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Title
Effect of nitrogen fertilization on atmospheric methane oxidation in boreal
forest soils
Permalink
https://escholarship.org/uc/item/48k10791
Journal
Chemosphere - Global Change Science, 2(2)
ISSN
1465-9972
Authors
Whalen, SC
Reeburgh, WS
Publication Date
2000-09-23
DOI
10.1016/S1465-9972(00)00003-9
Copyright Information
This work is made available under the terms of a Creative Commons Attribution
License, availalbe at https://creativecommons.org/licenses/by/4.0/
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Eect of nitrogen fertilization on atmospheric methane
oxidation in boreal forest soils
S.C. Whalen
*
, W.S. Reeburgh
Department of Earth System Science, University of California, Irvine, CA 92717-3100, USA
Received 7 May 1999; accepted 9 September 1999
Importance of this Paper: The ``greenhouse'' gas CH
4
is increasing in atmospheric concentration. Oxidation by met-
hanotrophic bacteria in upland soils is the only biological sink for atmospheric CH
4
. Methanotrophs are inhibited by high
soil N, raising concern that the contemporary worldwide increase in atmospheric N deposition will decrease soil CH
4
oxidation. The boreal forest occupies 13% of the earth's continental surface and is important in atmospheric CH
4
oxidation.
We studied atmospheric CH
4
oxidation in fertilized boreal forest plots to determine the N sensitivity of the methanotrophic
community and conclude that increased N deposition will not alter the rates of CH
4
oxidation in these soils.
Abstract
Field plots of aspen and black spruce in the Alaskan boreal forest were fertilized repeatedly with nitrogen during the
1993 summer growing season, and weekly determinations of the in¯uence of fertilization on atmospheric CH
4
oxidation
were made with static chambers. Repeated fertilization with (NH
4
)
2
SO
4
solution or nutrient media used to culture
methanotrophic or nitrifying bacteria gave a total addition of 140 or 580 kg N ha
ÿ1
. Time-integrated CH
4
oxidation
was not signi®cantly dierent in fertilized soils versus watered controls because CH
4
oxidation was localized in a
subsurface soil zone that was probably not penetrated by surface-applied aqueous phase fertilizer. Insensitivity of CH
4
oxidation by these soils to a high rate of N fertilization and the low current rate of atmospheric N deposition suggest
that future increases in atmospheric N deposition will not alter the sink strength of high latitude boreal forest soils in
the atmospheric CH
4
budget. Ó 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Methane oxidation; Forests; Nitrogen fertilization
1. Introduction
The atmospheric concentration of the radiatively
important greenhouse gas CH
4
has shown a well-docu-
mented, contemporary increase (Houghton et al., 1996).
Methane is second only to CO
2
as a greenhouse gas and
contributes approximately 20% to global warming
(Bouwman, 1990). Microbial oxidation in well-drained
soils is the only identi®ed biological sink for atmospheric
CH
4
and accounts for 3% to 9% of total annual atmo-
spheric CH
4
destruction (Prather et al., 1996). This is
similar in magnitude to the current atmospheric increase
of 37 Tg CH
4
yr
ÿ1
(Houghton et al., 1996). Conse-
quently, alterations of the soil sink strength are a sig-
ni®cant determinant of the rate of change in the
atmospheric CH
4
concentration (Prather et al., 1996)
and absence of this sink will cause the atmospheric CH
4
concentration to increase at 1.5 times the current rate
(Duxbury, 1994).
Methanotrophic bacteria are generally considered to
be responsible for atmospheric CH
4
oxidation (Conrad,
Chemosphere ± Global Change Science 2 (2000) 151±155
*
Corresponding author. Present address: Department of
Environmental Science and Engineering, CB #7400, University
of North Carolina, Chapel Hill, NC 27599-7400, USA. Fax:
+1-919-966-7911.
E-mail address: steve_whalen@unc.edu (S.C. Whalen).
1465-9972/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved.
PII: S 1 465-9972(00)00003-9
1996; King, 1997), although NH
4
-oxidizing bacteria are
capable of oxidizing CH
4
at rates lower than methano-
trophs (Jones et al., 1984; Bedard and Knowles, 1989).
Hence, soils with high nitri®cation rates show low CH
4
oxidation because CH
4
oxidation is dominated by nit-
ri®ers (Castro et al., 1995; Steudler et al., 1996).
In¯uences on rates of atmospheric CH
4
oxidation in
well-drained soils include soil temperature, moisture and
N status (reviewed by King, 1992, 1997; Mancinelli,
1995) as well as land use patterns (Ojima et al., 1993;
Willison et al., 1995; Hudgens and Yavitt, 1997; Boecyx
et al., 1998). Forest plots experimentally fertilized with
N frequently show depressed atmospheric CH
4
oxida-
tion relative to unfertilized controls (Steudler et al.,
1989; King and Schnell, 1994; Sitaula et al., 1995).
Consequently, the well-documented worldwide increase
in atmospheric N deposition on terrestrial and aquatic
environments (Matthews, 1994; Galloway et al., 1995)
may result in a decrease in atmospheric CH
4
oxidation
by upland forest soils (Castro et al., 1995).
The boreal forest occupies 13% of the earthÕs conti-
nental surface (Schultz, 1995), and upland boreal forest
soils are important in atmospheric CH
4
consumption
(Whalen et al., 1991). Agriculture is expanding world-
wide (Adams et al., 1990) and agricultural soils have
lower area-based rates of CH
4
consumption than aer-
ated forests, shrublands and savannah (reviewed by
King, 1997). Since boreal forests are generally unsuitable
for this land use, upland soils of this region will be
critical in slowing future increases in atmospheric CH
4
.
Accurate prediction of future trends in soil CH
4
oxida-
tion in these high latitude soils requires an improved
understanding of the microbial community response
to global change, including increased atmospheric N
deposition.
This study was aimed at determining the impact of
increased N input on atmospheric CH
4
oxidation by
boreal forest soils and to qualitatively determine the
relative importance of methanotrophs and nitri®ers in
oxidizing CH
4
in these soils. We experimentally fertilized
plots with (NH
4
)SO
4
solution to simulate increased at-
mospheric N input. Further, we attempted to enhance
populations of nitri®ers or methanotrophs in additional
plots by fertilizing with liquid media used to culture
these microbial groups. The rate of CH
4
oxidation
in treated plots was compared with that of watered
controls.
2. Materials and methods
2.1. Site description
Field sites were located along a successional sequence
in Bonanza Creek experimental forest (64°N, 148°W)
near Fairbanks, Alaska. The intermediate successional
stage was represented by a south-facing aspen (Populus
tremuloides; site AS2) community with a 3±5 cm ¯oor of
leaf litter and an insigni®cant understory. The advanced
successional stage was represented by a north-facing
black spruce (Picea mariana; site BS2) stand with a
continuous ground cover of feather mosses (Pleurozium
spp. and Hylocomium spp.) invaded by lowbush cran-
berry (Vaccinium vitis-idaea) and lichens. The organic
horizon extends to about 10 cm. These sites have no soil
zone of methanogenesis and are atmospheric CH
4
sinks
(Whalen et al., 1992).
2.2. Experimental
Methane ¯ux determinations were made using the
static chamber technique (Whalen et al., 1991). Each
chamber consisted of a skirted aluminum base perma-
nently seated in the soil and a removable aluminum lid
that utilized a water-®lled channel for a seal. Lids were
equipped with an o-seal ®tting to allow syringe sampling
of headspace gas and ®tted with a capillary bleed to
equilibrate with atmospheric pressure. Eight bases were
deployed in June 1993 at each site. Methane ¯uxes were
determined at each chamber on 8 July (Calendar day
(CD) 189). Soils enclosed within two randomly selected
bases at each site were then amended with 2-L of one of
the following: (a) a growth medium for methanogens
(King and Adamsen, 1992); (b) a growth medium for
nitri®ers (De Boer et al., 1991); (c) (NH
4
)
2
SO
4
in de-
ionized water; or (d) deionized water only. Methane
oxidation was measured weekly from CD 194 through
CD 243. Fertilization with the assigned treatment im-
mediately followed each CH
4
¯ux determination. Fer-
tilization added 73 kg NO
ÿ
3
-N ha
ÿ1
(methanogen
medium) or 18 kg NH
4
-N (nitri®er medium and
(NH
4
)
2
SO
4
treatment) to these soils on a weekly basis.
2.3. Methane analysis
Methane determinations were made by ¯ame ion-
ization detection gas chromatography with a precision
of <1% (Whalen et al., 1991). Calibration gases are re-
latable to mixtures obtained from the National Institute
of Standards and Technology.
2.4. Statistical analyses and calculations
Area-based rates of CH
4
oxidation were calculated
from the time-linear decrease in CH
4
concentration in
the chamber headspace during a 0.75 h deployment. Day
rates of CH
4
oxidation for each chamber were time-in-
tegrated over the experimental period and treatment
means within each site were compared by single factor
analysis of variance. Data satis®ed assumptions of
normality and homoscedasticity without transforma-
tion. A signi®cance level of a 0.05 was used.
152 S.C. Whalen, W.S. Reeburgh / Chemosphere ± Global Change Science 2 (2000) 151±155
3. Results and discussion
Day-rates of CH
4
oxidation at AS2 varied over a
factor of about 1.6 (0.46±0.76 mg m
ÿ2
d
ÿ1
) during the
study period (Fig. 1). Time-integrated rates of CH
4
ox-
idation varied from 29.2 to 32.3 mg m
ÿ2
and were not
signi®cantly dierent across treatments. Day-rates of
CH
4
oxidation at BS2 were somewhat higher and less
variable than for AS2 (Fig. 1). Methane oxidation rates
varied over a factor of 1.3, from 0.69 to 0.89 mg m
ÿ2
.
Time-integrated rates of CH
4
oxidation varied from 36.8
to 40.1 mg m
ÿ2
and were not signi®cantly dierent
across treatments. The temporal pattern of CH
4
oxida-
tion was generally similar across treatments within each
site. This probably re¯ects changes in soil moisture,
which controls diusion of CH
4
to the zone of oxida-
tion, and hence is a primary determinant of rates of
atmospheric CH
4
oxidation by soils (King, 1997). In-
creased N input to forests can increase the ®ne root
biomass (Magill et al., 1997), which may decrease air-
®lled porosity and limit rates of diusion and atmo-
spheric CH
4
oxidation. Any increase in ®ne root bio-
mass in response to fertilization here had no impact on
soil±atmosphere gas exchange, as time-integrated rates
of CH
4
oxidation did not dier signi®cantly between
control and fertilized plots at either site. The generally
higher rates of CH
4
oxidation at BS2 than at AS2 is
consistent with the higher gas-®lled porosity at BS2
(Whalen et al., 1992), which allows for more rapid dif-
fusion of atmospheric CH
4
into the soil.
The N load from repeated fertilization totaled about
140 kg NH
4
-N ha
ÿ1
for plots treated with (NH
4
)
2
SO
4
and the nitri®er medium and about 580 kg NO
ÿ
3
-N ha
ÿ1
for plots fertilized with the methanotroph medium. At-
mospheric CH
4
oxidation at both sites was unaected by
fertilization at these levels, which is consistent with a
similar observation for a boreal spruce forest fertilized
for three years with NH
4
NO
3
at 60 kg N ha
ÿ1
(Gulledge
et al., 1997). However, most studies report inhibition of
atmospheric CH
4
oxidation in N-fertilized forest soils.
Pine forests fertilized with NH
4
NO
3
at 30 and 90 kg N
ha
ÿ1
gave CH
4
oxidation rates that were 85% and 62%,
respectively, of unfertilized controls (Sitaula et al.,
1995), while a temperate softwood forest amended with
the same fertilizer at 120 kg N ha
ÿ1
showed a CH
4
ox-
idation rates that were 67% of unfertilized controls
(Steudler et al., 1989). Urea fertilization of a pine
plantation (180 kg N ha
ÿ1
) resulted in a 5- to 20-fold
decrease in atmospheric CH
4
oxidation relative to con-
trols (Castro et al., 1995), while application of urea,
KNO
3
or NH
4
Cl to forested peatland soils at 100 kg N
ha
ÿ1
signi®cantly depressed CH
4
oxidation (Crill et al.,
1994). Finally, fertilization of a mixed oak±pine forest
with only 10 kg NH
4
Cl-N ha
ÿ1
inhibited atmospheric
CH
4
oxidation by about 20% relative to control plots
(King and Schnell, 1994).
Sensitivity of atmospheric CH
4
oxidation in forest
soils to N fertilization may relate to the position of the
CH
4
oxidizing community in the soil column. Maximum
rates of atmospheric CH
4
oxidation are found 10±20 cm
below the soil surface at both sites, and 91% and 98% of
depth-integrated CH
4
oxidation at AS2 and BS2 occurs
at soil depths >10 cm (Whalen et al., 1992). Hence, it is
likely that liquid fertilizer additions did not penetrate to
the active CH
4
oxidizing zone. Other studies also point
to the locus of the CH
4
oxidizing zone as a key deter-
minant of soil sensitivity to N fertilization. Surface lo-
calization of fertilizer in conjunction with a subsurface
zone of CH
4
oxidation was oered as a possible expla-
nation for the lack of a fertilizer eect on CH
4
oxidation
in a humisol amended with 100 kg ha
ÿ1
urea-N (Dun-
®eld et al., 1995), while addition of NH
4
Cl, KNO
3
and
urea-N at this same level all inhibited CH
4
oxidation in
forest peat soils where CH
4
oxidation was largely re-
stricted to the 0±5 cm zone.
Nitri®ers are frequently located in the organic, NH
4
-
rich surface layers of forest soils (Lensi et al., 1991;
Schnell and King, 1994; Castro et al., 1995), suggesting
that the NH
4
-N and nitri®er medium solutions
Fig. 1. Methane oxidation at sites AS2 and BS2 in response to
fertilization with nitri®er medium (m), methanotroph medium
(s), (NH
4
)
2
SO
4
solution (M) or deionized water (). Each
point represents the mean rate of CH
4
oxidation from duplicate
chambers for each treatment. Standard deviations averaged
0.06 and 0.17 mg CH
4
m
ÿ2
d
ÿ1
at AS2 and BS2, respectively.
Error bars are eliminated for clarity.
S.C. Whalen, W.S. Reeburgh / Chemosphere ± Global Change Science 2 (2000) 151±155 153
contacted this component of the microbial community.
Methane is co-oxidized along with NH
4
-N by nitrifying
bacteria (Bedard and Knowles, 1989) and urea-N fertil-
ization (rapidly hydrolyzed to NH
4
-N; Zhengping et al.,
1996) in pine forest soil has been demonstrated to reduce
atmospheric CH
4
oxidation by shifting the relative im-
portance of methanotrophs and nitri®ers (Castro et al.,
1995). Hence, insensitivity of CH
4
oxidation at AS2 and
BS2 to amendment with NH
4
-N or the nitri®er medium
suggests that methanotrophs are primarily responsible
for CH
4
oxidation in these soils. This is consistent with
the observation that nitrifying activity is low in other
boreal forest soils (Gulledge et al., 1997). In contrast to
the nitri®er medium, the methanotroph medium proba-
bly never reached the target organisms. Hence, the rate of
CH
4
oxidation was unin¯uenced by this treatment.
Increased atmospheric N deposition is not likely to
reduce rates of atmospheric CH
4
oxidation in boreal
forest soils. The loading rate of 140 kg N ha
ÿ1
for re-
peated fertilization with NH
4
-N solution or nitri®er
medium is 70-fold higher than the estimated annual
deposition (wet plus dry) of 2 kg N ha
ÿ1
for remote
continental areas (Galloway, 1993). Moreover, experi-
mental conditions were considerably more favorable
than natural rainfall for delivery of nutrients to the ac-
tive methanotrophic zone located at >10 cm below the
soil surface. Each of the eight fertilizations simulated a
2.7 cm, high N rainfall while total annual precipitation
at these study sites is 30 cm equally divided between rain
and snow, and no single rain event exceeded 2 cm during
the year this study was conducted (NOAA, 1993). Ad-
ditionally, chamber bases extended to a depth of about
15 cm below the soil surface, ensuring that fertilizer did
not move laterally following application. In concert, low
annual precipitation and subsurface localization of the
zone of atmospheric CH
4
oxidation ensure that any re-
alistic increase in atmospheric N deposition will not alter
the sink strength of upland boreal forest soils in the
atmospheric CH
4
budget.
Acknowledgements
This work was supported by the US Environmental
Protection Agency and the National Institute for Global
Environmental Change. The Bonanza Creek Long Term
Ecological Research Program is supported by the Na-
tional Science Foundation. Chad Staiger assisted in
laboratory and ®eld analyses.
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