and restoration of floodplain carbon stocks
Human alterations of river corridors can reduce organic carbon stocks in
floodplain vegetation, downed wood, and soil (Hanberry et al., 2015;
Wohl et al., 2017). Alterations include floodplain drainage that reduces
primary productivity and soil moisture; deforestation that reduces
primary productivity; and removal of large, downed wood from the channel
and floodplain that reduces sediment trapping and eventual soil
development. In addition, flow regulation, artificial levee
construction, and channelization decrease channel-floodplain
connectivity, in turn reducing associated floodplain inundation and
deposition of sediment and organic matter.
Stream restoration can potentially enhance carbon stocks by restoring
processes that facilitate higher floodplain water tables and associated
reducing conditions in the soil, as well as greater floodplain primary
productivity and increased deposition of sediment and organic matter.
Access to soil moisture from raised water tables facilitates new
riparian vegetation growth that provides higher supply of organic matter
via leaf litter. The conditions optimal for promoting large floodplain
carbon stocks correspond to Stage 0 anastomosing wet woodland or
anastomosing grassed wetland in the Cluer and Thorne (2014) stream
evolution model. Reconfiguration and reconnection of river corridors to
achieve Stage 0 conditions has been increasingly applied in the United
States as part of stream restoration efforts within the past decade
(Booth et al., 2009; Powers et al., 2019; Mattern et al., 2020;
Flitcroft et al., 2022).
The process of stream restoration implementation has a non-zero carbon
footprint, estimated by Chiu et al. (2022) as 9-14 kg
CO2 per meter of stream restored. However, ecological
restoration can transform the relative proportions of landscapes
considered as carbon sources versus sinks and provide significant
capacity to more efficiently sequester, rather than emit, carbon over
decadal timescales (Zhou et al., 2020).
Commonly, only a small portion of the project budget for most stream
restoration projects is allocated to monitoring, and practically no
budget is allocated to measure carbon stocks. However, incentives exist
for practitioners to start measuring carbon. Other than the
informational value of quantification of carbon sequestered from the
atmosphere, carbon credits in the units of tons of carbon can be sold on
the carbon market (Wara, 2007; Schneider et al., 2019). This practice is
widely applied within industries of agriculture and forestry (Ribaurdo
et al., 2010; Paul et al., 2013), and floodplain restoration could also
qualify for carbon offsets (e.g., Matzek et al., 2015; Sapkota and
White, 2020).
To our knowledge, only one study thus far has directly examined the
effects of restoration on carbon stock in river corridors. Samaritani et
al. (2011) compared soil carbon stocks and fluxes in channelized and
restored portions of the Thur River in Switzerland in the context of
spatial heterogeneity and temporal variability but did not explicitly
compare total carbon stock between restored and degraded areas. The
restored floodplain had a larger range and higher heterogeneity of
organic carbon stocks and fluxes. Related studies indicate the effects
of human alterations on river corridor carbon stock by comparing altered
and natural river corridors in the same region. Cabezas and Comin (2010)
found that floodplain soils with natural land cover have higher organic
carbon stocks than agricultural portions of the floodplain along Spain’s
Middle Ebro River, a pattern similar to that from Lininger and Polvi
(2020), who showed decreasing floodplain SOC stocks with increasing
human alteration in Swedish river corridors.
To enhance our understanding of carbon sequestration potential in stream
restoration, we must address at least two questions: (1) Can measurable
quantities of carbon be added to floodplain soil through restoration,
and over what timescales?, and (2) What framework best facilitates
understanding and measuring carbon stocks in stream restoration?
Ideally, measurement of carbon in stream restoration would occur before
and after restoration takes place. As a surrogate for pre and post
restoration conditions, we use three alternative floodplain states to
evaluate floodplain SOC stocks: degraded, treatment, and reference.
Alternative states are self-reinforcing states of equilibrium that can
exist simultaneously under the same environmental conditions (Holling,
1973; May, 1977). We consider our categories of floodplain to
approximate alternative states of semi-equilibrium that have been
affected by different levels of human intervention. We use the
terminology of degraded, treatment, and reference to designate potential
near-endmembers and an intermediate position on a spectrum of
restoration, but recognize that 1) degraded and reference sites are not
exact endmember positions, 2) the treatment, or stream restoration
project category, is likely not in equilibrium and may fall anywhere
along the spectrum, and 3) reference conditions do not always provide
ideal comparisons because we may not be able to restore streams to a
selected reference state (Dufour and Piegay 2009), and it may not be
possible to find exact matches between reference, treatment, and
degraded sites with respect to the many environmental variables that can
influence river corridors and carbon stock.
We use the term treatment instead of restored because stream restoration
sites are not “restored” as soon as construction takes place. The term
degraded can encompass a range of impaired or impacted floodplain
conditions. Degraded sites represent intensive land uses that have
degraded natural floodplain processes over time, and commonly include
histories of levee construction, channel straightening, grazing,
agriculture, timber harvest, or other activities that disconnect
channels from their floodplains. We use degraded as a descriptive term
and recognize that degraded floodplains can fall within a spectrum of
conditions that may not be directly caused by one single type of
floodplain alteration. Rather, floodplains chosen for the degraded
category can represent the culmination of land or resource uses that may
have limited floodplain function over the past few centuries.
The primary objective of this study is to quantitatively compare
floodplain organic carbon stocks in degraded, treatment, and reference
stream corridors. Given the assumptions outlined above, we hypothesize
that degraded sites contain the least carbon, treatment sites contain an
intermediate amount of carbon, and reference sites contain the most
carbon. Our secondary objective is to use the data to examine factors
that influence floodplain SOC stock at sites across the regional scale
of the western United States.