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Scientific registration no: 2047
Symposium no: 26
Presentation: poster
Effects of an increased soil temperature on nitrous
oxide emissions from an arable Eutrochrept in
Southern Germany
Effet d'une augmentation de température sur l'émission
d'oxydes nitreux dans un Eutrochrept cultivé du sud de
l'Allemagne
KAMP Thomas (1), STEINDL Hubert (1), HANTSCHEL Ralph E. (2), BEESE
Friedrich (3), MUNCH Jean-Charles (1)
(1) GSF - Institute of Soil Ecology, 85764 Neuherberg, Germany
(2) Agiplan AG, 45470 Muelheim/Ruhr, Germany
(3) Institute of Soil Science and Forest Nutrition, 37077 Goettingen, Germany
Introduction
Theoretical and numerical analyses agree that rising trace gas emissions will cause a
global warming (Houghton et al. 1990, 1996) and most General Circulation Models
estimate in Central Europe an increasing temperature of 2-4°C during the next decades
(Rowntree et al. 1991; Harrison et al. 1995; Hollwurtl and Beinhauer 1995). As a
consequence, changes in evaporation, precipitation and soil moisture will undoubtedly
occur (Houghton et al. 1990; Anderson 1992). However, increasing temperatures have
significant impacts on agriculture and will affect the microbial activity and the
productivity of crops (Waggoner 1983; Rosenzweig and Parry 1993; Kutsch et al. 1995).
Among other trace gases, nitrous oxide is contributing to global warming (Yung et al.
1976) and is involved in the destruction of stratospheric ozone (Crutzen 1970). Its total
atmospheric emission is calculated as approximately 10-17 Tg N2O-N a-1 (Houghton et
al. 1996) with an increasing rate of 0.2 to 0.3% per year (Badr and Probert 1992). About
70% of the total emitted nitrous oxide is derived from soils (Bouwman 1990) and
recently Iserman (1994) calculated the share of agriculture as 81% of the anthropogenic
nitrous oxide emissions. In consequence Mosier et al. (1996) suggested to use
nitrification inhibitors mixed to N fertilizers to mitigate nitrous oxide emissions from
agricultural soils. Continuing, Beauchamp (1997) gave a review of several management
practices and procedures to control nitrous oxide emissions from agroecosystems.
1
Fig. 1 Simplified scheme
of the influence on N2O
emissions from arable
soils and its feedback via
changing temperature and
precipitation
due
to
climate change. A more
detailed scheme about the
variables
regulating
processes
that
form
nitrous
oxide
in
agroecosystems is given
by Benckiser (1994) for
denitrification and by
Beauchamp (1997) for
nitrification.
The production of nitrous oxide in soils is mostly depended on denitrification and
nitrification (Delwiche 1981; Firestone 1982; Kuenen and Robertson 1988; Rheinbaben
1990). Because these formations are biotic processes they will be controlled by factors
influencing the microbial activity. Climate is one of them. Therefore, a hypothetical
positive feedback is regulating the interaction between climate change as induced by
nitrous oxide emissions and the formation of nitrous oxide as influenced by global
warming (Fig. 1).
To proof the effects of a postulated increased temperature on soil biological processes in
agroecosystems a field experiment with special regard to nitrous oxide releases was
carried out. The soil warming experiment was related to the scenario SA90 of the IPCC
(Houghton et al. 1990) and an increase of the soil temperature of 3°C above ambient was
assumed.
Materials and Methods
The experiments were performed on the research station of the 'Forschungsverbund
Agrarökosysteme München' (FAM) in southern Germany, approximately 45 km north of
Munich in the Bavarian tertiary hillslopes. The research farm (N 48°30.0', E 11°20.7') is
located 454 m above sea level. The mean annual air temperature is about 7.4°C with
minimum in January (-4°C daily mean temperature) and maximum in August (22°C daily
mean temperature). The annual precipitation is 833 mm (60% in the summer months).
The studies were carried out on a fine-loamy district Eutrochrept (clay 18%, silt 57%
sand 25%) with a pH (CaCl2) of 5.2. Total organic C was 1.83% and total organic N
0.17%. Bulk density was 1.3 g cm-3.
In summer 1994 two heating grids were installed in a fallow (cut twice a year) and a
wheat field (100 kg N ha-1 as ammonium urea solution and white mustard as catch crop),
respectively. Each grid consisted of a 25 m2 (5 × 5 m) heated plot with a control plot of
the same size beside. The heating equipment was modified after a technique as reported
by Hantschel et al. (1995) and was previously described by Kamp and Steindl (1997).
2
The heating started in August 1994 after a short control period of investigation. The
plots in the fallow remained installed over the total investigation period whereas the plots
in the wheat field were shortly removed in autumn 1994 and 1995 and reinstalled after
tillage.
Monitoring of gas fluxes was carried out weekly from July 1994 to March 1996 using
the closed chamber technique (Hutchinson and Mosier 1981). In each plot five
permanent PVC rings were installed. Five gas samples were collected periodically from
the chambers' atmosphere within a period between 45 and 90 minutes. Nitrous oxide
concentration in the gas samples was analyzed using a gas chromatograph with a 63Ni
electron capture detector (ECD) connected to an autosampler with pneumatic multiport
valves (Loftfield et al. 1997). The chamber design and the calculations of N2O fluxes are
described by Flessa et al. (1995).
Accompany to the gas flux measurements soil samples were taken monthly down to 0.5
m depth in the experimental plots and analyzed gravimetrically for soil moisture (dried at
105°C).
Results and Discussion
Within the investigated period temperature and precipitation differed between the years.
The average of the daily mean air temperature in winter 1994/95 was 5.6°C whereas it
was -0.1°C in winter 1995/96. The daily mean air temperatures during the summer
months 1994 was 16°C and 14°C in 1995 and the total precipitation in the first winter
period was 372 mm whereas it was 214 mm in the second period. Fortunately in the
present investigation two different climatic years appeared: the warm and wet year
1994/95 and the cold and dry year 1995/96. Therefore, the effects of the soil warming
experiment could be proofed under different natural climatic fluctuations.
In January 1994 as well as in January and February 1995 snow was covering the fields
which insulated the soil and the plants against the cold air temperatures. However, due to
the heating of the wheat field snow melted and no snow cover occurred in the heated
plot. As a result the topsoil of the heated wheat field was exposed to the very cold wind.
Despite of heating this effect resulted in lower temperatures in the heated plot as against
in the control plot. We assume that without heating the soil temperature in the snowless
heated plot would have been 5 to 6°C lower. Surprisingly this effect was not observed in
the fallow field. Both control plot and heated plot of the fallow field were very closely
covered with different grasses and herbs which separated the topsoil and the snow cover
from each other. The effect was that the snow did not reach the ground and the
resistance wires, respectively, and therefore snow could not melt. For this reason the
temperature differences between control and heated plot could kept at approximately
3°C as desired.
Heating the topsoil (0.01 m) 3°C above ambient influenced the soil temperatures in the
lower horizons. The treatment resulted in temperature differences of approximately 1 to
1.8°C even at 1 m depth between the heated and the control plots. The soil temperatures
in the 1 m profile of the fallow plots is shown exemplary in figure 2 for a summer day
(31.07.95) and a late autumn day (14.11.95).
3
soil depth (m)
16
17
18
19
20
21
22
soil temperature (°C)
23
24
4
5
6
7
8
9
10
11
12
13
0.0
0.0
-0.2
-0.2
-0.4
-0.4
-0.6
-0.6
-0.8
-0.8
-1.0
31.07.95
soil depth (m)
soil temperature (°C)
15
-1.0
14.11.95
Fig. 2 Soil temperature in the 1 m profile of the control plot ( ) and the heated plot ( ) of the fallow
during the 31.07.95 and the 14.11.95.
soil depth (m)
10
12
14
16
18
20
22
soil moisture [% DS ]
24
26
28
16
18
20
22
24
26
28
30
32
34
36
0.0
0.0
-0.1
-0.1
-0.2
-0.2
-0.3
-0.3
-0.4
-0.4
-0.5
14.11.95
31.07.95
soil depth (m)
soil moisture [% DS ]
8
-0.5
Fig. 3 Soil moisture in the 0.5 m profile of the control plot ( ) and the heated plot ( ) of the fallow during
the 31.07.95 and the 14.11.95.
The same amounts of precipitation reached the topsoil in both plots, heated and control.
However, soil moisture in the heated plots decreased, by approximately 20% the soil
moisture in the control plots. Corresponding to the higher temperatures in lower
horizons soil moisture in the soil profile of the heated plots decreased as against in the
control plots. Even at 0.3 to 0.5 m depth we measured a lower soil moisture. For
example the soil moisture of the heated plot of the fallow was 84% and 80% on the
31.07.95 and the 14.11.95, respectively, of that of the control plot in this depth (Fig. 3).
We did not measure the evaporation nor the transpiration and we could not distinguish
between the direct effect of the higher temperature and the indirect effect due to a higher
water uptake of faster growing plants in the heated plot (data not shown). Nevertheless,
elevated soil temperatures of only 3°C obviously have a profound effect on soil water
balance. Throughout the year plants covered the fallow soil very closely. In contrast
wheat plants do not grow closely packed and evaporation might be higher than from the
fallow. In consequence soil water content in mid summer months 1995 was less in the
wheat field (control plot 13.6 %DS, heated plot 6.8 %DS) than in the fallow field (control
plot 19.4 %DS, heated plot 14.3 %DS). However, we found a clear negative relation
between soil temperature and soil moisture, and this would have a negative effect on
microbial activity.
4
12
12
2
-1
N O-N release (kg ha )
fallow
10
10
8
8
6
4
6
**
*
**
4
**
2
2
0
0
Sum 94
W in 94/95
Sum 95
field
W in 95/96
*
Sum 94
W in 94/95
*
Sum 95
W in 95/96
Fig.4 Cumulated release of nitrous oxide from the control plots (n) and heated plots (o) of the fallow and
the wheat field over the single seasons (summer from May to October and winter from November to April).
During the first summer while white mustard was growing as a catch crop no significant
differences in nitrous oxide releases between the two plots of the field occured. Nitrous
oxide releases from the fallow (0.73 kg N2O-N ha-1) were higher as from the field (0.62
kg N2O-N ha-1). The emissions of nitrous oxide between fallow and wheat field are
difficult to compare for the first summer period because the fallow was laid out only a
few weeks before the start of the experiment and the former field was well fertilized.
Nevertheless, in the following year the fallow was not fertilized while in the wheat field
100 kg N ha-1 was applied during the vegetation period. As a consequence in the control
plots the total summer releases of the wheat field (1.54 kg N ha-1) were three times
higher compared to the fallow (0.47 kg N ha-1) (Fig. 4). Surprisingly, the releases from
the heated plots increased in the fallow (1.40 kg N ha-1) and decreased in the wheat field
(0.66 kg N ha-1) under elevated soil temperatures. Significant higher emissions from the
heated fallow plot as against the control plot was even observed in the same range during
the first summer 1994.
It seems that the emissions from the fallow were considerably influenced by temperature
whereas they were impeded by the lower soil moisture in the heated field plot. And in
fact, cumulated summer releases from the wheat field could be explained by 94% with
increasing soil moisture and by 78% with decreasing soil temperature. In contrast 53% of
the nitrous oxide emissions from the fallow could be explained with increasing soil
temperatures whereas we found only a slight relation with soil moisture. Due to
decreasing soil moisture with increasing soil temperature two contrary factors were
controlling nitrous oxide releases from the investigated plots. Therefore, the direct
effects of temperature might be of importance in ecosystems where a closed vegetation
covers the soil as in the fallow. In a more open stand as it is in cropped fields soil
moisture might become more important for regulation of the emissions of nitrous oxide.
However, in both control plots 71% (fallow) and 60% (wheat field) of the total nitrous
oxide releases during the investigation period were measured in winter 1995/96. In
winter 1995/96 soil temperature in the heated fallow plot was mostly above 0°C and in
the control fallow plot several freezing thawing cycles with high short term peaks were
observed. In contrast, in the wheat field several high emissions of nitrous oxide occurred
in connection with freezing and thawing in both, control and heated plot. As mentioned
above soil of the heated wheat field was exposed to the cold wind due to a missing snow
cover. As a result the soil in the upper layer was frozen several times as well. The only
difference between the two plots of the wheat field was that the time when the freezing
thawing peaks of nitrous oxide appeared was quite unlike.
5
Conclusions
The hypothetically expected higher emissions of nitrous oxide from a warmer soil were
also affected by the lower soil moisture during the vegetation period in the heated plot of
the wheat field as against they occurred in the fallow.
The winter months will be the most sensitive season in relation to nitrous oxide emissions
when climate will change. The results indicate that if soil temperature is lower than the
freezing point or if temperatures are higher as 0°C most the time in the winter months
only little nitrous oxide releases from agriculture will occur. However, if warmer
conditions will become likely the mean winter temperatures in southern Germany will
rise to approximately 0°C and freezing-thawing events will change. If as predicted for
Central Europe, not only temperature will rise but also summer rainfall will increase,
which might compensate the higher evapotranspiration caused by a warmer soil surface
the overall effect must result in dramatically higher nitrous oxide losses from arable soils
which will sustain an advanced global warming.
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Keywords : climate change, nitrous oxide, soil warming, agriculture
Mots clés : changement climatique, oxyde d'azote, réchauffement du sol, agriculture
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