Temperature response of denitrification rate and greenhouse
gas production in agricultural river marginal wetland soils
S. A. F. BONNETT,
1,2
M. S. A. BLACKWELL,
3
R. LEAH,
2
V. COOK,
2
M. O’CONNOR
2,4
AND E. MALTBY
2,4
1
Department of Crops and Environment Sciences, Harper Adams University, Newport, UK
2
Institute for Sustainable Water, Integrated Management and Ecosystem Research (SWIMMER), University of Liverpool,
Liverpool, UK
3
Department of Sustainable Soils and Grassland Systems, Rothamsted Research North Wyke, Okehampton, UK
4
School of Environmental Sciences, University of Liverpool, Liverpool, UK
ABSTRACT
Soils are predicted to exhibit significant feedback to global warming via the temperature response of green-
house gas (GHG) production. However, the temperature response of hydromorphic wetland soils is compli-
cated by confounding factors such as oxygen (O
2
), nitrate (NO
3
À
) and soil carbon (C). We examined the
effect of a temperature gradient (2–25 °C) on denitrification rates and net nitrous oxide (N
2
O), methane
(CH
4
) production and heterotrophic respiration in mineral (Eutric cambisol and Fluvisol) and organic (Histo-
sol) soil types in a river marginal landscape of the Tamar catchment, Devon, UK, under non-flooded and
flooded with enriched NO
3
À
conditions. It was hypothesized that the temperature response is dependent
on interactions with NO
3
À
-enriched flooding, and the physicochemical conditions of these soil types. Deni-
trification rate (mean, 746 Æ 97.3 lgm
À2
h
À1
), net N
2
O production (mean, 180 Æ 26.6 lgm
À2
h
À1
) and
net CH
4
production (mean, 1065 Æ 183 lgm
À2
h
À1
) were highest in the organic Histosol, with higher
organic matter, ammonium and moisture, and lower NO
3
À
concentrations. Heterotrophic respiration
(mean, 127 Æ 4.6 mg m
À2
h
À1
) was not significantly different between soil types and dominated total
GHG (CO
2
eq) production in all soil types. Generally, the temperature responses of denitrification rate and
net N
2
O production were exponential, whilst net CH
4
production was unresponsive, possibly due to sub-
strate limitation, and heterotrophic respiration was exponential but limited in summer at higher tempera-
tures. Flooding with NO
3
À
increased denitrification rate, net N
2
O production and heterotrophic respiration,
but a reduction in net CH
4
production suggests inhibition of methanogenesis by NO
3
À
or N
2
O produced
from denitrification. Implications for management and policy are that warming and flood events may
promote microbial interactions in soil between distinct microbial communities and increase denitrification
of excess NO
3
À
with N
2
O production contributing to no more than 50% of increases in total GHG
production.
Received 21 May 2012; accepted 28 January 2013
Corresponding author: S. A. F. Bonnett. Tel.: 01952 815133; fax: +44 (0) 1952 814783; e-mail:
sbonnett@harper-adams.ac.uk
INTRODUCTION
Interference of the nitrogen (N) cycle has been identified
as one of three Earth-system processes, along with climate
change and biodiversity loss, that have transgressed bio-
physical thresholds, or planetary boundaries, on a global
scale (Rockstr€ om et al., 2009). These planetary boundaries
define the safe operating space for humanity with respect
to the Earth system and are associated with the planet’s
biophysical subsystems or processes (Rockstr€ om et al.,
2009). Large quantities of nitrate (NO
3
À
) in runoff origi-
nating from agricultural fertilizers are responsible for
eutrophication in freshwater bodies resulting in reduced
oxygen (O
2
) levels and associated fish kills. River marginal
wetlands are important components of the agricultural
landscape as an interface between agroecosystems and
freshwater environments, regulating the quantity of NO
3
À
in runoff that reaches surface waters by converting it to
© 2013 Blackwell Publishing Ltd 1
Geobiology (2013) DOI: 10.1111/gbi.12032