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.