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Keywords:

  • Andosol;
  • carbon dioxide;
  • gas concentration in soil profile;
  • Gray Lowland soil;
  • nitrous oxide

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

We measured nitrous oxide (N2O) and carbon dioxide (CO2) fluxes from the soil surface and in the soil through to a depth of 0.3 m, and their concentration profiles through to a depth of 0.6 m in both a Gray Lowland soil with macropores and cracks and an Andosol with undeveloped soil structure in central Hokkaido, Japan. The objective of the present study was to elucidate any differences in N2O production and flux in the soil profile between these two soil types. In the Gray Lowland soil, the N2O concentration above 0.4 m increased with an increase in soil depth. In the Andosol, there were no distinctive N2O concentration gradients in the topsoil when the N2O flux did not increase. However, the N2O concentration at a depth of 0.1 m significantly increased and this concentration was higher than the concentration below 0.2 m when the N2O flux greatly increased. Thus, the N2O concentration profiles were different between these two soils. The contribution ratios of the N2O produced in the top soil (0–0.3 m depth) to the total N2O emitted from the soil to the atmosphere in the Gray Lowland soil and the Andosol were 0.86 and 1.00, respectively, indicating that the N2O emitted from the soil to the atmosphere was mainly produced in the top soil. However, the contribution ratio of the subsoil to the N2O emitted from the Gray Lowland soil was higher than that of the Andosol. There was a significant positive correlation between the N2O flux through to a 0.3 m depth and the flux from the soil to the atmosphere in the Gray Lowland soil only. These results suggest that N2O production in the subsoil of the Gray Lowland soil could have been activated by NO3 leaching through macropores and cracks, and subsequently the N2O produced in the subsoil could have been rapidly emitted to the atmosphere through the macropores and cracks.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

Increased nitrous oxide (N2O) concentrations in the troposphere cause global warming and contribute to the depletion of stratospheric ozone (Prather et al. 2001). Enhanced N2O emissions from agricultural and natural ecosystems are believed to result mainly from the increased use of nitrogen (N) fertilizer. The contribution of agricultural activities to global N2O emission is estimated to be approximately 35% (Food and Agriculture Organization, International Fertilizer Association 2001; Prather et al. 2001). Therefore, elucidation of the mechanism of N2O emission from agricultural soil has been crucial to mitigate global N2O emission. Nitrification and denitrification by soil microbes are the dominant processes in the production of N2O in soils. These processes are strongly influenced by soil conditions, such as temperature, water content, nitrate (NO3) and ammonium (NH4+) concentrations, and organic matter content (Food and Agriculture Organization, International Fertilizer Association 2001). Marked increases in N2O emission rates have been observed immediately after the application of fertilizer and manure (Akiyama and Tsuruta 2002; Akiyama et al. 2000; Jambert et al. 1997; Lessard et al. 1996; Skiba et al. 1996). In addition, a number of studies have reported that high N2O emission rates are observed after heavy rain and irrigation (Koga et al. 2004; Kusa et al. 2002, 2006; Lessard et al. 1996; Mosier and Hutchinson 1981), suggesting that N2O emissions from the soil to the atmosphere are influenced strongly by N dynamics and the addition of water to soils. In a prismatic structured soil with interstitial pores, water moves vertically through macropores, bypassing the soil matrix within peds (Hasegawa 1986; Hayashi and Hatano 1999; Inoue 1988). Gas movement is primarily associated with macropores (Osozawa 1998). In addition, rainwater or snowmelt water is likely to mix with the soil solution in the topsoil and drain directly through the macropores in the subsoil when large drainage takes place in prismatic structured soil (Hayashi and Hatano 1999). However, in an Andosol characterized by the absence of cracks and fissures after drying, rainwater moves mainly by a matrix flow (Hasegawa and Eguchi 2002) and the movement of gas through macropores becomes minor (Osozawa 1998). Therefore, the movement of NO3 and the production and movement of N2O in soils are influenced by the soil structure. The concentration profiles of soil N2O have been used to estimate the depth of N2O production in soils (Goodroad and Keeney 1985; Hosen et al. 2000). An understanding of N2O movement in the soil profile is necessary to explain the production and emission of N2O in the soil.

Carbon dioxide (CO2) is another greenhouse gas produced by the respiration of soil microbes and roots in soils. Soil microbes and roots are distributed mainly in the top soil (Nakamoto 1993; Osozawa 1998). Therefore, most of the CO2 emitted is produced in the top soil (de Jong and Schappert 1971). In addition, comparisons of N2O and CO2 concentrations and fluxes among different soils in the soil profile could be useful to elucidate the influence of soil type on the mechanisms of N2O production and fluxes in the soil profile. In the present study, we measured N2O and CO2 fluxes from the soil surface and both N2O and CO2 concentrations and fluxes in the soil profiles in a Gray Lowland soil with macropores and an Andosol without macropores, where N2O emissions during the pluvial period and after heavy rains were higher than those observed immediately after fertilizer application (Kusa et al. 2002, 2006). The objective of the present study was to elucidate any differences in N2O production and flux in the soil profile between these two soil types.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

Experimental sites

The experimental sites were a 2.0 × 104 m2 onion (Allium cepa L.) field in Mikasa City (43°14′N, 141°50′E) and a 1.8 × 104 m2 maize (Zea mays L.) field at the National Agricultural Research Center for Hokkaido Region in Sapporo City (43°00′N, 141°24′E) located in central Hokkaido, Japan. In the onion field, the soil was Humic Gray Lowland soil (Japanese Society of Pedology 2003); the soil texture at a depth of 0–0.48 m was silty clay (SiC) and there were interstitial pores in the subsoil (Table 1). Saturated hydraulic conductivity was low at a depth of 0–0.28 m and was higher below 0.28 m (Table 1) because of the presence of macropores (Hayashi and Hatano 1999). Subsurface drains were installed at a depth of 0.8–1.0 m and the groundwater table was at a depth of 0.7–0.8 m throughout the year. Chemical fertilizer was applied to the field at a rate of approximately 300 kg N ha−1 at the end of April. Onion seedlings were transplanted at the beginning of May and harvesting was carried out in both early and mid September (Kusa et al. 2002). In the maize field, the soil was Silandic Andosol (Japanese Society of Pedology 2003). The soil texture at a depth of 0–0.30 m was clay loam (CL), which is rich in humus. There was an impermeable layer 1.3 m below ground level, and consequently the groundwater table temporarily rose to near the ground surface level during the snowmelt period and after heavy rains (Kanazawa et al. 1999). Respective saturated hydraulic conductivity at a depth of 0–0.30 m was lower than that at 0.30–0.47 m (Table 1). Composted cattle manure was applied to the field at a rate of 300 kg N ha−1 (fresh weight 3.0 kg m−2) every year in mid May. After furrowing, chemical fertilizer was applied to the rows at a rate of 130 kg N ha−1. The row width was 0.75 m and the inter-row width was 0.25 m. Maize was sown in mid May and harvested at the end of September. Monitoring of gas emission rates and other factors in the maize field was conducted only between the plants in each row (Kusa et al. 2006). For 3 years (1998–2000), N2O and CO2 fluxes were usually measured every week on the same day during the snow-free season (from the end of May to October in the Gray Lowland soil and from June to the end of September in the Andosol); in addition N2O fluxes were measured every week during the snow-free season from 1995 to 1997 in the Gray Lowland soil (Tables 2,3).

Table 1.   Soil texture, structure and saturated hydraulic conductivity of the study site
HorizonDepth (m)TextureStructureSaturated hydraulic conductivity (m s−1)
GradeSizeType
  1. ND, no data.

Humic Gray Lowland soil
 Ap0–0.28SiCStrongMediumSubangular blockly1.0 × 10−7
 B0.28–0.48SiCStrongMediumSubangular blockly1.8 × 10−6
 C10.48–0.68HCStrongCoarsePrismlike4.6 × 10−6
 C20.68–1.0+SiCMassive2.2 × 10−4
Andosol
 Ap0–0.3CLGranular3.3 × 10−6
 AB0.3–0.37LiCModerateMediumSubangular blockly2.2 × 10−5
 B0.37–0.47LiCModerateMediumSubangular blockly2.3 × 10−5
 BC10.47–0.75CLWeakCoarseSubangular blockly4.3 × 10−5
 BC20.75–0.9LiCWeakCoarseSubangular blocklyND
 C0.9–1.0+SLMassiveND
Table 2.   Cumulative flux, production and mass balance of N2O during the study period
YearPeriodCumulative N2O flux during study periodMass of N2O in the topsoil (above 0.3 m)N2O production by topsoilContribution ratio of topsoil
Surface flux (F0, mg N m−2)Through to 0.3 m (F0.3, mg N m−2)Beginning (Ms, mg N m−2)End (Me, mg N m−2)(P, mg N m−2)(P/F0)
  1. F0 was measured using the chamber method and F0.3 was measured using the gradient method. Values are mean ± standard deviation.

Gray Lowland soil
 1995 6/13–10/28760 ± 14086 ± 60.350.556700.89
 1996 7/2–10/31310 ± 3371 ± 40.261.022400.77
 1997 6/13–10/23450 ± 20056 ± 50.320.563900.88
 1998 6/23–10/27430 ± 7665 ± 40.200.833700.85
 1999 5/26–10/20930 ± 25080 ± 70.690.648500.91
 2000 5/30–10/241190 ± 450160 ± 100.210.7010300.86
  Average680 ± 9687 ± 30.340.725900.86
Andosol
 1998 6/15–9/29630 ± 881.3 ± 0.10.238.376401.01
 1999 6/6–9/131980 ± 2306.2 ± 0.60.120.3019801.00
 2000 7/17–9/181430 ± 1407.8 ± 1.10.200.3314200.99
  Average1350 ± 945.1 ± 0.40.183.0013501.00
Table 3.   Cumulative flux, production and mass balance of CO2 during the study period
YearPeriodCumulative CO2 flux during the study periodMass of CO2 in the topsoil (above 0.3 m)CO2 production by topsoilContribution ratio of topsoil
Surface flux (F0, g C m−2)Through to 0.3 m (F0.3, g C m−2)Beginning (Ms, g C m−2)End (Me, g C m−2)(P, g C m−2)(P/F0)
  1. F0 was measured using the chamber method and F0.3 was measured using the gradient method. Values are mean ± standard deviation.

Gray Lowland soil
 1998 6/23–10/27360 ± 2922 ± 10.320.233300.94
 1999 5/26–10/20410 ± 2644 ± 20.120.133700.89
 2000 5/30–10/24430 ± 2716 ± 10.120.164100.96
  Average400 ± 1627 ± 10.220.223700.93
Andosol
 1998 6/15–9/29380 ± 94.8 ± 0.10.340.703800.99
 1999 6/6–9/13540 ± 97.1 ± 0.30.251.315300.99
 2000 7/17–9/18340 ± 63.7 ± 0.30.530.473200.99
  Average420 ± 55.2 ± 0.10.370.834100.99

N2O and CO2 concentrations in the soil profile

After the polyvinyl chloride pipes (soil–air sampling tubes: the inside diameter was 0.013 m and the outside diameter was 0.016 m) were installed into the soil, silicon stoppers threaded with rubber tubes with three-way cocks were connected to the top of the soil–air sampling tubes. The depths of the soil–air sampling tubes installed were 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6 m. Twenty soil–air sampling tubes were installed at a depth of 0.05 and at a depth of 0.1 m and 10 tubes were installed at various depths between 0.2 and 0.6 m. Gas samples from the enclosed atmosphere in the soil–air sampling tubes were withdrawn using a 10 mL syringe; all gas samples from the same depth were transferred into a 1 L Tedlar Bag and mixed. The ambient air above the soil surface was also sampled to obtain the concentration at a depth of 0 m. The N2O concentrations in the gas samples were measured using a gas chromatograph equipped with an electron capture detector (GC-14B; Shimadzu, Kyoto, Japan). The CO2 concentrations were analyzed using an infrared gas analyzer (ZFP-5; Fuji Electric Company, Tokyo, Japan).

Measurement of soil physical properties and rainfall

The soil temperature was measured at a depth of 0.1 m using a digital thermometer. Three undisturbed soil samples (0–0.05 m and 0.05–0.1 m) were collected using three 100 mL steel cylinders at each sampling date; the air-filled porosity, water-filled pore space (WFPS) and the relative gas diffusion coefficient (D/D0) were measured using the method reported by Osozawa (1998).

To obtain the air-filled porosity and gas diffusion coefficient in the soil profile, undisturbed soil samples were collected once using three 100 mL steel cylinders from the Gray Lowland soil in October 1996 and from the Andosol in May 1998 (Gray Lowland soil, 0.05–0.1, 0.15–0.2, 0.23–0.28, 0.32–0.37, 0.43–0.48 and 0.54–0.59 m; Andosol, 0.13–0.18, 0.31–0.36, 0.40–0.45 and 0.58–0.63 m). The air-filled porosity and D/D0 of these samples were measured at a water suction of −0.098 (water saturated), −0.31, −0.98, −3.1, −9.8 and −31 kPa. The D/D0 of the water-saturated sample was assumed to be 0. Regression curves were obtained from the relationship of the soil water suction to the D/D0 (Table 4) and air-filled porosity. Two tensiometers were installed at depths of 0.1, 0.2, 0.3, 0.4 and 0.5 m and the soil water suction was measured at each sampling date (Hasegawa and Kasubuchi 1988). Changes in the air-filled porosity and the D/D0 in the soil profile were calculated using the value obtained from the soil water suction and the regression curves.

Table 4.   Regression curves of the soil water suction (pF)–D/D0 in the soil profile
Depth (m)Regression curveR2Mean square of residual
  1. Y denotes the value of D/D0, x denotes the soil water suction (pF), pF = log (−10.2 ϕ), ϕ is the soil water suction (kPa)

Gray Lowland soil
 0.15–0.20Y = 4.6 × 10−4x3−2.2 × 10−3x2 + 3.5 × 10−3x+6.0 × 10−50.9735.0 × 10−8
 0.23–0.28Y = 1.0 × 10−3x3−3.7 × 10−3x2 + 4.9 × 10−3x−2.0 × 10−50.9722.0 × 10−7
 0.32–0.37Y = 9.8 × 10−4x3−3.5 × 10−3x2 + 4.7 × 10−3x−2.0 × 10−50.9997.8 × 10−9
 0.43–0.48Y = 1.1 × 10−3x3−3.8 × 10−3x2 + 5.3 × 10−3x−1.0 × 10−50.9911.4 × 10−7
 0.54–0.59Y = 4.0 × 10−3x3−1.1 × 10−2x2 + 9.5 × 10−3x−1.7 × 10−40.9944.3 × 10−7
Andosol
 0.13–0.18Y = −8.0 × 10−5x3 + 2.2 × 10−3x2−3.4 × 10−3x+1.3 × 10−30.9981.0 × 10−7
 0.31–0.36Y = −5.2 × 10−4x3 + 4.2 × 10−3x2−6.7 × 10−4x−1.2 × 10−30.9931.3 × 10−6
 0.40–0.45Y = −9.6 × 10−4x3 + 5.8 × 10−3x2−1.6 × 10−3x−1.1 × 10−30.9832.9 × 10−6
 0.58–0.63Y = −5.0 × 10−5x3 + 8.6 × 10−4x2 + 1.8 × 10−3x−1.2 × 10−30.9933.5 × 10−7

Rainfall data for the Gray Lowland soil and the Andosol sites were recorded at the Iwamizawa Weather Station (43°12.6′N, 141°47.3′E) (Sapporo Distinct Meteorological Observatory 1995–2000) and the National Agricultural Research Center for Hokkaido Region, respectively.

N 2O and CO 2 fluxes in the soil profile and from the soil surface to the atmosphere

The N2O and CO2 fluxes through a depth of 0.3 m in the soil profile were calculated by the following equation using Fick’s law (gradient method; Granli and Bøckman 1994) as follows:

  • image(1)

where F0.3 is the gas flux (mg m−2 s−1) in the soil through to a depth of 0.3 m, D is the gas diffusion coefficient (m2 s−1), ρ is the gas density (ρCO2 = ρN2O = 1.98 × 106 [mg m−3]), [dC/dz] is the gas concentration gradient (m2 m−3), D/D0 is the relative gas diffusion coefficient at a depth of 0.3 m (these values were calculated from the regression curves of the soil water suction –D/D0 of the Ap horizon at a depth of 0.23–0.28 m in the Gray Lowland soil and at a depth of 0.13–0.18 m in the Andosol; Table 4), D0 is the N2O or CO2 air inter-diffusion coefficient (m2 s−1), C0.2 and C0.4 are the gas concentrations at depths of 0.2 and 0.4 m (m3 m−3), respectively, z is the distance from 0.4 to 0.2 m, and T is the soil temperature between 0.2 and 0.4 m (°C), which was presumed to be 20°C. D0 values under an air pressure of 1 atm and a soil temperature of 20°C were calculated using the following equation (Pritchard and Currie 1982):

  • image(2)

where DS (N2O) and DS (CO2) (m2 s−1, in standard condition) represent 0.143 × 10−4 and 0.139 × 10−4, respectively (Pritchard and Currie 1982).

In our previous paper (Kusa et al. 2008), we revealed that the gradient method was useful for measuring N2O fluxes from the soil surface into the atmosphere (flux from the soil surface). However, there were differences in the CO2 and extremely high N2O fluxes between the chamber and gradient methods when the production and consumption of these gases were active in the soil above the installed location of the soil–air sampling tube. Therefore, the N2O and CO2 fluxes from the soil surface were measured by a closed-chamber method. Cylindrical stainless steel chambers, 0.3 m in diameter and 0.35 m high for the Gray Lowland soil and 0.2 m in diameter and 0.2 m high for the Andosol, were used. Fifteen minutes after placement of the chamber, a gas sample was taken from the enclosed atmosphere. Mean gas emission rates from four replicates in the Gray Lowland soil and from two replicates in the Andosol were calculated. The gas sampling method and calculation of the gas fluxes were described in detail in our previous papers (Kusa et al. 2002, 2008). The cumulative gas fluxes during the study period were calculated through linear interpolation.

Mass balance analysis

Hosen et al. (2000) showed that N2O consumption in the top soil (above 0.24 m) does not have much effect on the N2O emission rate. Although CO2 can be dissolved in soil water, Osozawa (1998) reported that the CO2 runoff volume by water percolation was very small. Therefore, mass balance analyses were conducted to estimate the production of N2O and CO2 in the topsoil (0–0.3 m) using the following equation:

  • image

where P is the N2O and CO2 production (mg m−2) in the topsoil during the study period, and F0 and F0.3 are the cumulative N2O and CO2 fluxes (mg m−2) from the soil surface through to a depth of 0.3 m during the study period. Ms and Me signify the mass of N2O and CO2 (mg m−2), respectively, in the topsoil at the beginning and end of the investigation, and are the product of air-filled porosity (m3 m−3), gas concentration (mg m−3) and depth (m). The contribution ratios of the gas production (P/F0) in the topsoil to the gas emitted from the soil surface to the atmosphere were estimated.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

Seasonal patterns of rainfall and soil physical properties

The frequency of rainfall in both the Gray Lowland and Andosol sites increased after July every year (Figs 1,2). The mean WFPS values from 1998 to 2000 at depths of 0–0.05 and 0.05–0.1 m were 45 and 59% in the Gray Lowland soil and 48 and 57% in the Andosol, respectively (Figs 3,4). At a depth of 0–0.1 m, D/D0 values were below 0.02 when the WFPS values were above 60%. The values of D/D0 in the Andosol were higher than those in the Gray Lowland soil when the WFPS values were below 60% (Fig. 5). The mean values of soil water suction at depths of 0.2, 0.3 and 0.6 m from 1998 to 2000 were −15.6, −10.9 and −3.2 kPa in the Gray Lowland soil and −10.5, −9.9 and −3.6 kPa in the Andosol. There were no significant differences in WFPS (paired t-test: 0–0.05 m |t| = 2.02, = 0.05, = 61 and 0.05–0.1 m |t| = 1.07, = 0.29, = 59) and soil water suction (paired t-test: 0.2 m |t| = 1.82, = 0.08, = 24, 0.4 m |t| = 1.28, = 0.21, = 24 and 0.6 m |t| = 0.11, = 0.91, = 21) between the Gray Lowland soil and the Andosol. Soil water suction increased with an increase in soil depth. The value of WFPS increased and the soil water suction decreased after rainfall (Figs 1–5). In addition, the soil temperature at a depth of 0.1 m increased from spring to summer and decreased after summer (Figs 3,4).

image

Figure 1.  Seasonal patterns in (a) rainfall and N2O fluxes from the soil surface and through to a depth of 0.3 m in the soil profile, (b,c) N2O concentrations in soil air (at depths of 0.05, 0.1, 0.3 and 0.5 m), (d) soil water suction (at depths of 0.2, 0.3 and 0.6 m) in the Gray Lowland soil from 1995 to 2000. Chemical fertilizer was applied at the end of April. The surface fluxes were reported by Kusa et al. (2002).

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image

Figure 2.  Seasonal patterns in (a) rainfall and N2O fluxes from the soil surface and through to a depth of 0.3 m in the soil profile, (b,c) N2O concentration in soil air (at depths of 0.05, 0.1, 0.3 and 0.5 m), (d) soil water suction (at depths of 0.2, 0.3 and 0.6 m) in the Andosol from 1998 to 2000. Chemical fertilizer was applied in mid May. The surface fluxes were reported by Kusa et al. (2006).

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image

Figure 3.  Seasonal patterns in (a) rainfall and water-filled pore space (WFPS) of the soil surface, (b) CO2 fluxes from the soil surface and through to a depth of 0.3 m in the soil profile, (c) CO2 concentrations of soil air (0.1, 0.3 and 0.5 m depths), (d) soil temperature at a depth of 0.1 m in the Gray Lowland soil from 1998 to 2000. The WFPS (0–0.05 m) and the soil temperature were reported by Kusa et al. (2002).

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image

Figure 4.  Seasonal patterns in (a) rainfall and water-filled pore space (WFPS) in the soil surface, (b) CO2 fluxes from the soil surface and through to a depth of 0.3 m in the soil profile, (c) CO2 concentrations in soil air (0.1, 0.3 and 0.5 m depths), (d) soil temperature at a depth of 0.01 m in the Andosol from 1998 to 2000. The WFPS (0–0.05 m), the surface fluxes and the soil temperature were reported by Kusa et al. (2006).

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image

Figure 5.  Relationships between the relative gas coefficient (D/D0) and soil moisture (water-filled pore space [WFPS]) from 1998 to 2000. The WFPS (0–0.05 m) was reported by Kusa et al. (2002, 2006).

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In 1998–2000, the mean values of soil air porosity at depths of 0–0.05, 0.05–0.1, 0.3 and 0.6 m were 32, 23, 5.5 and 4.9% in the Gray Lowland soil the values were 35, 29, 8.7 and 7.6% in the Andosol, respectively. The values of soil air porosity in the Andosol were higher than those in the Gray Lowland soil (paired t-test: depth of 0–0.05 m |t| = 2.65, = 0.01, = 61, depth of 0.05–0.1 m |t| = 4.60, < 0.01, = 59, depth of 0.3 m |t| = 1.84, = 0.08, = 18 and depth of 0.6 m |t| = 3.48, < 0.01, = 21). The mean values of D/D0 at depths of 0–0.05, 0.05–0.1, 0.3 and 0.6 m (1998–2000) were 0.097, 0.048, 0.003 and 0.003 in the Gray Lowland soil and 0.150, 0.080, 0.004 and 0.003 in the Andosol, respectively (Fig. 6). The values of D/D0 above 0.3 m in the Andosol were higher than those in the Gray Lowland soil, but there was no significant difference at a depth of 0.6 m (paired t-test: depth of 0–0.05 m |t| = 4.41, < 0.01, = 61, depth of 0.05–0.1 m |t| = 5.12, < 0.01, = 57, depth of 0.3 m |t| = 2.39, = 0.03, = 18 and depth of 0.6 m |t| = 1.03, = 0.31, = 24).

image

Figure 6.  Seasonal patterns in the relative gas coefficient (D/D0) for the Gray Lowland soil and the Andosol in 1998.

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Concentration and flux of N2O in the soil profile

The cumulative N2O flux from the soil to the atmosphere during the study period from 1995 to 2000 ranged from 310 to 1190 mg N m−2 in the Gray Lowland soil and the cumulative flux from 1998 to 2000 ranged from 630 to 1980 mg N m−2 in the Andosol (Table 2). There was no significant difference in the cumulative N2O flux between the Gray Lowland soil and the Andosol from 1998 to 2000 (t-test: |t| = 1.11, = 0.33, = 3). In addition, a significant increase in N2O fluxes occurred during the increasing frequency of rainfall in the Gray Lowland soil (Fig. 1a) and after heavy rainfall (above 80 mm day−1) in the Andosol (Fig. 2a).

In both soils, the N2O concentrations at a depth of 0.05 m were always higher than those of the ambient air, which is approximately 0.3 p.p.m.v (10−6 m3 m−3 =p.p.m.v). A significant increase in soil N2O concentration occurred after July in the Gray Lowland soil, when the frequency of rainfall increased and after heavy rainfall occurred (above 80 mm day−1) in the Andosol. These increases in concentration were greater than those that took place in June after the application of fertilizer (Figs 1,2). The seasonal pattern in the N2O concentration in the soil was similar to the N2O flux from the soil to the atmosphere. The mean values of the N2O concentration in the soil at depths of 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6 m were 2.2, 5.8, 15, 22, 54, 62 and 59 p.p.m.v in the Gray Lowland soil and 8.4, 18, 7.7, 7.2, 11, 16 and 10 p.p.m.v in the Andosol, respectively. The maximum concentrations of N2O at these depths were 21, 37, 83, 140, 240, 430 and 370 p.p.m.v in the Gray Lowland soil and 93, 250, 55, 18, 59, 110 and 35 p.p.m.v in the Andosol, respectively. In the Gray Lowland soil, the N2O concentrations above 0.4 m increased with an increase in soil depth; however, there was no increase in concentration below a depth of 0.4 m (Fig. 7). Furthermore, the N2O concentration gradients of the soil profile increased from August to October (Figs 1,7). In the Andosol, there were no N2O concentration gradients in the topsoil in June when the N2O flux did not increase. However, the N2O concentration at a depth of 0.1 m significantly increased (above 40 p.p.m.v), and this concentration was higher than the concentration at a depth of 0.2 m when the N2O flux greatly increased (September 1998, July 1999, July 2000 and September 2000) (Figs 2,8).

image

Figure 7.  Monthly average concentrations of N2O and CO2 in the soil profile from the Gray Lowland soil from 1995 to 2000 (N2O) and from 1998 to 2000 (CO2).

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image

Figure 8.  Monthly average concentrations of N2O and CO2 in the soil profile from the Andosol from 1998 to 2000.

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The mean value of the N2O fluxes in the soil through to a depth of 0.3 m in the Gray Lowland soil was 0.026 mg N m−2 h−1, which was approximately 10-fold higher than that in the Andosol (0.002 mg N m−2 h−1). The N2O fluxes in the soil through to a depth of 0.3 m were much lower than those from the soil to the atmosphere (Figs 1,2). There was a significant positive correlation between the N2O flux at a depth of 0.3 m and the N2O flux from the soil to the atmosphere in the Gray Lowland soil (= 0.54, < 0.01, = 90). However, there was no significant correlation in the Andosol.

The cumulative N2O flux in the soil through to a depth of 0.3 m during the study period ranged from 56 to 160 mg N m−2 (mean value: 87 mg N m−2) in the Gray Lowland soil and from 1.3 to 7.8 mg N m−2 (mean value: 5.1 mg N m−2) in the Andosol (Table 2). The cumulative N2O flux of the Gray Lowland soil was significantly higher than that of the Andosol in 1998–2000 (t-test: |t| = 3.21, < 0.05, = 3). In both soils, the cumulative N2O fluxes in the soil through to a depth of 0.3 m were lower than those from the soil to the atmosphere. The N2O produced in the soil above a depth of 0.3 m during the study period was 240–1030 mg N m−2 (mean value: 590 mg N m−2) in the Gray Lowland soil and 640–1980 mg N m−2 (mean value: 1350 mg N m−2) in the Andosol (Table 2). The contribution ratios of the N2O produced in the topsoil (above a depth of 0.3 m) to the emitted N2O from the soil to the atmosphere were 0.77–0.91 in the Gray Lowland soil and 0.99–1.01 in the Andosol (Table 2). These contribution ratios of the Gray Lowland soil were significantly higher than the ratios of the Andosol from 1998 to 2000 (t-test: |t| = 6.40, < 0.01, = 3). In other words, the proportion of the N2O produced in the subsoil (below a depth of 0.3 m) to the N2O emitted from the soil to the atmosphere was 9–23% in the Gray Lowland soil and 0–1% in the Andosol.

Concentration and flux of CO2 in the soil profile

The cumulative CO2 fluxes from the soil to the atmosphere in the Gray Lowland soil and the Andosol during the study period were 360–430 g C m−2 and 340–540 g C m−2, respectively (Table 3). There was no significant difference in the CO2 emission from the soil into the atmosphere between the Gray Lowland soil and the Andosol (t-test: |t| = 0.20, = 0.85, = 3). In both soils, the CO2 flux increased in July and August with an increase in soil temperature (Figs 3,4).

The mean values of the CO2 concentration in the soil at depths of 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6 m were 2.1, 3.5, 7.6, 12, 20, 21 and 21 × 103 p.p.m.v in the Gray Lowland soil and 3.5, 6.4, 10, 13, 17, 22 and 21 × 103 p.p.m.v in the Andosol, respectively. In both soils, the CO2 concentrations at a depth of 0.05 m were always higher than the concentrations of the ambient air (0.36 × 103 p.p.m.v). The CO2 concentration gradient in the soil profile also increased from July to September with an increase in soil temperature. The seasonal pattern of CO2 concentration in the soil was similar to that of the CO2 flux from the soil surface (Figs 3,4). The CO2 concentration in the soil above a depth of 0.4 m increased with an increase in depth; however, the concentration below a depth of 0.4 m did not increase (Figs 7,8).

The mean value of the CO2 fluxes in the soil through to a depth of 0.3 m was 5.5 mg C m−2 h−1 in the Gray Lowland soil and 2.6 mg C m−2 h−1 in the Andosol. The CO2 fluxes in the soil through to a depth of 0.3 m were much lower than those from the soil to the atmosphere (Figs 3,4). There was no significant correlation between the CO2 flux in the soil through to a depth of 0.3 m and the CO2 flux from the soil to the atmosphere in either soil.

The range of the cumulative CO2 flux in the soil through to a depth of 0.3 m during the study period was 16–44 g C m−2 (mean value 27 g C m−2) in the Gray Lowland soil and 3.7–7.1 g C m−2 (mean value 5.2 g C m−2) in the Andosol, and this cumulative CO2 flux of the Gray Lowland soil was higher than that of the Andosol from 1998 to 2000 (t-test: |t| = 2.51, = 0.07, = 3) (Table 3). In both soils, the cumulative CO2 fluxes in the soil through to a depth of 0.3 m were lower than those from the soil to the atmosphere. The CO2 produced in the soil above a depth of 0.3 m during the study period was 330–410 g C m−2 (mean value 370 g C m−2) in the Gray Lowland soil and 320–530 g C m−2 (mean value 410 g C m−2) in the Andosol. The contribution ratios of the CO2 produced in the topsoil (above a depth of 0.3 m) to the CO2 emitted from the soil into the atmosphere were 0.89–0.96 in the Gray Lowland soil and 0.99 in the Andosol (Table 3). These contribution ratios of the Andosol were significantly higher than the ratios of the Gray Lowland soil from 1998 to 2000 (t-test: |t| = 2.88, = 0.04, = 3). In other words, the proportions of the CO2 produced in the subsoil (below a depth of 0.3 m) to the CO2 emitted from soil to the atmosphere were 4–11% in the Gray Lowland soil and 1% in the Andosol.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

N 2O concentration in the soil profile

In both Gray Lowland and Andosol soils, the seasonal pattern of the N2O flux from the soil surface was similar to that of the N2O concentration in the soil from depths of 0.05–0.6 m (Figs 1,2). In the Gray Lowland soil, the N2O concentration gradient from the surface soil to a depth of 0.6 m increased when the N2O flux significantly increased (Figs 1,7). In contrast, the N2O concentration gradient of the surface soil increased when the N2O flux significantly increased in the Andosol (Figs 2,8). These results suggest that the N2O produced in the soil profile was emitted into the atmosphere. In several studies, a similar N2O flux from the soil into the atmosphere and a similar concentration gradient in the soil profile have been reported after fertilizer application and irrigation (Clough et al. 2006; Goodroad and Keeney 1985; van Groenigen et al. 2005; Hirose and Tsuruta 1996; Lessard et al. 1996; Li et al. 2002; Mosier and Hutchinson 1981; Müller et al. 2004). In our previous paper, we reported that a significant amount of the N2O emission from the Gray Lowland soil and the Andosol that occurred during the increasing frequency of rainfall and after heavy rainfall was derived from the denitrification process (Kusa et al. 2002, 2006). A number of reports have suggested that the effect of soil moisture on N2O production by the denitrification process was greater than the effect of the NO3 concentration in the soil. This is because N2O can be produced by denitrification in subsoil (below a depth of 0.2 m) with high soil moisture levels and a low NO3 concentration (van Groenigen et al. 2005; Li et al. 2002; Müller et al. 2004). In both the Gray Lowland soil and the Andosol, a significant increase in N2O flux with an increase in soil moisture and after heavy rainfall, and with increasing N2O concentrations in the top soil and decreasing soil water suction, occurred at the same time (Figs 1,2). These results suggest that denitrification is the main process responsible for the production of N2O in the soil.

Reported maximum N2O concentrations around a depth of 0.1 m are 0.9–180 p.p.m.v (Arah et al. 1991; Goodroad and Keeney 1985; van Groenigen et al. 2005; Itahashi et al. 1998; Jacinthe and Lal 2004; Lessard et al. 1996; Li et al. 2002; Mosier and Hutchinson 1981; Müller et al. 2004). In our study, the maximum N2O concentration at a depth of 0.1 m in the Gray Lowland soil was 37 p.p.m.v (this N2O flux was 1.5 mg N m−2 h−1) (Fig. 1); this value remained within the reported maximum N2O concentrations (Arah et al. 1991; Goodroad and Keeney 1985; van Groenigen et al. 2005; Itahashi et al. 1998; Jacinthe and Lal 2004; Lessard et al. 1996; Li et al. 2002; Mosier and Hutchinson 1981; Müller et al. 2004) and was similar to a report from a corn field in Colorado (the N2O concentration at a depth of 0.1 m was approximately 40 p.p.m.v and the N2O flux was approximately 2.3 mg N m−2 h−1) (Mosier and Hutchinson 1981). In contrast, the maximum N2O concentrations around a depth of 0.1 m were approximately 0.4–4.2 p.p.m.v in Japanese Andosols, when maximum N2O fluxes (0.04–0.2 mg N m−2 h−1) were measured just after fertilizer application (Hirose and Tsuruta 1996; Li et al. 2002; Tsuruta 1997; Yoh et al. 1997). The N2O concentrations in Japanese Andosols were lower than those in other soils (Arah et al. 1991; Goodroad and Keeney 1985; van Groenigen et al. 2005; Jacinthe and Lal 2004; Lessard et al. 1996; Mosier and Hutchinson 1981; Müller et al. 2004), and this result is consistent with the values reported by Akiyama and Tsuruta (2003), who concluded that N2O emissions from Japanese Andosols were lower than those from other soils in Japan and the world. The reason for the low N2O concentrations in Japanese Andosols was pointed out to be high gas diffusivity resulting from high porosity and low N2O production by denitrification (Li et al. 2002). However, the Andosol at our study site had a maximum N2O concentration at a depth of 0.1 m of 250 p.p.m.v and this concentration was higher than previously reported values, particularly from Japanese Andosols (Figs 1,2). Therefore, considerable N2O may have been emitted from the Japanese Andosols, which had high groundwater levels and a high soil moisture level after heavy rainfall (as did our study site). This is because the N2O concentration in the soil surface might have increased as a result of denitrification after heavy rain on these soils.

The N2O concentration profiles in the soils were different between the Gray Lowland soil and the Andosol (Figs 7,8). The N2O concentration profiles in some studies (Arah et al. 1991; Burton and Beauchamp 1994; van Groenigen et al. 2005; Jacinthe and Lal 2004; Li et al. 2002; Mosier and Hutchinson 1981; Müller et al. 2004; Yoh et al. 1997) were similar to the profile in the Gray Lowland soil of our study site, where the concentration in the soil increased in the deeper layer. In Japanese Andosols, it has been reported that N2O concentrations in the soil surface (depth of 0.1–0.2 m) are higher than the concentrations in the deeper layers (Hirose and Tsuruta 1996; Itahashi et al. 1998; Tsuruta 1997). Although this result is consistent with our study, the N2O concentration profile in the soil varied according to the seasons and was different among the Japanese Andosols (Li et al. 2002; Verchot et al. 1999; Yoh et al. 1997). In this way, there are no consistent results with regard to N2O concentration profiles in the soil.

CO 2 concentration in the soil profile

In both soil types, the CO2 concentration in the soil increased from spring to summer and decreased in autumn. The seasonal pattern of this concentration was similar to that of the CO2 flux from the soil to the atmosphere (Figs 3,4). The CO2 concentrations at a depth of 0.05 m were always higher than the concentrations in the ambient air. Similar types of results have frequently been reported (Hendry et al. 1999; Jacinthe and Lal 2004; de Jong and Schappert 1971; Osozawa 1998). In addition, the peak of the CO2 concentration in the soil profile gradually drops from a depth of 0.2–0.4 m with growing plants and the CO2 concentration increases with the depth in both fallow and cultivated soils after the autumn season (Hendry et al. 1999; Jacinthe and Lal 2004; de Jong and Schappert 1971; Osozawa 1998). Similar results were recorded at our study sites (Figs 3,4,7,8).

N 2O and CO2 concentrations and fluxes after rainfall

The CO2 concentration in the soil was greatly influenced by rain. This is because, at first, the CO2 concentration in the surface soil increased just after rainfall when the soil porosities were filled by rainwater. Thus, the CO2 concentration in the surface soil could have been vertically diffused after drainage and evaporation from the soil surface, resulting in a decrease in CO2 concentrations around the soil surface (Osozawa 1998). In the Andosol, the CO2 concentration at a depth of 0.1 m increased after heavy rainfall (the precipitation during a week before the day of investigation exceeded 80 mm in July 1999, July 2000 and September 2000), but the CO2 fluxes decreased (Figs 3,4). This may have occurred because gas diffusion from the soil into the atmosphere could have been restricted by rainfall, as reported by Osozawa (1998). This is because the D/D0 of the surface soil at this time was below 0.02 (Figs 3,4,5), which might have restricted gas diffusion from the soil into the atmosphere (Hatano 1997). However, the N2O concentrations in the surface soil and the N2O flux from the soil surface increased at the same time (Figs 1,2), suggesting that anaerobic conditions with increasing soil moisture levels and the restriction of gas diffusion could have accelerated the production of N2O by denitrification in the surface soil.

Effect of soil structure on the production and emission of N2O

The production of N2O in the lower soil profile has been reported in several studies when NO3 leached from the surface layer after rain (Goodroad and Keeney 1985; van Groenigen et al. 2005; Müller et al. 2004). This suggests that a N2O production spot in the soil could be greatly influenced by water movement and the NO3 concentration in the soil. In the Gray Lowland soil of our study, the NO3 in the surface soil leached through macropores after rain (Hayashi and Hatano 1999), and the total N concentration of the groundwater rapidly increased after applications of slurry in the grassland (Kanazawa et al. 1999) adjoining the Andosol site of our study. Therefore, it appears that the NO3 in the surface soil leached with rainwater in both soils. In the Gray Lowland soil of our study site, the NO3 concentrations of the soil solution at a depth of 0.7 m were always below 3 mg N L−1, whereas the concentrations of the pipe drain were always approximately 10 mg N L−1 (Hayashi and Hatano 1999); therefore, the subsoil around the macropores could have been in contact with high concentrations of NO3. On another front, it has been reported that NO3 concentrations in the soil solution at a depth of 0.8 m and those in seepage water were comparable in another Andosol in Hokkaido, Japan (Suzuki and Shiga 2004). Hasegawa and Eguchi (2002) found that rainwater moved mainly by a matrix flow in an Andosol without cracks and fissures in Tsukuba, Japan. Therefore, the NO3 concentration in the surface soil could have been higher than in the subsoil because water and NO3 infiltrated from the surface soil to the subsoil by matrix flow. This suggests that the activity of N2O production in the subsoil of the Gray Lowland soil was higher than that in the Andosol and corresponded with the difference in the N2O concentration profiles in the soil (Figs 1,2) and with the contributing ratio of N2O production in the top soil between both soils (Table 2).

An important factor regarding N2O emission from the soil into the atmosphere is that it is not only the activity of N2O production in the soil, but also gas diffusivity. Under usual upland soil moisture conditions, the gas diffusivity of an Andosol with high porosity is higher than that in a Gray Lowland soil; however, gas movement through macropores and cracks is not active in Andosols (Osozawa 1998). In contrast, gas movement through macropores and cracks is dominant in a Gray Lowland soil (Osozawa 1998). Although the gas diffusivity estimated by the value of D/D0 in the surface soil (0–0.1 m) disturbed by the plowing of the Andosol was higher than that of the Gray Lowland soil (Fig. 6), there were no differences in the value of D/D0 at a depth of more than 0.3 m between the Andosol and the Gray Lowland soil. However, the value of D/D0, measured using a 100 mL core, could not take into account gas flowing through macropores and cracks. This suggests that the gas flowing in the subsoil of the Gray Lowland soil with macropores and cracks was higher than the value of D/D0, and thus the gas diffusivity around macropores and cracks in the subsoil of the Gray Lowland soil might be higher than that in the Andosol. Therefore, the mobility of N2O in the subsoil around the macropores and cracks of the Gray Lowland soil might be higher than that in the Andosol. In addition, the N2O produced in the subsoil of the Andosol without macropores and cracks might have been reduced to N2 before it was emitted to the atmosphere. These results indicate that differences in water mobility, NO3, O2 and N2O in the soils, particularly in the subsoil, between the Gray Lowland and the Andosol might be the reason for variation in the seasonal pattern of N2O fluxes, the N2O concentration profile in the soil, and the ratio of the contribution of the subsoil to N2O production in the soil between both soils (Figs 7,8; Table 2).

The CO2 emitted from the soil into the atmosphere is produced by the respiration of plant roots and soil microbes (Smith et al. 2003). Despite the differences in soil type, CO2 could be produced in the top soil because plant roots and soil microbes were distributed in the top soil (Nakamoto 1993; Osozawa 1998). More than 90% of the CO2 emitted from the soil to the atmosphere was produced in the top soil in both soil types (Table 3). Unlike N2O, there was no significant correlation between the CO2 flux through to a 0.3 m depth and the flux from the soil into the atmosphere in either soil. In this way, our results correspond to previous reports (Nakamoto 1993; Osozawa 1998; Smith et al. 2003).

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

In the Gray Lowland soil and in the Andosol, N2O and CO2 were mainly produced in the top soil (0–0.3 m depth). Seasonal patterns in the CO2 concentration profile were similar in both soil types. However, the N2O concentration profile was different between the two soils. In addition, the ratio of the contribution of the subsoil to the N2O production in the soil of the Gray Lowland soil was higher than that of the Andosol because the N2O production in the subsoil around macropores and cracks of the Gray Lowland soil might have been activated by leaching of NO3 through macropores and cracks. Subsequently the N2O produced in the subsoil could have been rapidly emitted from the soil into the atmosphere through macropores and cracks. This suggests that variations in the N2O concentration profile between the two soils are caused by differences in the soil structure, in particular because of the presence of macropores and cracks in the soil structure, which influenced the production and movement of N2O in the soil.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

We thank Dr Ohashi, Dr Yoshida, Dr Hayakawa, Dr Kanazawa and Dr Tsuruta for their contribution in operating the gas chromatograph and for their valuable suggestions. This study was partly supported by Japanese Grants-in-Aid for Science Research from the Ministry of Education (08456038) and by a Global Environment Research Fund from the Environment Agency (Ministry) of Japan B-51 (6)

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  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References
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