E�ects of soil compaction on N2O emission in agricultural soil

B.K. Sitaula a,*, S. Hansen b, J.I.B. Sitaula c, L.R. Bakken a

a Department of Soil and Water Sciences, Agricultural University of Norway, Box 5028, N-1432, �As, Norwayb Norwegian Centre for Ecological Agriculture, N-6630, Tingvoll, Norway

c Department of Biotechnological Sciences, Agricultural University of Norway, Box 5040, N-1432, �As, Norway

Received 2 March 2000; accepted 23 May 2000

Importance of this paper: Agricultural soil is an important source of atmospheric N2O. Soil compaction created by

tractor tra�c is favourable for N2O production in soil due to its e�ect on soil aeration. The high input of mineral nitrogen (a

common practice of conventional agricultural) to the compacted soil would emit N2O emission more vigorously. In this

paper, we have presented the result of four year long ®eld research investigating the e�ect of soil compaction on N2Oemission in relation to factors of potential explanatory importance. Since agricultural modernisation (with increased

tractor tra�c) is taking place world-wide, such investigation will have important implications for the global N2O budget.

This paper may also provide a scienti®c basis for encouraging ecological agriculture that causes the least soil compaction.

Abstract

We have studied the e�ect of soil compaction on N2O ¯uxes in relation to gas di�usion and N fertilization in the

®eld, and N2O release rates in laboratory incubated soil samples. The fertilization and soil compaction ®eld experiment

was established in 1985, and the gas ¯uxes were measured in the period from 1992 to 1994. N2O emission was higher in

compacted than in uncompacted soil. This compaction e�ect was four times higher in the NPK-fertilized treatment

compared to the unfertilized one. Soil compaction decreased gas di�usivity and this may have contributed for increased

N2O emission. This increased N2O emission due to soil compaction in the ®eld became non-signi®cant after the

compacted soil was sieved (2-mm mesh) and N2O emission rates were measured in laboratory incubations. The sieving

presumably removed di�usion barriers and increased the oxygen supply compared with that under the soil compaction

in ®eld. This reversibility of ®eld compaction e�ects indicates that the soil compaction does not permanently increase

the biological potential for N2O production in the soil. Ó 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Compaction; Di�usivity; Nitrogen fertilizer; Nitrous oxide

1. Introduction

Soil compaction by tractor tra�c is a widespread

problem in modern agriculture (H�akansson et al., 1988;

Hansen, 1996). In Western Norway, large areas of soil

are easily compacted, partly because of the humid cli-

mate, and partly because of ®ne soil texture and high

organic material content (Hansen et al., 1993). The

possible e�ects of soil compaction are decrease in gas

di�usivity (Ball et al., 1999), modi®cation of soil mi-

crohabitats (occurrence of higher percentage of small

pores) and increased probability of anaerobic conditions

(Hansen et al., 1993). These conditions, created by soil

compaction, are favourable for N2O production from

both nitri®cation and denitri®cation (Rosswall et al.,

1989). Little work has been reported on the e�ect of soil

compaction on N2O emission at the process level. In-

creased denitri®cation, N2O emission and decreased soil

NOÿ3 , due to soil compaction, has been reported earlier

Chemosphere ± Global Change Science 2 (2000) 367±371

* Corresponding author. Tel.: +47-64-94-82-12; fax: +47-64-

94-82-11.

E-mail address: bishal.sitaula@ijvf.nlh.no (B.K. Sitaula).

1465-9972/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved.

PII: S 1 4 6 5 - 9 9 7 2 ( 0 0 ) 0 0 0 4 0 - 4

(Bakken et al., 1987; Maidl and Fischbeck, 1987; Han-

sen et al., 1993). However, to our knowledge, there are

very few works on e�ects of compaction on N2O emis-

sion (e.g., Ball et al., 1999), and no work is on its

residual e�ects once the physical e�ects of compaction

are eliminated.

It has previously been shown, many times, that the

N2O emission in soils is increased by nitrogen input, due

to increased supply of substrate for both nitri®cation

and denitri®cation (Eichner, 1990; Mosier et al., 1991;

Hansen et al., 1993; Sitaula et al., 1995). Therefore, both

compaction and N fertilization are favourable for N2O

production in soil. The combined e�ects of these two

factors (compaction and N fertilization) are of great

interest since these two factors are superimposed, si-

multaneously, on the agricultural soil. We hypothesised

that this combination would increase N2O production

rates higher than the production modulated by a single

factor. We have investigated this by measuring N2O

emission rates in compacted and uncompacted soil,

under-unfertilized and NPK-fertilized treatments, for

four years (1991±1994). We also measured the soil

compaction e�ects on gas di�usivities in the ®eld.

The repeated soil compaction for long period may

have other long lasting e�ects (modi®cation of biological

potentials) in addition to the physical e�ects. The re-

peated soil compaction might a�ect the biological po-

tentials, as has been observed for reduced methane

oxidation potentials in the same experimental site (Si-

taula et al., 2000). We investigated this other e�ect of

compaction for N2O emission as well, by measuring

N2O production rates after eliminating the physical ef-

fect created by the soil compaction. If the increased N2O

emission by soil compaction could be found to be a re-

sult of physical e�ects (restricted di�usion) only, this

anthropogenic increase of N2O emission (due to soil

compaction) might be decreased by minimising soil

compaction.

2. Materials and methods

2.1. Site description

N2O ¯uxes were measured in a ®eld experiment in

Surnadal, Norway for four years (1991±1994). More

detail on the ®eld site is given by Hansen et al. (1993). In

brief, the soil is a typic udorthents (USDA system of soil

classi®cation) developed on ¯uvial deposits. In 1991, the

crop grown was green fodder (Hansen et al., 1993). In

1992, the ®eld was ploughed and the crop was grown

(barley with ley under-sown). The ley consisted of tim-

othy (Phleum pratense) and clover (Trifolium pratense,

T. repens and T. hybridum) and remained throughout the

rest of the experimental period.

2.2. Experimental design

The experiment had a split-plot factorial design with

two replicates, soil compaction on main plots and fer-

tilization on small plots (2.8 m ´ 8 m, two sample areas

2 m ´ 1 m at each plot with 5.5 m between). For each ¯ux

measurement, soil cover chambers were placed ran-

domly within each sampling plot.

Soil compaction treatment comprised one pass of a

four ton tractor, wheel by wheel, each spring, and two

passes after each harvest. The rear wheels were double-

settings with a total tyre width of 140 cm (in¯ation

pressure of 57 kPa). In front, there were low-pressure

tyres with a total width of 100 cm.

The fertilization treatment consists of NPK (mineral-

fertilizer) and a UNF (unfertilized) treatment. The four

years of NPK fertilization was done with NPK mineral-

fertilizer containing 18% N as NH4NO3: The N appli-

cation rates were; in 1991:140 kg N haÿ1, in 1992:83 kg

N haÿ1, in 1993:120 kg N haÿ1 divided into two appli-

cations (70 + 50 kg N) and in 1994:211 kg N haÿ1 divided

in two applications (123 + 88 kg N).

2.3. Field measurements

In 1991, N20 ¯uxes were measured shortly after snow

melt (23, 24 and 26 April), before and after ploughing

(4 and 7 June) and 12 times after compaction and fer-

tilization treatments in June and July). In 1992, we

started measuring gas ¯uxes shortly after the snow melt

(18 May) and continued until 26 June. Ten measure-

ments were taken during this period. Similarly, nine

measurements were made in 1993. In 1994, four mea-

surements were made.

Gas ¯uxes were measured by soil cover chambers as

described by Hansen et al. (1993). For each combination

of fertilization and compaction treatments, there were

four parallel ¯ux measurements taken on each day of

measurement. All gas samples were analysed by gas

chromatography within seven days of sampling (Sitaula

et al., 1992).

In situ di�usivity was measured using the method of

Ball et al. (1994) but modi®ed to use freon-22 as dif-

fusing gas instead of krypton-85, as described in Ball

et al. (1997). Two measurements were taken only once,

in June 1993, in both compacted and uncompacted plots

(Fig. 2).

The volumetric moisture content of the top 20 cm of

the soil layer, was measured on each gas sampling date

using the TRIME-system digital moisture meter (IMKO

GmbH, Ettingen, Germany) connected to time-domain-

re¯ectrometry (TDR) probes.

In 1993 and 1994, the NH�4 and NOÿ3 content of soil

(0±20 cm) was determined in a composite soil sample for

each treatment collected with a soil auger. The soil

samples were taken on each date of gas measurement,

368 B.K. Sitaula et al. / Chemosphere ± Global Change Science 2 (2000) 367±371

close to the spots (5±10 cm away), where each gas ¯ux

measurement was done. The gravimetric soil moisture

content was determined to express NOÿ3 and NH�4content of the soil on a dry weight basis. The soil sam-

ples were analysed for NOÿ3 and NH�4 concentrations by

standard methods (20 g soil in 50 ml 2 M KCl extrac-

tions and ¯ow injection analysis by FIA-star 5010 ana-

lyser, Tecator, Sweden).

2.4. Laboratory incubation experiment

In the incubation experiments, compacted and un-

compacted soils (from both unfertilized and NPK-fer-

tilized plots) were compared. A composite sample of

approximately 1 kg was taken from compacted and

uncompacted treatments from 0±10 cm soil depth. The

compacted and uncompacted samples were mixed sep-

arately and sieved (4 mm). The soils had a moisture

content of 45% � 4% (v/v, mean � S.D.) in the ®eld and

were subjected to air drying down to 35% (v/v) (con-

trolled by frequent weighing during drying). N2O pro-

duction rates at 15°C were measured by incubating 20 g

soil sub-samples in 120 ml serum bottles capped with

butyl rubber stopper (type 20±B3P, Chromacol, Lon-

don). During the following incubation, N2O concen-

trations were measured every 12 h for three days.

2.5. Statistical analyses

The main e�ects and interactions of soil compac-

tion, fertilization and date were tested with analyses of

variance (ANOVA) and the Newman±Keul's test. The

interaction between replicate and compaction was used

as an error term to test the e�ect of compaction.

Studentised residuals were used to test the normality

of the distributions with residual plots and procedure

univariate (SAS Institute, 1988). Fluxes of N2O were

log-normally distributed, and the data were natural

log-transformed before the statistical analyses were

run.

3. Results and discussion

3.1. Gas di�usion

Soil compaction decreased gas di�usivity (Fig. 1). In

situ di�usivity of freon-22 at 5±10 cm soil depth was 1.9

mm2 sÿ1 in uncompacted and 1.5 mm2 sÿ1 in compacted

soil; and at 10±18 cm soil depth 1.2 and 0.6 mm2 sÿ1,

respectively. Similarly, the air permeability was de-

creased at 5±10 cm depth by compaction (data not

shown). This decrease in gas di�usivity due to compac-

tion, is likely to restrict the oxygen supply (this agrees

with an earlier study on the same site by Hansen and

Bakken, 1993). This is not surprising, since soil com-

paction reduces the total pore volume (Breland and

Hansen, 1996), and a signi®cant reduction in the per-

centage of air ®lled pore space due to soil compaction

was observed in the same experimental site (Hansen

et al., 1993). This disturbed ``physical'' condition, with

low oxygen availability is favourable for N2O produc-

tion (Rosswall et al., 1989).

3.2. N2O emission

Soil compaction resulted in increased N2O emission

and this compaction e�ect was more pronounced in

NPK fertilization treatments (Fig. 2). In unfertilized

plots, N2O emission rate was increased by 44% due to

compaction, whereas in NPK-fertilized treatment the

average N2O emission rate was increased by 170%

(P < 0.05). This means that the e�ect of compaction on

increased N2O emission, was about four times greater in

NPK-fertilized treatment. Since soil compaction reduces

the total pore volume (Breland and Hansen, 1996), a

higher occurrence of anaerobic sites can be expected in a

compacted soil, especially with a high soil moisture

status. A high moisture content (>45% V/V) prevailed

during most of our measurement dates. These conditions

would favour a greater loss of nitrate through denitri-

®cation, more obviously in compacted soil. The average

soil NOÿ3 (mg N kgÿ1 dry soil) in NPK treatment were

22 in compacted and 24 in uncompacted soil. In UNF

treatment, average soil NOÿ3 content (mg N kgÿ1 dry

soil) were 1.5 in compacted and 1.7 in uncompacted

Fig. 1. In situ gas di�usivities (average values for each treat-

ment � S.D., n� 2) as in¯uenced by soil compaction.

B.K. Sitaula et al. / Chemosphere ± Global Change Science 2 (2000) 367±371 369

treatment. This indicated lower soil nitrate content in

compacted treatment, although di�erence was less pro-

nounced. Increased denitri®cation and decreased soil

NOÿ3 , due to soil compaction, has been reported earlier

(Bakken et al., 1987; Maidl and Fischbeck, 1987).

The rate of N2O production is regulated by the

supply of substrates (NH�4 ;NOÿ3 ) to the N2O generating

process (nitri®cation and denitri®cation). The average

soil ammonium contents (NH�4 ±N kgÿ1 dry soil) for

NPK and UNF treatments for compacted soil and un-

compacted soil were 22 and 4, 23 and 3.5, receptively.

NPK fertilization signi®cantly increased the soil NH�4content and N2O emission, as was expected due to the N

input in mineral form (NH4NO3). Despite similar NH�4content in both compacted and uncompacted treat-

ments, N2O emissions rates were higher in the com-

pacted soils. The high nitrogen input (through NPK

fertilization) to a favourable physical condition for N2O

production created by soil compaction (retarded gas

di�usion as discussed above) may have resulted in four-

times increase in N2O production rates in the combi-

nation of NPK fertilized and compacted treatment

compared to the rates in compacted and unfertilized

treatment combination. This indicates that soil com-

paction is an important factor that modulates the N

e�ects, resulting in signi®cantly higher N2O production.

Since soil compaction caused by tractor tra�c is a

widespread problem in modern agriculture, the modu-

lating e�ects of compaction on N-fertilized soil should

be taken in to account when estimating N2O emission

from agricultural soil.

3.3. Incubation experiment

The result of incubation experiment showed no sig-

ni®cant di�erence in N2O production rates between

compacted and uncompacted soils, when the physical

e�ect of ®eld compaction was minimized by sieving

(Fig. 3). The unfertilized N2O production (calculated

from Fig. 3), in the previously compacted soil

(0.15 � 0.07S:E: ng N2O±N dÿ1 gÿ1 soil dry weight,) was

not statistically di�erent from that in the uncompacted

soil (0.20 � 0.08S:E: ng N2O±N dÿ1 gÿ1 soil dry weight).

Similarly, the NPK-fertilized N2O emission rate, in the

previously compacted soil (0.25 � 0.06S:E: ng N2O±N dÿ1

Fig. 3. Time dependent increase of N2O in the headspace of

serum bottles (mean values for each treatment � S.D., n� 4)

containing sieved sub-samples.

Fig. 2. N2O emission rates (mean values for each treatment � S.E., n� 4±16 measurements) in Surnadal ®eld experiments as in¯uenced

by soil compaction.

370 B.K. Sitaula et al. / Chemosphere ± Global Change Science 2 (2000) 367±371

gÿ1 soil dry weight), was not statistically di�erent from

that in the uncompacted soil (0.30 � 0.07S:E: ng N2O±N

dÿ1 gÿ1 soil dry weight). This may indicate that the soil

compaction e�ects seen in the ®eld ¯ux data (Fig. 1), are

due to restriction of di�usion. The sieving of compacted

soil samples in the laboratory removed di�usion re-

striction arising from the compaction. Thus, increased

N2O emission, as a result of soil compaction, may be

minimised by searching and adopting suitable farm

management practices that minimize soil compaction.

Acknowledgements

The work was funded by the research council of

Norway (NFR) within the frame work of the commis-

sion of the European communities project (No. EV5V-

CT91-0052).

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Bishal K. Sitaula is a senior researcher at the Department ofSoil and Water Sciences, the Agricultural University of Norway(AUN). His research covers several aspects of soil science andsoil microbiology, especially greenhouse gas ¯uxes in soil, re-search methods, and interdisciplinary research on soil degra-dation.

Sissel Hansen is a senior researcher at the Norwegian Center ofEcological Agriculture, Tingvoll. Her research is focussed uponnutrient cycling in ecological farming systems.

JIB Sitaula's research covers the microbiological aspects ofgreenhouse gas ¯uxes in soil.

Lars R. Bakken is professor at the Department of Soil andWater Sciences, AUN. His work covers several aspects of soilmicrology, trace gas ¯uxes, microbial nitrogen transformations,and modelling these processes.

B.K. Sitaula et al. / Chemosphere ± Global Change Science 2 (2000) 367±371 371