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Effect of Mechanized Tillage Operations on Soil Physical Properties and Greenhouse Gases Fluxes in Two Agricultural Fields



Nsalambi V. Nkongolo, Kanta Kuramochi and Hatano Ryusuke
 
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ABSTRACT

Soil management practices may affect greenhouse gases emissions and exacerbate global warming. We studied the short-term effect of mechanized tillage operations on soil properties and CO2, CH4, NO and N2O fluxes in a corn and soybean fields. The study was conducted from June to December 2001 at Hokkaido University in Sapporo (Japan). The soil of the experimental site is classified as Eutric Fluvisols (FAO). Two plots of 20 m long by 30 m width each were isolated in fields planted to corn (Zea mays) and soybean (Glucine max). Plot interrows were compacted by 1, 2, 3 and 4 cycles a tractor. Soil and air samples were collected for measuring CO2, CH4, NO and N2O fluxes and other soil properties. Results showed that soil volumetric water content (θv), bulk density (ρb), the pore tortuosity factor (τ) and Soil Penetration Resistance (SPR) increased while air-filled porosity (ƒa), Total Pore Space (TPS) and the soil gas diffusion coefficient (Ds/Do) decreased linearly with increasing tractor cycle in both corn (p< 0.0001) and soybean (p< 0.01) fields. In corn field, CO2 (p< 0.0011), NO (p< 0.0257) and N2O (p< 0.0116) fluxes increased quadratically with increasing tractor cycle. In soybean field, CO2 and CH4 fluxes increased while N2O and NO fluxes decreased linearly with increasing tractor cycle. CO2 (r = 0.45, p< 0.003) and N2O (r = 0.45, p< 0.003) fluxes were significantly correlated with soil penetration resistance in corn and soybean field, respectively. Increasing tractor cycle deteriorated soil physical properties and increased greenhouse gas fluxes. More studies are needed to determine if these effects are permanent or only temporary on both soil and gas fluxes.

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Nsalambi V. Nkongolo, Kanta Kuramochi and Hatano Ryusuke, 2008. Effect of Mechanized Tillage Operations on Soil Physical Properties and Greenhouse Gases Fluxes in Two Agricultural Fields. Research Journal of Environmental Sciences, 2: 68-80.

DOI: 10.3923/rjes.2008.68.80

URL: https://scialert.net/abstract/?doi=rjes.2008.68.80

INTRODUCTION

Global atmospheric concentrations of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years. The increase in carbon dioxide concentration is due primarily to fossil fuel use and land-use change, while methane and nitrous oxide increases are primarily caused by agriculture (IPCC, 2007). Agricultural practices such as tillage have been shown to change emissions of N2O and the consumption of patterns of CH4 in agricultural soils (Teepe et al., 2004). Tractor traffic during tillage operations is one of the practices that influence the exchange of CO2, CH4, NO and N2O between the soil and the atmosphere as during such traffic, depending on the moisture level, soil compaction increases (Meek, 1994; Rollerson, 1990). In fact, compaction packs the primary soil particles (sand, silt, clay) and soil aggregates closer together and dramatically alter the balance between solids, air-filled and water-filled pore space (Allbrook, 1986; Bruand and Cousin, 1995). By increasing the portion of water-filled pores, compaction makes the soil prone to denitrification and therefore increases N2O losses (Ball et al., 2000; Douglas and Crawford 1993). There are numerous studies on the effects of soil compaction on soil properties (Greene and Stuart, 1985; Rollerson, 1990; Meek, 1994). However, less work has been reported on the effect of tractor compaction on gases fluxes. Among these few studies, Flessa et al. (2002) quantified N2O and CH4 fluxes for ridges, uncompacted interrows and tractor-compacted interrows from potato (Solanum tuberosum) fields. They found that N2O emissions were highest for the tractor compacted soil. However, the major fraction of the total CH4 uptake (+86%) occurred on the ridges. Ruser et al. (1998) observed that the gaseous fluxes of N2O and CH4 fluxes from potato field were strongly affected by ridge-till practices; this produced areas with increased (ridges) and strongly reduced (tractor-compacted interrows) soil porosity. Hansen et al. (1993) compared tractor-compacted and uncompacted soils. They found that N2O emissions (approximately 35%) increased due to soil compaction. Tractor “trips” during farming operations affect soil properties which lead to greenhouse emissions. Unfortunately the magnitude of these emissions is not still well quantified as many of these studies are conducted either at the beginning, middle or end of the growing season. However, in order to accurately predict the total emissions from agricultural systems, contribution at each stage of farming operations should be known. The objective of this study was therefore to assess the short-term (at early stage of field operations) effect of tractor induced compaction on soil properties and gases fluxes in a corn and a soybean fields of northern Hokkaido.

MATERIALS AND METHODS

Experimental Site
This study was conducted at Hokkaido University Experimental Farm in Sapporo, Hokkaido, Japan (43° 11’ N, 141° 30’ E), from early June to late December 2001. Sapporo, Japan's third largest city enjoys a mild climate with a year-round average temperature of 9.1°C. The average temperature was -3.5°C in January and 20.3°C in July 2001. The soil of the experimental site is classified as Typic Fluvaquents (Soil Taxonomy), Eutric Fluvisols (FAO). The physical and chemical properties of different horizons were reported by Hayashi and Hatano (1999). Soil texture consists of 25.4% sand, 47.0% silt and 27.6% clay. The saturated hydraulic conductivity is 2.99 x 10-5cm s-1. The carbon and nitrogen contents were 2.1 and 0.16%, respectively. Field preparation began in April and in May, two plots of 30 m long by 20 m width were isolated in fields cropped to corn (Zea mays) and soybean (Glucine max). These fields were established maintained by the Crop Production Laboratory, Faculty of Agriculture, Hokkaido University. The corn field was fertilized with N, 130; P2O5, 180; K2O, 100 and MgO, 40 kg ha-1 while soybean received N, 32; P2O5, 100; K2O, 80 and MgO, 24 kg ha-1. In June 2001, plots interrows in both soybean and corn fields were compacted by 1, 2, 3 and 4 cycles (1 cycle = 2 passes) with a 2.4 tons Fordson Major tractor (as during regular tillage operations) (Fig. 1). The ridges of crop rows were not compacted. Immediately after tractor compaction, Soil Penetration Resistance (SPR) was measured to a depth of 100 cm and soil samples were taken in both interrows and ridges. A second measurement of SPR, sampling for soil properties and greenhouse gas fluxes was conducted three weeks later in August 2001.

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Fig. 1: Experimental site, showing gas sampling chamber, compacted-non compacted interrows and ridges

Measurement of Soil Chemical Properties
Soil samples were taken at each sampling locations immediately after measurements of greenhouse gases emissions, for analyses of chemical properties. Soil samples were collected at 5 cm depth from the soil surface with a 5.1 cm height and 5 cm diameter aluminum cylinder. The properties studied were soil pH (H2O and KCl), electrical conductivity (EC), nitrite (NO2¯), nitrate (NO3¯) and ammonium (NH4+). For analyses of NO2¯ and NO3¯, 10 g of field moist soil sample was extracted by 50 mL of deionized water (1:5 = soil: water) and concentrations of the above anions were determined by ion exchange chromatography. This extract was also used to measure pH (H2O) and EC. For NH4+ determination, 7 g of field moist sample was extracted using 70 mL of 2 M KCl. pH (KCl) was measured using this extract and soil NH4+ was determined by colorimetry with indophenol-blue.

Measurements of Soil Physical Properties
For soil physical properties, soil cores (3 replicates for each of the 5 tractor cycles) were taken in each of corn and soybean fields at 5 cm depth from the soil surface with a 5 cm diameter and a 5.1 cm height cylinder (volume = 100 cm3). Cores fresh weights were first measured then their bottom covered with a filter paper. The filter paper was strongly held with rubbed elastic. Cores without their top covers were thereafter transferred onto a tension table. The top of the tension table was covered with a plastic paper to prevent evaporation. Cores were saturated for comparison purpose between calculated Total Pore Space (TPS) to that determined as core volumetric water content at saturation. However, in this report only TPS values calculated were used. After 72 h of saturation, cores fresh weights were again measured. They were then transferred into an oven to be dried at 105°C for 72 h. Soil bulk density (ρb), Total Pore Space (TPS), volumetric water content (θv), air-filled porosity a), relative gas diffusion coefficient (Ds/Do) and the pore tortuosity factor (τ) were later calculated as follows:

Bulk Density (ρb)

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(1)

where, ρb (kg m-3) is the soil bulk density, Ms (kg) is the mass of dry solids determined after drying the soil sample to constant weight at 105°C and Vt (m3) is the total volume of soil and thus Vt is the volume of cylinder.

Image for - Effect of Mechanized Tillage Operations on Soil Physical Properties and Greenhouse Gases Fluxes in Two Agricultural Fields
(2)

where:

Vs (m3) : Volume of soil solids
Vw (m3) : Volume of water
Va (m3) :

Volume of the air fractions successively.

Total Pore Space (TPS)

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(3)

where, TPS (m3 m-3) is the total pore space or the total space of soil filled with fluid (air + water).

Gravimetric Water Content (θg)

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(4)

where:

θg (kg soil water kg-1 soil) : Gravimetric water content or mass of water present in each unit mass of the dry soil,
Mt (kg) :

Weight of the moist soil sample as taken from the field.

Volumetric Water Content (θv)

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(5)

where:

θv (m3 soil water m-3 soil) : Volumetric water content or the volume of water present in a unit volume of the sample.
ρw :

Density of water taken as equals to 1000 kg m-3.

Air-Filled Porosity (ƒa)

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(6)

where, ƒa (m3 soil air m-3 soil) is air-filled porosity or the portion of the pore space filled with air (air space).

Relative Gas Diffusivity (Ds/Do)

Relative gas diffusivity was calculated using Buckinghan (1904) equation:

Image for - Effect of Mechanized Tillage Operations on Soil Physical Properties and Greenhouse Gases Fluxes in Two Agricultural Fields
(7)

where:

Ds/Do(m2 sec-1. m-2 sec) : Relative gas diffusion coefficient
Ds : Gas diffusion coefficient in the soil (m3 soil air m-1 soil s-1)
Do :

Gas diffusion coefficient in free air (m2 air s-1).

Pore Tortuosity (τ)

The pore tortuosity factor was calculated by comparing Reible and Shair (1982).

Image for - Effect of Mechanized Tillage Operations on Soil Physical Properties and Greenhouse Gases Fluxes in Two Agricultural Fields
(8)

where:

τ (m m-1) : Pore tortuosity factor.

Water Filled Pore Space (WFPS)

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(9)

where:

WFPS (%) : Percentage of the total pore space filled with water.

Gas Sampling for CO2, CH4, NO and N2O Flux Measurements
CO2, CH4, NO and N2O emissions from tractor-compacted interrows and non-compacted ridges were measured using a closed-chamber technique. This technique has also been used by Tokuda and Hayatshu (2000) and (2004). The chambers were circular with steel frames. The top of each chamber had a gas sampling tube and a bag to control air pressure inside the chamber. The height and diameter of the chamber were 0.35 and 0.30 m, respectively. At each sampling time, 3 chambers (each chamber corresponding to a replicate) spaced 10 m were installed in the soil in the interrow or ridge and kept for 20 min and then samples of the enclosed atmosphere were withdrawn by a 50 mL syringe and transferred into a 1L Tedlar ® Bag with non-sorbant walls. A total of 30 samples (3 replicates x 5 tractor cycles x 2 fields) were taken in both corn and soybean fields. The air temperature inside the chambers was recorded using a digital thermometer. Ambient air between 0 and 2 m from the soil surface was collected and its mean concentration was used as a background concentration for calculation of gas fluxes. Immediately after sampling, a gas chromatography with an electron capture detector and FID used for N2O and CH4 analyses, respectively. NO flux was analyzed by chemo-iluminescence with a nitrogen oxide analyzer (Kimoto, Model 265 P) and an infra-red analyzer was used for CO2. Fluxes were calculated using the equation:

Image for - Effect of Mechanized Tillage Operations on Soil Physical Properties and Greenhouse Gases Fluxes in Two Agricultural Fields
(10)

were:

F :

Gas production rate

ρ :

Gas density (mg m-3) under standard conditions

V (m3) and A (m2) :

Volume and bottom area of the chamber

ΔC/Δt :

Ratio of change in the gas concentration inside the chamber;

T : Absolute temperature
α : Transfer coefficient (12/44 for CO2, 12/16 for CH4, 14/30 for NO and 28/44 for N2O).

A positive value indicates gas emission from the soil, while a negative value indicates gas uptake. The detectable limits were 0.1 mg C m-2 h-1 for CO2, 0.01 μg C m-2 h-1 for CH4 and 0.1 μg N m-2 h-1 for NO and N2O. Soil temperature was measured at 5 cm and 10 cm from the top soil layer, using a digital thermometer. Statistix 8.0 statistical package was used to calculate summary of simple statistics, analysis of variance, polynomial contrasts, correlation matrix and linear regression.

RESULTS

Effect of Tractor Cycle on Soil Chemical Properties
Soil chemical properties as affected by tractor load and cycle (Table 1 and 2) for corn and soybean, respectively. At 5% probability level, tractor load and cycle did not affect any of the soil chemical properties studied. In magnitude, values of chemical properties observed in ridges were similar to those found in tractor-compacted interrows, except for NO3¯ which tended to increase with tractor cycles.

Effect of Tractor Cycle on Soil Physical Properties
Table 3 and 4 show the effect of tractor load and cycle on soil physical properties. All soil physical properties studied were significantly affected by tractor cycle. Volumetric water content (θv), bulk density (ρb) and pore tortuosity (τ) increased, whereas air-filled porosity (ƒa), Total Pore Space (TPS) and the gas diffusion coefficient (Ds/Do) decreased linearly with increasing tractor cycle. In comparison to all compacted interrows, average ridge values for θv, ρb and τ were lower while those for ƒa, TPS and Ds/Do were higher. In addition, in magnitude, values of θv, ρb, τ, ƒa, TPS and Ds/Do were similar in both corn and soybean fields. However, for tractor-compacted interrows, average values of θv, ρb and τ were higher in corn while those for ƒa, TPS and Ds/Do were higher in soybean field.

The effect of tractor load and number of cycle on soil resistance (Table 5 and 6) to penetration in July 2001 for corn and soybean, respectively. Tractor cycle linearly increased soil resistance to penetration for both sampling dates and in both fields, but the effect was prevalent only in the top 20 cm of the soil profile. Below this depth, the relationship was no longer prevalent. In addition, in magnitude, values of soil resistance to penetration measured immediately after compaction treatments were as twice as high in comparison to those measured three weeks later. Finally, in comparison to tractor-compacted interrows, SPR values measured on the ridges were lower.

Effect of Tractor Cycle on Greenhouse Gas fluxes
The effect of tractor load and number of cycle on greenhouse gas fluxes (Table 7 and 8) for corn and soybean fields, respectively. Except for CH4 which failed to respond, all greenhouse gas fluxes were significantly affected by tractor load and cycle.

Table 1: Soil chemical properties in a cornfield as affected by mechanized tillage operations
Image for - Effect of Mechanized Tillage Operations on Soil Physical Properties and Greenhouse Gases Fluxes in Two Agricultural Fields
ns = non significantly different at LSD = 0.05

Table 2: Soil chemical properties in a soybean field as affected by mechanized tillage operations
Image for - Effect of Mechanized Tillage Operations on Soil Physical Properties and Greenhouse Gases Fluxes in Two Agricultural Fields
ns = no significant

Table 3: Soil physical properties in a cornfield as affected by mechanized tillage operations
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*, **, ***, **** = significantly different at 5, 1, 0.1 and 0.01%, respectively. ns = no significant

Table 4: Soil physical properties in a soybean field as affected by mechanized tillage operations
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*, **, ***, **** = significantly different at 5, 1, 0.1 and 0.01%, respectively. ns = no significant

Table 5: Soil resistance to penetration (kg cm-2) in a corn field as affected by mechanized tillage operations
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*, **, ***, **** = significantly different at 5, 1, 0.1 and 0.01%, respectively. ns = no significant

Table 6: Soil resistance to penetration (kg cm-2) in a soybean field as affected by mechanized tillage operations
Image for - Effect of Mechanized Tillage Operations on Soil Physical Properties and Greenhouse Gases Fluxes in Two Agricultural Fields
*, **, ***, **** = Significantly different at 5, 1, 0.1 and 0.01%, respectively. ns = no significant

Table 7: Greenhouse gas fluxes in a cornfield as affected by mechanized tillage operations
Image for - Effect of Mechanized Tillage Operations on Soil Physical Properties and Greenhouse Gases Fluxes in Two Agricultural Fields
*, **, ***, **** = significantly different at 5, 1, 0.1 and 0.01%, respectively. ns = no significant

Table 8: Greenhouse gas fluxes in a soybean field as affected by mechanized tillage operations
Image for - Effect of Mechanized Tillage Operations on Soil Physical Properties and Greenhouse Gases Fluxes in Two Agricultural Fields
*, **, ***, **** = significantly different at 5, 1, 0.1 and 0.01%, respectively. ns = no significant

In addition, except for NO fluxes which increased linearly in the soybean field, all gas fluxes increased quadratically with increasing tractor cycle. In soybean field, CO2 and N2O fluxes measured in the ridges were lower than those obtained in tractor-compacted interrows. However, NO and CH4 fluxes were higher in the ridges than tractor-compacted interrows. There was no specific trend for the relationship between ridges and compacted interrows fluxes in the corn field, but after computing the average values for all tractor-compacted interrows and comparing them with ridge values, the same trend as in the soybean field was found. Among ridges, CO2 and N2O fluxes were higher in the corn as compared to soybean while NO fluxes dominated in soybean. A close examination of the means also reveal that in both fields, the highest CO2 and N2O fluxes were obtained after 2 and 4 cycles of interrows compaction, respectively. The highest fluxes for NO were obtained after 2 cycles in corn, but in the ridge for soybean. In cornfield, CH4 was consumed in both ridges and tractor-compacted interrows. However, in soybean field, CH4 was emitted in non-compacted ridges and consumed in tractor-compacted interrows. Finally, uptake of N2O (negative fluxes) was observed in non-compacted ridge of soybean field, indicating that denitrification was enhanced as a result of soil compaction.

Correlation between Soil Physical Properties and Greenhouse Gas Fluxes
The relationship between CO2 fluxes and soil penetration resistance (SPR) measured at 2.5 cm depth in the cornfield (Fig. 2). CO2 fluxes were also significantly correlated with SPR measured at 5 cm (r = 0.58, p = 0.029) and 10 cm (r = 0.58, p = 0.044) depth. CH4 fluxes were only correlated with SPR measured at 15 cm depth (r = 0.62, p = 0.014). The relationship between N2O fluxes and SPR measured at 2.5 cm depth for the soybean field (Fig. 3). N2O fluxes were also significantly correlated with SPR measured at 15 cm (r = 0.64, p = 0.011) and 30 cm (r = 0.52, p = 0.045) depth. CO2 was only correlated with SPR measured at 20 cm (r = 0.59, p = 0.021).

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Fig. 2: Relationship between Carbon dioxide (CO2) fluxes and soil penetration resistance in a cornfield

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Fig. 3: Relationship between Nitrous oxide (N2O) fluxes and soil penetration resistance in soybean field

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Fig. 4: Relationship between Nitrous oxide (NO) fluxes and the pore tortuosity factor in soybean field

In addition, NO was either positively correlated with ƒa (r = 0.68, p = 0.005), Ds/Do (r = 0.71, p = 0.003) and TPS (r = 0.70, p = 0.004), or negatively correlated with ρb (r = -0.69, p = 0.0044), θv (r = 0.67, p = 0.006), WFPS (r = -0.67, p = 0.0067) and with pore tortuosity in Fig. 4.

DISCUSSION

The average values for bulk density, volumetric water content and pore tortuosity were higher in tractor-compacted interrows as compared to rigdes. These results agree with those reported by Canqui et al. (2004) who found that wheel traffic reduced Ksat by three times and increased bulk density by 6%. Our results are however opposed to those reported by Ginting and Eghball (2005) who found that wheel traffic had no significant effect on a specific soil physical property [(bulk density, soil moisture and water filled porosity (WFP)] and N2O fluxes. The lack of difference in bulk density for example in Ginting and Eghball (2005) study could be due to their depth of soil bulk density measurements (20 cm) as compared to our depth of sampling (5 cm). In fact, it has been suggested that small depth increments might detect bulk density differences that would be obscured in a large depth increment samples (Unger, 1991). Logsdon and Cambardella (2000) indicated that changes in no-till bulk density at the 0- to 12-cm depth was partially due to biopores from surface-feeding earthworms (Lumbricus terrestris L.) that were observed in the no-till field but not in the disk field. The air-filled porosity, total pore space and the gas diffusion coefficient were higher in ridges as compared to tractor-compacted interrows. These results agree with those of Ruser et al. (1998) who reported that ridge-till practice produced areas with increased (ridges) and strongly reduced (interrow soil compacted by tractor traffic) soil porosity. The air-filled porosity and soil gas diffusion coefficient were lowest and soil penetration resistance of 0-10 cm depth highest in the 4 cycles tractor-compacted interrows. This treatment also corresponded to the highest N2O fluxes in both corn and soybean fields. These results agree with those of Klemedtsson et al. (1988) who suggested that the highest N2O production should occur in the presence of low concentrations of O2, at the transition between aerobic and anaerobic conditions. Flessa et al. (2002) and Ruser et al. (1998) also found that soil compaction was an important factor for increased N2O emissions from ridge-tilled potato fields. Teepe et al. (2004) reported that high N2O emissions which occurred after compaction were restricted to short periods at the sandy loam and silty clay loam sites whereas emissions at the silt site remained high throughout the entire growing season. Hansen et al. (1993) compared tractor-compacted and uncompacted soils and found increased N2O emissions (approximately 35%) due to soil compaction. However, emission rates reported by these authors are considerably higher as compared to flux rates measured in the present study. The higher N2O fluxes in these studies can be explained by the much stronger soil compaction (e.g., a bulk density of 1.56 g cm-3 for tractor-compacted soil) and greater WFPS (mean of 85% for tractor-compacted soil) in Ruser et al. (1998) for example. In our study, the highest bulk density observed for the 4 cycles tractor-compacted interrows was less than unity and the corresponding WFPS below 65%. In non-compacted ridges, even though the averages air-filled porosity and gas diffusion coefficient were highest, denitrification could still happen, perhaps at a lower level in comparison to compacted soil. In fact, Rolston (1981) reported that in aerobic soils, anaerobiosis can still occur at microsites where consumption of oxygen exceeds the oxygen-supply via diffusion. In addition, uptake of N2O (negative fluxes) was observed in the ridges of soybean This behavior is unusual as in most studies, soils have been reported a source of N2O (Ball et al., 2000; Matson et al., 1990). However, several studies where soils have acted occasionally as sinks have also been reported. Donoso et al. (1993) found that in contrast with a significant emission in the rainy season, the soil of a scrub-grass savannah of Venezuela acted as a sink for N2O in the dry season. Cicerone et al. (1978) found a significant sink activity in wet grass-covered soil of Michigan. Blackmer and Bremner (1976) found that cultivated soils of Iowa acted as sinks for atmospheric N2O at certain times during spring. Ryder (1981) reported that the soil acts as both a source and sink for atmospheric N2O depending on soil condition and the amount of nitrogenous fertilizer applied, the sink activity was observed in conditions conducive to microbial reduction of N2O (i.e. very low nitrate in the soil). Matson and Vitousek (1987) suggested that even though the overall average fluxes measured in La Selva, Costa Rica were positive, under certain conditions uptake of N2O occurred in this tropical soils. The mechanism by which soil acts as a sink for N2O is not known. It has been suggested that the net flux of N2O to the atmosphere results from its production by nitrifying and or denitrifying bacteria. N2O consumption is therefore likely due to the reduction of N2O to N2 (Donoso et al., 1993). It has also been reported that N2O production was somewhat higher and N2O uptake somewhat lower in the more disturbed communities and that N2-fixing cyanobacteria could both produce and consume N2O (http://www.ceh.ac.uk/award3.html). In this study, N2O uptake was observed in soybean field. Soybean is a N2-fixing legume in symbiosis with bacteria living in its roots. Even though we did not investigate the nature of bacterial flora in our soil, it may be also thought that soybean, through its bacterial symbiosis, contributed to this phenomena. Another explanation may be a temporal N deficit in the soil. In fact, the soybean crop received a starter dose of N of 32 kg N. This amount might have been taken up by the soybean plants during early growth when the root nodules were not established and the rhizobium bacteria where not actively fixing N2. During such periods with low nitrate availability, the soil may consume atmospheric N2O. All soil physical properties studied were significantly correlated with either CO2, CH4, N2O or NO with correlation coefficients ranging from 0.30 to 0.70. Correlation between soil physical properties and gas fluxes have also been reported by Ball et al. (1997) who found significant relationships between N2O fluxes and air permeability, the soil gas diffusion coefficient and tortuosity. Hu et al. (2001) also reported a significant relationship between the soil gas diffusion coefficient and CH4 fluxes.

SUMMARY

Tractor compaction increased soil resistance to penetration, water, bulk density and pore tortuosity while reducing air-filled porosity, total pore space and the soil gas diffusion coefficient. Changes in soil physical properties resulted in increased CO2, NO and N2O emissions. This work helped identify rarely measured soil physical properties such as Ds/Do and τ which significantly influence soil gas exchange. More studies are needed to determine if these effects are permanent or only temporary on both soil and gas fluxes.

ACKNOWLEDGMENTS

The authors wish to thank the Japan Society for the Promotion of Science (JSPS) which enabled Dr. Nkongolo to join the Laboratory of Soil Science, Graduate School of Agriculture at Hokkaido University.

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