Uptake of soil-derived carbon into plants: implications for disposal of

Subscriber access provided by University of South Dakota

Environmental Processes

Uptake of soil-derived carbon into plants: implications for disposal of nuclear waste Soroush Majlesi, Jukka Juutilainen, Anne Kasurinen, Promise Mpamah, Tatiana Trubnikova, Markku J. Oinonen, Pertti Martikainen, and Christina Biasi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b06089 • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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 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 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.

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 22

Environmental Science & Technology

1 2 3

Uptake of soil-derived carbon into plants: implications for disposal of nuclear waste

4

Authors: Soroush Majlesia*, Jukka Juutilainena, Anne Kasurinena Promise Mpamaha, Tatiana Trubnikovaa,

5

Markku Oinonenb, Pertti Martikainena, Christina Biasia*

6

aUniversity

7

70211 Kuopio, Finland

8

bLaboratory

of Eastern Finland, Department of Environmental and Biological Sciences, P.O. Box 1627, FI-

of Chronology, Finnish Museum of Natural History, P.O. Box 64, FI-00014 Helsinki, Finland

9 10

Abstract

11

Radiocarbon (14C) is potentially significant in terms of release from deep geological disposal of

12

radioactive waste and incorporation into the biosphere. In this study we investigated the transfer of soil-

13

derived C into two plant species by using a novel approach, where the uptake of soil-derived C into newly

14

cultivated plants was studied on 8000-year leftover peat in order to distinguish between soil-derived and

15

atmospheric C. Two-pool isotope mixing model was used to reveal the fraction of soil C in plants. Our

16

results indicated that although the majority of plant C was obtained from atmosphere by photosynthesis,

17

a significant portion (up to 3-5%) of C in plant roots was derived from old soil. We found that uptake of

18

soil C into roots was more pronounced in ectomycorrhizal Scots pine than in endomycorrhizal reed

19

canary grass, but nonetheless, both species showed soil-derived C uptake in their roots. Although plenty

20

of soil-derived C was available in canopy air for re-assimilation by photosynthesis, no trace of soil-derived

21

C was detected in aboveground parts, possibly due to the open canopy. The results suggest that the

22

potential for contamination with 14C is higher for roots than for leaves.

23

Key words: Radiocarbon; Soil-derived C; Two-pool isotope mixing model; radioecology; radioactive

24

waste

25 26 27 28 29 30 31 32 33

*Corresponding

authors: [email protected], [email protected]

34 1 ACS Paragon Plus Environment


Page 2 of 22

35

Introduction

36

Radioactive waste is considered as one of the major concerns of the nuclear power

37

industry. Safe disposal of spent fuel is a crucial step to ensure the protection of humans

38

and the environment in the long term. Countries such as Finland and Sweden have

39

selected deep geological disposal for management of long-lived radioactive waste. The

40

above method is considered by many as the best option to guarantee the long-term

41

safety of radioactive wastes (1-3) and a deep geological repository is currently being

42

constructed in Finland. However, the long-lived radioactive waste must be isolated from

43

humans and the environment for a very long time, and understanding of the processes

44

and pathways of radionuclide behavior still needs to be improved for adequate

45

assessment of possible risks in the distant future.

46

Among the radionuclides possibly released into the environment, radiocarbon (14C) is of

47

great significance because of its high potential to enter the biosphere. About one fifth of

48

the body mass of all living organisms consists of carbon (C), and

49

from nuclear waste mixes with common stable isotopes of C found in nature (12C, 13C).

50

This radionuclide can thus, theoretically, move from deep geological disposals to the

51

biosphere and be incorporated into plants, animals and to the human food chain.

52

Radiocarbon has a relatively long half-life of 5730 years, which additionally contributes

53

to the risk of significant effects from this radionuclide in the food chain.

54

The releases of 14C from repositories could be in the form of gases (14CO2, 14CH4) or as

55

aqueous species (4). Both gaseous and aqueous 14C species can end up in living matter

56

through, e.g., photosynthesis by plants (CO2 uptake), oxidation and assimilation of CH4

57

by methanothrophs or direct uptake of water and inorganic and organic C species by

58

plants and animals. The 14C taken up by organisms can also re-enter the soil since plant

59

litter and debris of soil animals and microorganisms are the major sources of soil

60

organic matter. Exposure of humans could results from uptake by contaminated plants

61

and consequent transfer of 14C into the food chain.

62

Traditionally it is believed that C uptake from soil or belowground sources into plants

63

or plant roots is low and therefore of less significance compared to stomatal

64

atmospheric CO2 uptake from the air (4). However, even small portions of soil-derived

65

14C

66

Thus, it is important to accurately assess the amount of C that plants utilize from the

ending up in plants could significantly increase transfer of

2 ACS Paragon Plus Environment

14C

14C

possibly released

into the biosphere.

Page 3 of 22


67

soil. Particularly the potential uptake from old native soil is relevant for studies on

68

geological disposal of nuclear waste. Direct incorporation of soil CO2 (gaseous and/or

69

dissolved) and other species of dissolved inorganic C (e.g. bicarbonate) by roots is

70

possible (5-7). In addition, plants are known to be able to take up organic carbon and

71

nitrogen forms from the soil, with mycorrhizal fungi possibly facilitating the uptake of

72

organic substrates (8, 9). Carbon uptake by the roots is believed to be only a few percent

73

of the total C in the plant (10, 11), but the uncertainty of the estimates is large.

74

Furthermore, indirect uptake of soil-derived CO2 via leaf-level photosynthesis might

75

play a role. The rate of re-assimilation of soil-derived CO2 by plant photosynthesis is

76

poorly known, but it may be a more important source of soil-derived C than root uptake.

77

Up to a few percent of soil-derived C in plant canopy atmosphere has been assumed to

78

be reassimilated by plants (11).

79

The key problems with analyzing the possible transfer of soil-derived C into plants are

80

how to distinguish soil-derived C from other sources of C, and how to determine the

81

contribution of soil-derived C in the foliar uptake of C. The most reliable methods to

82

distinguish the C sources in plants are isotope methods (12). Continuous and pulse

83

labelling methods are frequently used but these techniques are intrusive (12).

84

Additionally, experimental labelling of soil with

85

Soil–plant systems with natural differences in isotope signature overcome the

86

limitations of labelling studies but such systems are rare.

14C

poses risks to the environment.

87 88

This paper addresses transfer of soil-derived C into plants by using a new approach to

89

distinguish between soil-derived and atmospheric C (13). The principal idea of this

90

approach is to study uptake of soil C into plants in cultivated cutaway peatlands, where

91

a distinct natural

92

between the “normal” 14C levels in air and the up to 8000-year-old leftover peat, which

93

is strongly depleted in 14C (due to decay of 14C with time). This large difference in 14C is

94

due to the fact that the top layers of the peat have been removed by peat extraction,

95

leaving peat that is depleted in

96

opportunity to calculate, with high accuracy, the contributions of atmospheric C and

97

soil-derived C in the plants growing on the leftover peat. Furthermore, it is possible to

98

quantify the proportion of soil-derived CO2 in canopy air and thus estimate uptake of 14C

14C

pattern exists: there is a very large difference in

14C

compared to the modern


14C

14C

content

level. This gives an


99

via leave photosynthesis. This approach allows us to introduce a robust tool to follow

100

the fate of soil C, here with a natural distinct 14C signal. The aims of this study were to

101

investigate 1) uptake of soil C via roots, 2) foliar uptake of C released from soil as CO2

102

and 3) differences in C uptake of soil-derived C between two plant species. Soil C acts

103

here as a proxy of

104

incorporated into soil organic matter. All results are also relevant for basic studies on C

105

cycling since more information on internal re-cycling of C is needed for the scientific

106

community dealing with C dynamics and e.g. climate change research. The study was

107

carried out in Linnansuo (Eastern Finland) and the experiments were performed on a

108

cutaway peat soil cultivated with reed canary grass (Phalaris arundinacea L.) and Scots

109

pine (Pinus sylvestris L.). The field studies were complemented with laboratory

110

incubation studies with plant seedlings under controlled conditions.

14C

possibly released during disposal of nuclear waste and

111 112

Materials and Methods

113 114

Field experiments - study site

115

The study was carried out in Linnansuo on an 8000-year cutover peatland complex (62◦

116

30_ N, 30◦ 30_ E) located in the rural area of the city of Joensuu in eastern Finland in

117

2009. According to climatic data available for the region, the mean annual temperature

118

and precipitation in the region is 2.1 ◦C and 669 mm, respectively (14). In its pristine

119

state, the peatland was classified as an ombrotrophic Sphagnum fuscum bog (14). Peat

120

extraction, including drainage, began in 1978 (14). After the end of peat extraction, in

121

2001, when the thickness of the remaining peat was 20-85 cm, the cutover peatland was

122

cultivated with reed canary grass (RCG, a species used for bioenergy production). The

123

carbon content of the residual peat at the surface was 39 ± 18% (15). The variability of

124

soil C and the organic matter content was rather high (42-63%), because sand was

125

mixed into the surface peat when digging the drainage ditches, in order to improve soil

126

aeration and thus plant growth. The C/N ratio of the peat was 40.3 and its pH was 4.3-

127

6.3 (14). The site served as the primary research site in this study and was chosen to

128

study transfer of C from soil into grass roots, leaves and the canopy air. The dense


Page 4 of 22

Page 5 of 22


129

growth of RCG generates canopy air, which might contain a considerable amount of soil-

130

derived CO2.

131

The second site used for the study was a smaller, forested cut-away peatland adjacent to

132

the RCG cultivation. There, Scots pine (Pinus sylvestris L.) were planted in 2001. The

133

trees were three to at maximum five meters tall with rather low average stand density,

134

so the canopy formed by the trees was relatively open. The plants of both species had

135

been growing for 8 years at the time of the study.

136

Both plant species are common in Finland and are adapted to the Finnish climatic

137

conditions; these plants are thus also likely to grow on repositories in the future.

138

Importantly, RCG is a perennial grass species whereas Scots pines are woody plants.

139

Scots pines form ectomycorrhizas in their fine root systems whereas RCG are

140

endomycorrhizal plants (16) and might thus behave differently with respect to C

141

transfer from belowground sources.

142 143

Field Sampling

144 145

Soil and Biomass

146

Soil samples were taken to the depth of 15cm from two different strips on the RCG and

147

Pine sites, and divided into 0-5 cm, 5-10 cm and 10-15 cm sections, by using a metal soil

148

corer with a diameter of 7 cm (n=3 for RCG and n=2 for Pine). We sampled sections

149

based on depth, since no horizons were present in the residual peat. One soil core taken

150

from the RCG site extended to the bottom of the peatland, down to 45 cm, and was also

151

divided in 5 cm increments. In the laboratory, roots were carefully removed before 14C

152

dating. For analysis of mycorrhizal colonization in roots, intact soils cores (n= 4 for each

153

site) were sampled separately and roots were not removed until analysis (see below).

154

Plant samples were taken on two different strips of each site. Samples from

155

aboveground parts of RCG were once pooled (n=6) and once, in a single replicate, also

156

taken at several heights (8, 40 and 80cm). The Scots pine samples were only taken at

157

different heights (55, 70, 110, 115, 190 and 260cm; n=6). Only single needles from the

158

tips of the pine twigs were sampled, to ensure analysis of 5 ACS Paragon Plus Environment

14C

in young needles. Stems


159

and leaves of the plants were dried and root samples were additionally carefully

160

washed before drying and sent to the Radiocarbon Dating Laboratory in Poznan, Poland,

161

for 14C dating.

162 163

Mycorrhizal colonization level

164

The total mycorrhizal colonization level was calculated as total number of mycorrhizal

165

short roots divided by total number of all short roots in the sample x 100. Samples came

166

from Linnansuo as soil core samples (n=4). The roots were washed for mycorrhizal

167

status analyses. For RCG roots, root clearing before staining was performed: roots were

168

kept in 10 % KOH for 24 hours at room temperature, then rinsed in 1% HCl to make

169

them acidic again before staining. The samples were stained for 24 hours at room

170

temperature with lactophenol blue stain (Diagnostica Merck, E. Merck, 64271

171

Darmstadt Germany) and then rinsing overnight (12 h) at room temperature in

172

deionized water to remove excess stain before microscopy. Measurement was done

173

with 10 x 1 cm long root pieces on a glass slide. Samples were examined by Motic AE21

174

inverted research microscope (Motic Microscopes, China).

175

In Scots pine root analysis, 1-m-long fine root (≤ 2 mm in diameter) samples were

176

analysed for total mycorrhizal colonization level. Pine fine roots were studied with a

177

stereomicroscope under a 7.5x-50 x magnification (Motic SMZ-168, Motic Microscope,

178

China). All short roots (mainly 1st order roots) were examined for the presence of

179

mycorrhizal stuctures on the basis of their gross morphological features. In general,

180

non-mycorrhizal roots were mainly monopodial, and rarely dichotomous in their

181

ramification pattern, the visibility of the cortical cells was clear, and there could be root

182

hairs emanating from the short root surface. In mycorrhizal short roots, the ramification

183

pattern was usually dichotomous/coralloid/tubercle-like and rarely monopodial, roots

184

did not have root hairs, but sometimes hyphae/extramatrical mycelium/rhizomorps

185

emanating from the fungal mantle covering the short roots, and only in some thin-

186

mantled mycorrhizas, root cortical cells could be seen through mantle.

187 188


Page 6 of 22

Page 7 of 22


189

Carbon dioxide

190

Carbon dioxide in canopy air was sampled at the heights of 8, 40 and 80cm from RCG

191

cultivations, and at 55, 70, 110, 115, 190 and 260cm from pine cultivations (the same

192

heights that were used for sampling leaves). Carbon dioxide was sampled with the

193

molecular sieve technique (17), where CO2 molecules are trapped (adsorbed) in pores

194

in the sieve. Type 13X zeolite sieves (Merck 1.05703.1000) were used in this study, and

195

they were packed in glass containers with approximate volume of 100 ml.

196

sampling, air was pumped through the molecular sieves for about five minutes. After

197

sampling, CO2 was released in the Laboratory of Chronology in Helsinki, Finland for

198

14CO 2

199

than 95%. Amount of fractionation by the molecular sieves is fairly limited (17) but this

200

is, nevertheless, taken into account when post-processing the raw 14C data as mentioned

201

below (by using the 13C value).

For

analysis under very high temperature and under-pressure, with efficiency greater

202 203

Laboratory studies

204

To complement the field studies, laboratory experiments were carried out in a

205

greenhouse at the Research Garden of the University of Eastern Finland in Kuopio using

206

RCG and Scots pine seedlings. All plants (RCG and Scots pine) were grown from seeds

207

sown into pots in the greenhouse. The C uptake experiment started in April 2009 and

208

ended in September 2009 (5 months in 2009). During the experiment, all plants were

209

kept in two separate rooms with equal size of 68.2 m2. In total, the temperature differed

210

between these two rooms (by 3 ͦC max.; on average 1.5 ͦC). However, statistical tests did

211

not detect any effect of temperature on C transfer, so we consider all samples as one

212

treatment. Both species were grown in flower pots on old peat collected from an

213

adjacent cut-away peatland (Linnansuo) which was still under peat harvesting, and thus

214

not influenced by modern plants. A total of 400 plastic pots (OS Plastic A/S Danmark,

215

11mm in diameter), as well as equivalent number of small flat plates were carefully

216

washed and dried for the experiment.

217

Peat and fine sand (0.5-1.2mm) were carefully mixed at the ratio of 1:1 to ensure

218

homogeneous soil material. The sand was added to increase aeration of the peat and to

219

aid root penetration. Pots filled with 450 g of fresh peat-sand mixture in each pot were 7 ACS Paragon Plus Environment


220

kept in the research garden from sowing the seeds to the end of the experiment. Both

221

plant species were grown from seeds collected from the Natural Resources Institute of

222

Finland (LUKE, Finland).

223

In total, 154 and 112 pots, respectively, were planted with RCG and pine in the two

224

chambers. We planted about 300 RCG seeds per square meter (similar to the field

225

density) and four Scots pine seeds per pot. Scots pine was later thinned to one seedling

226

per pot. The pots were watered once a day. One week after the pots were thinned, the

227

addition of fertilizer (about 0.5dL of 0.1% Taimi-Superex fertilizer, NPK 19-4-20,

228

Kekkilä, Finland) was started. The fertilization was done once a week.

229

Throughout the experimental period, temperature and air humidity were monitored.

230

The average minimum/maximum temperatures in the two rooms were 18/20 and

231

18/23oC (on average 19 and 20.5 oC, respectively). The minimum/maximum relative

232

humidities in the chambers were 75/80% respectively. Artificial light in the

233

greenhouses turned off when the solar radiation exceeded 400 W/m2; the photoperiod

234

was 16 hours.

235 236 237

Soil and biomass

238

All RCG leaves and pine needles from the pots were collected at the end of the

239

laboratory experiment. Like in the field experiment, they were dried at 65 ○C, weighed

240

and a subsample was sent for 14C analysis to the Poznan Radiocarbon Dating laboratory

241

in Poland. After cutting off the shoots, the residual content of the pots (roots and soil)

242

was shaken carefully to remove as much soil as possible for soil analysis while roots

243

were carefully washed over a sieve (mesh size 8) to remove the remaining soil

244

materials. Finally, the washed roots were dried before 14C analysis.

245

Radiocarbon analysis

246

All radiocarbon dating of solid samples (soil, plants) were carried out by accelerator

247

mass spectrometer (AMS) in the Poznan Radiocarbon Dating Laboratory in Poland, and

248

of gas samples by AMS in the Laboratory of Chronology, Helsinki, Finland (17,18). AMS

249

is a technique for measuring the concentration of a single isotope (14C) that is rare in the 8 ACS Paragon Plus Environment

Page 8 of 22

Page 9 of 22


250

presence of more abundant isotopes. With this method, it is possible to analyze the

251

proportion of 14C with high precision (

Uptake of soil-derived carbon into plants: implications for disposal of

Recommend Documents