Drainage Ditches Contribute Considerably to the CH4 Budget of a Drained and a Rewetted Temperate Fen

TL;DR: In this article, the authors assessed the CH4 emissions of a drained and a rewetted temperate fen including emissions of managed and unmanaged drainage ditches over the course of 2.5 years, covering three vegetation periods.

Abstract: Small water bodies including drainage ditches can be hotspots for methane (CH4) emissions from peatlands. We assessed the CH4 emissions of a drained and a rewetted temperate fen including emissions of managed and unmanaged drainage ditches over the course of 2.5 years, covering three vegetation periods. Ditch CH4 emissions in the rewetted fen were significantly higher than in the drained fen. In the rewetted fen ditches contributed up to 91% of the annual CH4 budget, despite covering only 1.5% of the area. In the drained fen CH4 emissions were solely made up of ditch emissions. When including CH4 uptake by the peat soil, the CH4 balance of the drained fen was neutral. Dissolved organic carbon concentrations likely had an enhancing effect on CH4 emissions while nitrate and sulfate in the ditch water seem to have had an inhibitory effect. Air and water temperature controlled seasonal variability of ebullitive as well as diffusive CH4 emissions. Ebullition contributed less than 10% to the overall CH4 budget in the ditches. Drainage ditches represent a hotspot of CH4 emissions and need therefore be taken into account when assessing the success of rewetting projects of peatlands.

Summary

Introduction

• Peatlands are a globally important carbon store (Treat et al. 2019 ) that is turned into a strong source of greenhouse gases (GHGs) when drained, and faces other threats, for instance, from global warming (Loisel et al. 2020) .
• Due to the high availability of organic substrate in the soil, water-logged areas in drained or rewetted peatlands can become hotspots for emissions of the GHG methane (CH 4 ).
• Small water bodies play an important role in the global carbon cycle (Bastviken et al.
• The authors determine the effects of climatic (air temperature, water temperature, air pressure), biotic (DOC, nutrients) and morphological (water depth, orientation) variables on CH 4 emissions from ditches and evaluate the importance of ebullitive CH 4 uxes in relation to diffusive uxes.

Site description

• The two studied fens are located 8 km apart in the valleys of the two rivers Recknitz (drained fen) and Trebel (rewetted fen) in north-eastern Germany.
• The peat in both sites is mainly of sedge and reed origin (Jurasinski et al. 2020) .
• PW is dominated by sedges (Carex acutiformis Ehrh.) and occasional great willowherb (Epilobium hirsutum L.) and grey sallow (Salix cinerea L.).

Study setup

• Five collars for the measurement of soil CH 4 exchange were installed along a boardwalk at both sites.
• Additionally, air pressure, vapour pressure and sunshine duration was obtained from three different weather stations in proximity of the soil sampling site (Warnemünde − 40 km NW, Barth − 30 km N, Greifswald, 40 km E), run by the german weather service (DWD).
• Accordingly, the depth of the ditches varies throughout the year, ranging from 10 to 70 cm.
• The ditches at PW are often covered entirely with vegetation during the summer months, with Stratiotes aloides L. being dominant in PW-o and Typha latifolia L. and Lemna minor L. being dominant in PW-p.

Flux measurements

• The chamber was equipped with a temperature and humidity sensor as well as with a fan powered by a 9 V battery mounted inside the chamber lid.
• CH 4 concentration measurements were carried out insitu with laser spectrometers ('Ultra-Portable Greenhouse Gas Analyzer', Los Gatos Research, Mountain View, USA and 'GasScouter', Picarro, Santa Clara, USA) connected to the chamber with exible polyurethane tubes (inner diameter: 4 mm).
• The diameter of the soil chamber was 0.65 m. Diffusive uxes from the ditches and the soil surface at both sites were performed between April 1st 2018 and September 29th 2020.
• Diffusive uxes were estimated using the uxx function of the package ux (Jurasinski et al. 2014) for R (R development core team 2020).

Ebullitive CH 4 uxes

• Ebullitive CH 4 emissions were assessed during the vegetation period of 2018.
• Bubble traps were installed oating in the middle of the ditches ( ve measurement points at each ditch).
• After sampling the authors re lled the bubble trap completely with water.
• Final ebullition uxes were calculated as such:.
• When there was no gas sample taken the arithmetic mean of the CH 4 concentration from all gas samples was taken to estimate the ebullitive CH 4 emission.

Greenhouse gas budgets

• For each measurement day, one ux value per ux subset (ditch or soil) and each site (PD or PW) was randomly chosen.
• This was repeated 100 times to obtain 100 different ux time series.
• Then, the area-under-curve (auc.mc function from the R package ux, Jurasinski et al. 2014) was calculated 100 times for each ux time series each time leaving out one ux value, leading to a total of 10 000 different CH 4 balances.
• The CH 4 balances were calculated per season (i.e. vegetation period (April -September) and non-vegetation period (October -March).
• To estimate the contribution of CH 4 emitted from ditches to total ecosystem emissions the authors manually determined the area covered by ditches by digitizing them within a randomly-chosen 1 km² area around the soil sampling site using aerial imagery.

Water and sediment characteristics

• Water samples of ditch water and groundwater at the soil sampling site were taken to assess potential in uences of chemical properties (i.e. nutrients, DOC) on CH 4 emissions.
• On each sampling occasion, one sample was taken for every ux measurement location in the ditch (n = 20).
• Groundwater samples were taken at the central site every four week between April 2018 and September 2020.
• All water samples were directly ltered in the eld with syringe lter units (pore size 0.45 µm, Sartorius, Göttingen, Germany) and afterwards stored cool (~ 5°C) or frozen until analysis.
• DOC, DIC and TN were analysed using a Dimatoc 2100 (Dimatec, Essen, Germany In June 2018 sediment samples were taken in two depths (0-5 cm, 10-20 cm) from the sediment surface of each ditch.

Statistical analyses

• All statistical analyses and visualizations were carried out with R 4.0.2 (R development core team 2020).
• The entire dataset and any subsets were tested for normality and homogeneity of variance using Shapiro-Wilk tests and Levene's tests, respectively.
• In order to assess relationships between water chemical variables (pH, O 2 saturation, water temperature, electrical conductivity, redox potential and salinity) and CH 4 uxes all values were averaged by date, because not every ux measurement had an associated measurement of water chemical variables.
• Transformed diffusive ux values were subsequently directly linked to environmental variables from the weather station with multiple linear regressions (wind direction, wind speed, air temperature, air pressure, vapour pressure and pressure change over different time intervals).
• Additional variables, such as concentration values of nutrients and DOC that were either only available at certain dates were merged with daily average ebullitive and average non-transformed diffusive CH 4 uxes.

Environmental variables and ditch characteristics

• According to the weather station data air temperature differed only slightly between PD and PW.
• During the two-year study period PD was much drier than PW, receiving only 973 mm in comparison to 1173 mm at PW.
• Nonetheless, there were pronounced dry spells in both peatlands during the summer months of 2018 and 2019 (Fig. 4 ).
• Generally, the amplitude of water table uctuations in the ditches was lower in PW.
• Regardless of the site, ditches with an orthogonal orientation to the drainage direction (PW-o and PD-o) showed DOC concentrations that were almost double those of the ditches with parallel orientation to drainage direction (Table 2 ).

Diffusive uxes

• All ditches were strong sources of CH 4 during the measurement period.
• Diffusive CH 4 uxes from ditches were generally signi cantly higher than soil uxes from the adjacent peatlands.
• The average diffusive CH 4 uxes from the ditches were higher in summer than in winter by many orders of magnitude and they differed strongly between different ditches (Fig. 5 ).
• Also, within one site there were large differences between ditches.

CH 4 budgets

• Seasonal CH 4 budgets were roughly ten times lower in winter than in summer.
• Again, air temperature showed the strongest relationship with diffusive CH 4 uxes.
• Also water depth, wind speed and vapour pressure were signi cantly related.
• Thus, it is likely that single high uxes at PW dominated the multiple linear regression models.
• Nutrient concentrations varied among ditches of different orientation at both sites (Table 2 ).

Discussion

• In general, diffusive CH 4 emissions from ditches were much higher than those from the peat soils in both the drained (PD) and the rewetted peatland (PW).
• Overall, the magnitude of the diffusive ditch CH 4 emissions in their study compare well to the few other studies that exist, depending on whether the surrounding peatland was in a rather natural state (PW) or drained for agriculture (PD).
• Average emissions in summer were 5 to 10 times higher than average winter emissions for PD and PW.
• Yet, winter emissions were consistent and not negligible.
• Instead, CH 4 concentrations in the bubbles and the associated ebullitive uxes were rather comparable with values from subarctic peatlands (Wik et al., 2013) .

Drivers of ditch CH 4 emissions

• Diffusion as well as ebullition uxes were explained best by temperature, as was also found in other small water bodies recently (Audet et al. 2020) .
• Ditches with increased DOC concentrations (PD-o, PW-p, PW-o, Table 2 ) showed signi cantly higher diffusive and ebullitive CH 4 emissions, with PW-o standing out with uniquely high diffusive and ebullitive uxes and DOC concentrations.
• The opposite pattern is observed in the ditches (Table 2 ), which suggests fast carbon turnover in the ditch water.
• Across both sites, the ditches with higher phosphate concentrations showed higher diffusive and ebullitive CH 4 emissions.
• Apart from nutrient and substrate availability in the water column, the nutrient status of the sediment is important for methanogenesis.

Importance of ditch emissions for total ecosystem budgets

• The annual CH 4 budgets reported for PWp and PWo were larger than values from other studies (Peacock et al. 2017 , Schrier-Uijl et al. 2010) .
• The relative importance of ditch emissions for total ecosystem CH 4 budgets was higher than values presented in the scarce studies that exist (Hyvönen et al. 2013) .
• Moreover, CH 4 concentrations in the samples obtained from the bubble traps in their study were low compared to other studies (e.g. Maeck et al.
• Thus, ebullition would still be of minor importance.
• CH 4 emissions from ditches and soils continued during winter, although at a much lower rate.

Conclusions

• Ditches can play an important role in the overall GHG budgets of peatlands.
• Here, the authors showed that active and inactive ditches in drained and rewetted temperate fens can act as hotspots for CH 4 emissions.
• Emissions from ditches in the rewetted fen were much higher than from ditches at the drained site.
• In drained fens high CH 4 emissions from ditches add to the high CO 2 emissions from the drained peat soils.
• There, emissions from drainage ditches need to be included in the calculations of GHG budgets.

Figures

Figure 4

Figure 5

Figure 9

Figure 1

Figure 7

Figure 2

Table 1 Nutrient contents [% dry weight] of sediment samples in ditches (± denotes one standard deviation, n = 4 per ditch)

Table 2 Nutrient concentrations [mg l− 1] in the ditch water. (± denotes one standard deviation)

Full text

Page 1/31
Drainage Ditches Contribute Considerably to the
CH4 Budget of a Drained and a Rewetted Temperate
Fen
Daniel Köhn (  j.daniel.koehn@gmail.com )
University of Rostock https://orcid.org/0000-0001-5435-8831
Carla Welpelo
Johann Heinrich von Thünen -Institut Institut für Agrarklimaschutz: Johann Heinrich von Thunen -
Institut Institut fur Agrarklimaschutz
Anke Günther
University of Rostock: Universitat Rostock
Gerald Jurasinski
University of Rostock
Research Article
Keywords: Methane (CH4), peatland rewetting, restoration, ditches, methane budget, ebullition
Posted Date: February 19th, 2021
DOI: https://doi.org/10.21203/rs.3.rs-223145/v1
License:   This work is licensed under a Creative Commons Attribution 4.0 International License. 
Read Full License

Page 2/31
Abstract
Small water bodies including (former) drainage ditches can be hotspots for methane (CH
4
) emissions
from peatlands. We assessed the CH
4
emissions of a drained and a rewetted temperate fen including
emissions of active and former drainage ditches over the course of 2.5 years, covering three vegetation
periods. Ditch CH
4
emissions in the rewetted fen were signicantly higher than in the drained fen. In the
rewetted fen ditches contributed up to 91 % of the annual CH
4
budget, despite covering only 1.5 % of the
area. In the drained fen CH
4
emissions were solely made up of ditch emissions. When including CH
4
uptake by the peat soil, the CH
4
balance of the drained fen was neutral. Dissolved organic carbon
concentrations likely had an enhancing effect on CH
4
emissions while nitrate and sulphate in the ditch
water seem to have had an inhibitory effect. Air and water temperature controlled seasonal variability of
ebullitive as well as diffusive CH
4
emissions. Ebullition contributed less than 10 % to the overall CH
4
budget in the ditches. Drainage ditches represent a hotspot of CH
4
emissions and need therefore be taken
into account when assessing the success of rewetting projects of peatlands.
Introduction
Peatlands are a globally important carbon store (Treat et al. 2019) that is turned into a strong source of
greenhouse gases (GHGs) when drained, and faces other threats, for instance, from global warming
(Loisel et al. 2020). Peatland rewetting represents an ecient way to reduce or stop GHG emissions. Due
to the high availability of organic substrate in the soil, water-logged areas in drained or rewetted
peatlands can become hotspots for emissions of the GHG methane (CH
4
).
Small water bodies play an important role in the global carbon cycle (Bastviken et al. 2011; DelSontro et
al. 2016; Holgerson and Raymond 2016). However, only few studies have so far examined the importance
of CH
4
emissions from drainage ditches in peatlands. Drainage ditches can be important hotspots for
CH
4
emissions in wetlands (Schrier-Uijl et al. 2011, 2010), sometimes contributing a major part of the
total regional CH
4
budget (Schrier-Uijl et al. 2010). Also in agricultural landscapes drainage ditches may
contribute signicantly to the landscape carbon budget via high CH
4
emissions (Peacock et al. 2017). In
this context, ebullition is often mentioned as an important pathway of CH
4
emissions in various aquatic
ecosystems (Baulch et al. 2011; Bastviken et al. 2004; Repo et al. 2007; Yang et al. 2020).
The major biotic factor driving high CH
4
emissions is thought to be the trophic state of the water body
(Schrier-Uijl et al. 2011). Phosphate (PO
4
) and reduced iron in the ditch water are indicators for anaerobic
conditions and can explain a large proportion of the variance in CH
4
emissions (Schrier-Uijl et al. 2011). In
connection with the trophic status of water bodies, the oxygen concentration in the water column is a
good indicator for CH
4
emissions (Liikanen et al. 2003). Since methanogenesis depends on small organic
carbon molecules, either carbon dioxide (CO
2
), hydrogen (H) or acetate (H
3
C) as a substrate (Kelly and

Page 3/31
Chynoweth 1981), the concentrations of dissolved organic matter (DOM) or dissolved organic carbon
(DOC) are important drivers of CH
4
emissions in small water bodies (Bastviken et al. 2004; Zhou et al.
2019). However, it is often unclear whether the organic matter in ditches mainly derives from high
biomass production within the ditch or from allochthonous DOC that was potentially leached at high
rates from surrounding decomposing peat as was shown in a mesocosm experiment (Laine et al. 2014).
High nutrient inputs from surrounding agriculturally-used peat soils can cause eutrophication in the
ditches and thereby enhance plant and algal biomass production and subsequent depletion of oxygen
from biomass decomposition (Zhou et al. 2019). Increased nutrient concentrations and experimental
warming showed an increase in CH
4
emissions from small water bodies in a study on CH
4,
ebullition from
lake mesocosms (Davidson et al. 2018). This relationship was also shown in natural northern lakes and
ponds (DelSontro et al. 2016). Shallow water bodies such as most ditches are highly susceptible to
warming and eutrophication because of their small water volume and climate warming is expected to
globally increase the CH
4
emissions via ebullition by up to 51 % (Aben et al. 2017).
Here, we study the importance of ditch CH
4
emissions in regional GHG budgets and the drivers for
temporal variation in two peatlands with differing land use. We determine the effects of climatic (air
temperature, water temperature, air pressure), biotic (DOC, nutrients) and morphological (water depth,
orientation) variables on CH
4
emissions from ditches and evaluate the importance of ebullitive CH
4
uxes
in relation to diffusive uxes. Using a 2.5 year time series of oating chamber measurements and closed
chamber measurements in the adjacent peatland, we assess the interannual variability of CH
4
uxes and
seasonal CH
4
budgets.
Materials And Methods
Site description
The two studied fens are located 8 km apart in the valleys of the two rivers Recknitz (drained fen) and
Trebel (rewetted fen) in north-eastern Germany. The average annual mean temperature is 9.1°C (DWD
raster data, Krähenmann et al. 2016). The drained fen (PD, 54.13194° N, 12.62889° E, elevation a.s.l = 20
m) is an extensively used grassland which is harvested once a year for fodder production. The rewetted
fen (PW, 54.10111° N, 12.73944° E, elevation a.s.l. = 2 m) has been rewetted in 1997 after being used as
intensive grassland for decades. After rewetting, the water table in PW now uctuates around the soil
surface. Peat thickness is around 5 m in PD and approx. 6 m in PW. The peat in both sites is mainly of
sedge and reed origin (Jurasinski et al. 2020). The vegetation at PD can be characterised as a uniform
grassland dominated by
Ranunculus repens
L. and
Deschampsia cespitosa
(L.) P. Beauv. PW is
dominated by sedges (
Carex acutiformis
Ehrh.) and occasional great willowherb (
Epilobium hirsutum
L.)
and grey sallow (
Salix cinerea
L.). Especially around ditches and former peat cuttings large areas of reed
(
Phragmites australis
Trin. ex Steud.) and occasional cattail (
Typha latifolia
L.) can be found.
Study setup

Page 4/31
At each study site (PD and PW) a soil measurement site was established inside a fenced area (12 x 30 m)
between April and June 2017 (Fig.1). Five collars for the measurement of soil CH
4
exchange were
installed along a boardwalk at both sites. The soil collars also included vegetation. Weather stations
inside both fenced areas recorded air temperature, humidity, photosynthetic photon ux density (PPFD),
wind speed, wind direction and precipitation (logged with CR300, Campbell Scientic, Bremen, Germany).
Additionally, air pressure, vapour pressure and sunshine duration was obtained from three different
weather stations in proximity of the soil sampling site (Warnemünde − 40 km NW, Barth − 30 km N,
Greifswald, 40 km E), run by the german weather service (DWD). For analyses, the values of all three
weather stations were averaged.

We selected two ditches to measure diffusive and ebullitive CH
4
exchange from the water surface in close
proximity to each soil sampling site (~ 300–400 m distance, Fig.1). At each site, one of the selected
ditches runs parallel to the drainage direction (PD-p, PW-p) and one ditch runs orthogonal to the drainage
direction (PD-o, PW-o) towards the main river. In all four ditches ve sampling spots were established at
approximately 10 m from each other (20 ditch sampling locations in total, Fig.1).
The ditches at PD are relatively uniform with a width of approximately 2 m and are regularly excavated in
summer (own observations). Accordingly, the depth of the ditches varies throughout the year, ranging
from 10 to 70 cm. During summer, the ditches are often covered by common duckweed (
Lemna minor
L.)
Further, water starwort (
Callitriche palustris
L.) was abundant. At PW the ditches are not managed and,
thus, do not vary in depth over the year. PW-o, however, is signicantly deeper than PW-d with average
depths of 104 cm and 38 cm, respectively. Also, PW-o is much wider than PW-d with approximately 4 m
and 2 m, respectively. The ditches at PW are often covered entirely with vegetation during the summer
months, with
Stratiotes aloides
L. being dominant in PW-o and
Typha latifolia
L. and
Lemna minor L.
being dominant in PW-p. The banks of both ditches in PW are dominated by
Phragmites australis
(Cav.)
Trin. ex Steud.
Flux measurements
Diffusive CH
4
uxes
Diffusive emissions of CH
4
from the ditches were measured with a oating chamber. The oating
chamber was constructed using a bucket (diameter = 20 cm, height = 25 cm), coated with reective
material to reduce heating inside the chamber (Fig.2). The chamber was equipped with a temperature
and humidity sensor as well as with a fan powered by a 9 V battery mounted inside the chamber lid. The
chamber was placed inside a oat (square 50 × 40 cm, Styrodur, BASF, Ludwigshafen am Rhein,
Germany) and connected to a 1.5 m long handle. CH
4
concentration measurements were carried out in-
situ with laser spectrometers (‘Ultra-Portable Greenhouse Gas Analyzer’, Los Gatos Research, Mountain

Page 5/31
View, USA and ‘GasScouter’, Picarro, Santa Clara, USA) connected to the chamber with exible
polyurethane tubes (inner diameter: 4 mm). Measurements lasted 180 s.

Diffusive CH
4
ux measurements on the soil surface at PD and PW were carried out with circular exible
chambers constructed out of polyurethane walls varying in height between 0.9 and 1.4 m. The diameter
of the soil chamber was 0.65 m. The soil chamber was also equipped with three fans at the chamber top
ensuring constant mixing of the air inside the chamber. Diffusive uxes from the ditches and the soil
surface at both sites were performed between April 1st 2018 and September 29th 2020.
Diffusive uxes were estimated using the
uxx
function of the package
ux
(Jurasinski et al. 2014) for R
(R development core team 2020). The slope between all concentration points was calculated and the
median slope was used for ux estimation (median-based regression, Siegel 1982). All diffusive ux
measurements were visually checked for signs of ebullition (i.e. strong, sudden increase in CH
4
concentrations). If an ebullition event was identied during a diffusive ux measurement, it was excluded
from the calculation of annual CH
4
balances (157 uxes excluded, 302 uxes remaining at ditches in PD
and 182 uxes excluded, 374 uxes remaining at ditches in PW).
Ebullitive CH
4
uxes
Ebullitive CH
4
emissions were assessed during the vegetation period of 2018. Bubble traps were installed
oating in the middle of the ditches (ve measurement points at each ditch). The bubble traps were
constructed from inverted polypropylene funnels (15 cm diameter opening) connected to a 120 ml
syringe that functioned as the gas reservoir, similar to the approaches of Molongoski & Klug (1980) and
Baulch et al. (2011). The funnel and the syringe were attached to each other with an insoluble adhesive
sealant and a three-way stop cock allowed sampling at the top of the trap (Fig.3). To prevent large water
insects such as water scavenger beetles (Hydrophilidae) from entering the bubble trap we covered the
opening of the funnel with a net (polyvinyl chloride, net width 5 mm). The traps were provided with a
20×30×5 cm cuboid oat (Styrodur, BASF, Ludwigshafen am Rhein, Germany). The bubble traps were
xed in place by cables running between the oat and both banks of the ditch to prevent any disturbance
to the sediment.
To prepare for gas collection all bubble traps were lled with water completely. During the time in which
the trap is deployed, rising bubbles are trapped in the funnel and replace the water inside the trap. After
approximately two weeks (11–14 days) the volume of the accumulated gas in the trap was noted by
reading the printed scales on the syringes. Gas samples were taken from the headspace collected in the
syringe without disturbing the bubble trap by laying a portable aluminium footbridge across the ditch.
Because PW-o was too wide to reach both banks, the bridge was instead placed onto a small,
permanently-installed wooden platform inside the ditch. Gas samples were taken with a 60 ml syringe
and immediately transferred to 12 ml exetainers (Labco, Lampeter, UK). The nal sample volume was