Hansen S1., Bleken M. A2., Sitaula B.K3.
1
)Bioforsk, Norwegian Institute for Agricultural and Environmental Research, Organic Food
and Farming Division, N-6630 Tingvoll, tlf. +47 404 81 549, fax. +47 71534405,
e-mail: Sissel.hansen@bioforsk.no, corresponding author.
2)
Norwegian University of Life Sciences, Dept. of Plant & Environmental Sciences, N-1432
Ås, tlf. +47 64965612, e-mail: marina.bleken@umb.no
3)
Norwegian University of Life Sciences, Dept. of International Environment and Development
Studies, N-1432 Ås, tlf. +47 64965325, e-mail: bishal.sitaula@umb.no
Effect of soil compaction and fertilization practise on N2O emission
and CH4 oxidation
Summary
The effects of fertilization and tractor traffic on N2O emission and CH4 uptake in an agricultural soil
were studied in a long-term field trial with different fertilization and soil compaction. The soil was a
well-drained sandy loam and the crop rotation was rich in leys and legumes adapted to organic
farming practise. The fertilization treatments reported here are: Conventional fertilization practise;
compound fertilizer with NH4NO3 (NPK), cattle slurry high, cattle slurry level adjusted to organic
fertilization practise (CSO), and an unfertilized treatment. The soil was experimentally compacted by
two passes with a tractor, wheel by wheel, shortly before fertilization. Gas fluxes at the soil surface
were measured by the soil cover method.
Conventional fertilization practise (NPK) resulted in 2.1 to 3.4 times higher N2O emissions than with
CSO in uncompacted and compacted soil, respectively, in year 7, and 1.0 and 3.0 times higher in year
9. The accumulated CH4 uptake was reduced by 52 % by soil compaction, 50 % on average by
fertilization, and 78 % by soil compaction and fertilization combined. Fertilization with NH 4NO3 or cattle
slurry had similar effects.
Key words: GWP, ammonium nitrate, cattle slurry,
Introduction
The ability of organic agriculture to limit the contribution of agriculture to the greenhouse effect
depends on a successful circulation and utilisation of nutrients. Management activities such as manure
application, fertilization, crop rotation, green manure, foddering strategies, tillage and soil compaction
by tractor traffic all affect the circulation of nutrients and thus the greenhouse gas emissions from
agriculture. This paper presents some results concerning N2O emission and CH4 oxidation in a dairy
farming system in Norway, as affected by soil compaction and organic or conventional fertilization
practise on a sandy loam soil. The treatments were selected as virtual models of fertilization in a range
of dairy farms within conventional and organic practise. Accumulated fluxes are reported from the
fertilization treatments: compound fertilizer with NH4NO3, cattle slurry high, cattle slurry level adjusted
to organic fertilization practise, and an unfertilized treatment
Material and methods
Gas fluxes were measured during the 7th, 8th, 9th and 10th years of a long-lasting field experiment with
different fertilization and soil compaction treatments. Details are described by Hansen et al. (1993)
and Sitaula et al. (2000). The field experiment was located in Surnadal, Norway 25 m a.s.l., 63º 00' N,
8º 88' E, in a moist and chilly climate. Normal precipitation during the growing season May to
September is 64, 86, 117, 120 and 173 mm per month, and the average temperatures in the
corresponding months are 9, 12, 13.5, 13.2, and 9.4oC. The soil was a well-drained sandy loam soil;
the topsoil contained 2.2 % organic carbon and 0.17% organic nitrogen. The crop rotation was
adapted to organic dairy farming and had a high frequency of legumes in grassland and green fodder.
Crops from year 1 to 10 were: 1, green fodder; 2, barley; 3, grassland; 4, grassland; 5, grassland; 6,
oats; 7, green fodder; 8, barley; 9, grassland; 10, grassland.
The experiment had a split-plot factorial design with two replicates, soil compaction on main plots and
fertilization on small plots (2.8 m x 8 m). In the compacted treatment the soil was experimentally
compacted by two passes with a four tonne tractor, wheel by wheel, shortly before fertilization. The
rear wheels were double-settings with a total tyre width of 140 cm (inflation pressure of 57 kPa). In
front there were low pressure tyres with a total width of 100 cm. Average bulk densities (7-11 cm
depth) were 1.21 g cm-3 in uncompacted soil and 1.30 g cm-3 in compacted soil. In years with
grassland there were two harvests; the field was fertilized in spring and shortly after the first harvest.
The fertilization treatments reported here are NPK, CSH, CSO as given in Table 1, and an unfertilized
treatment. CS was diluted with water up to 200% of the original volume.
Table 1. Amount of mineral nitrogen (NH4-N + NO3-N) and total nitrogen (Total N) applied (kg N ha-1
yr-1). NPK = compound fertilizer with NH4NO3, CSH = cattle slurry high, CSO = cattle slurry level
adjusted to organic fertilization practise.
Fertilization
Year
NPK
7
8
9
CSH
10
7
8
9
CSO
10
7
8
9
10
NH4-N+NO3-N
140 83 70+50 123+88
120 26 51+30
74+52
50 15 30+20 17+18
Total N
140 83 70+50 123+88
190 62 80+48 123+88
80 34 48+32 45+30
Gas fluxes at the soil surface were measured by the soil cover chambers (thin-walled tin cans, 22.5
cm inner diameter, 23 cm high). For each treatment, there were four parallel flux measurements taken
on each day of measurement. The flux was estimated by the increased concentration 3 hours after
placement of the soil cover chambers. Gas fluxes were mainly measured in the first half of the growth
season in the years 7, 8, 9 and 10 (Hansen et al. 1993, Sitaula et al. 2000).
The area under the flux curves (straight lines between data points) were used to estimate the
accumulated N2O emission and CH4 uptake shortly after fertilization and afterward in early summer; in
year 7, 4 June to 8 July, in year 9, 8 May to 23 June and in late summer year 9; 22 July to 7
September.
In addition to these measurements, N2O-emissions from plots fertilized with a combination of cattle
slurry and NPK-fertilizer (CSNPK) were measured in some of the dates in year 9. (1, 2, 4, 7 June, 22,
26, 29 July, 2 August and 7 September). On the CSNPK treatment (73 + 51) kg total-N ha-1 and (37 +
27) kg mineral-N ha-1 was applied this year.
The main effects and interactions of soil compaction, fertilization and date were tested with analyses
of variance (ANOVA). The interaction replicate * compaction was used as an error term to test the
effect of compaction. N2O flux was log-normally distributed, and the data were log-transformed to
natural logarithms before the statistical analyses were run. The CH4 flux was so close to normal
distribution that they were treated statistically as if that were the case. CH4 uptake rates were
calculated in accordance with first-order kinetics.
The average percentage fertilizer-derived N2O emissions was calculated as the N2O emission in
fertilized treatments minus N2O emission in unfertilized treatment, divided by the amount of mineral N
(total-N) applied with spring fertilization.
When the effect of N2O emissions on global warming potential was compared with CH4 oxidation, N2O
was calculated as 310 CO2 equivalents and CH4 as 23 CO2 equivalents.
Results
Conventional fertilization practise (NPK) resulted in 2.1 to 3.4 times higher N2O emissions than with
organic fertilization practise (CSO) in uncompacted and compacted soil, respectively, in year 7, and
1.0 and 3.0 times higher in year 9 (Table 2). The share of applied nitrogen that was lost as N 2Oemissions was larger in year 7 than in year 9 (Table 3). This was most evident in uncompacted soil.
The N20 emission rate from CSNPK treatment (combination of cattle slurry and NPK-fertilizer) in year
9 was 144 % of CSO and 48 % of the NPK treatment in compacted soil and 128 and 55% in
uncompacted soil.
Table 2. Accumulated CH4 uptake and N2O emissions. Early summer: from 4 June to 8 July in year 7
and from 8 May to 23 June in year 9. Late summer: from 22 July to 7 September. Treatment averages
± standard error. Fertilization abbreviations as in Table 1.
CH4 (g ha-1)
Year
Time period
7
Early
summer
N2O (kg N ha-1)
9
Early
summer
7
Early
summer
9
Early
summer
9
Late
summer
Uncompacted
NPK
CSH
CSO
Unfertilized
62
56
64
97
±
±
±
±
6
4
6
6
42
54
43
68
±
±
±
±
14
6
11
8
5.3
3.6
2.5
0.6
±
±
±
±
1.6
0.4
0.4
0.1
0.9
0.4
0.9
0.3
±
±
±
±
0.09
0.01
0.68
0.08
1.0
0.8
0.4
0.1
±
±
±
±
0.28
0.66
0.30
0.05
30
11
24
68
±
±
±
±
4
6
7
5
29
26
31
47
±
±
±
±
1
0
7
11
7.4
2.7
2.2
0.6
±
±
±
±
1.1
0.4
0.4
0.1
3.4
1.0
1.1
0.7
±
±
±
±
1.7
0.6
0.5
0.1
2.1
0.3
0.5
0.1
±
±
±
±
0.85
0.01
0.20
0.03
Compacted
NPK
CSH
CSO
Unfertilized
Table 3. Accumulated N2O emissions during early summer (periods as in Table 2) in percentage of
mineral N and of total N applied in spring. Fertilization abbreviations as in Table 1.
Year
7
9
tot-N min-N
tot-N
min-N
Uncompacted
NPK
CSH
CSO
3.36
1.56
2.38
3.36
2.47
3.80
0.84
0.07
1.18
0.84
0.12
1.88
Compacted
NPK
CSH
CSO
4.81
1.08
1.94
4.81
1.72
3.10
4.38
0.77
1.67
4.38
1.21
2.66
The accumulated CH4 uptake by soil for the period 4 June to 8 July in year 7 ranged from 10 to 100 g
CH4 ha-1 (Table 2). Both fertilization (p < 0.01) and soil compaction (p = 0.06) reduced CH 4 uptake. In
year 7 the CH4 uptake rate was reduced by 52% by soil compaction, 50% by fertilization and 78% by
the combination of soil compaction and fertilization. The same pattern as was observed in year 7 was
confirmed in years 8, 9, and 10 (Table 4). There was no significant difference in CH4 uptake between
fertilization with NH4NO3 or with cattle slurry. Neither did the amount of cattle slurry affect CH 4 uptake
significantly. There was no interaction between fertilizers and soil compaction with respect to CH 4
uptake.
Table 4. . CH4 uptake rates as influenced by fertilization and soil compaction for years 8 to10
(treatment averages  standard error). Fertilization abbreviations as in Table 1 (Sitaula et al. 2000a).
CH4 flux (g CH4 m-2 h-1)*)
Fertilization*)
Year 8
NPK, 83 kg N ha-1
CSH, 62 kg N ha-1
CSO, 34 kg N ha-1
Unfertilized
Uncompacted
Compacted
5.5  1bA
4.1  0.9bA
5.1 ± 0.9bA
9.3 ± 1.2aA
2.1 ± 0.7bB
3.1 ± 0.7bA
2.7 ± 0.8bB
6.1 ± 0.8aB
Year 9
NPK, 120 kg N ha-1
CSH,128 kg N ha-1
CSO, 82 kg N ha-1
Unfertilized
4.2 ± 0.4bA
4.9 ± 0.5abA
4.5 ± 0.4abA
6.1 ± 0.5aA
2.7 ± 0.3bB
3.0 ± 0.4bB
2.2 ± 0.4bB
4.1 ± 0.8aB
Year 10
NPK,209 kg N ha-1
CSH, 211 kg N ha-1
CSO, 75 kg N ha-1
Unfertilized
6.2 ± 0.8abA
3.6 ± 1.4bA
8.5 ± 1.1aA
9.0 ± 0.9aA
2.2 ± 0.6abB
0.9 ± 0.8bA
1.3 ± 0.9bB
4.2 ± 1.0aB
*)
Within each compaction treatment (columns), the fertilization treatments are compared with lower
case letters (abc) and within each fertilization treatment (rows), the compaction treatments are
compared with upper case letters (AB). Values followed by the same letter in the same column in
lower case or rows in upper case are not significantly different (Newman-Keuls test,  = 0.05).
Discussion
Fertilization with NH4NO3 on compacted soils with a high moisture content rapidly led to increased N2O
emissions, whilst this was not observed in the organic treatment fertilized with cattle slurry. Because
NO3-N is the main source for denitrification, this is not surprising. The rather high N2O-N emissions (45 % of fertilizer-N) during one to one-and-a-half month after fertilization can be explained by the high
water content. Especially in year 7 the soil had a high moisture content with up to 81 and 73 % waterfilled pore space in compacted and uncompacted soil, respectively (Hansen et al. 1993).
The content of NO3 in the cattle slurry was low, and we did not observe such a rapid increase in N2O
emissions. Because of likely denitrification during and after nitrification, higher N2O emissions from
cattle slurry treatments late in the season could be expected. This was not confirmed (Table 2).
A combination of cattle slurry and NO3-fertilization is assumed to create a larger risk of N2O emissions
because readily decomposable carbon in the cattle slurry is an energy source for denitrification of NO3
as suggested by Stevens and Laughin (2001). However, we did not observe this. On the contrary, only
half the N2O emissions were observed in the combined fertilization treatment (CSNPK) compared with
NPK-treatment. This can probably be explained by the fact that readily decomposable C was not a
limiting factor in the present field situation, with several fertility-building crops in the rotation.
Biological nitrogen fixation is an important source of nitrogen in organic farming systems. In this field
experiment the yearly biological nitrogen fixation was estimated to be 70 and 50 kg ha-1 in CSO in the
uncompacted and compacted treatment, respectively, and correspondingly 40 and 20 kg in the NPKtreatment (Hansen 1993). However, because of the uncertainties in these estimates and uncertainties
in how much of the nitrogen from biological nitrogen fixation that emitted as N 2O, this N-source has not
been adjusted for in the calculation of N2O emission from fertilized N. Thus the percentage of N2O-N
from fertilizer/manure-N is likely overestimated in the treatments with the highest biological nitrogen
fixation. This will not change the emissions per ha, however. Even if the NPK fertilization is rather low
compared with today’s practise in conventional dairy farming, the emission per ha was higher with
conventional fertilization than with fertilization adapted to organic dairy farming. This is in contrast to
the findings on a field experiment in Finland (Syväsalo et.al. 2006), but in accordance with the study of
two farming systems in Germany (Flessa et al. 2001).
To improve the comparison between the conventional and organic farming systems, better estimates
on the effect of biological nitrogen fixation are needed as well as calculations of N2O emissions per
fodder unit produced.
There were negative N-balances (N applied in fertilizer and/or manure minus N in harvested crops)
with CSO fertilization when biological nitrogen fixation was not taken into account in the calculations
(Hansen 1993). Both soil compaction and increased application levels decreased the N-efficiency and
resulted in a surplus of N. This is in accordance with the findings that increased manure loading
decreases the N efficiency of the farm (Bleken et al. 2005) and augments the risk for N2O emissions
(Olesen et al. 2006).
As shown in Tables 3, the N2O emissions as percentage of N applied with cattle slurry were higher
with CSO than CSH treatments. In CSH treatment the soil had little infiltration capacity and a large
part of the surplus N in the CSH treatments was probably volatilized as ammonia. Following
redeposition, this ammonia is likely to contribute to further N2O emission elsewhere.
Both ammonia application (as mineral fertilizer or cattle slurry) and soil compaction decreased the
activity of the CH4 oxidisers, and thus the soil’s ability to serve as a sink of CH4 (Tables 2 and 4).
Several papers provide a possible explanation for decreased CH4 uptake caused by input of inorganic
N. NH3 and CH4 compete for the same active site of the monooxygenases, the enzymes catalysing the
first oxidation step of CH4 and NH4+ in CH4-oxidising and ammonium-oxidising bacteria (See Sitaula et
al 2000a). It should be noted, however, that the N input may have decreased the CH4 oxidation rate,
probably at a very early stage of the field experiment, and this fertilizer effect is not reversed when
fertilizer application is stopped. Furthermore, adding more fertilizers to the same soil does not seem to
have the same inhibiting effect on the CH4 oxidation rate remaining after the first fertilizer additions,
indicating long-lasting effects of the initial N input (Sitaula et al. 2000). Hence the inhibition in CH4
oxidation due to NH4+ availability might be a plausible mechanism only in the early stage of N input.
The reduction in CH4 oxidation rates persisted beyond the physical effect of gas diffusion caused by
soil compaction. Thus, both soil compaction and fertilization seems to have a long lasting inhibitory
effects on soil CH4 oxidising potentials.
N2O emissions affected the global warming potential (GWP) more than CH4 oxidation did. On average
in year 7 CSO, uncompacted and compacted treatment, 720 times more CO2 equivalents were
emitted as N2O, than absorbed in soil through CH4 oxidation. The corresponding value for NPK
treatment was 1860 times.
Factors that reduce N2O emissions from organic farming systems, such as low manure load, avoiding
hot spots of N, crop rotation and soil management that facilitates good soil structure may also improve
the conditions for CH4 oxidation.
Conclusion
Conventional fertilization practise using compound fertilizer with NH4NO3 (NPK) resulted in one to two
times higher N2O emissions per ha than cattle slurry level adjusted to organic fertilization practise
(CSO) in uncompacted soil. Soil compaction through tractor traffic in this moist soil increased the N2O
emission from NPK treatment more than from CSO treatment, and the N2O emission was three times
higher from NPK than CSO treatments in compacted soil. The effect of methane oxidation on reduced
GWP was negligible compared with increased GWP caused by N2O emission.
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