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Influence of hydromorphic soil conditions on greenhouse gas emissions
and soil carbon stocks in a Danish temperate forest
Jesper Riis Christiansen
⇑, Per Gundersen, Preben Frederiksen, Lars Vesterdal
Division of Ecosystem and Biomass Science, Forest & Landscape Denmark, Faculty of Life Sciences, University of Copenhagen, Rolighedsvej 23, DK-1958 Frederiksberg C, Denmark
a r t i c l e
i n f o
Article history:
Received 15 March 2012
Received in revised form 15 June 2012 Accepted 27 July 2012
Keywords:
Nitrous oxide Methane Soil hydrology Hydromorphic soils Carbon sequestration
a b s t r a c t
Recent research has shown that wet or hydromorphic soils in forests are hotspots for greenhouse gas (GHG) emission of methane (CH4) and nitrous oxide (N2O), and that emission of these gases may offset
the mitigation potential from carbon (C) sequestration. However, quantitative evidence at the forest scale is limited. We investigated the role of hydromorphic soils for N2O and CH4fluxes at the forest district
level (Barritskov, 348 ha) by mapping the distribution of upland and hydromorphic soils, measuring the soil carbon and nitrogen stocks and field fluxes of N2O and CH4for a period of 2 years as well as in
laboratory experiments.
Field exchange rates of N2O (mean ± standard error of the mean(SE),lg N2O–N m2h1) were similar
for hydromorphic (3.8 ± 1.2) and upland soils (3.8 ± 0.4). Although both soil types displayed net CH4
oxi-dation the average rate (lg CH4–C m2h1) was significantly lower in hydromorphic soils (5.8 ± 2)
compared to the upland soils (23 ± 1.2). Soil water content (SWC) was, as expected, higher in hydromor-phic soils which was consistent with lower uptake of CH4as well as significantly larger soil carbon stocks
in O horizon plus 0–30 cm mineral soil (86 ± 6 versus 66 ± 5 Mg C ha1in hydromorphic versus upland).
Oxidation rates of CH4in laboratory incubations at ambient concentration (2lL L1) were similar in the two soil types, but the hydromorphic soils oxidised CH4fastest when incubated at 10,000lL L1CH4:
only hydromorphic soils produced CH4. Potential N2O production did not differ between soil types and
N2production was significantly higher in hydromorphic soils, which also had a higher pH > 6.
Based on four scenarios, we assessed how reduced ditching might affect the emissions of N2O and CH4
from upland soils. The CH4sink of the soil decreased in all four reduced ditching scenarios from 1.3 to
7 Mg CO2-equivalent (eqv) y1. The emissions of N2O and CH4in the current state and all scenarios
com-prised only a minute fraction (<1%) of the global warming potential (GWP) of carbon stored in the soil. We conclude that hydromorphic soils are potential hotspots for CH4production and reduced uptake of
atmospheric CH4, but their limited area covered by such soils at Barritskov implies that upland soils are
most important in terms of soil C stock and the non-CO2GHG budget. Ceased drainage activities in upland
soils are expected to increase the likelihood of CH4emissions and reduce soil CH4uptake.
Ó2012 Elsevier B.V. All rights reserved.
1. Introduction
Forests may sequester carbon dioxide (CO2) from the
atmo-sphere reducing the human-induced greenhouse effect. In the northern hemisphere forests annually remove an estimated 2.2– 2.6 Pg of atmospheric CO2(Goodale et al., 2002), equal to 7–8% of
the total global human CO2emission of fossil fuels in 2010 (Peters
et al., 2012). The sequestration of atmospheric greenhouse gases (GHGs) in forest vegetation and soil is partly offset by soil emissions of the strong GHGs N2O and CH4, 298 and 21 times
more effective as GHG than CO2, respectively. Soil hydrology is
considered one of the most important drivers of GHG emissions and it has been proposed that hotspots of emissions from hydro-morphic forest soils may constitute an important but underesti-mated contribution to the GHG balance of ecosystems (Jungkunst and Fiedler, 2007; Grunwald et al., 2012). In a recent assessment of the European GHG balance oxidation of atmospheric CH4in
up-land soils (forest and heathup-lands) was equivalent to an annual CO2
sequestration of 4 Tg while upland forest and heathland soils were only a minor source for N2O compared to arable lands (
Schu-lze et al., 2010). In a comparison of CH4fluxes comprising arable,
grassland and forests soils from around the world it was found that forest soils generally were stronger net sinks for atmospheric CH4
than the other land use types (Boeckx and Van Cleemput, 2001). Thus, experimental data suggest that forests are an important sink for atmospheric CH4. Although there has been increasing research
0378-1127/$ - see front matterÓ2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.foreco.2012.07.048
⇑Corresponding author. Present address: Belowground Ecosystem Group, Department of Forest Sciences, Faculty of Forestry, Forest Sciences Centre, Univer-sity of British Columbia, 2424 Main Mall, Vancouver, British Columbia, Canada V6T 1Z4. Tel.: +1 778 788 6184; fax: +45 3533 1517.
E-mail address:[email protected](J.R. Christiansen).
Contents lists available atSciVerse ScienceDirect
Forest Ecology and Management
focus to understand the controls on GHG dynamics in soils, consid-erable uncertainty still exists to what degree N2O and CH4fluxes
from the soil offset the CO2 sink of forest soils especially when
accounting for emissions of the strong GHGs CH4 and N2O from
the wetter parts of the forest (Grunwald et al., 2012).
Hydromorphic soils in forests are often located in depressions in the landscape where runoff water is concentrated. They are charac-terised by high groundwater table, morphological features such as gley and elevated stocks of soil carbon compared to soils on higher ground (Christiansen et al., 2012). Comparing drained and un-drained deciduous forests in Sweden von Arnold et al. (2005a) showed that CH4 emissions in an undrained alder forest were
one to two magnitudes of order higher than in adjacent drained plots. However, the wet soils under alder showed a smaller N2O
emission than the drained alder and birch soils. Furthermore, upscaling field observations of N2O and CH4fluxes show that even
small areas of hydromorphic forest soils are hotspots for CH4and
N2O emissions. Emissions of the strong GHGs from these areas will
negatively affecting the carbon sequestration potential of forests (Fiedler et al., 2005; Jungkunst and Fiedler, 2005, 2007).
Thus, the distribution of water in the landscape also dictates the spatial occurrence of GHG uptake and production. Recently, Grun-wald et al. (2012)attempted to elucidate the role of the hydromor-phic forest soils for the European CH4 balance. They found that
inclusion of hydromorphic forest soils in inventory calculations doubled net CH4emissions from natural systems in Europe.
How-ever, they also emphasised that the large uncertainty of the conti-nental CH4budgets stems from the lack of knowledge regarding
the spatial distribution of hydromorphic soils. The same applies to N2O emissions as distributions of hydromorphic and upland
soils also drive N2O emission fingerprints from larger forest areas
(Christiansen et al., 2012). Thus, improved knowledge of spatial variability in soil hydrology together with field observations of N2O and CH4will provide more robust estimates of CH4and N2O
fluxes from forests.
The need to account for the role of hydromorphic forest soils must also be seen in the light of future land-use change in forests and of projected climate change (Gundersen et al., 2012). Current land management trends in Denmark and parts of Europe are to re-store natural conditions by afforestation and forest restoration leading, amongst other effects, to re-establishment of forest hydrology to pre-drainage conditions (Stanturf and Madsen, 2002). Understanding the role of hydromorphic soils is underlined by the fact that the impacts of forest cover changes on greenhouse gas fluxes in Nordic riparian forests remains unclear although these forests are hotspots for C and N turnover in the environment (Gundersen et al., 2010). Furthermore, winter precipitation has been suggested to increase for large parts of northern Europe in the near future which could lead to increased soil wetness during this period with unknown consequences for the GHG balance in European forests (Christensen et al., 2007).
While it has been shown that drainage of organic deciduous and coniferous forest soils decreased CH4 emissions, while
(simulta-neously) increasing CO2and N2O emissions and resulting in higher
global warming potential of the soil compared to a non-drained site (von Arnold et al., 2005a,b), little is known regarding the ef-fects of rewetting previously drained soils. Without the possibility of large scale landscape manipulations studying the processes of N2O and CH4formation and consumption for hydromorphic soils
in laboratory conditions compared to field observations may pro-vide us with clues to understand how rewetting of previously wet soils might affect the regulation and emission of these impor-tant GHGs from forest soils.
With this in mind we set out to investigate the importance of hydromorphic forest soils for N2O and CH4emissions in a typical
Danish deciduous forest district. The objectives of this study were;
(1) to map hydromorphic and upland soils in the forest to develop a strategy for representative sampling and upscaling of GHG fluxes based on available spatial data in GIS, (2) to measure the fluxes of N2O and CH4in the chosen hydromorphic and upland study sites in
the field and laboratory, (3) to estimate the soil carbon pools in the chosen hydromorphic and upland study sites and (4) to assess how reduced drainage of upland forest soils could affect the GHG bud-get of N2O and CH4at the forest district level.
2. Materials and methods
2.1. Study area
The study was carried out in the period from March 2008 to May 2010 in the forests of Sønderskov, Barrit Tykke and Klakring Skovhaver (a total forested area of 348 ha) located close to the town of Barrit on the northern shore of Vejle Fjord in Jutland, Denmark (55°410N, 9°550E). The forests are part of the Barritskov
estate and documented as forests since 1770, possibly with for-ested cover dating further back in time (Lone Nørgaard Telling, pers. comm.). The dominant tree species are (in order of areal) European beech (Fagus sylvatica), pedunculate oak (Quercus robur), and ash (Fraxinus excelsior).
The climate is temperate with a mean annual temperature of 7.9°C and precipitation of 670 mm y1for the period 1961–1990
(DMI, 2000). The area is gently sloping with a southward aspect to-ward Vejle Fjord and consists mainly of glacial till deposits. It is intersected by north-to-south oriented narrow valleys or gulleys with illuvial deposits of sand and gytja. A previous soil survey of the forests (Jellesen et al., 2001) characterised the soils as Typic Paleudalf (Sønderskov), Typic Epiaquent and Humaqueptic Endoa-quent (Barrit Tykke) and Typic Epiaqoll (Klakring Skovhaver) (Soil Survey Staff, 1998). The clay, silt and sand content varied between 12% and 20%, 15% and 19%, 61% and 70%, respectively, with a range of 1.6–4% soil organic carbon in the A horizon. A detailed spatial investigation of the internal drainage conditions in the soils of the forest revealed that 51% of the glacial till soils or upland soils (comprising an area of 324 ha) suffer from poor internal drainage resulting in pseudogley (Jellesen et al., 2001).
We also included data on potential denitrification from two additional sites. Strødam, an unevenly aged mixed deciduous for-est, is located in northern Zealand, Denmark. The soil is sandy loa-my till and there is a steep hydrological gradient within the study site. The second additional site, Vestskoven, was afforested in 1971 with pedunculate oak. The soil at this site is classified as a Mollic Hapludalf (Soil Survey Staff, 1998) and consists of clayey loamy till and quite variable in moisture content. Further details about the latter two sites are given inChristiansen et al. (2012).
2.2. Identification of representative study sites
The GIS analyses were performed with ArcMap 9.3 (ESRI, Redlands, CA, USA). The spatial reference for each data layer was standardised and the spatial precision errors that occurred during this procedure were assessed not to have any practical significance for the purpose of the analyses.
The aim of this GIS analysis was to map the hydromorphic and upland soils in order to devise a strategy for sampling and upscal-ing of GHG exchange and soil carbon stocks to forest district level that encompassed all dominant tree species types and represented averagely aged stands.
the hydromorphic sites and then selected the remaining sites. We defined the hydromorphic soils to be present in the areas with soil material originating from fluvial processes while upland soils occur in all areas defined as glacial till soils (Geological Survey of Den-mark and Greenland, 2008). On subsequent field trips we verified this definition by using soil augers to sample soil to 100 cm depth. A schematic flow chart of the GIS analysis with criteria for selec-tion of the 12 study sites is given in Supplementary material (Fig. A1). Eight of the sites were characterised as upland soils and four sites as hydromorphic soils (Table 1). We validated the results of the GIS analysis in the field and altered the status of one oak site from upland soil to hydromorphic soil, increasing the total number of wet sites to five.
By identifying suitable study sites within the forest, it was pos-sible to optimise the collection regarding the number of samples and obtain the broadest spatial representation of GHG fluxes and soil carbon stocks.
2.3. Soil sampling and analyses
Forest floor and mineral soil were sampled at two points next to each of the three chambers per study site. Forest floors were sam-pled in September 2008 just before the onset of foliar litterfall for deciduous species, i.e. when forest floor mass was at a minimum. The forest floor was defined as the organic material above the min-eral soil and was sampled on an area basis using a 25 cm25 cm wooden frame. Forest floors were dried to constant weight at 55°C
and hand-sorted to remove herbaceous litter and roots if present before weighing. The two subsamples per chamber were subse-quently ground and pooled leaving three samples per study site for chemical analysis. Intact cores of mineral soil were sampled using an auger with an internal diameter of 4.5 cm (Westman, 1995). To determine bulk density of the mineral soil fraction cores were divided into three segments: 0–5 cm, 5–15 cm, and 15– 30 cm, and passed through a 2 mm sieve to remove stones and gravel. In three of the study sites, one upland and two hydromor-phic sites, we sampled the soil to a depth of 100 cm.
Fine and coarse roots were removed by hand. The sieved sam-ples were dried at 55°C and weighed. Subsamples were dried at
105°C for correction of weight. Stone content ranged within
1–7%. Following determination of bulk density the six soil cores per study site were pooled to one composite sample per depth seg-ment. A subsample of the mineral soil samples were finely ground in an agate mortar for carbon and nitrogen analysis. Ground sam-ples of forest floor material and mineral soil were analysed for total C and N by dry combustion (Dumas method, VarioMax CH ana-lyzer, Elementar Analysensysteme GmbH, Hanau, Germany) at Agrolab, Institut Koldingen, Sarstedt, Germany.
Forest floor C contents were calculated by multiplying C concentrations with forest floor mass. Mineral soil organic carbon
content (SOC) for the fraction P2 mm was not assessed. There
was no inorganic C (CaCO3) to a depth of 30 cm depth and all
mea-sured C was consequently considered to be organic. Soil organic C contents in (Mg ha1) for each of the three soil layers were
calcu-lated by correcting for the fragment of the sampleP2 mm content
and extrapolating to a hectare basis using the fine fraction bulk density according to the equation
is the relative volume of the fraction P2 mm (%),didenotes the thickness of layeriin cm,Cidenotes the C concentration of layeri (mg g1), and 101is a unit factor (109mg Mg1
108cm2ha1).
Soil pH was determined for all mineral soil samples by mixing 12 g of dried soil sample with 30 mL 0.01 M CaCl2. The pH was
measured in the supernatant using a video titrator (Radiometer, Copenhagen, Denmark).
2.4. Greenhouse gas measurements and flux calculation
Measurements of GHG fluxes were conducted approximately on a monthly basis during a period of 2 years from March 2008 to May 2010. At each sampling occasion volumetric soil water content (SWC) (Theta probe ML2x, Delta T Devices, UK) and soil tempera-ture (model 550B, UEi, Beaverton, Oregon, USA) were also mea-sured 0–5 cm from each static chamber.
The net soil surface exchange of CH4and N2O was measured
with non-mixed closed static chambers that were installed in per-manent locations throughout the study period. Three chambers were installed at random positions within each study site. The chamber collar (inner diameter of 30.5 cm) was inserted 10 cm in the soil and headspace volumes ranged from 6 to 8 L. For chambers placed in the wet parts of the forest a platform was placed in the vicinity of the chamber so that gas samples could be withdrawn from the chamber without disturbing the soil around the chamber. During sampling of headspace air, a lid was placed on top of the collar and sealed with a silicon rubber ring around the edge of the lid ensuring gas-tight conditions. Chamber headspace samples were withdrawn with 60 mL plastic syringes through a butyl rub-ber septum in the middle of the lid at times 0, 30, 60 and 120 min. From March 2009 the enclosure time was reduced to 60 min and samples taken every 20th minute at 0, 20, 40 and 60 min. At each headspace sampling the syringe was used to mix the headspace by pumping three times before fully filling the syringe. Pressure changes in a manually sampled chamber headspace has been re-ported to lead to overestimation of the estimated diffusive flux (Bekku et al., 1995), but we did not observe any changes, e.g. non-linear behaviour of concentrations over time in headspace concentrations that could be attributed to mass flow caused by depressurisation of the chamber headspace. Headspace samples were transferred to non-evacuated 2.7 mL crimped vials with a bu-tyl rubber septum by flushing the vial with 58 mL of the sample in the syringe and pressurising the vial with the remaining 2 mL.
Gas samples were stored for a maximum of 5 days in vials be-fore analysis. Gas samples were analysed on a Shimadzu GC-2014 gas chromatograph (Shimadzu, Kyoto, Japan) equipped with elec-tron capture and flame ionisation detectors set at 300°C and
200°C, respectively. The carrier gas was 100% N2with a flow rate
of 25 mL min1. Methane and N
2O were analysed in separate
col-umns set in a constant oven temperature of 40°C. The column
used for CH4was a 60/80 Carboxen 1000 (15 ft, 1/8 in.). For N2O
an 80/100 Hayesep Q (2.5 m, 1/8 in.) column was used. An auto-sampler equipped with a syringe extracted 1.6 mL sample from
Table 1
Main characteristics of the twelve study sites at Barritskov selected through the GIS analysis.
the vial and injected 0.75 mL into each column used for CH4and
N2O.
All gas fluxes were calculated by linear regression of gas con-centrations versus time. For this study we did not estimate the minimal detectable flux and instead usedR2 of linear regression as a threshold for a significant flux. Regressions with anR2-value above 0.85 were accepted for flux calculations. For regression anal-yses resulting inR2-values below 0.85, the increase or decrease in N2O and CH4concentrations was smaller than we most likely could
detect with our gas chromatographic setup and fluxes were set to zero. We always rejected chamber enclosure data that appeared chaotic in nature. Fluxes were expressed as
l
g N2O–N m2h1orl
g CH4–C m2h1. We estimated an annual budget in kg CH4ha1or kg N2O ha1by assuming that the flux rate measured for a given
date represented the entire period until the next sampling date. We converted the annual budgets of CH4and N2O to CO2
-equiva-lents by a factor of 21 and 298, respectively (Forster et al., 2007).
2.5. Laboratory incubations
Intact soil cores from the top mineral soil (0–5 cm) were sam-pled in the middle of each chamber for all twelve study sites using corers with a diameter of 5 cm in November 2009 and 2011 at Strødam/Vestskoven and Barritskov, respectively. Upon arrival to the laboratory the intact soil were kept in the corners in a dark cool room at 4°C until analysis. We divided the intact soil cores in five
parts and removed stones and larger root fragments. Two parts were air dried and used for the potential CH4 oxidation
experi-ment, one part was used for the potential CH4production
experi-ment, and the last two parts for the potential denitrification determination. To determine the absolute SWC 20–30 g of fresh and air dried soil were dried at 55°C for 48 h.
For incubation, the soil was transferred to screw cap glass incu-bation bottles (approximately 120 mL) that were sealed with butyl rubber septums. The headspace of the incubation bottles were determined by the difference in weight for a bottle with soil (and substrate solution) only and the same bottle with soil (and sub-strate solution) filled with water to the brim. For all three CH4
experiments we used three control bottles to check the back-ground of the experiments. Any rates smaller than the control rates were set to zero. All rates were determined with linear regression between concentration and time and expressed as
l
g N2O–N g dry soil1d1or
l
g CH4–C g dry soil1d1.2.5.1. Potential CH4oxidation
Two experiments were performed to measure the CH4oxidation
potential of the soils from Barritskov in order to target high and low-affinity methanotrophic bacteria. Prior to the CH4 oxidation
experiments we air dried the fresh soil for 24 h to decrease the SWC and create more optimal conditions for CH4 oxidation
(Gulledge and Schimel, 1998). We incubated 10 g of air dried soil in each bottle. In one set of bottles the initial CH4concentration
in the headspace was maintained at ambient level (2
l
L L1) totarget high-affinity methanotrophs (Reay et al., 2005). In another set of bottles we increased the headspace concentration to 10,000
l
L L1 by injecting 1 mL of 100% CH4 to targethigh-affinity methanotrophs (Reay et al., 2005). Bottles were thoroughly mixed by rolling prior to sampling. For the high-affinity methano-troph experiment we sampled at 0, 24 and 48 h after bottle closure and extracted 600
l
L from the headspace and manually injected500
l
L into the gas chromatograph. For the low-affinitymethano-troph experiment we sampled at 0, 24, 74, 146 and 251 h after addition of pure CH4. We extracted 300
l
L from the headspaceand injected 200
l
L into a crimp sealed vial containingatmo-spheric air. The samples were subsequently analysed on the gas chromatograph using the autosampler system.
2.5.2. Potential CH4production
We added 3 mL of anoxic 10 mM sodium acetate solution to 10 g of fresh soil in an incubation bottle. MilliQ water was used and the solution was anoxified in an ultrasonic bath for 10 min before addition to the bottles. We then evacuated the closed incu-bation bottle with a vacuum pump to a constant partial vacuum of ca.700 mbar for 3 min. Subsequently, we flushed the bottles with pure N2for 3 min and assumed we had created an O2free
head-space. Following this step we pressurised the bottle with 10 mL a H2/CO2 gas mixture (80%/20% v/v) obtaining an overpressure of
about 15 mbar. Our protocol was slightly modified from that of Wagner et al. (2007). We sampled headspace at 0, 24, 240 and 540 h after addition of the H2/CO2 gas mixture. We extracted
600
l
L from the headspace, rejected 100l
L and injected theremaining 500
l
L in a crimp sealed vial containing atmosphericair. The samples were subsequently analysed on the gas chromato-graph using the autosampler system.
2.5.3. Potential denitrification
The rate of potential denitrification was determined using the acetylene inhibition technique where addition of the acetylene gas inhibits the reduction of N2O to N2 (Yoshinari et al., 1977).
To two sets of incubation bottles with 10 g of fresh soil we added 15 mL of a solution consisting of 1 mM KNO3, 0.5 mM glucose,
0.5 mM sodium acetate, 0.5 mM sodium succinate (Bernsteinsä-ure). Similar to the CH4 production experiment anoxified MilliQ
water was used in preparing the solutions and bottles were evacu-ated and flushed. In one set of bottles with soils from all the study sites 10 mL of acetylene was added and for the second set of bottles we added 10 mL N2. The samples were incubated under constant
shaking for 2.5 h and gas samples were extracted at 30, 60, 90, 120 and 150 min after addition of gas and transferred to crimp-sealed vials and subsequently analysed on the gas chromatograph. Fluxes of potential N2O obtained without acetylene were
sub-tracted from the N2O fluxes with acetylene for each sample to
ob-tain a flux interpreted as the potential rate of complete denitrification to N2expressed as
l
g N2–N g dry soil1d1.2.6. Reduced soil drainage scenario analysis
2.6.1. Affected forest area
This analysis served to provide the data on the spatial extent of soils affected by reduced drainage scenarios. The aforementioned GIS map layers were supplemented with the following information and assumptions:
A digitised map of internal soil drainage type in the top 120 cm of the forest soils divided in two classes: (A) temporary water saturation and (B) no signs of temporary saturation (Jellesen et al., 2001).
Ceased drainage in class A would affect the whole area, whereas ceased drainage in class B would affect the soil closest to the ditches within an impact zone.
For class B areas we defined two scenarios (impact zones) in the case of reduced drainage: A 2 m zone and a 30 m zone around the ditches (zones intersecting with hydromorphic soils were excluded here, since already represented in class A).
2.6.2. Impact on GHG emissions
In order to calculate the effect on the N2O and CH4net exchange
by reduced soil drainage in the upland soils classes A and B we ex-plored the relationship between the mean N2O or CH4exchange
rate and soil moisture content for a given date and established a re-sponse function between GHG exchange rate and soil moisture content. Thus, we aggregated data on soil group (hydromorphic and upland) levels only and assumed the relationship to be valid independent of location.
To date we have no knowledge of studies in Danish forests that have quantified the large scale impact on soil hydrology after ceased effect of ditching (although this currently occurs in many forest districts). Thus, we attained a conservative approach and cal-culated the effect on GHG exchange by two levels of increased soil moisture content for each of the impact scenarios in Section2.6.1. We assumed that ceased maintenance of ditches would increase SWC by 5% for the upland soils (classes A and B) and as a maximum response increased the SWC by 16% equalling the mean SWC ob-served for the hydromorphic soils.
In total we calculated the impact on the GHG exchange for six dif-ferent scenarios, including a scenario without the presence of hydro-morphic soils, the current distribution of hydrohydro-morphic and upland soils and the four combinations of reduced drainage discussed above (i.e. 5% and 15% SWC increases in both 2 and 30 m impact zones).
2.7. Statistics
For statistical analyses the SAS software version 9.2 (SAS Insti-tute Inc., Cary, North Carolina, USA, 2008) was used and signifi-cance was accepted at p60.05. The statistical analyses in this
study focused on determining the effect of soil type between the upland (N= 7) and hydromorphic study sites (N= 5). For the fol-lowing parameters we used repeated measures ANOVA in PROC MIXED to test treatment effect time series of soil surface net ex-change of N2O and CH4, SWC and temperature in 0–5 cm. The
chamber measurements represented the subject repeated over time and the stand was used to account for the random variation of the tested parameter in the repeated measure analyses.
We used one way ANOVA to test ‘treatment’ effect (hydromor-phic versus upland soils) on mineral soil pH, pools of organic carbon and total nitrogen in the organic horizon and mineral soil (0–30 cm) as well as the C:N ratio of the organic horizon and in the mineral soil (0–30 cm). Common for both analyses was that the type of soil was the independent variable. In order to comply with the assumptions of the statistical test, i.e. variance homogeneity between groups and normal distribution of residuals, N2O fluxes, SWC and temperature
were square-root transformed, CH4 fluxes were transformed to
(CH4+ 350)1.5and C:N ratio of the organic horizon was log10
trans-formed. The transformation was adopted from the automatic rou-tine ‘‘Guided data analysis’’ in the SAS software version 9.2. We did not test for tree species differences for any of the treated data as we did not have a fully balanced collection of tree species repre-sented on both upland and hydromorphic soils.
The differences in potential denitrification and CH4oxidation and
production was tested using Mann–Whitney rank sum test because these data were not normally distributed or there was no variance heterogeneity between groups (hydromorphic and upland soils).
3. Results
3.1. Upland versus hydromorphic soils
3.1.1. Soil moisture and field GHG fluxes
As expected the mean SWC was20% higher (Fig. 1A,p< 0.001) in the hydromorphic soils and was associated with a fourfold lower
mean CH4uptake (Fig. 1D,p< 0.001) compared to the upland soils
(Table 2). All the study sites on upland soils were net sinks for CH4
whereas the study site with oak was the only net emitter for the hydromorphic soil group and ash and beech were small net sinks (Fig. 1D). Methane fluxes tended to decrease in summer periods and increase during autumn and winter (Fig. 2C and D) although the temporal variation in flux magnitude was around 20
l
gCH4–C m2h1. Furthermore, there was a high degree of
covari-ance between CH4fluxes (mean, min and max) in the
hydromor-phic and upland soil types during the study period (Fig. 2C and D). We observed the majority of CH4 emission events in spring
when SWC was still high while the number of emission events sharply decreased in the summer and the SWC in the hydromor-phic soils presumably fell below a threshold for CH4production.
There was no significant effect of soil water regime on N2O
fluxes. Although we did not evaluate tree species differences statis-tically, some notable differences were observed. While the beech stands did not differ, the emission of N2O under ash was twice as
high in the upland soils as for the hydromorphic, whereas the opposite trend was observed for oak (Fig. 1C). The most conspicu-ous difference in regard to CH4fluxes was the net emission of CH4
under oak for the hydromorphic soils compared to the highest re-corded CH4uptake for the oak on upland soils (Fig. 1D). These
re-sults potentially point to important tree species specific differences in these different soil environments which deserve further atten-tion for following studies. However, as menatten-tioned earlier our study design did not offer an opportunity to test for tree species differences.
We did not observe any temporal trend in N2O fluxes in either
upland or hydromorphic soils (Fig. 2A and B), however, there was a tendency for higher maximum fluxes of N2O in the hydromorphic
soils (Fig. 2B).
3.1.2. Incubation studies
The upland and hydromorphic soils did not differ significantly in potential methane oxidation (mean ± standard error of the mean(SE)) (5 ± 0.5 and 7 ± 2 ng CH4–C g dw1d1, respectively)
when incubated at atmospheric levels of CH4(Fig. 3A). When
incu-bated at 10,000
l
L L1all soils showed higher CH4oxidation ratesthan during incubation at ambient methane concentration. How-ever, the hydromorphic soils had a significantly (p< 0.001) higher CH4oxidation potential (7.5 ± 2.9
l
g CH4–C g dw1d1) thanup-land soils (0.05 ± 0.03
l
g CH4–C g dw1d1,Fig. 3B). Similarly, weobserved that the hydromorphic soils potentially produced signif-icantly (p< 0.001) more CH4(0.001 ± 0.0003
l
g CH4–C g dw1d1),at least one order of magnitude, than the upland soils (0.00008 ± 0.00005
l
g CH4–C g dw1d1). Potential CH4oxidationrates were in general markedly higher than CH4production
poten-tial (Fig. 3A–C).
No differences were observed in potential N2O production
be-tween upland and hydromorphic soils (0.16 ± 0.02 and 0.20 ± 0.03
l
g N2O–N g dw1d1,Fig. 3D) at Barritskov. However,our acetylene inhibition experiment for Barritskov indicated that the hydromorphic soils (0.35 ± 0.05
l
g N2–N g dw1d1) weremore efficient at reducing N2O to N2 than upland soils
(0.05 ± 0.015
l
g N2–N g dw1d1,Fig. 3E). The potential N2Opro-duction rates at Strødam and Vestskoven were similar between up-land and hydromorphic soils as observed for Barritskov (Fig. 3D). Also, we found no significant difference in potential N2production
between upland and hydromorphic soils, although the rate of N2
production was threefold higher for the hydromorphic soils at Vestskoven (Fig. 3E).
3.1.3. Soil carbon and nitrogen stocks and pH
0–30 cm (Fig. 1E); about 20 Mg ha1higher for the hydromorphic
soils compared to the upland soils (Table 3). There was also a trend toward higher N stock in the hydromorphic soils, albeit not signifi-cant (Table 3,p= 0.075). Furthermore, the tree species showed the same order in terms of C and N stocks in both upland and hydromor-phic soils, with ash having the highest and oak lowest stocks (Fig. 1E and F). Thus, C:N ratios in the mineral soil and O-horizon were sim-ilar regardless of tree species (Fig. 1G and H) and soil moisture regime.
Mineral soil pH was significantly (p< 0.001) lower (4.2 ± 0.2) in the upper 5 cm (Table 2) for the upland soils compared to the hydromorphic soils (6.2 ± 0.4). Consistently lower soil pH in upland
soils was also observed for the layers 5–15 cm (p< 0.001), 15– 30 cm (p< 0.001) and 30–100 cm (data not shown).
3.2. Reduced drainage scenarios
3.2.1. Forested area affected by reduced drainage
The GIS analysis was used to determine the size of areas to be af-fected and not afaf-fected by reduced drainage scenarios (Table 3). The area of soils with signs of permanent of temporary saturation (hydromorphic soils + till soils class A) was 190 ha or 28% of the for-ested area (Table 3). For the 2 m impact zones the area assessed to be impacted by reduced drainage increased by 3 ha only. Increasing
Fig. 1.Mean values ± standard error of the mean for (A) soil water content (vol.%), (B) soil temperature (°C), (C) N2O flux (lg N2O–N m2h1), (D) CH4flux (lg CH4–
C m2h1), (E) soil carbon stock (Mg C ha1), (F) soil nitrogen stock (Mg N ha1), (G) mineral soil C/N ratio and (H) organic horizon CN ratio divided in the three investigated
the impact zone to 30 m would increase the affected area to 214 ha or 31% of the forested area (sum of column 2 and 7 inTable 3).
3.2.2. Impact on GHG emissions
There was no positive relationship between SWC and N2O
fluxes in either upland or hydromorphic soils (data not shown).
Therefore, an increasing area of poorly drained soils or hydromor-phic soils would, at Barritskov, not result in increased emissions of N2O. However, for CH4there was a significant positive relationship
between SWC in 0–5 cm and CH4emission fluxes (Fig. 4). We
ap-plied this relationship to assess how increased SWC would increase CH4fluxes from the soil.
This relationship suggests that the CH4uptake would decrease
with increasing SWC and, given sufficient moisture, result in CH4
emission. The magnitude of the CH4budget for the forest would
then depend on the spatial coverage of increasingly hydromorphic soils (Table 2).
Exclusion of the hydromorphic soils currently present at Barrits-kov from the calculation would increase the annual CH4sink by
1 Mg CO2-eqv from the current estimate of17.6 Mg y1(Fig. 5).
A 5% higher SWC in the all currently well-drained soils (Table 2) de-creased the CH4sink by1.7 and2.2 Mg CO2-eqv y1compared to
the current estimate (Fig. 5) for the 2 m and 30 m impact zones, respectively. A further increase in the SWC of 16% for these soils de-creased the CH4sink further by6.3 and8.0 Mg CO2-eqv y1for
the 2 m and 30 impact zones, respectively.
Although N2O emissions (53 Mg CO2-eqv y1) from the forest
comprise the largest proportion of the non-CO2 GHG emissions,
the total emission of CO2-equivalents originating from N2O and
CH4increases from 4% to 22% by increasing the SWC of the forest
soils.
Currently, the estimate of non-CO2 GHG emissions of
0.10 ± 0.06 Mg CO2-eqv ha1y1 comprises a minute fraction
(0.5%) of the difference in soil carbon stock (forest floor and min-eral soil to 30 cm) between upland and hydromorphic soils, i.e. 20 Mg C ha1(Table 2). Even the scenario including the maximum
spatial coverage of affected soils and increase of SWC matching the hydromorphic soil group would not lead to any significant offset of a potential gain in the soil carbon pool in this forest district.
Fig. 2.Temporal trends in N2O and CH4fluxes at Barritskov. The symbols connected by the black line represent mean ± sem for upland and hydromorphic soils. Dashed lines
above and below represent maximum and minimum fluxes measured on each sampling occasion. Unit of fluxes arelg N2O–N m2h1andlg CH4–C m2h1. Black dots
represent average CH4emission across the soil type. Table 2
Environmental variables, gas fluxes, C and N stocks and C:N ratios (mean ± standard error) for upland and hydromorphic soils at Barritskov. Significant differences are highlighted in bold and trends (not significant) in italics.
Parameter Unit Upland Hydromorphic p -Value Soil moisture (0–
5 cm)
vol.% 33.8 ± 0.5 50.2 ± 1 <0.001
Soil temperature (0–5 cm)
°C 9.4 ± 0.2 9 ± 0.2 0.370
N2O flux lg N2O–
N m2h1
3.8 ± 0.4 3.8 ± 1.2 0.180 CH4flux lg CH4–
C m2h1 23.4 ± 1.2 5.8 ± 2.0 <0.001
Soil pH in CaCl2(0–
5 cm)
– 4.2 ± 0.2 6.2 ± 0.4 <0.001
Soil pH in CaCl2(5–
15 cm)
– 4.1 ± 0.2 6.2 ± 0.4 <0.001
Soil pH in CaCl2
(15–30 cm)
– 4.3 ± 0.2 6.2 ± 0.4 <0.001
C stock (O horizon) Mg ha1 3.7 ± 0.5 2.5 ± 0.6 0.162
N stock (O horizon) Mg ha1 0.11 ± 0.02 0.06 ± 0.02 0.121
C stock (0–30 cm) Mg ha1 62.7 ± 4.9 83.9 ± 5.8
0.019
N stock (0–30 cm) Mg ha1 4.9 ± 0.5 6.6 ± 0.6
0.075
C stock (O + 0– 30 cm)
Mg ha1 66.4 ± 4.7 86.4 ± 5.6
0.020
N stock (O + 0– 30 cm)
Mg ha1 5.0 ± 0.5 6.7 ± 0.6 0.076
4. Discussion
4.1. Upland versus hydromorphic soils
The significantly higher SWC of the hydromorphic soils agrees with our expectations when we designed the study setup with the GIS analysis. The temporal variations of SWC in the upland and hydromorphic soils were similar but availability of water in the landscape depressions systematically increased the wetness of the soil. The similar temporal variation and magnitude of soil temperatures also point to the same microclimatic conditions in the hydromorphic and upland soils.
4.1.1. N2O fluxes
Surprisingly, the mean N2O fluxes were similar for both the
hydromorphic and upland soils (3.8
l
g N2O–N m2h1) atBarrits-kov (Fig. 1A and B), and in addition the potential rates were similar (Fig. 3D). Previously, we found considerably higher N2O emissions
(8–15
l
g N2O–N m2h1) at the Strødam site at soil moisturelev-els corresponding to those in the hydromorphic soils at Barritskov (Christiansen et al., 2012). However,Beier et al. (2001)also did not detect differences between the upland and wetter soils in another
Fig. 3.Comparison of mean ± standard error potential rates of (A) CH4oxidation at a starting concentration of 2lL L1, (B) CH4oxidation at a starting concentration of
10,000lL L1, (C) CH4production, (D) N2O production and (E) indirect measure of N2production for Barritskov and the Strødam/Vestskoven locations. Rates are inlg CH4–
C g dw1d1or
lg N2O/N2–N g dw1d1. Data for Barritskov are given by tree species butp-values represent the overall difference between upland and hydromorphic soils
at this site only. Bars represent standard error of the mean. Note the differences in units ony-axis for (A–C).
Table 3
Areal coverage of hydromorphic and upland till soils in% of total forest area (348 ha) at Barritskov. The upland soils are further divided in classes (A) temporary saturation and (B) well-drained. It was assumed that the entire area of soils belonging to class A was affected if ditching terminated whereas for class B only an impact zone surrounding the ditches was affected. Columns 4–7 represent the GIS analysis used to determine the area of class B till soils affected by two impact zone widths, e.g. 2 and 30 m.
Hydromorphic soils
Till soils (current drainage status)
Class B 2 m impact zone
Class B 30 m impact zone Class A
temporary saturation
Class B well drained
Not affected by ditching
Affected by ditching
Not affected by ditching
Affected by ditching
7 48 45 44 1 31 14
Fig. 4.Relationship between mean soil water content (vol.%) and mean CH4fluxes
Danish beech forest with a comparable glacial till soil (mean rates of 5.7
l
g N2O–N m2h1).Both soil types at Barritskov had mineral soil C:N ratios of 12– 13 which are favourable for N2O emissions (Pilegaard et al., 2006).
Higher emission rates could thereby be expected especially when combined with an optimal SWC of 60% water filled pore space (Davidson et al., 2000). Low N2O emissions from the hydromorphic
soils may, however, be explained by the high soil pH in hydromor-phic soils. At pH > 6 (Table 2), the produced N2O is reduced to N2by
nitrous oxide reductase (Simek and Cooper, 2002) as is also indi-cated by the similar or higher potential N2emissions (Fig. 3E)
com-pared to the potential N2O emissions (Fig. 3D) from the
hydromorphic Barritskov soils.
At lower pH nitrous oxide reductase is inhibited and the ratio of N2O/N2 from denitrification increases (Weslien et al., 2009).
Accordingly, the denitrification product was dominated by N2O
in all upland soils with lower pH than hydromorphic soils (Fig. 3D and E). For the Strødam soils (lowest pH) the potential rates were more dominated by N2O in both upland and
hydromor-phic soils than for Barritskov and Vestskoven soils (Fig. 3D and E), which is in line with the higher N2O emissions observed in the field
at Strødam.
4.1.2. CH4fluxes
The CH4fluxes for the upland soils were lower, but in the same
range as recently published fluxes for upland European forest soils (mean ± SE: 46 ± 20
l
g CH4–C m2h1; Skiba et al., 2009). Thesimilarities in temporal variation of CH4fluxes (Fig. 2) indicate that
the differences between upland and hydromorphic soils are mainly driven by differences in the magnitude of SWC. Soil water slows down diffusion of atmospheric CH4in the soil matrix and the
up-take of atmospheric CH4is reduced compared to a well-drained
soil.
Soils from Barritskov oxidised CH4under ambient
concentra-tion levels in the laboratory (Fig. 3A). This suggests that Type II or high-affinity CH4oxidising bacteria were present (Hanson and
Hanson, 1996). During incubation SWC of the upland and hydromorphic soil samples were adjusted by pre-drying to obtain similar physical conditions in the soil, e.g. for diffusion. Accord-ingly, the high-affinity CH4 oxidation between hydromorphic
and upland soils (Fig. 3C and D) were similar, which supports our conclusion that the higher SWC under field conditions caused the lower oxidation rates of CH4 in the hydromorphic soils
(Fig. 2D). Compared to previous studies of high-affinity CH4
oxida-tion our rates were lower.Menyailo et al. (2008)found up to 10 times higher rates in Siberian forest soils, and high-affinity CH4
oxidation rates in soils under sessile oak (Quercus petraea) in the Gisburn Experimental Forest in the UK were on average six times higher (Reay et al., 2005). Similar high rates were found for soils under beech in Germany (Degelmann et al., 2010).
The much higher low-affinity CH4 oxidation rates (Fig. 3B)
measured at 5000 times ambient CH4concentration level for both
upland and hydromorphic soils are in accordance with other stud-ies (Reay et al., 2005). By incubating the soil samples at 10,000
l
L L1CH4 we solely target low-affinity methanotrophsthriving under conditions with elevated CH4 concentrations, as
they are the only active methane oxidisers at concentrations above 2500
l
L L1(Bender and Conrad, 1992, 1995). Thus, low-affinitymethanotrophs are present in both soil types, but more abundant in hydromorphic soils which fits well with the significantly higher CH4production rate in hydromorphic compared to upland soils.
Even under optimal conditions for CH4 production there is not
much evidence to support that the upland soils produce CH4. Only
17 out of 370 CH4flux measurements showed positive fluxes, all
occurring during the early spring or winter when SWC was high. For the hydromorphic soils 63 out of 262 measurements showed CH4 emission. Thus, our laboratory incubation studies and field
data comply.
4.1.3. Carbon stocks
The total C stock in forest floor and mineral soil to 30 cm for the whole forest district based on upland soil C contents only amounts to only 23.1 Gg C. Inclusion of the hydromorphic soils in the assess-ment increases the total C stock to 23.6 Gg C, or by only 2%. Although the hydromorphic soils have significantly higher C con-tents than upland soils their limited spatial coverage reduces their influence on soil C stock at the forest (district) level.
Carbon stocks in mineral soil (to 30 cm) for upland soils were of the same order of magnitude as the ca. 55 Mg C ha1reported for
the upper 40 cm in Danish well-drained Alfisols by Vejre et al. (2003), but the C stocks in hydromorphic soils were clearly higher. Reduced oxygen availability hampering decomposition is believed to be the main factor responsible for the higher C stocks in hydro-morphic soils. In a study of Danish soil databases, Krogh et al. (2003) attributed their higher average forest soil C stock to 1 m (167 Mg ha1) compared to the 125 Mg ha1 reported by Vejre
et al. (2003)to the fact that their database also included poorly drained soils. Forest floor C stocks were quite similar between soil moisture regimes and corresponded well to the stock of approx. 5 Mg ha1reported byVejre et al. (2003).
The difference in C stock between moisture regimes may be inter-preted as the potential C sequestration following reduced drainage. Based on this inference and an assumption of 50–100 years needed for SOC stocks to reach a new steady state level, the rate of C sequestration would amount to 0.7–1.5 Mg CO2ha1yr1. This rate
of soil C sequestration is within the range reported for, e.g. land-use change from cropland to forest (Poeplau et al., 2011; Vesterdal et al., 2007), but higher than rates reported from forest soil inventories (e.g. Berg et al., 2009). Decreased CH4 oxidation resulting from
Fig. 5.Scenario analyses of the net CH4budget for the entire forest at Barritskov in
Mg CO2-equivalents (eqv) y1± standard error of the mean. Scenarios are (a) no
reduced ditching would offset the potential C sequestration by 5–23 kg CO2ha1y1 or 0.7–3% in the upland soils assuming
50–100 years to steady state in soil C as mentioned above.
4.2. The importance of hydromorphic soils
The preliminary GIS analysis of the soil types at Barritskov showed that the hydromorphic soils occupied only 7% of the entire forested area of 348 ha, however, we deliberately chose to over-represent these soil types in the final selection of study sites in order to establish a better data basis to compare upland and hydro-morphic soils. We based our mapping on the national geological survey of the general soil types, but this classification is relatively coarse and it has been shown earlier that even relatively small areas (5–10%) of the catchment can have a large impact on GHG budgets. Thus, it is likely we slightly underestimated the spatial coverage of hydromorphic soils.
The GIS analysis showed that in a worse case scenario we can expect a decrease in the CH4oxidation in the soil at the forest
dis-trict level at Barritskov (Fig. 5) equivalent to 8 Mg CO2y1 or
23 kg CO2ha1y1 by making the upland soils wetter. Currently,
even a cautious estimate of non-CO2GHG emissions (N2O + CH4)
from the entire forest at Barritskov (36 ± 22 Mg CO2-eqv y1 or
104 ± 31 kg CO2-eqv ha1y1) comprises a minute fraction (0.5%
y1) of the difference in soil C stock between upland and
hydromorphic soils.
If the upland soils that become wet in the reduced drainage scenario were to increase the C content at a rate similar to the estimated equivalent change in non-CO2 flux it would take
200 years to reach the C content of the hydromorphic soils. Thus, if the soil C reaches this level before 200 years, reduced drainage will have an overall positive effect on the soil GHG balance. However, if the build-up takes in excess of 200 years it cannot fully off-set the non-CO2 emission change in this forest
district.
Even for the scenario including the maximum spatial coverage of affected soils and an increase of SWC matching the hydromor-phic soil type the total emissions of 44 ± 25 Mg CO2-eqv y1 or
124 ± 37 kg CO2-eqv ha1y1will not lead to any significant offset
of the difference in soil carbon pools.
The results of this study suggest that the global warming poten-tial from soil emissions will increase by an increasing proportion of hydromorphic soils, but not to such a degree that it can offset the entire C sink in vegetation and soil for this type of forest. On the other hand, the practice of ditching has most likely facilitated an increase in the CH4sink of the forest and a termination of this
prac-tice by filling up or ceasing maintenance of ditches will increase the areas of wet soils in the future.
Our results indicate that increased wetness in these upland soils would decrease the oxidation rate of atmospheric CH4rather than
induce net emission of CH4 (Figs. 3 and 4). It is likely that the
hydromorphic soils will exhibit enhanced CH4 emissions under
wetter soil conditions, but the results do not indicate that these soils would become overall net emitters of CH4.
An unknown in our assessment of the area affected by reduced ditching is the lateral drainage effect of ditches. We used the Skaggs method originally developed to assess the lateral impact zone of ditches in peat soils (Skaggs et al., 2005). The calculation requires the lateral hydraulic conductivity which is seldom known and hence we used the vertical conductivity for a similar soil type. Whether the minimal estimated impact zone of 2 m is underesti-mated remains unknown, however we did arbitrarily set a maxi-mum lateral impact zone of 30 m which we assume covered the extremes of the lateral impact in these soil types.
5. Conclusions
We studied the importance of hydromorphic soils on the GHG budgets and dynamics of CH4and N2O as well as soil C and N stocks
at the forest district level in a typical Danish deciduous forest. Hydromorphic soils had significantly higher soil water content, higher soil pH, higher soil organic C pool in the top 30 cm, lower field scale CH4uptake, higher CH4production potential and larger
proportion of N2O reduced to N2in comparison to upland soils.
However, field N2O fluxes and rates of potential N2O production
were similar in hydromorphic and upland soils in this forest. The empirical evidence compiled in this study suggests that hydromor-phic soils are indeed capable of producing more CH4than upland
soils as observed in the laboratory. This may also be the case for N2O, but the difference in observed field and laboratory emissions
was not significant. On the other hand the hydromorphic soils also displayed characteristics that offset the enhanced production of N2O and CH4, such as the observed rates of low-affinity CH4
oxida-tion capable of oxidising large quantities of CH4. Also, the
signifi-cantly higher reduction potential of N2O to N2 suggests that the
hydromorphic soils are capable of limiting the net emission of N2O. Our results indicated tree species specific effects on the
GHG exchange that deserve further studies to identify possible mitigation options through tree species choice in managed forests. Our analysis of the impact of reduced ditching showed that the CH4sink decreased due to expected increased soil water content.
In this specific study we conclude that the hydromorphic soils are hotspots of CH4production and oxidation but not for N2O. In
that the area covered by hydromorphic soils is limited at Barritskov implies that the upland soils within this forest are most important for the soil C stock as well as the non-CO2GHG budget. However,
our analyses also suggest that increased wetness in upland forest soils, e.g. due to ceased drainage activities, can impose an impact on the GHG budget by increasing the likelihood of CH4emissions
and reducing soil CH4uptake.
Future research focusing on a spatial prediction of where hydro-morphic soils naturally exist in forests and the extent to which the hydrological regime of these soils can be expected to move toward a hydromorphic state if drainage practices stop or precipitation in-creases is important to improve our estimates of GHG budgets from forests.
Acknowledgements
We are very grateful for the support and fieldwork carried out by Trine Møller Kopp at Barritskov. Also, we would like to extend our gratitude to Thomas Harttung, owner of Barritskov, for grant-ing us unlimited access to forests durgrant-ing this study. We also thank Lone Nørgaard Telling at Barritskov for support. The work pre-sented in this paper was part of the project ‘‘Målrettet Naturnær Skovdrift og Hydrologi som optimeringsredskaber for C-indhold i vedmasse og skovjord’’ in collaboration with Green Carbon Initia-tive c/o Barritskov Land- og Skovbrug Skovbrug supported by The Danish Nature Agency. We thank the two anonymous reviewers who provided constructive criticism that helped improve the pa-per. Also a thank you to Andy Howe for help with correcting the language in the manuscript.
Appendix A. Supplementary material
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