Riverine Export of Aged Carbon Driven by Flow Path Depth and

Subscriber access provided by READING UNIV

Article

Riverine export of aged carbon driven by flow path depth and residence time Rebecca T Barnes, David Ellison Butman, Henry Wilson, and Peter A. Raymond Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04717 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 14, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

1

Environmental Science & Technology

Riverine export of aged carbon driven by flow path depth and residence time

2 3

Rebecca T. Barnes1*, David E. Butman2, Henry F. Wilson3, Peter A. Raymond4

4 5

1

Environmental Program, Colorado College, Colorado Springs, CO 80903, USA

6

2

School of Environmental and Forest Sciences, University of Washington, Seattle, WA 98195,

7

USA

8

3

9

Canada

Brandon Research Centre, Agriculture and Agri-Food Canada, Brandon, Manitoba R7A 5Y3,

10

4

11

*corresponding author: [email protected]

School of Forestry & Environmental Sciences, Yale University, New Haven, CT 06115, USA

12

ACS Paragon Plus Environment


13

Abstract

14

The flux of terrestrial C to rivers has increased relative to pre-industrial levels, a fraction of

15

which is aged dissolved organic C (DOC). In rivers, C is stored in sediments, exported to the

16

ocean, or (bio)chemically processed and released as CO2. Disturbance changes land cover and

17

hydrology, shifting potential sources and processing of DOC. To investigate the likely sources of

18

aged DOC, we analyzed radiocarbon ages, chemical, and spectral properties of DOC and major

19

ions from nineteen rivers draining the coterminous U.S. and Arctic. DOC optics indicated that

20

the majority is exported as aromatic, high molecular weight, modern molecules while aged DOC

21

tended to consist of smaller, microbial degradation products. Aged DOC exports, observed

22

regularly in arid basins and during base flow in arctic rivers, are associated with higher

23

proportion of mineral weathering products, suggesting deeper flows paths. These patterns also

24

indicate potential for production of microbial byproducts as DOC ages in soil and water with

25

longer periods of time between production and transport. Thus, changes in hydrology associated

26

with landscape alteration (e.g. tilling or shifting climates) that can result in deeper flow paths or

27

longer residence times will likely lead to a greater proportion of aged carbon in riverine exports.

28 29


Page 2 of 25

Page 3 of 25

30 31


Introduction To predict how the carbon (C) cycle will respond to global change drivers, we need to

32

understand how organic matter pools at the land-water interface will shift in response to drivers

33

such as altered precipitation and increased agricultural development1. The terrestrial landscape

34

exports approximately 2.7 Pg C yr-1 to aquatic systems2, 3, a value that has increased by ~0.1 –

35

0.2 Pg C yr-1 due to anthropogenic activities3. Inland waters both process and transport organic

36

matter, storing C in sediment4 and releasing it to the atmosphere as CO2 and CH42, 5-7. The fate of

37

organic carbon within inland waters is largely determined by its chemical composition and the

38

hydrology of the system. For example, the chemical composition of dissolved organic matter will

39

determine, in part, how susceptible it is to microbial metabolism8 and UV oxidation9. Further,

40

studies have illustrated that the majority of annual dissolved organic C (DOC) export occurs

41

during high flow events, in both temperate10, tropical11, and arctic12 systems. During these

42

periods of high flow, it is likely that shorter transit times lead to increased export of less

43

processed DOC to the coastal ocean.

44

Stream DOC is dominated by terrestrial sources in most systems13 and thus if the age and

45

nature of the DOC varies between systems or through time it follows that DOC source regions

46

(or processing) are different or shifting. The majority of this DOC is modern in age, reflecting its

47

dominant sources: terrestrial vegetation and surface soils14. However, rivers also export aged

48

DOC, organic carbon that is stored in terrestrial sinks, including: shales15, peatlands16,

49

permafrost17, as well as non-terrestrial sources such as precipitation18, 19 and petroleum products

50

ranging from soaps to motor oil20. A recent synthesis illustrated that human landscape

51

disturbance, i.e. urban and agricultural development, is associated with the export of aged carbon

52

from streams and rivers21. Further, several studies have documented relationships between flow



53

conditions and DOC age; though some systems export older carbon during baseflow17 and others

54

during high flow22, 23. This aged carbon, once thought recalcitrant, is biologically available

55

within aquatic systems 24-26, fueling CO2 production and evasion. Given that shifts in

56

precipitation are expected to be a dominant driver in future riverine C fluxes27, 28, it is critical that

57

we understand how this changing hydrology will affect C source areas, export and, cycling.

58

Changes in hydrology are likely to affect flow paths and thus the nature and amount of

59

carbon exported. Kaiser and Kalbitz29 describe a conceptual model that explains the vertical

60

profile observations of soil organic matter (SOM) characteristics. Several studies have

61

documented an increase in the age of SOM with depth30 which cannot be explained by the simple

62

leaching of organic matter decomposition products in surface horizons to depth31. In addition to

63

the physio-chemical stripping of dissolved organic matter (DOM) components (fractional

64

sorption and co-precipitation32), DOM undergoes microbial processing during transport29. Thus

65

given that exported DOM reflects both the sources and cycling of SOM33, riverine chemistry

66

should reflect differences in flow path depth. For example, surficial and shallow flow paths will

67

export large amounts of modern, plant-derived DOM, while deeper flows paths are more likely

68

to have an aged, microbial DOM signature.

69

Deeper portions of the soil column will likely have a greater proportion of organic matter

70

protected via organo-mineral associations, which are key to retaining C within soils34, 35 as

71

organo-mineral associations (cation bridging, ligand exchange, cation-anion exchange, hydrogen

72

bonds, Van der Waal forces, etc.) are more important in predicting stability and turnover of SOM

73

than the molecular structure of the OM35, 36. Tipping et al.37 found that SOM associated with

74

minerals had an average residence time of 100-200 years, while SOM not associated with

75

minerals turned over in 20-30 years. As such, it follows that older DOC should be exported with


Page 4 of 25

Page 5 of 25


76

relatively greater mineral weathering products (e.g. base cations, Si). In addition, many microbial

77

byproducts are non-aromatic and recalcitrant38 and when in the deeper portion of the soil more

78

likely protected from further decomposition, increasing the likelihood that these molecules will

79

age in place. Weathering and pedogenic processes stratify soils structure into distinct vertical

80

horizons, that create large ranges of physical/chemical environments experiences by microbes.

81

Overtime, changes in soil mineralogy and the release of metal cations like Fe and Al, produce

82

binding sites on the remaining organo-mineral complexes that can control the stabilization of soil

83

organic matter, as well as alter the quality and quantity of DOM. The alteration of soil

84

mineralogy and the release of Fe and Al over time produce binding sites on mineral surfaces and

85

organo-mineral complexes that not only stabilize SOM but alter the quality and quantity of DOM

86

remaining in soil39-41. For example, reducing conditions in soil can result in the export of Fe2+

87

and DOC to streams42. As water moves through this portion of the soil it flushes both particulates

88

and dissolved constituents into ground and/or surface waters, where additional OM may be

89

released from particles43.

90

Given that multiple factors of global change (climate, land cover and land use, etc.)

91

directly and indirectly shift flow paths, and thus the nature of carbon exports, we examine the

92

spatiotemporal variability in the stoichiometry of DOC and mineral weathering products (base

93

cations) within large river basins draining the coterminous United States and Arctic. Given the

94

conceptual model of Kaiser and Kalbitz29, we hypothesize that more labile, older DOC will be

95

exported with relatively greater amounts of weathering products (i.e. a low DOC to base cation

96

ratio). These exports will be associated with more mineral rich and/or deeper flow paths and

97

longer residence times associated with arid climates44, temporal changes in water routing over

98

the hydrograph (e.g. baseflow17), or alteration to the landscape (e.g. agricultural liming45).



99 100

Materials & Methods To examine these questions, we examined the riverine export of DOC to coastal waters

101 102

from thirteen major watersheds of the U.S.44 and the six largest Arctic watersheds that comprise

103

the Arctic Great Rivers Observatory (Arctic-GRO). The U.S. watersheds are monitored as part of

104

the National Stream Quality Accounting Network (NASQAN) program and in 2009 additional

105

DOC quality and age were measured on 6-11 of the monthly samples. This one year of data,

106

describing the coterminous U.S. watersheds, allows us to examine how large spatial variability

107

(different climates, ecosystems, levels of development) shifts dissolved weathering product and

108

organic carbon fluxes. The Arctic-GRO data (2003-2016) provides a way to examine six systems

109

with strong seasonal shifts in hydrology (i.e. freshet vs. baseflow) draining relatively organic rich

110

soils.

111

The U.S. watershed data describe samples collected across the 2009 hydrograph from 13

112

rivers that drain 79% of the land area and make up 90% of the freshwater discharge in the

113

coterminous U.S.44. Watersheds range in size from 2.9 million km2 (Mississippi) to 29,973 km2

114

(Potomac) and encompass a range of climate regimes with average precipitation ranging from

115

>1400 mm yr-1 in southeastern U.S. watersheds to less than 350 mm yr-1 in the high plains of the

116

Colorado River basin. As such, water yield varies significantly across these systems from

117

monthly lows of 1 mm yr-1 in the Rio Grande (Texas) to highs of 2615 mm yr-1 in the Altamaha

118

River (Georgia). Additional information (e.g. land use, population density) about these sites is

119

available in Tables 1a-b in Butman et al.44

120 121

The six Arctic watersheds (Yenisey, Lena, Ob’, Mackenzie, Yukon, Kolyma) comprise more than 50% of the land area draining to the Arctic Ocean, suggesting that differences in


Page 6 of 25

Page 7 of 25


122

permafrost extent, microbial community, soil characteristics, and bedrock weathering amongst

123

the basins should be reflected in this dataset. Five of the six basins’ headwaters are within boreal

124

forest, draining a tundra dominated landscape before emptying into the Arctic Ocean. In contrast,

125

the Yukon River follows a different trajectory, with its headwaters in tundra, moving south into a

126

forest dominated landscape. Continuous permafrost coverage varies significantly between

127

systems with the Kolyma almost entirely underlain with permafrost (99%) and the Ob’ having

128

just 1% of its area occupied by permafrost46. In all six systems, water and constituent fluxes

129

associated with organic matter are greatest during the spring freshet12. There is significant

130

variability in the strength of the seasonality in discharge and DOC fluxes, with 50% or greater of

131

annual DOC flux occurring during this ~two month period of time in the Yenisey, Lena,

132

Kolyma, and Yukon while less than 30% of the annual total occurred during this period of time

133

in the Ob’ and the Mackenzie46. The average water yield at the time of sampling ranges from 210

134

± 120 mm yr-1 (Mackenzie) to 460 ± 460 mm yr-1 (Lena) with relatively larger seasonal

135

variability than the NASQAN rivers. The fourteen years of sampling across the hydrograph in

136

these systems, provides a robust way to compare spatial variation amongst the catchments as

137

well as how seasonal shifts in hydrology affect the nature and concentration of dissolved carbon

138

and base cation exports.

139

Sampling protocols were similar between projects allowing us to compare the data.

140

Sampling of these large rivers consisted of integrated grab samples occurring at gauged sites or

141

near gauging stations (in most cases maintained by the USGS), providing concomitant discharge

142

and water quality measurements. The NASQAN and majority of Arctic (2002-2011) samples

143

represent a composite of depth integrated samples from multiple points across the channel, while



144

more recent sampling efforts in the Arctic (2012-2016) represent surface samples from multiple

145

points across the channel.

146

Sample processing for carbon constituents was similar (all samples filtered through 0.7

147

µm pre-combusted GF/F filters) and for the most part conducted in the same group of labs at the

148

USGS, Yale University, and Woods Hole. There were a few differences in the processing or

149

handling of samples. While all samples were filtered through the same filters, the NASQAN

150

samples were shipped unfiltered on ice within 24 hours of sampling and immediately filtered

151

upon arrival, while the Arctic samples were filtered in the field and then shipped. The largest

152

difference in carbon constituent analysis is the measurement of DOC concentrations. DOC

153

concentration for NASQAN rivers was measured using the persulfate wet oxidation on an OI

154

Analytical Model 700 TOC Analyzer47, while Arctic samples were analyzed using a Shimadzu

155

TOC analyzer12. All carbon quality measurements were made consistently across the two studies.

156

Briefly, radiocarbon isotope (∆14C-DOC) measurements determined the average age of DOC

157

exported and all samples were processed using established methods14, 48, which involve oxidizing

158

the DOC with UV light, converting it to CO2 which was then trapped and cryogenically purified.

159

The sample was then sent to the National Ocean Sciences Accelerator Mass Spectrometry

160

(NOSAMS) for isotopic analysis. The specific UV absorbance at 254 nm (SUVA254) was used as

161

a proxy for the aromaticity of dissolved organic matter, therefore sample absorbance at 254 nm

162

was measured using a UV-Vis Spectrometer and then normalized to DOC concentrations and

163

reported in L/(mg C*m) 49. Organic acid fractions were chromatographically separated using

164

columns packed with Amberlite™ XAD-8 and XAD-4 resins47. These operationally defined

165

dissolved organic matter fractions: larger molecular weight hydrophobic organic acid (HPOA),

166

smaller molecular weight hydrophilic molecules (HPI), and transphilic acids (TPIA)47 can


Page 8 of 25

Page 9 of 25


167

provide information about carbon quality for reactivity, bacterial mineralization, and

168

photodegradation. The sum of the four major base cations (Ca2+, Na+, K+, Mg2+), as reported in

169

meq L-1, was used as a proxy for mineral soil weathering. In the Arctic river systems, water

170

isotopic (δ18O-H2O) measurements were used as proxy for shifts in flow paths, given systematic

171

seasonal shifts of the system (i.e. freshet always has more depleted δ18O values as compared to

172

later season base flow samples)50. For a more detailed field and lab protocol descriptions

173

associated with Arctic-GRO please see: carbon12, major ion chemistry51, and water isotope

174

analyses50. Carbon sampling protocols for the NASQAN sites are given in Butman et al.44 and

175

major ions and discharge measurements were downloaded from the USGS National Water

176

Inventory System (www.usgs.gov/nwis).

177

All statistical analyses were conducted in R (3.3.2, 2016 R Foundation for Statistical

178

Modeling) and variables were transformed to meet requirements of normality. For example, the

179

ratio of DOC to the sum of base cations was transformed using log10. Statistical relationships

180

were considered significant at p

Riverine Export of Aged Carbon Driven by Flow Path Depth and

Recommend Documents