Agricultural and Forest Meteorology 151 (2011) 1440–1452
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Agricultural and Forest Meteorology
j ourna l ho me pag e: www.elsevier.com/locate/agrformet
Seasonal variation and fire effects on CH
4
, N
2
O and CO
2
exchange in savanna soils
of northern Australia
Stephen J. Livesley
a,b,∗
, Samantha Grover
c
, Lindsay B. Hutley
c
, Hizbullah Jamali
b
,
Klaus Butterbach-Bahl
d
, Benedikt Fest
b
, Jason Beringer
c
, Stefan K. Arndt
b
a
School of Geography and Environmental Science, Monash University, VIC, Australia
b
Department of Forest and Ecosystem Science, The University of Melbourne, VIC, Australia
c
School of Environmental and Life Sciences, Charles Darwin University, NT, Australia
d
Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, Kreuzeckbahnstr 19, 82467 Garmisch-Partenkirchen, Germany
a r t i c l e i n f o
Article history:
Received 14 August 2010
Received in revised form
29 December 2010
Accepted 2 February 2011
Keywords:
Methane
Nitrous oxide
Carbon dioxide
Nitrification
Fire
Savanna woodland
Termites
Soil gas diffusion
a b s t r a c t
Tropical savanna ecosystems are a major contributor to global CO
2
, CH
4
and N
2
O greenhouse gas
exchange. Savanna fire events represent large, discrete C emissions but the importance of ongoing soil-
atmosphere gas exchange is less well understood. Seasonal rainfall and fire events are likely to impact
upon savanna soil microbial processes involved in N
2
O and CH
4
exchange. We measured soil CO
2
, CH
4
and
N
2
O fluxes in savanna woodland (Eucalyptus tetrodonta/Eucalyptus miniata trees above sorghum grass)
at Howard Springs, Australia over a 16 month period from October 2007 to January 2009 using manual
chambers and a field-based gas chromatograph connected to automated chambers. The effect of fire on
soil gas exchange was investigated through two controlled burns and protected unburnt areas. Fire is
a frequent natural and management action in these savanna (every 1–2 years). There was no seasonal
change and no fire effect upon soil N
2
O exchange. Soil N
2
O fluxes were very low, generally between -1.0
and 1.0 g N m
-2
h
-1
, and often below the minimum detection limit. There was an increase in soil NH
4
+
in the months after the 2008 fire event, but no change in soil NO
3
-
. There was considerable nitrification
in the early wet season but minimal nitrification at all other times.
Savanna soil was generally a net CH
4
sink that equated to between -2.0 and -1.6 kg CH
4
ha
-1
y
-1
with no clear seasonal pattern in response to changing soil moisture conditions. Irrigation in the dry
season significantly reduced soil gas diffusion and as a consequence soil CH
4
uptake. There were short
periods of soil CH
4
emission, up to 20 g C m
-2
h
-1
, likely to have been caused by termite activity in,
or beneath, automated chambers. Soil CO
2
fluxes showed a strong bimodal seasonal pattern, increasing
fivefold from the dry into the wet season. Soil moisture showed a weak relationship with soil CH
4
fluxes,
but a much stronger relationship with soil CO
2
fluxes, explaining up to 70% of the variation in unburnt
treatments. Australian savanna soils are a small N
2
O source, and possibly even a sink. Annual soil CH
4
flux measurements suggest that the 1.9 million km
2
of Australian savanna soils may provide a C sink of
between -7.7 and -9.4 Tg CO
2
-e per year. This sink estimate would offset potentially 10% of Australian
transport related CO
2
-e emissions. This CH
4
sink estimate does not include concurrent CH
4
emissions
from termite mounds or ephemeral wetlands in Australian savannas.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
The increased concentration in the atmosphere of greenhouse
gases including carbon dioxide (CO
2
), methane (CH
4
) and nitrous
oxide (N
2
O) as a result of anthropogenic activity is widely recog-
nised as the cause of global climate warming and an increase in the
∗
Corresponding author at: Dept. of Resource Management and Geography,
The University of Melbourne, Burnley campus, Richmond, Melbourne, VIC 3121,
Australia. Tel.: +61 439 615 772.
E-mail address: sjlive@unimelb.edu.au (S.J. Livesley).
intensity and frequency of extreme weather events (IPCC, 2007).
These gases are all naturally present in the atmosphere and cycle
between the land, the ocean and the atmosphere over a range of
time scales. Global climate models have been developed as tools
to predict global biogeochemical cycling and climate systems, but
their efficacy is limited by our understanding of the underlying pro-
cesses which drive the cycling of greenhouse gases (Beringer et al.,
2011), this is particularly so for CH
4
and N
2
O.
Savanna ecosystems are a significant global biome and cover
approximately one sixth of the earth’s land surface and have been
estimated to produce 30% of the earth’s total net primary produc-
tivity (Grace et al., 2006). Tropical savanna ecosystems have been
0168-1923/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.agrformet.2011.02.001