Carbon exchange of grazed pasture on a drained peat soil

Carbon exchange of grazed pasture on a drained peat soil

J OO S T P. N I E V E EN *, DAV I D I . C AMP B E L L w , L OU I S A . S CH I P P E R * and I AN J . B L A I R w*Landcare Research, Private Bag 3127, Hamilton 2001, New Zealand, wDepartment of Earth Sciences, University of Waikato,

Private Bag 3105, Hamilton 2001, New Zealand

Abstract

Land-use changes have contributed to increased atmospheric CO2 concentrations.

Conversion from natural peatlands to agricultural land has led to widespread

subsidence of the peat surface caused by soil compaction and mineralization. To study

the net ecosystem exchange of carbon (C) and the contribution of respiration to peat

subsidence, eddy covariance measurements were made over pasture on a well-

developed, drained peat soil from 22 May 2002 to 21 May 2003. The depth to the water

table fluctuated between 0.02m in winter 2002 to 0.75m during late summer and early

autumn 2003. Peat soil moisture content varied between 0.6 and 0.7m3m�3 until the

water table dropped below 0.5m, when moisture content reached 0.38m3m�3. Neither

depth to water table nor soil moisture was found to have an effect on the rate of night-

time respiration (ranging from 0.4–8.0lmolCO2m�2 s�1 in winter and summer,

respectively). Most of the variance in night-time respiration was explained by changes

in the 0.1m soil temperature (r25 0.93). The highest values for daytime net ecosystem

exchange were measured in September 2002, with a maximum of �17.2 lmol

CO2m�2 s�1. Grazing events and soil moisture deficiencies during a short period in

summer reduced net CO2 exchange. To establish an annual C balance for this ecosystem,

non-linear regression was used to model missing data. Annually integrated (CO2)

C exchange for this peat–pasture ecosystem was 45� 500kgCha�1 yr�1. After including

other C exchanges (methane emissions from cows and production of milk), the net

annual C loss was 1061� 500kgCha�1 yr�1.

Keywords: carbon, drainage, eddy covariance, methane, pasture, peat

Received 3 August 2004; revised version received 22 September 2004 and accepted 28 October 2004

Introduction

Worldwide, drainage of organic soils for agricultural or

forestry purposes have led to vast subsidence of the

peat surface, due to changes in physical conditions and

acceleration of microbial decomposition. Land-use

changes like this have contributed to increased atmo-

spheric CO2 concentrations (Watson et al., 1996). A

significant proportion of the global carbon (C) pool is

stored as peat, with high latitude peatlands in the

northern hemisphere alone estimated to contain one

third of the world’s soil C pool (Gorham, 1991).

Widespread drainage of temperate peatland ecosystems

for agricultural purposes may have had, and may

continue to have, significant impacts on the global C

balance. Once drained for agriculture, forestry or fuel

production, large amounts of C are lost to the atmo-

sphere through mineralization, estimated at 30–

370MtCyr�1 globally (Armentano, 1980).

Reported rates of peat subsidence vary greatly at

different sites. For boreal conditions, Minkkinen (1999)

reported average subsidence values for old shallow

drained peatlands varied between 10 and 30 cm over

50–60 years (0.2–0.6 cmyr�1), and between 0 and 25 cm

for 15–25 years (0–1.7 cmyr�1) for old drained mires,

depending on the site type and predrainage peat

thickness. Schothorst (1976, 1977) reported subsidence

rates of up to 4m over 130 years (3.1 cmyr�1) for

drained peatlands in the Netherlands. Average sub-

sidence rates of 2–4 cmyr�1 over the last 100 years were

found for English fens (Hutchinson, 1980, French &

Pryor, 1993), and 2.5 cm yr�1 over the last 70 years in

the Florida Everglades (Stephens et al., 1984). In the

Waikato region of New Zealand, Schipper & McLeod

(2002) demonstrated an average subsidence rate of

3.4 cmyr�1 during 40 years of dairy farming.Correspondence: David I. Campbell, fax 1 64-7-856 0115,

e-mail: [email protected]

Global Change Biology (2005) 11, 607–618, doi: 10.1111/j.1365-2486.2005.00929.x

r 2005 Blackwell Publishing Ltd 607

It is unclear how much of the subsidence is due to

either microbial respiration or compaction/shrinkage.

Eggelsman (1976) found that as time progressed after

drainage, subsidence was increasingly due to respira-

tion. Stephens & Speir (1969) estimated respiration

accounted for as much as 75% of total subsidence in the

Everglades agricultural area. In New Zealand, Schipper

& McLeod (2002) attributed 37% of the subsidence to

respiration during the first 40 years of development.

Glenn et al. (1993) calculated that only 10% of the

subsidence could be attributed to the respiration of

drained peat soils in Canada. Measurements made by

Dirks et al. (2000), suggested a minor contribution of

respiration to subsidence for intensively managed peat

pastures in the Netherlands. The differences in the

proportion of respiration between these studies might

be due to differences in peat structure, C :N ratio,

drainage depth, climate, and cultivation practice

(Kasimir-Klemedtsson et al., 1997) or chemical proper-

ties of the peat-forming plants (Kuder et al., 1998).

To comply with the Kyoto protocol, an accurate C

balance is of great importance for New Zealand.

Additionally, concerns increase over the subsidence of

farmed organic soils in New Zealand. Systematic

drainage may not only lead to increased greenhouse

gas emissions, but may also increase the risk of surface

flooding, or increase the costs of draining land near to

sea level. Our objectives were to determine the net flux

density of C from these peat soils used for dairy

farming. We report on the year-round measurements of

net ecosystem CO2 exchange using the eddy covariance

(EC) technique, and on the magnitude of daytime and

night-time CO2 flux densities, and identify environ-

mental factors and management practices that regulate

CO2 exchange at paddock and farm scales. Finally, an

annual C balance is established, taking into account

estimates of other important C sinks/sources, such as

methane emissions from cows and the production of

milk, in order to assess the contribution of respiration to

subsidence for this well-developed peatland system.

Materials and methods

Site description

When the Waikato River, on the North Island of New

Zealand, took its final course some 18 000 years ago,

low-lying land was left behind in between old river

courses. In the low areas, impervious fine pumiceous

silt in combination with a high water table favoured

development of swamp vegetation. Over time, exten-

sive peat bogs were formed by the accumulation of

organic matter, primarily of vascular plants such

as wire rush (Empodisma minus) and cane rush

(Sporodanthus ferrugineus). In the early 1900s, around

110 000 ha of peat bog covered the Waikato Region

(Clarkson, 2002). Since then the area has been drained

extensively and much of the peatland has been

converted into pasture.

Measurements for this study were made at the

Rukuhia peatland (6400 ha), southwest of Hamilton in

the Waikato region, New Zealand (latitude 371510S,

longitude 1751140W, elevation 40m), from 22 May 2002

to 21 May 2003. This agricultural area consists mainly of

pasture for dairy farming and was one of the first peat

bogs in the Waikato to be converted to dairying in the

1940s. Pastures were developed on 10–12m deep, rush

and sedge peat. The measurement site was situated on

a dairy farm with drainage ditches (o2m deep)

surrounding clusters of paddocks and shallow drains

(o0.4m) crossing single paddocks (see Fig. 1). The

vegetation at the site was a mixture of perennial

ryegrass (Lolium perenne) and white clover (Trifolium

repens). The paddock, in which the measurements took

place, and the surrounding paddocks were grazed at

irregular intervals, about eight times during the study

period. Footprint analysis (Schuepp et al., 1990)

estimated a fetch distance ranging from 150 to 500m,

in most cases sufficient for multi-paddock measure-

ments (Fig. 1). The dry bulk density of the peat soil at

the site was 336 kgm�3.

Instrumentation

A 4.5m tower was used to accommodate an EC system

at a height of 4.25m. The system consisted of: a 3-D

380 m

60 m

To main drains

Farm road

EC tower

Fig. 1 Farm drainage diagram, showing the 1m deep farm

drains surrounding 380m� 60m paddocks (thick lines) and the

0.3m deep ‘spinner’ drains crossing individual paddocks (thin

lines). Farm drains discharge to the regional drainage system

shown by the arrows. The dashed circle indicates the minimum

flux source area around the eddy covariance tower (radius

150m) from which at least 80% of the measured flux density

originates (Schuepp et al., 1990).

608 J . P. N I E V E EN et al.

r 2005 Blackwell Publishing Ltd, Global Change Biology, 11, 607–618

sonic anemometer (CSAT3, Campbell Scientific Inc.,

Logan, UT, USA) and an air intake tube mounted on the

frame of the CSAT3 and leading down to a closed-path

infrared CO2/H2O gas analyser, IRGA (LI-6262, LI-

COR Inc., Lincoln, NE, USA). At the same height, air

temperature and humidity (HMP 35C,Vaisala, Finland)

were measured, and atmospheric pressure was mea-

sured near the tower base (PTB100, Vaisala, Finland).

The IRGA was housed in a temperature-controlled

part of an insulated aluminium box, which further

contained: two 12V DC suction pumps (TD–4X2N,

Brailsford & Co., Rye, NY, USA) in parallel to sample

the air; power supplies; relays; solenoids; a laptop

computer and a control data logger (CR10X, Campbell

Scientific Inc.). Flow rate through the system was

8Lmin�1, resulting in a delay of about 15 samples

between the CSAT3 and the LI-6262 signals. The

reference cell of the IRGA was kept free of water

vapour and CO2 by a steady flow of nitrogen gas at a

rate of 40 cm3min�1.

To overcome condensation in the sample gas flow

path, the 5.2m intake tube (0.004m i.d., Dekabon type

1300 Olex Cables Inc., Melbourne, Australia) was

encircled with three sets of nichrome heater wire at

different heights along the tube and insulated with

closed cell tubing (Aeroflex International, Samutpra-

kan, Thailand). Heat generated inside the aluminium

box convected up the insulating tubing to 3m height to

warm the sampling tube. In addition, the heater wire

could be controlled by the data logger to maintain the

temperature inside the tube above the dew-point

temperature of the air.

The gas analyser was calibrated every 24 h by

automatic shunting of bottled zero N2 and span

(368.0 � 0.1 ppmv CO2 in air) gases to the intake end

of the sample line, with solenoid valves under the

control of a data logger. A dew-point hygrometer

provided a reference humidity measurement (Dew-10,

General Eastern Instruments, Woburn, MA, USA). Its

accuracy was frequently checked with a dew-point

generator (LI-610, LI-COR Inc.).

Incoming global radiation (LI-200SA, LI-COR Inc.)

and photosynthetic active radiation (LI-190SA, LI-COR

Inc.) were measured at 4.5m height. A separate mast,

close to the main tower, was used to mount a net

radiometer (Q6.7.1, Radiation and Energy Balance

Systems, Seattle, WA, USA), a cup anemometer and a

wind vane (A101M & W200P, Vector Instruments,

Clwyd, UK) all at 1.5m. Soil heat flux was measured

with two soil heat flux plates (2�HFT3, Radiation and

Energy Balance Systems) buried at 0.05m. Soil tem-

peratures were monitored at 0.025, 0.1 and 0.2m

respectively, using one four-junction averaging thermo-

couple (TCAV, Campbell Scientific Inc.) and two

thermistors. The soil volumetric water content was

measured using two water content reflectometers

(CS615, Campbell Scientific Inc.), one installed horizon-

tally at 0.05m depth; the other vertically in the layer

0–0.3m. The water table was monitored using a pres-

sure sensor (SS3, Instrument Services & Developments,

Rangiora, New Zealand).

Data acquisition and processing

A CR10X data logger sampling at 10Hz measured the

signals from the IRGA and ultrasonic anemometer.

These data were downloaded every minute to a laptop

computer and stored on the computer’s hard drive in

half-hourly binary data files using a custom software

program, and backed up weekly on a recordable CD.

All supporting instruments were measured with two

additional CR10X data loggers and stored as 30-min

averages. These averages were downloaded hourly to

the same laptop computer and stored on the hard drive.

Back ups were made via a telemetry system (PCAny-

where 9.2.1, Symantec Corporation, Cupertino, CA,

USA) allowing access to both stored and real-time data.

A Matlab 5.2 (The Mathworks Inc., Natick, MA, USA)

software program was used to post-process the

accumulated EC data files. The raw data were checked

for spikes, either identified by a CSAT3 diagnostic sig-

nal or apparent high values of the vertical wind speed

(4100ms�1), sonic temperature (460 1C), and CO2 or

H2O concentration (1200mmolmol–1 and 50 gm�3 re-

spectively). Following Schmid et al. (2000), large short-

lived departures from the mean (‘soft spikes’) were

identified in an iterative procedure. Both types of spikes

were removed from the dataset. If after filtering less

than 67% of the data remained, the covariances were

treated as missing for a given 30min period.

After filtering, density corrections (Webb et al., 1980),

co-ordinate rotation corrections (McMillen, 1986), sonic

temperature corrections (Schotanus et al., 1983) and

frequency response corrections (Moore, 1986) were

applied to calculate half hourly averaged flux densities.

CO2 storage corrections were calculated (Hollinger

et al., 1995), but were found to be several orders of

magnitude smaller than the measured flux density.

Results and discussion

Meteorological and soil conditions

Figure 2 summarizes the meteorological conditions

during the measuring period. The annual 30-year

normal of the total incoming global radiation (K#) was

4% higher than the measured 5137.9MJm�2 yr�1. The

total of 1281mm precipitation during the measurement

CAR BON EXCHANGE ON A DRA IN ED P EAT SO I L 609


5

10

15

20

25

May 02 Jun 02 Jul 02 Aug 02 Sep 02 Oct 02 Nov 02 Dec 02 Jan 03 Feb 03 Mar 03 Apr 03 May 03 Jun 03

Date

Tem

per

atu

re (

°C)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Dep

th t

o w

ater

tab

le (

m)

0

10

20

30

40

50

60

70

Pre

cip

itat

ion

(m

m)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Vo

lum

etri

c m

ois

ture

co

nte

nt

(m3

m−3

)

0

5

10

15

20

25

Glo

bal

rad

iati

on

(M

J m

−2 d

ay−1

)2002−2003

Normal

(a)

(b)

(c)

(d)

Fig. 2 Annual trends of (a) average 30-year normal and measured daily incoming global radiation, (b) daily precipitation, and average

daily soil volumetric moisture content in the top 0.3m (solid line), (c) average daily depth to the water table, (d) average daily soil

temperature at 0.1m depth (bold line) and air temperature at 4.25m height (thin line).



period was comparable to the 30-year normal of

1252mm. While May, June, and July 2002 were wetter,

August, September, and October were drier than

normal. February and April 2003 were extremely dry

(Fig. 2b) with average rainfall for these months less

than 50% of the 30-year normal.

The dry period is also clearly visible in the decline in

volumetric soil moisture content (yv) and depth to the

water table (DWT) (Fig. 2b and c). Daily average yvranged between 0.6 and 0.7m3m�3 until the soil dried

out in February 2003. During this month the average yvreached a minimum value of 0.38m3m�3 in the top

0.3m and as low as 0.3m3m�3 at 0.05m depth. In the

subsequent months, the average yv gradually increased

again to 0.62 m3m�3 by the end of May 2003. DWT

ranged from 0.02m briefly in July 2002 to 0.75m in

March, April and May 2003, with an average of 0.46m.

Daily average soil temperature at 0.1m depth (Ts)

ranged from 19.4 1C in January 2003 to 10.9 1C in

August 2002 (Fig. 2d). The average annual soil

temperature was within 1.3 1C of the 30-year normal

temperature at 0.1m, measured at a nearby weather

station. Except for the summer months (December 2002

to March 2003) the monthly average soil temperatures

were higher than the monthly 30-year normal soil

temperatures (by 1.7–4.2 1C). During the summer

months, the 0.1m average soil temperatures were

within 1.1 1C of the normal.

Air temperature (Ta) at 4.25m height showed an

average daily maximum in March 2003 of 22.0 1C and a

minimum of 5.8 1C in August 2002. Although the

annual mean air temperature equalled the 30-year

normal (13.8 1C), the measured monthly average tem-

peratures from October 2002 to April 2003 were lower

(by 0.1–1.4 1C) than the 30-year normal monthly

averages and were higher from May 2002 to September

2002 (by 0.3–2 1C).

In summary, the measurement period was character-

ized by near normal air temperatures, and higher soil

temperatures most of the time, except during summer

and non-limiting soil moisture conditions from May

2002 to early January 2003. A short, relatively dry

period in January and February caused a soil moisture

deficit, but this was restored by rain events during late

February and March 2003.

Night-time net ecosystem exchange

Respiration rates of peat increase with soil temperature,

hence the low CO2 emissions from boreal peat

ecosystems (Glenn et al., 1993; Silvola et al., 1996) and

the higher oxidation rates in the lower latitudes (Tate,

1980). Respiration rates from peat soils are commonly

dependent on DWT since aerobic microbial activity

increases with a decrease in DWT (Andreis, 1976;

Stephens et al., 1984; Hodge, 2002). Figure 2d and c,

respectively, shows the daily averages of soil tempera-

ture and DWT during the field study.

To establish a relationship between total ecosystem

respiration (Re) and soil temperature, measured half

hourly averaged net ecosystem CO2 exchange (NEE)

was used (Fig. 3). Measurements were only included

when average friction velocity (u*) exceeded 0.15m s�1

and K # was zero. We fitted a Lloyd & Taylor (1994,

Eqn (11)) model:

Re ¼ R10 e308:56 1

56:02� 1Ts�227:13ð Þ; ð1Þ

where Ts is in Kelvin, and determined an R10 of

2.32 � 0.02mmol CO2m�2 s�1 (mean � SE; r25 0.51;

n5 1990). This factor is the sum of plant and soil

respiration at 10 1C. Due to the nature of night-time EC

measurements and varying contribution of plant

respiration to Re, there was a considerable amount of

scatter in the data. To overcome this, the data were

averaged in 1 1C bins and plotted, and the R10 value

was recalculated to be 2.44 � 0.06mmol CO2m�2 s�1

(mean � SE; r25 0.93).

Unexpectedly, we did not observe increases in night-

time respiration as DWT increased. Silvola et al. (1996)

previously found an increase in CO2 emissions from

peat soil when DWT increased to depths of 0.3–0.4m.

Here, as the DWT grew the air-filled porosity would

increase and support greater aerobic degradation of

peat. While DWT varied greatly at our field site it was

rarely water logged due to the extensive drainage

system (see Figs 1 and 2). Increasing the DWT had little

impact on yv until DWT fell below about 0.5m during

January 2003 (Figs 2 and 4). Therefore, oxygen avail-

0

2

4

6

8

10

8 10 12 14 16 18 20 220

0.1

0.2

0.3

0.4

NE

E (

µm

ol m

−2 s

−1)

Soil temperature (°C)

NE

E (

mg

m−2

s−1

)

Fig. 3 Measured 30min averages of the night-time net ecosys-

tem CO2 exchange (NEE) vs. 0.1m soil temperature (Ts), where

friction velocity, u*40.15m s�1. Solid curve represents the Lloyd

& Taylor (1994) model fitted to 1 1C binned 30min averages

(r25 0.93).

C A R BON EXCHANGE ON A DRA IN ED P EAT SO I L 611


ability in peat would have been fairly constant and with

it the effect on soil respiration. Even when the DWT fell

below about 0.5m and yv declined to a minimum of

0.38m3m�3 in the top 0.3m, we found no evidence that

either DWT or yv reduced respiration rates, in contrast

to the findings of others (Baldocchi, 1997; e.g. Hunt

et al., 2002). This left soil temperature as the major

driving variable for respiration.

The R10 we determined is different from that

measured in other peat studies. Smith (2003) measured

an R10 of 1.6 mmolCO2m�2 s�1 for a nearby peat

bog with soil moisture contents ranging between

0.04–0.27m3m�3. Here, among other reasons the

relatively drier peat might have limited respiration.

Lohila et al. (2003) measured an R10 of 3.1 � 0.34mmol

CO2m�2 s�1 (mean � SE) from the peat soil under

pasture (yv5 0.2–0.55m3m�3). For a waterlogged

Dutch restored peat bog, on the other hand, Nieveen

et al. (1998) observed a total ecosystem R10 of

1.3mmolCO2m�2 s�1, which may indicate anaerobic

conditions.

Between May 2002 and May 2003 nocturnal respira-

tion (soil and plant) averaged 3.19 mmolCO2m�2 s�1

with average night-time soil temperatures at 0.1m

depth ranging between 9.1 1C and 22 1C. Average noc-

turnal respiration rates were lowest (0.4mmolCO2

m�2 s�1) in August 2002 (Ts5 11.3 1C; yv5 0.72m3m�3),

and greatest in summer (9.3mmolCO2m�2 s�1) after a

rain event following a dry period in early March 2003

(Ts5 21.2 1C; yv5 0.43m3m�3).

Lohila et al. (2003) found higher total ecosystem

respiration rates from pasture on peat in southern

Finland with values ranging between 5.2 and 20.5 mmol

CO2m�2 s�1 (Ts (4 cm)5 8–31 1C; yv5 0.2–0.55m3m�3).

Smith (2003) measured average respiration of 1.77mmol

CO2m�2 s�1 and a maximum of 6.0 mmolCO2m

�2 s�1

from a peat bog with restiad vegetation in New Zealand

(Ts (8 cm)5 4–20 1C; yv5 0.04–0.27m3m�3).

Daytime net ecosystem exchange

The climate of the Waikato allows for year-round

growth and grazing of pasture. The highest values for

daytime NEE were found in September 2002, with a

maximum of �17.2mmol CO2m�2 s�1 around noon.

This was higher than observed using chambers for

pasture on peat soil in Finland, where maximum

uptake only reached �9.3mmol CO2m�2 s�1 (Maljanen

et al., 2001). However, respiration rates at Rukuhia were

much higher as a result of higher soil temperature.

Grazing took place at irregular intervals on the

paddocks within the flux footprint. In some cases, the

effect of grazing was clearly visible in the daytime NEE.

In others, the relative source area or footprint of the

measured flux density suppressed this effect. Grazing

time and intensity also decreased during the milking

season, causing the grazing impact to diminish. Figure

5 shows the NEE light response for 2 months. In early

August 2002, paddocks within the flux footprint area

were intensively strip-grazed for 6 consecutive days,

removing most of the above-ground photosynthetic

tissue. This area was not grazed again until October

2002, which resulted in a considerably higher NEE in

September 2002 than August. Following Goudriaan et al.

(1985), data were fitted with an asymptotic exponential

allowing for saturation at higher PPFD, given by

NEE ¼ Re � Am þ Rdð Þ 1e�aPPFDAm þ Rd

� �� Rd

� �; ð2Þ

0.4

0.5

0.6

0.7

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

So

il m

ois

ture

co

nte

nt

(m3

m−3

)

Depth to water table (m)

Fig. 4 Average daily soil volumetric moisture content in the

top 0.3m vs. daily average depth to the water table.

− 20

− 15

− 10

− 5

0

5

100 500 1000 1500 2000

− 0.8

− 0.6

− 0.4

− 0.2

0

0.2

0.4

NE

E (

µm

ol m

−2 s

−1)

NE

E (

mg

m−2

s−1

)

PPFD (µmol m−2 s−1)

Fig. 5 Daytime 30min averages of the net ecosystem CO2

exchange (NEE) as a function of photosynthetic photon flux

density (PPFD) for August 2002 after intensive grazing (solid

circles) and September 2002 with no grazing (open circles). Data

for both months are fitted with an asymptotic exponential

function (August 2002 r25 0.83; September 2002 r25 0.91).



where Am is the maximum NEE at high photosynthetic

photon flux density (mmolm�2 s�1), PPFD the photo-

synthetic photon flux density (mmolm�2 s�1), and a the

initial light use efficiency for fixing CO2 (mmolCO2mmol�1 PPFD). PPFD was calculated from K# (Varlet-

Grancher et al., 1981), due to a faulty quantum sensor. Rd

is the dark respiration of the plants. As Rd is included in

the measured Re, the Rd parameter in Eqn (2) should be

small compared to Re. For both months, NEE did

not meet its asymptotic value (Am) or light saturation

with a PPFD of 1850mmolm�2 s�1. The initial light

use efficiency was higher in September (0.027

mmolmmol�1) than in August (0.015mmolmmol�1). Simi-

lar a values to September were found for other closed

grassland canopies (Ruimy et al., 1995) and a grass-

dominated restored peat bog in the Netherlands

(Nieveen et al., 1998). In August, a was probably lower

due to a decrease in leaf area index (LAI) and damage to

the canopy from grazing.

During the summer months, yv in the top 0.3m

gradually declined from a maximum of 0.74m3m�3 in

December 2002 to a minimum of 0.36m3m�3 (Fig. 2b).

Past measurements in the same location showed

moisture content around 0.2m3m�3 to be the wilting

point (Joe, 1986). Figure 6 presents the daytime, half-

hourly NEE data plotted against the photosynthetic

photon flux density for December 2002 and February

2003. These data were fitted with an asymptotic

exponential as given in (2). Both fits show light

saturation at PPFD � 1500mmolm�2 s�1, and a similar

initial light use efficiency 0.023 � 0.002 mmol mmol�1

and 0.021 � 0.002mmol mmol�1 (mean � SE) for Decem-

ber and February respectively. However, Am in Febru-

ary 2003 was about 45% lower than Am in December

2002, despite the grass being grazed more frequently in

December. There were similar respiration rates for both

months. This demonstrates the likely response of the

grass to limiting soil moisture conditions during

February 2003 (Fig. 2b).

The hypothesis that soil moisture limited photo-

synthesis (and, therefore, NEE) in February was

supported by a comparison of the measurements, bin-

averaged by time of the day (u*40.15m s�1), in

December 2002 and February 2003. In December 2002

the ensemble average daytime NEE peaked around

noon (Fig. 7e), in phase with the daily trend in PPFD

(Fig. 7a). For February 2003, however, the peak NEE

shifted towards early morning and declined about 2/3

in magnitude. This could be due to the higher D late

morning and afternoon (Fig. 7d), causing lower

stomatal conductances when D is higher than 1 kPa.

However, Fig. 7f shows the diurnal course of NEE and

D for two specific days in December 2002 and February

2003. While D followed the same course and had the

same magnitude for both days, NEE showed the same

trends as the ensemble average NEE for the respective

months (Fig. 7e). This was most likely due to lower

stomatal conductances in February caused by low soil

water status (Fig. 7c). Similar response of the vegeta-

tion to high D was reported for water-stressed

deciduous savannah (Verhoef et al., 1996) and tussock

grassland in New Zealand during a summer drought

(Hunt et al., 2002).

C balance

The primary objective of this study was to determine

net annual flux of C from this peat–pasture system used

for dairying. The EC technique, unfortunately, never

achieves 100% data coverage for a variety of reasons

(e.g. Baldocchi, 2003). To establish the CO2 component

of the annual C balance, gap-filling strategies were

needed. For the year-round measurement period, valid

data were obtained for 45% of the half-hourly periods.

About 30% of the rejected data were measurements

during periods of light wind and low friction velocity

(u*o0.15m s�1).

Non-linear regression methods were used to model

missing and unreliable data (Falge et al., 2001). Night-

time data were gap-filled using Eqn (1) and R105

2.44 mmolCO2m�2 s�1, with soil temperature at 0.1m as

the forcing variable. This relationship was also used to

correct for periods with insufficient turbulent mixing

(u*o0.15m s�1). Missing half-hourly daytime data

were filled by establishing a model for gross ecosystem

CO2 production (GEP) from NEE as a function of PPFD,

− 20

− 15

− 10

− 5

0

5

100 500 1000 1500 25002000

− 0.8

− 0.6

− 0.4

− 0.2

0

0.2

0.4

NE

E (

µm

ol m

−2 s

−1)

NE

E (

mg

m−2

s−1

)

PPFD (µmol m−2 s−1)

Fig. 6 Daytime 30min averages of the net ecosystem CO2

exchange (NEE) as a function photosynthetic photon flux

density (PPFD) for December 2002 with abundant soil moisture

(open circles) and February 2003 with lower soil moisture (solid

circles). Data for both months are fitted with an asymptotic

exponential function (December 2002 r25 0.67; February 2003

r25 0.68).

C A R BON EXCHANGE ON A DRA IN ED P EAT SO I L 613


and modelled total ecosystem respiration Re using Eqn

(1) and R105 2.44 mmolCO2m�2 s�1. GEP is given by

(GEP5Re�NEE). NEE was determined as a function of

PPFD for each month, to incorporate the effect of

different environmental factors, such as soil water

deficiencies and grazing cows, on photosynthesis. We

are aware that using monthly-pooled data to develop

light response curves could be too long a period,

because of changing environmental factors and plant

conditions that alter the light response. However, our

ecosystem is constantly changing due to grazing of the

farm paddocks and different growth stages of the

different paddocks. This is in contrast to other slower

growing/changing systems, such as: prairie or tussock

grasslands, and commercial crops. The continuously

varying flux density source area, sometimes by the

hour, will, in our opinion, overshadow any other factor,

justifying the use of monthly pooled data.

Figure 8 shows the averaged NEE for the year from

June 2002 to May 2003. Here, positive values indicate

net loss of CO2 during that month, while negative

values mean net sequestration. During 7 months of the

year, the peat–pasture ecosystem was a net source of

CO2, with March 2003 being the largest source of

0

500

1000

1500

2000

PP

FD

(µ

mo

l m−2

s−1

)

17

18

19

20

Ts

(°C

)

0.3

0.4

0.5

0.6

0.7

0.8

Θv

(m3

m−3

)

− 10

− 5

0

5

10

0 4 8 12 16 20 24

NE

E (

µm

ol m

−2 s

−1)

− 10

− 5

0

5

10

NE

E (

µm

ol m

−2 s

−1)

Time (h)

0 4 8 12 16 20 24

Time (h)

0

0.5

1

1.5

D (

kPa)

0

0.5

1

1.5

2

D (

kPa)

(a)

(c)

(e)

(b)

(d)

(f)

Fig. 7 Diurnal courses, bin-averaged by the time of the day and filtered for friction velocity, u*4 0.15ms�1, for December 2002

(circles, solid line) and February 2003 (diamonds, dashed line) of (a) photosynthetic photon flux density (PPFD), (b) 0.1m soil

temperature (Ts), (c) soil volumetric moisture content in the top 0.3m (yv), (d) saturation vapour pressure deficit (D) and (e) net

ecosystem CO2 exchange (NEE). Also shown, in (f), are daytime net ecosystem CO2 exchange (NEE) and saturation vapour pressure

deficit (D) for 29 December 2002 (circles) and 15 February 2003 (diamonds).



1.37mmol CO2m�2 s�1 (1.42 gCm�2 day�1), and August

2002 the smallest with 0.05mmolCO2m�2 s�1 (0.05 g

Cm�2 d�1). For 5 months of the year, the peat–

pasture ecosystem acted as a net sink for CO2. The

sink strength ranged from �1.67mmolCO2m�2 s�1

(�1.73 gCm�2 day�1) in September 2002 to �0.33mmol

CO2m�2 s�1 (�0.34 gCm�2 d�1) in December 2002.

Clearly visible is the effect of grazing on the average

CO2 exchange in August 2002 as described previously.

Furthermore, a switch from sink to a source of CO2

corresponds to the drought period in January and

February 2003.

For the entire measurement period (22 May 2002 to 21

May 2003), the annual CO2 balance for this peat-pasture

ecosystem was 164 kg CO2ha�1 yr�1 (45 kgCha�1 yr�1;

i.e. a source of CO2). Due to the instrumental set-up, the

annual CO2 balance might not be representative for the

whole farm. The height of the EC sensors caused the

flux source area to be limited to a maximum 500-m

radius around the tower. This meant that for certain

wind directions flux densities were biased towards

more local conditions. This was especially the case for

the measurements in August 2002, where grazing had a

large impact on NEE (Fig. 5). Here, a prevailing

westerly wind captured much of the grazed paddock

in the flux source area during the whole month. By

modelling the complete month of August 2002 using

the GEP–PPFD relationship for the total data set

and the measured soil temperature to calculate Re, the

areal annual exchange becomes �731 kgCO2ha�1 yr�1

(�199 kgCha�1 yr�1), i.e. a C sink. This shows the

significant effect of a single grazing event.

Quantifying errors in annual C exchange is compli-

cated, in part because of a lack of independent data for

verification. The effects of random errors caused by

sampling inadequacies and regression-based gap-filling

methodologies tend to diminish as the length of data set

increases (Falge et al., 2001). Baldocchi et al. (2001) used

the method of Moncrieff et al. (1996) to estimate an

annual random error contribution of � 530 kgCha�1

yr�1 for closed path EC systems similar to that used in

this study. Similar confidence ranges have been found

for a number of other studies at ideal sites (Baldocchi,

2003) using a variety of methodologies, so we adopt an

indicative ‘confidence range’ for annual summed NEE

of � 500 kgCha�1 yr�1.

Systematic errors in EC flux data are difficult to

detect. However, EC system design and data processing

focus on eliminating these sources of error, for e.g. the

application of spectral and air density corrections, and

rejecting measurements during periods of light winds.

A common check on EC measurements of the con-

vective energy fluxes is energy balance closure. In this

study, we achieved an energy balance closure ratio (the

sum of half-hour latent, lE, and sensible, H, heat fluxes

divided by the available energy, calculated as the

difference between the net radiation, Rn, and soil heat,

G, fluxes) of 80%, suggesting that H and lE were

systematically underestimated by approximately 20%

(ignoring likely errors in Rn and G). If CO2 exchanges

are similarly affected (Goulden et al., 1996; Twine et al.,

2000), our measured exchange of 45 kgCha�1 yr�1 may

be systematically underestimated by about 9 kgCha�1

yr�1. However, Baldocchi (2003) cautions against

adjusting NEE for lack of energy balance closure.

Applying an indicative confidence range (Baldocchi

et al., 2001), our annual C exchange measured by EC is

45 � 500 kgCha�1 yr�1. It is not possible to state cate-

gorically whether the peat-pasture system was a net

source or sink of CO2 for the period June 2002–

May 2003.

There are other C sinks and sources that should be

included in the annual farm C balance. Figure 9 depicts

the major components of the C balance, which are: CO2

exchange through photosynthesis and respiration as

measured; CH4 emissions through the digestion of

grass by cows; the removal of C through the export of

milk from the farm.

In New Zealand, Laubach & Kelliher (2004) recently

measured an average CH4 emission rate of grazing

dairy cows of 330 � 20 g CH4 per cow per day (mean

� 2� SE). At an average 3.1 cowsha�1 for the farm

this means an additional loss of 278 � 17 kgC

ha�1 yr�1. Several authors have reported on the CH4

emissions from drained organic soils (Glenn et al., 1993;

e.g. Nykanen et al., 1995; Van den Pol-van Dasselaar

− 2

− 1.5

− 1

− 0.5

0

0.5

1

1.5

− 2

− 1.5

− 1

− 0.5

0

0.5

1

1.5

Jun

02

Jul 0

2

Aug

02

Sep

02

Oct

02

Nov

02

Dec

02

Jan

03

Feb

03

Mar

03

Apr

03

May

03

Month

NE

E (

µm

ol C

O2

m−2

s−1

)

NE

E (

g C

m−2

day

−1)

Fig. 8 Monthly variation in net ecosystem exchange (NEE)

expressed in mmol CO2m�2 s�1 and gCm�2 d�1 for the period

between June 2002 and May 2003. Negative values indicate

fixation of CO2 or C, while positive values denote a net release of

CO2 or C.

C AR BON EXCHANGE ON A DRA IN ED P EAT SO I L 615


et al., 1998). All found that drained organic soils used

for farming were a negligible source or sink of CH4.

In the year June 2002–May 2003, the farm produced

345 000 kg milk solids from 1200 cows grazing 390 ha.

Each kilogram of milk solids is equivalent to 0.834 kg

of C (Wells, 2001). Therefore C export from the farm

as milk was 738 kgCha�1 yr�1. There is negligible error

in this term because of accurate farm production

accounting.

C exported as CH4 and milk is obtained from pasture

(and a small proportion of crops) grown on the farm. A

New Zealand cow requires dry matter containing

approximately 7 kg C (assume � 10%) to produce 1 kg

of milk solids, so that a total of 6192 � 620 kg

Cha�1 yr�1 in dry matter was consumed. No supple-

mentary feed was imported onto the farm.

Dry matter produced by cows in the form of faeces

(6192–738–2785 5176 � 620 kgCha�1 yr�1) is assumed

to decompose and respire completely, and is measured

as part of NEE by the EC system, along with soil, grass

and cow respiration.

Thus, after about 60 years of dairying, total C

exchange was 1061 � 500 kgCha�1 yr�1 (451 7381

278 kgCha�1) for this well-developed peatland system.

This loss is substantially less than that reported by

Schipper & McLeod (2002), who estimated long-term

losses of 3700 kgCha�1 yr�1 (95% confidence interval of

2500–5000 kgCha�1 yr�1) by comparing cores taken

from a peat bog with those from adjacent farmland.

However, this long-term rate included initial drainage

and establishment of pasture when rates were likely to

be much greater.

Summary

Pastures on peat in the Waikato region have been used

for dairy farming for over 60 years. We estimated C loss

of 1061 � 500 kgCha�1 yr�1, significantly less than

average losses of 3700 kgCha�1 yr�1 estimated for the

first 40 years following pasture development at a

nearby farm (Schipper &McLeod, 2002). Taken together

these studies indicate that initially high C losses

following conversion to pasture decline with time.

We determined that only 45 � 500 kgCha�1 yr�1 was

lost through the imbalance in CO2 exchange, measured

by EC. A single severe grazing event near the EC tower

in August 2002 had a dramatic effect on annual CO2

exchange, and we estimated that without this event,

annual CO2 losses would have been reduced by

approximately 244 kgCha�1 yr�1.

Respiration was found to be independent of the soil

moisture content in a range between 0.38 and

0.67m3m�3, but highly dependent on soil temperature.

Fitted with a Lloyd & Taylor-type model an R10 value of

2.44 mmolCO2m�2 s�1 was established. Most of the

variation in daytime CO2 exchange was explained by

either grazing events or by limiting soil moisture

conditions slowing down photosynthesis.

Of the three pathways for C loss from the pasture/

soil system, CH4 was of a similar magnitude

(278 � 17 kgCha�1 yr�1) to CO2, and the C content of

milk solids was significantly larger (738 kgCha�1 yr�1).

While the magnitudes of CH4 and milk production are

likely to remain relatively constant, or increase only

slightly with intensification of farming practices, the net

exchange of CO2 is strongly affected by both climate

and grazing management, so will vary considerably

from year to year.

Acknowledgements

The authors would like to thank the Wallace Corporation for ac-cess to the field site, in particular Nick Rowe and other farm stafffor general logistics, and Terry Truitt for providing farm pro-duction statistics. We would also like to thank Hort Research, inparticular Mark Astill and Philip Prendergast for kindly lendingus their CSAT3, and John Hunt for his useful comments on thismanuscript. We acknowledge that the suggestions of threeanonymous referees have significantly improved this paper.

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Carbon exchange of grazed pasture on a drained peat soil

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