Influence of hydromorphic soil condition (1)

Forest Ecology and Management

journalhomepage:www.elsevier.com/locate/foreco

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

article info

abstract

Article history: Recent research has shown that wet or hydromorphic soils in forests are hotspots for greenhouse gas Received 15 March 2012

4 ) and nitrous oxide (N 2 O), and that emission of these gases may offset Accepted 27 July 2012

(GHG) emission of methane (CH

Received in revised form 15 June 2012

the mitigation potential from carbon (C) sequestration. However, quantitative evidence at the forest scale is limited. We investigated the role of hydromorphic soils for N 2 O and CH 4 fluxes at the forest district level (Barritskov, 348 ha) by mapping the distribution of upland and hydromorphic soils, measuring

Keywords: the soil carbon and nitrogen stocks and field fluxes of N 2 O and CH 4 for a period of 2 years as well as in

Nitrous oxide

laboratory experiments.

Methane

Field exchange rates of N 2 O (mean ± standard error of the mean(SE), l gN 2 O–N m h ) were similar

Soil hydrology for hydromorphic (3.8 ± 1.2) and upland soils (3.8 ± 0.4). Although both soil types displayed net CH 4 oxi- Hydromorphic soils

dation the average rate ( l g CH 4 –C m h

Carbon sequestration phic soils which was consistent with lower uptake of CH 4 as well as significantly larger soil carbon stocks

in O horizon plus 0–30 cm mineral soil (86 ± 6 versus 66 ± 5 Mg C ha

in hydromorphic versus upland). Oxidation rates of CH 4 in laboratory incubations at ambient concentration (2 l LL ) were similar in the two soil types, but the hydromorphic soils oxidised CH 4 fastest when incubated at 10,000 l LL CH 4 : only hydromorphic soils produced CH 4 . Potential N 2 O production did not differ between soil types and N 2 production 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 N 2 O and CH 4 from upland soils. The CH 4 sink of the soil decreased in all four reduced ditching scenarios from 1.3 to

7 Mg CO 2 -equivalent (eqv) y . The emissions of N 2 O and CH 4 in 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 CH 4 production and reduced uptake of atmospheric CH 4 , 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-CO 2 GHG budget. Ceased drainage activities in upland soils are expected to increase the likelihood of CH 4 emissions and reduce soil CH 4 uptake. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction more effective as GHG than CO 2 , 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- sphere reducing the human-induced greenhouse effect. In the

Forests may sequester carbon dioxide (CO 2 ) from the atmo-

morphic forest soils may constitute an important but underesti- northern hemisphere forests annually remove an estimated 2.2–

mated contribution to the GHG balance of ecosystems ( Jungkunst

2.6 Pg of atmospheric CO 2 ( Goodale et al., 2002 ), equal to 7–8% of

and Fiedler, 2007; Grunwald et al., 2012 ). In a recent assessment

of the European GHG balance oxidation of atmospheric CH 4 in up- et al., 2012 ). The sequestration of atmospheric greenhouse gases

the total global human CO 2 emission of fossil fuels in 2010 ( Peters

land soils (forest and heathlands) was equivalent to an annual CO 2 (GHGs) in forest vegetation and soil is partly offset by soil

emissions of the strong GHGs N 2 O and CH 4 , 298 and 21 times

were only a minor source for N 2 O compared to arable lands ( Schu- lze et al., 2010 ). In a comparison of CH 4 fluxes comprising arable, grassland and forests soils from around the world it was found that

⇑ Corresponding author. Present address: Belowground Ecosystem Group, Department of Forest Sciences, Faculty of Forestry, Forest Sciences Centre, Univer-

forest soils generally were stronger net sinks for atmospheric CH 4 sity of British Columbia, 2424 Main Mall, Vancouver, British Columbia, Canada V6T

than the other land use types ( Boeckx and Van Cleemput, 2001 ). 1Z4. Tel.: +1 778 788 6184; fax: +45 3533 1517.

Thus, experimental data suggest that forests are an important sink E-mail address: jrc@life.ku.dk (J.R. Christiansen).

for atmospheric CH 4 . Although there has been increasing research 0378-1127/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.foreco.2012.07.048

J.R. Christiansen et al. / Forest Ecology and Management 284 (2012) 185–195

focus to understand the controls on GHG dynamics in soils, consid- (1) to map hydromorphic and upland soils in the forest to develop

erable uncertainty still exists to what degree N 2 O and CH 4 fluxes

a strategy for representative sampling and upscaling of GHG fluxes

from the soil offset the CO 2 sink of forest soils especially when

based on available spatial data in GIS, (2) to measure the fluxes of

N 2 O and CH 4 in the chosen hydromorphic and upland study sites in the wetter parts of the forest ( Grunwald et al., 2012 ).

accounting for emissions of the strong GHGs CH 4 and N 2 O from

the field and laboratory, (3) to estimate the soil carbon pools in the Hydromorphic soils in forests are often located in depressions in

chosen hydromorphic and upland study sites and (4) to assess how the landscape where runoff water is concentrated. They are charac-

reduced drainage of upland forest soils could affect the GHG bud- terised by high groundwater table, morphological features such as

get of N 2 O and CH 4 at the forest district level. 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)

2. Materials and methods

showed that CH 4 emissions in an undrained alder forest were one to two magnitudes of order higher than in adjacent drained

2.1. Study area

plots. However, the wet soils under alder showed a smaller N 2 O

emission than the drained alder and birch soils. Furthermore, The study was carried out in the period from March 2008 to

upscaling field observations of N 2 O and CH 4 fluxes show that even

May 2010 in the forests of Sønderskov, Barrit Tykke and Klakring

Skovhaver (a total forested area of 348 ha) located close to the N 2 O emissions. Emissions of the strong GHGs from these areas will

small areas of hydromorphic forest soils are hotspots for CH 4 and

town of Barrit on the northern shore of Vejle Fjord in Jutland, negatively affecting the carbon sequestration potential of forests

Denmark (55°41 0 N, 9°55 0 E). The forests are part of the Barritskov ( Fiedler et al., 2005; Jungkunst and Fiedler, 2005, 2007 ).

estate and documented as forests since 1770, possibly with for- Thus, the distribution of water in the landscape also dictates the

ested cover dating further back in time (Lone Nørgaard Telling, spatial occurrence of GHG uptake and production. Recently, Grun-

pers. comm.). The dominant tree species are (in order of areal) wald et al. (2012) attempted to elucidate the role of the hydromor-

European beech (Fagus sylvatica), pedunculate oak (Quercus robur),

phic forest soils for the European CH 4 balance. They found that

and ash (Fraxinus excelsior).

inclusion of hydromorphic forest soils in inventory calculations The climate is temperate with a mean annual temperature of doubled net CH 4 emissions from natural systems in Europe. How-

for the period 1961–1990 ever, they also emphasised that the large uncertainty of the conti-

7.9 °C and precipitation of 670 mm y

( DMI, 2000 ). The area is gently sloping with a southward aspect to- nental CH

4 budgets stems from the lack of knowledge regarding the spatial distribution of hydromorphic soils. The same applies

ward Vejle Fjord and consists mainly of glacial till deposits. It is

intersected by north-to-south oriented narrow valleys or gulleys to N 2 O emissions as distributions of hydromorphic and upland

with illuvial deposits of sand and gytja. A previous soil survey of soils also drive N 2 O emission fingerprints from larger forest areas

the forests ( Jellesen et al., 2001 ) characterised the soils as Typic ( Christiansen et al., 2012 ). Thus, improved knowledge of spatial

Paleudalf (Sønderskov), Typic Epiaquent and Humaqueptic Endoa- variability in soil hydrology together with field observations of

quent (Barrit Tykke) and Typic Epiaqoll (Klakring Skovhaver) ( Soil

Survey Staff, 1998 ). The clay, silt and sand content varied between fluxes from forests.

N 2 O and CH 4 will provide more robust estimates of CH 4 and N 2 O

12% and 20%, 15% and 19%, 61% and 70%, respectively, with a range The need to account for the role of hydromorphic forest soils

of 1.6–4% soil organic carbon in the A horizon. A detailed spatial must also be seen in the light of future land-use change in forests

investigation of the internal drainage conditions in the soils of and of projected climate change ( Gundersen et al., 2012 ). Current

the forest revealed that 51% of the glacial till soils or upland soils land management trends in Denmark and parts of Europe are to re-

(comprising an area of 324 ha) suffer from poor internal drainage store natural conditions by afforestation and forest restoration

resulting in pseudogley ( Jellesen et al., 2001 ). leading, amongst other effects, to re-establishment of forest

We also included data on potential denitrification from two hydrology to pre-drainage conditions ( Stanturf and Madsen,

additional sites. Strødam, an unevenly aged mixed deciduous for- 2002 ). Understanding the role of hydromorphic soils is underlined

est, is located in northern Zealand, Denmark. The soil is sandy loa- by the fact that the impacts of forest cover changes on greenhouse

my till and there is a steep hydrological gradient within the study gas fluxes in Nordic riparian forests remains unclear although

site. The second additional site, Vestskoven, was afforested in 1971 these forests are hotspots for C and N turnover in the environment

with pedunculate oak. The soil at this site is classified as a Mollic ( Gundersen et al., 2010 ). Furthermore, winter precipitation has

Hapludalf ( Soil Survey Staff, 1998 ) and consists of clayey loamy till been suggested to increase for large parts of northern Europe in

and quite variable in moisture content. Further details about the the near future which could lead to increased soil wetness during

latter two sites are given in Christiansen et al. (2012) . this period with unknown consequences for the GHG balance in

European forests ( Christensen et al., 2007 ).

2.2. Identification of representative study sites While it has been shown that drainage of organic deciduous and

coniferous forest soils decreased CH 4 emissions, while (simulta-

The GIS analyses were performed with ArcMap 9.3 (ESRI,

Redlands, CA, USA). The spatial reference for each data layer was global warming potential of the soil compared to a non-drained

neously) increasing CO 2 and N 2 O emissions and resulting in higher

standardised and the spatial precision errors that occurred during site ( von Arnold et al., 2005a,b ), little is known regarding the ef-

this procedure were assessed not to have any practical significance fects of rewetting previously drained soils. Without the possibility

for the purpose of the analyses.

of large scale landscape manipulations studying the processes of The aim of this GIS analysis was to map the hydromorphic and N 2 O and CH 4 formation and consumption for hydromorphic soils

upland soils in order to devise a strategy for sampling and upscal- in laboratory conditions compared to field observations may pro-

ing of GHG exchange and soil carbon stocks to forest district level vide us with clues to understand how rewetting of previously

that encompassed all dominant tree species types and represented wet soils might affect the regulation and emission of these impor-

averagely aged stands.

tant GHGs from forest soils. We combined map layers (digital elevation map ( National With this in mind we set out to investigate the importance of

Survey and Cadastre, 2012 ), national soil map 1:25,000 ( Geological

Survey of Denmark and Greenland, 2008 ), forest map and ditch Danish deciduous forest district. The objectives of this study were;

hydromorphic forest soils for N 2 O and CH 4 emissions in a typical

map) and defined criteria for the site selection. First we selected

187 Table 1

J.R. Christiansen et al. / Forest Ecology and Management 284 (2012) 185–195

content (SOC) for the fraction P2 mm was not assessed. There Main characteristics of the twelve study sites at Barritskov selected through the GIS

analysis. was no inorganic C (CaCO 3 ) to a depth of 30 cm depth and all mea-

sured C was consequently considered to be organic. Soil organic C Soil type

contents in (Mg ha ) for each of the three soil layers were calcu- Upland

Tree species

Parent material

Year of planting

Beech

Glacial till

lated by correcting for the fragment of the sample P2 mm content

and extrapolating to a hectare basis using the fine fraction bulk

density according to the equation

Oak

Glacial till

Glacial till

SOC i ¼ q

d i 2mm

Hydromorphic Beech

Alluvial sand

where

q i is the bulk density of the <2 mm fraction in g cm ,d i,2 mm

is the relative volume of the fraction P2 mm (%), d i denotes the Oak

Ash

Alluvial sand

Alluvial sand

thickness of layer i in cm, C i denotes the C concentration of layer i (mg g ), and 10

8 cm 2 ha ). the hydromorphic sites and then selected the remaining sites. We

is a unit factor (10

mg Mg

Soil pH was determined for all mineral soil samples by mixing

12 g of dried soil sample with 30 mL 0.01 M CaCl . The pH was defined the hydromorphic soils to be present in the areas with soil

material originating from fluvial processes while upland soils occur measured in the supernatant using a video titrator (Radiometer, Copenhagen, Denmark).

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.

2.4. Greenhouse gas measurements and flux calculation

A schematic flow chart of the GIS analysis with criteria for selec- tion of the 12 study sites is given in Supplementary material

Measurements of GHG fluxes were conducted approximately on (Fig. A1) . Eight of the sites were characterised as upland soils and

a monthly basis during a period of 2 years from March 2008 to May four sites as hydromorphic soils ( Table 1 ). We validated the results

2010. At each sampling occasion volumetric soil water content of the GIS analysis in the field and altered the status of one oak site

(SWC) (Theta probe ML2x, Delta T Devices, UK) and soil tempera- from upland soil to hydromorphic soil, increasing the total number

ture (model 550B, UEi, Beaverton, Oregon, USA) were also mea- of wet sites to five.

sured 0–5 cm from each static chamber. By identifying suitable study sites within the forest, it was pos-

The net soil surface exchange of CH 4 and N 2 O was measured sible to optimise the collection regarding the number of samples

with non-mixed closed static chambers that were installed in per- and obtain the broadest spatial representation of GHG fluxes and

manent locations throughout the study period. Three chambers soil carbon stocks.

were installed at random positions within each study site. The chamber collar (inner diameter of 30.5 cm) was inserted 10 cm in

2.3. Soil sampling and analyses 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

Forest floor and mineral soil were sampled at two points next to vicinity of the chamber so that gas samples could be withdrawn each of the three chambers per study site. Forest floors were sam-

from the chamber without disturbing the soil around the chamber. pled in September 2008 just before the onset of foliar litterfall for

During sampling of headspace air, a lid was placed on top of the deciduous species, i.e. when forest floor mass was at a minimum.

collar and sealed with a silicon rubber ring around the edge of The forest floor was defined as the organic material above the min-

the lid ensuring gas-tight conditions. Chamber headspace samples were withdrawn with 60 mL plastic syringes through a butyl rub-

wooden frame. Forest floors were dried to constant weight at 55 °C ber septum in the middle of the lid at times 0, 30, 60 and 120 min. and hand-sorted to remove herbaceous litter and roots if present

From March 2009 the enclosure time was reduced to 60 min and before weighing. The two subsamples per chamber were subse-

samples taken every 20th minute at 0, 20, 40 and 60 min. At each quently ground and pooled leaving three samples per study site

headspace sampling the syringe was used to mix the headspace by for chemical analysis. Intact cores of mineral soil were sampled

pumping three times before fully filling the syringe. Pressure using an auger with an internal diameter of 4.5 cm ( Westman,

changes in a manually sampled chamber headspace has been re- 1995 ). To determine bulk density of the mineral soil fraction cores

ported to lead to overestimation of the estimated diffusive flux were divided into three segments: 0–5 cm, 5–15 cm, and 15–

( Bekku et al., 1995 ), but we did not observe any changes, e.g.

30 cm, and passed through a 2 mm sieve to remove stones and non-linear behaviour of concentrations over time in headspace gravel. In three of the study sites, one upland and two hydromor-

concentrations that could be attributed to mass flow caused by phic sites, we sampled the soil to a depth of 100 cm.

depressurisation of the chamber headspace. Headspace samples Fine and coarse roots were removed by hand. The sieved sam-

were transferred to non-evacuated 2.7 mL crimped vials with a bu- ples were dried at 55 °C and weighed. Subsamples were dried at

tyl rubber septum by flushing the vial with 58 mL of the sample in 105 °C for correction of weight. Stone content ranged within

the syringe and pressurising the vial with the remaining 2 mL. 1–7%. Following determination of bulk density the six soil cores

Gas samples were stored for a maximum of 5 days in vials be- per study site were pooled to one composite sample per depth seg-

fore analysis. Gas samples were analysed on a Shimadzu GC-2014 ment. A subsample of the mineral soil samples were finely ground

gas chromatograph (Shimadzu, Kyoto, Japan) equipped with elec- in an agate mortar for carbon and nitrogen analysis. Ground sam-

tron capture and flame ionisation detectors set at 300 °C and ples of forest floor material and mineral soil were analysed for total

200 °C, respectively. The carrier gas was 100% N 2 with a flow rate

C and N by dry combustion (Dumas method, VarioMax CH ana- of 25 mL min . Methane and N 2 O were analysed in separate col- lyzer, Elementar Analysensysteme GmbH, Hanau, Germany) at

umns set in a constant oven temperature of 40 °C. The column Agrolab, Institut Koldingen, Sarstedt, Germany.

used for CH 4 was a 60/80 Carboxen 1000 (15 ft, 1/8 in.). For N 2 O Forest floor C contents were calculated by multiplying C

an 80/100 Hayesep Q (2.5 m, 1/8 in.) column was used. An auto- concentrations with forest floor mass. Mineral soil organic carbon

sampler equipped with a syringe extracted 1.6 mL sample from

J.R. Christiansen et al. / Forest Ecology and Management 284 (2012) 185–195

the vial and injected 0.75 mL into each column used for CH 4 and

2.5.2. Potential CH 4 production

N 2 O. We added 3 mL of anoxic 10 mM sodium acetate solution to All gas fluxes were calculated by linear regression of gas con-

10 g of fresh soil in an incubation bottle. MilliQ water was used centrations versus time. For this study we did not estimate the

and the solution was anoxified in an ultrasonic bath for 10 min

minimal detectable flux and instead used R 2 of linear regression

before addition to the bottles. We then evacuated the closed incu-

bation bottle with a vacuum pump to a constant partial vacuum of above 0.85 were accepted for flux calculations. For regression anal- yses resulting in R 2 -values below 0.85, the increase or decrease in

as a threshold for a significant flux. Regressions with an R 2 -value

pure N 2 for 3 min and assumed we had created an O 2 free head- N 2 O and CH 4 concentrations was smaller than we most likely could

space. Following this step we pressurised the bottle with 10 mL a detect with our gas chromatographic setup and fluxes were set to

H 2 /CO 2 gas mixture (80%/20% v/v) obtaining an overpressure of zero. We always rejected chamber enclosure data that appeared

about 15 mbar. Our protocol was slightly modified from that of

chaotic in nature. Fluxes were expressed as l gN 2 O–N m h or

Wagner et al. (2007) . We sampled headspace at 0, 24, 240 and

l g CH 4 –C m h . We estimated an annual budget in kg CH 4 ha 540 h after addition of the H 2 /CO 2 gas mixture. We extracted

or kg N 2 O ha by assuming that the flux rate measured for a given

600 l L from the headspace, rejected 100 l L and injected the

date represented the entire period until the next sampling date.

remaining 500 l L in a crimp sealed vial containing atmospheric

air. The samples were subsequently analysed on the gas chromato- lents by a factor of 21 and 298, respectively ( Forster et al., 2007 ).

We converted the annual budgets of CH 4 and N 2 O to CO 2 -equiva-

graph using the autosampler system.

2.5. Laboratory incubations

2.5.3. Potential denitrification

Intact soil cores from the top mineral soil (0–5 cm) were sam- The rate of potential denitrification was determined using the pled in the middle of each chamber for all twelve study sites using

acetylene inhibition technique where addition of the acetylene corers with a diameter of 5 cm in November 2009 and 2011 at

gas inhibits the reduction of N 2 O to N 2 ( Yoshinari et al., 1977 ). Strødam/Vestskoven and Barritskov, respectively. Upon arrival to

To two sets of incubation bottles with 10 g of fresh soil we added the laboratory the intact soil were kept in the corners in a dark cool

15 mL of a solution consisting of 1 mM KNO 3 , 0.5 mM glucose, room at 4 °C until analysis. We divided the intact soil cores in five

0.5 mM sodium acetate, 0.5 mM sodium succinate (Bernsteinsä- parts and removed stones and larger root fragments. Two parts

ure). Similar to the CH 4 production experiment anoxified MilliQ were air dried and used for the potential CH

4 oxidation experi-

water was used in preparing the solutions and bottles were evacu-

ment, one part was used for the potential CH production experi- ated and flushed. In one set of bottles with soils from all the study

4 sites 10 mL of acetylene was added and for the second set of bottles ment, and the last two parts for the potential denitrification determination. To determine the absolute SWC 20–30 g of fresh

we added 10 mL N 2 . The samples were incubated under constant shaking for 2.5 h and gas samples were extracted at 30, 60, 90,

and air dried soil were dried at 55 °C for 48 h. For incubation, the soil was transferred to screw cap glass incu-

120 and 150 min after addition of gas and transferred to crimp- sealed vials and subsequently analysed on the gas chromatograph.

bation bottles (approximately 120 mL) that were sealed with butyl rubber septums. The headspace of the incubation bottles were

Fluxes of potential N 2 O obtained without acetylene were sub-

2 O fluxes with acetylene for each sample to ob- substrate solution) only and the same bottle with soil (and sub-

tracted from the N

determined by the difference in weight for a bottle with soil (and

tain a flux interpreted as the potential rate of complete strate solution) filled with water to the brim. For all three CH

d . experiments we used three control bottles to check the back-

denitrification to N 2 expressed as l gN

4 2 –N g dry soil

ground of the experiments. Any rates smaller than the control rates

2.6. Reduced soil drainage scenario analysis were set to zero. All rates were determined with linear regression

between concentration and time and expressed as l gN 2 O–

2.6.1. Affected forest area

N g dry soil

d or l g CH 4 –C g dry soil d .

This analysis served to provide the data on the spatial extent of soils affected by reduced drainage scenarios. The aforementioned

2.5.1. Potential CH 4 oxidation GIS map layers were supplemented with the following information

Two experiments were performed to measure the CH 4 oxidation

and assumptions:

potential of the soils from Barritskov in order to target high and

low-affinity methanotrophic bacteria. Prior to the CH 4 oxidation

experiments we air dried the fresh soil for 24 h to decrease the of the forest soils divided in two classes: (A) temporary water

saturation and (B) no signs of temporary saturation ( Jellesen ( Gulledge and Schimel, 1998 ). We incubated 10 g of air dried soil

SWC and create more optimal conditions for CH 4 oxidation

et al., 2001 ).

in each bottle. In one set of bottles the initial CH 4 concentration

ceased drainage in class B would affect the soil closest to the target high-affinity methanotrophs ( Reay et al., 2005 ). In another

l LL ) to

ditches within an impact zone.

set of bottles we increased the headspace concentration to

case of reduced drainage: A 2 m zone and a 30 m zone around affinity methanotrophs ( Reay et al., 2005 ). Bottles were thoroughly

l LL by injecting 1 mL of 100% CH 4 to target high-

the ditches (zones intersecting with hydromorphic soils were mixed by rolling prior to sampling. For the high-affinity methano-

excluded here, since already represented in class A). troph experiment we sampled at 0, 24 and 48 h after bottle closure and extracted 600 l L from the headspace and manually injected

The 2 m impact zone of ditches was calculated using the meth-

500 l L into the gas chromatograph. For the low-affinity methano-

odology of Skaggs et al. (2005) using data on hydrological conduc- troph experiment we sampled at 0, 24, 74, 146 and 251 h after

tivity from a comparable loamy soil profile in Denmark

addition of pure CH 4 . We extracted 300 l L from the headspace

(Frederiksborg in Gundersen et al. (2009) ). The 30 m impact zone

and injected 200 l L into a crimp sealed vial containing atmo-

was chosen as a ‘‘maximum influence’’ scenario of reduced soil spheric air. The samples were subsequently analysed on the gas

drainage around ditches. Roads and tracks (25 ha out of the chromatograph using the autosampler system.

373 ha) were excluded from the analysis.

J.R. Christiansen et al. / Forest Ecology and Management 284 (2012) 185–195

2.6.2. Impact on GHG emissions

mean CH 4 uptake ( Fig. 1

D, p < 0.001) compared to the upland soils

( Table 2 ). All the study sites on upland soils were net sinks for CH 4 by reduced soil drainage in the upland soils classes A and B we ex-

In order to calculate the effect on the N 2 O and CH 4 net exchange

whereas the study site with oak was the only net emitter for the

hydromorphic soil group and ash and beech were small net sinks rate and soil moisture content for a given date and established a re-

plored the relationship between the mean N 2 O or CH 4 exchange

( Fig. 1 D). Methane fluxes tended to decrease in summer periods sponse function between GHG exchange rate and soil moisture

C and D) although content. Thus, we aggregated data on soil group (hydromorphic

and increase during autumn and winter ( Fig. 2

the temporal variation in flux magnitude was around 20 l g

and upland) levels only and assumed the relationship to be valid

h . Furthermore, there was a high degree of covari- independent of location.

CH 4 –C m

ance between CH 4 fluxes (mean, min and max) in the hydromor- To date we have no knowledge of studies in Danish forests that

C and have quantified the large scale impact on soil hydrology after

phic and upland soil types during the study period ( Fig. 2

D). We observed the majority of CH 4 emission events in spring ceased effect of ditching (although this currently occurs in many

when SWC was still high while the number of emission events forest districts). Thus, we attained a conservative approach and cal-

sharply decreased in the summer and the SWC in the hydromor- culated the effect on GHG exchange by two levels of increased soil

phic soils presumably fell below a threshold for CH 4 production.

There was no significant effect of soil water regime on N 2 O We assumed that ceased maintenance of ditches would increase

moisture content for each of the impact scenarios in Section 2.6.1 .

fluxes. Although we did not evaluate tree species differences statis- SWC by 5% for the upland soils (classes A and B) and as a maximum

tically, some notable differences were observed. While the beech response increased the SWC by 16% equalling the mean SWC ob-

stands did not differ, the emission of N 2 O under ash was twice as served for the hydromorphic soils.

high in the upland soils as for the hydromorphic, whereas the In total we calculated the impact on the GHG exchange for six dif-

opposite trend was observed for oak ( Fig. 1 C). The most conspicu- ferent scenarios, including a scenario without the presence of hydro-

ous difference in regard to CH 4 fluxes was the net emission of CH 4 morphic soils, the current distribution of hydromorphic and upland

under oak for the hydromorphic soils compared to the highest re- soils and the four combinations of reduced drainage discussed above

corded CH 4 uptake for the oak on upland soils ( Fig. 1 D). These re- (i.e. 5% and 15% SWC increases in both 2 and 30 m impact zones).

sults potentially point to important tree species specific differences in these different soil environments which deserve further atten-

2.7. Statistics tion for following studies. However, as mentioned earlier our study design did not offer an opportunity to test for tree species

For statistical analyses the SAS software version 9.2 (SAS Insti-

differences.

tute Inc., Cary, North Carolina, USA, 2008) was used and signifi- We did not observe any temporal trend in N 2 O fluxes in either cance was accepted at p 6 0.05. The statistical analyses in this

A and B), however, there was study focused on determining the effect of soil type between the

upland or hydromorphic soils ( Fig. 2

a tendency for higher maximum fluxes of N 2 O in the hydromorphic upland (N = 7) and hydromorphic study sites (N = 5). For the fol-

soils ( Fig. 2 B).

lowing parameters we used repeated measures ANOVA in PROC MIXED to test treatment effect time series of soil surface net ex-

3.1.2. Incubation studies

change of N 2 O and CH 4 , SWC and temperature in 0–5 cm. The The upland and hydromorphic soils did not differ significantly chamber measurements represented the subject repeated over

in potential methane oxidation (mean ± standard error of the time and the stand was used to account for the random variation

d , respectively) of the tested parameter in the repeated measure analyses.

mean(SE)) (5 ± 0.5 and 7 ± 2 ng CH 4 –C g dw

when incubated at atmospheric levels of CH 4 ( Fig. 3 A). When incu- We used one way ANOVA to test ‘treatment’ effect (hydromor-

bated at 10,000 l LL all soils showed higher CH 4 oxidation rates

phic versus upland soils) on mineral soil pH, pools of organic carbon than during incubation at ambient methane concentration. How- and total nitrogen in the organic horizon and mineral soil (0–30 cm)

ever, the hydromorphic soils had a significantly (p < 0.001) higher as well as the C:N ratio of the organic horizon and in the mineral soil

CH 4 oxidation potential (7.5 ± 2.9 l g CH 4 –C g dw d ) than up-

(0–30 cm). Common for both analyses was that the type of soil was

land soils (0.05 ± 0.03 l g CH 4 –C g dw d , Fig. 3 B). Similarly, we

the independent variable. In order to comply with the assumptions observed that the hydromorphic soils potentially produced signif- of the statistical test, i.e. variance homogeneity between groups and

icantly (p < 0.001) more CH 4 (0.001 ± 0.0003 l g CH 4 –C g dw d ),

normal distribution of residuals, N 2 O fluxes, SWC and temperature

at least one order of magnitude, than the upland soils

were square-root transformed, CH 4 fluxes were transformed to

(0.00008 ± 0.00005 l g CH 4 –C g dw d ). Potential CH 4 oxidation

rates were in general markedly higher than CH 4 production poten- formed. The transformation was adopted from the automatic rou-

(CH 4 + 350) 1.5 and C:N ratio of the organic horizon was log 10 trans-

tial ( Fig. 3 A–C).

tine ‘‘Guided data analysis’’ in the SAS software version 9.2. We No differences were observed in potential N 2 O production be- did not test for tree species differences for any of the treated data

hydromorphic soils (0.16 ± 0.02 and as we did not have a fully balanced collection of tree species repre-

tween upland

and

D) at Barritskov. However, sented on both upland and hydromorphic soils.

0.20 ± 0.03 l gN 2 O–N g dw d , Fig. 3

our acetylene inhibition experiment for Barritskov indicated that

The differences in potential denitrification and CH 4 oxidation and

the hydromorphic soils (0.35 ± 0.05 l gN 2 –N g dw d ) were

production was tested using Mann–Whitney rank sum test because more efficient at reducing N 2 O to N 2 than upland soils these data were not normally distributed or there was no variance

(0.05 ± 0.015 l gN 2 –N g dw d , Fig. 3 E). The potential N 2 O pro-

heterogeneity between groups (hydromorphic and upland soils). duction rates at Strødam and Vestskoven were similar between up- land and hydromorphic soils as observed for Barritskov ( Fig. 3 D).

Also, we found no significant difference in potential N 2 production

3. Results between upland and hydromorphic soils, although the rate of N 2 production was threefold higher for the hydromorphic soils at

3.1. Upland versus hydromorphic soils

Vestskoven ( Fig. 3 E).

3.1.1. Soil moisture and field GHG fluxes

3.1.3. Soil carbon and nitrogen stocks and pH

Hydromorphic soils had a significantly higher soil organic C stock in the hydromorphic soils and was associated with a fourfold lower

Fig. 1

A, p < 0.001)

in 0–30 cm as well as a higher stock of C stock in the O-horizon plus

J.R. Christiansen et al. / Forest Ecology and Management 284 (2012) 185–195

Fig. 1. Mean values ± standard error of the mean for (A) soil water content (vol.%), (B) soil temperature (°C), (C) N 2 O flux ( l gN 2 O–N m h ), (D) CH 4 flux ( l g CH 4 – Cm

h ), (E) soil carbon stock (Mg C ha ), (F) soil nitrogen stock (Mg N ha ), (G) mineral soil C/N ratio and (H) organic horizon CN ratio divided in the three investigated tree species at Barritskov. Grey bars show data for upland soils and white bars for hydromorphic soils. For the C and N stocks bars are divided in the organic and mineral soil fraction down to 1 m depth. Values are given by tree species basis but p-values represent the overall difference between upland and hydromorphic soils.

0–30 cm ( Fig. 1 E); about 20 Mg ha

soils was also observed for the layers 5–15 cm (p < 0.001), 15– soils compared to the upland soils ( Table 3 ). There was also a trend

higher for the hydromorphic

30 cm (p < 0.001) and 30–100 cm (data not shown). toward higher N stock in the hydromorphic soils, albeit not signifi- cant ( Table 3 , p = 0.075). Furthermore, the tree species showed the

3.2. Reduced drainage scenarios

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. 1 E 3.2.1. Forested area affected by reduced drainage and F). Thus, C:N ratios in the mineral soil and O-horizon were sim-

The GIS analysis was used to determine the size of areas to be af- ilar regardless of tree species ( Fig. 1

fected and not affected by reduced drainage scenarios ( Table 3 ). The regime.

G and H) and soil moisture

area of soils with signs of permanent of temporary saturation Mineral soil pH was significantly (p < 0.001) lower (4.2 ± 0.2) in

(hydromorphic soils + till soils class A) was 190 ha or 28% of the for- the upper 5 cm ( Table 2 ) for the upland soils compared to the

ested area ( Table 3 ). For the 2 m impact zones the area assessed to hydromorphic soils (6.2 ± 0.4). Consistently lower soil pH in upland

be impacted by reduced drainage increased by 3 ha only. Increasing

191 Table 2

J.R. Christiansen et al. / Forest Ecology and Management 284 (2012) 185–195

Therefore, an increasing area of poorly drained soils or hydromor- 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 phic soils would, at Barritskov, not result in increased emissions of

highlighted in bold and trends (not significant) in italics. N 2 O. However, for CH 4 there was a significant positive relationship between SWC in 0–5 cm and CH 4 emission fluxes ( Fig. 4 ). We ap- Parameter

p- Value

plied this relationship to assess how increased SWC would increase

CH 4 fluxes from the soil.

Soil moisture (0– vol.%

5 cm) This relationship suggests that the CH 4 uptake would decrease

with increasing SWC and, given sufficient moisture, result in CH 4 (0–5 cm)

Soil temperature

emission. The magnitude of the CH 4 budget for the forest would N 2 O flux

then depend on the spatial coverage of increasingly hydromorphic Nm h soils ( Table 2 ).

CH 4 flux

Cm h Exclusion of the hydromorphic soils currently present at Barrits- Soil pH in CaCl 2 (0–

l g CH 4 –

kov from the calculation would increase the annual CH 4 sink by 5 cm)

1 Mg CO 2 ( Fig. 5 ). Soil pH in CaCl 2 (5–

A 5% higher SWC in the all currently well-drained soils ( Table 2 ) de- 15 cm)

creased the CH 4 2 -eqv y compared to Soil pH in CaCl 2 –

(15–30 cm) the current estimate ( Fig. 5 ) for the 2 m and 30 m impact zones, C stock (O horizon)

respectively. A further increase in the SWC of 16% for these soils de- N stock (O horizon)

creased the CH 4 2 -eqv y for C stock (0–30 cm)

the 2 m and 30 impact zones, respectively. N stock (0–30 cm)

Although N 2 O emissions (53 Mg CO 2 -eqv y ) from the forest 30 cm)

comprise the largest proportion of the non-CO 2 GHG emissions, N stock (O + 0–

C stock (O + 0– Mg ha

the total emission of CO 2 -equivalents originating from N 2 O and 30 cm)

CH 4 increases from 4% to 22% by increasing the SWC of the forest C:N (O horizon)

C:N (0–30 cm) –

Currently, the estimate of non-CO 2 GHG emissions of

comprises a minute fraction the impact zone to 30 m would increase the affected area to 214 ha

0.10 ± 0.06 Mg CO 2 -eqv ha

(0.5%) of the difference in soil carbon stock (forest floor and min- or 31% of the forested area (sum of column 2 and 7 in Table 3 ).

eral soil to 30 cm) between upland and hydromorphic soils, i.e.

20 Mg C ha

( Table 2 ). Even the scenario including the maximum

3.2.2. Impact on GHG emissions spatial coverage of affected soils and increase of SWC matching the

hydromorphic soil group would not lead to any significant offset of fluxes in either upland or hydromorphic soils (data not shown).

There was no positive relationship between SWC and N 2 O

a potential gain in the soil carbon pool in this forest district.

Fig. 2. Temporal trends in N 2 O and CH 4 fluxes 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 are l gN 2 O–N m h and l g CH 4 –C m h . Black dots represent average CH 4 emission across the soil type.

J.R. Christiansen et al. / Forest Ecology and Management 284 (2012) 185–195

Fig. 3. Comparison of mean ± standard error potential rates of (A) CH 4 oxidation at a starting concentration of 2 l LL , (B) CH 4 oxidation at a starting concentration of 10,000 l LL , (C) CH 4 production, (D) N 2 O production and (E) indirect measure of N 2 production for Barritskov and the Strødam/Vestskoven locations. Rates are in l g CH 4 – C g dw

d or l gN 2 O/N 2 –N g dw d . Data for Barritskov are given by tree species but p-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 on y-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 Till soils (current

Class B 2 m impact Class B 30 m

soils drainage status)

zone

impact zone

Class A Class B Not

Affected Not

Affected

temporary well

affected by

affected

by

saturation drained by

ditching by

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

Fig. 4. Relationship between mean soil water content (vol.%) and mean CH 4 fluxes ( l g CH 4 –C m h and hydromorphic soils were similar but availability of water in ) based on the time series for upland and hydromorphic soils.

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

A and B), and in addition the potential rates were similar the hydromorphic and upland soils.

kov ( Fig. 1

( Fig. 3 D). Previously, we found considerably higher N 2 O emissions

(8–15 l gN 2 O–N m h ) at the Strødam site at soil moisture lev-

2 O fluxes 4.1.1. N els corresponding to those in the hydromorphic soils at Barritskov

Surprisingly, the mean N 2 O fluxes were similar for both the

( Christiansen et al., 2012 ). However, Beier et al. (2001) also did not

hydromorphic and upland soils (3.8 l gN 2 O–N m h ) at Barrits-

detect differences between the upland and wetter soils in another

J.R. Christiansen et al. / Forest Ecology and Management 284 (2012) 185–195

193 Soils from Barritskov oxidised CH 4 under ambient concentra-

tion levels in the laboratory ( Fig. 3 A). This suggests that Type II or high-affinity CH 4 oxidising 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 CH 4 oxidation between hydromorphic

and upland soils ( Fig. 3

C and D) were similar, which supports our conclusion that the higher SWC under field conditions caused the lower oxidation rates of CH 4 in the hydromorphic soils ( Fig. 2 D). Compared to previous studies of high-affinity CH 4 oxida- tion our rates were lower. Menyailo et al. (2008) found up to 10 times higher rates in Siberian forest soils, and high-affinity CH 4 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 CH 4 oxidation rates ( Fig. 3 B) measured at 5000 times ambient CH 4 concentration 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 LL CH 4 we solely target low-affinity methanotrophs

thriving under conditions with elevated CH 4 concentrations, as they are the only active methane oxidisers at concentrations above 2500 l LL ( Bender and Conrad, 1992, 1995 ). Thus, low-affinity

methanotrophs are present in both soil types, but more abundant Mg CO 2 -equivalents (eqv) y

Fig. 5. Scenario analyses of the net CH 4 budget for the entire forest at Barritskov in

± standard error of the mean. Scenarios are (a) no in hydromorphic soils which fits well with the significantly higher hydromorphic soils, (b) the current situation and four scenarios (c–f) representing two different increases in soil water content paired with two different impact zones

CH 4 production rate in hydromorphic compared to upland soils. scenarios around ditches in well-drained soils. Dashed lines represent the

Even under optimal conditions for CH 4 production there is not estimated mean value of the scenario and the bar-width represents the standard

much evidence to support that the upland soils produce CH 4 . Only error of the estimate.

17 out of 370 CH 4 flux 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

Danish beech forest with a comparable glacial till soil (mean rates CH 4 emission. Thus, our laboratory incubation studies and field

of 5.7 l gN 2 O–N m h ).

data comply.

Both soil types at Barritskov had mineral soil C:N ratios of 12–

13 which are favourable for N 2 O emissions ( Pilegaard et al., 2006 ).

4.1.3. Carbon stocks

Higher emission rates could thereby be expected especially when 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- soils may, however, be explained by the high soil pH in hydromor-

( Davidson et al., 2000 ). Low N 2 O emissions from the hydromorphic

ment increases the total C stock to 23.6 Gg C, or by only 2%.

Although the hydromorphic soils have significantly higher C con- nitrous oxide reductase ( Simek and Cooper, 2002 ) as is also indi-

phic soils. At pH > 6 ( Table 2 ), the produced N 2 O is reduced to N 2 by

tents than upland soils their limited spatial coverage reduces their

cated by the similar or higher potential N 2 emissions ( Fig. 3

E) com-

influence on soil C stock at the forest (district) level.

Carbon stocks in mineral soil (to 30 cm) for upland soils were of hydromorphic Barritskov soils.

pared to the potential N 2 O emissions ( Fig. 3

D) from the

the same order of magnitude as the ca. 55 Mg C ha reported for At lower pH nitrous oxide reductase is inhibited and the ratio of

the upper 40 cm in Danish well-drained Alfisols by Vejre et al. N 2 O/N 2 from denitrification increases ( Weslien et al., 2009 ).

(2003) , but the C stocks in hydromorphic soils were clearly higher.

Reduced oxygen availability hampering decomposition is believed in all upland soils with lower pH than hydromorphic soils

Accordingly, the denitrification product was dominated by N 2 O

to be the main factor responsible for the higher C stocks in hydro- ( Fig. 3

D and E). For the Strødam soils (lowest pH) the potential morphic soils. In a study of Danish soil databases, Krogh et al.

(2003) attributed their higher average forest soil C stock to 1 m phic soils than for Barritskov and Vestskoven soils ( Fig. 3

rates were more dominated by N 2 O in both upland and hydromor-

D and E),

(167 Mg ha ) compared to the 125 Mg ha reported by Vejre

et al. (2003) to the fact that their database also included poorly at Strødam.

which is in line with the higher N 2 O emissions observed in the field

drained soils. Forest floor C stocks were quite similar between soil moisture regimes and corresponded well to the stock of approx.

4.1.2. CH 4 fluxes

reported by Vejre et al. (2003) . The CH 4 fluxes for the upland soils were lower, but in the same

5 Mg ha

The difference in C stock between moisture regimes may be inter- range as recently published fluxes for upland European forest soils

preted as the potential C sequestration following reduced drainage.

l g CH 4 –C m h ; Skiba et al., 2009 ). The

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 the differences between upland and hydromorphic soils are mainly

similarities in temporal variation of CH 4 fluxes ( Fig. 2 ) indicate that

sequestration would amount to 0.7–1.5 Mg CO 2 ha yr . This rate driven by differences in the magnitude of SWC. Soil water slows

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., take of atmospheric CH 4 is reduced compared to a well-drained

down diffusion of atmospheric CH 4 in the soil matrix and the up-

2007 ), but higher than rates reported from forest soil inventories soil.

(e.g. Berg et al., 2009 ). Decreased CH 4 oxidation resulting from

J.R. Christiansen et al. / Forest Ecology and Management 284 (2012) 185–195

reduced ditching would offset the potential C sequestration by

5. Conclusions