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Published online 27 October 2005
Published in Soil Sci Soc Am J 69:1977-1986 (2005)
DOI: 10.2136/sssaj2005.0035
© 2005 Soil Science Society of America
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Soil Biology & Biochemistry

Continuous, Automated Nitrous Oxide Measurements from Paddy Soils Converted to Upland Crops

Seiichi Nishimuraa,*, Takuji Sawamotoa,b, Hiroko Akiyamaa, Shigeto Sudoa, Weiguo Chenga and Kazuyuki Yagia

a National Institute for Agro-Environmental Sciences, 3-1-3 Kannondai, Tsukuba, Ibaraki 305-8604, Japan
b Rakuno Gakuen Univ., 582 Midori-cho, Bunkyodai, Ebetsu, Hokkaido 069-8501, Japan

* Corresponding author (ssnn{at}niaes.affrc.go.jp)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Since rotation of irrigated rice (Oryza sativa L.) and upland crop cultivations is a major land use management widely conducted in Japan, dynamics of greenhouse gas emission from drained paddy fields are required to be clarified. In the present study, the seasonal courses of N2O flux from a drained paddy field were measured continuously over one and a half years with an automated monitoring system at an interval of six times per day. The experimental paddy field with Gray lowland soil was drained and cultivated with two upland cropping systems: single cropping of upland rice and double cropping of soybean [Glycine max (L.) Merrill] and wheat (Triticum aestivum L.). Unlike many of the previous studies, the increase in N2O flux after fertilizer application was not distinctive, which suggests that the contribution of nitrification to the N2O flux is relatively small in this field. Temporal peaks of N2O flux were often observed after heavy rainfalls and harvest of the crops, which lasted for a few to approximately 15 d. In all the crop cultivations, high peaks of N2O flux were observed when the crops were in the flowering to ripening stages, which suggests significant influence of crop development on N2O flux. N2O production in the rhizosphere or possible pathway for transport through the plant body is possible loss mechanisms. In particular, the amounts of N2O flux during the flowering and ripening stages of the summer crops were significant and were most responsible for the cumulative N2O emissions. Cumulative N2O emissions increased 4.0 to 5.3 times by the land use change from paddy rice cultivation to upland crop cultivations.

Abbreviations: GC, gas chromatograph • SW, double cropping of soybean and wheat • UR, single cropping of upland rice • WFPS, water-filled pore space


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
NITROUS OXIDE is recognized as a major greenhouse gas, which has a 296-times higher global warming potential than CO2 in a period of 100 yr (Intergovernmental Panel on Climate Change, 2001). It also participates in the destruction of stratospheric ozone (World Meteorological Organization, 1999). Atmospheric N2O concentration has been increasing by about 0.8 nmol mol–1 per year during the 1980s (Intergovernmental Panel on Climate Change, 1996). Nitrous oxide is produced in soil by various kinds of soil microorganism as a byproduct of nitrification and an intermediate product of denitrification processes (Bouwman, 1990; Hutchinson and Davidson, 1993), and thus fertilized agricultural soil is recognized as one of the major sources of N2O, accounting for about 13 (Olivier et al., 1998) or 24% (Mosier et al., 1998; Kroeze et al., 1999) of the annual global emission from all the sources. Among agricultural fields, upland agricultural fields are recognized as being a major source of N2O, and thus field measurements of N2O emission from upland agricultural fields have been extensively conducted in previous studies.

A significant increase in N2O flux may be most typically found after fertilizer application, which has been reported in many previous studies (e.g., Conrad et al., 1983; Lessard et al., 1996; Dobbie et al., 1999; Akiyama et al., 2000; Dobbie and Smith, 2003; Hou and Tsuruta, 2003; Sehy et al., 2003). However, this may not always be the case; some previous experiments conducted in the northern part of Japan have found a less distinctive increase in N2O flux after fertilizer application (Sawamoto and Hatano, 2000; Kusa et al., 2002; Koga et al., 2004). Other temporal peaks of N2O flux have often been found after rainfall (e.g., Conrad et al., 1983; Mosier et al., 1986; Lessard et al., 1996; Sehy et al., 2003), thawing (Goodroad and Keeney, 1984; Cates and Keeney, 1987; Flessa et al., 1995; Koga et al., 2004), crop harvest (Sehy et al., 2003; Nishimura et al., 2004), and manure incorporation (Hou and Tsuruta, 2003; Koga et al., 2004) among others.

So as to restrict the overproduction of rice, a production adjustment policy has been implemented by the Japanese government since 1975. According to this policy, temporal cultivation of upland crops for a few or several years in drained paddy fields has been recommended to farmers. Nowadays, various kinds of upland crops, particularly cereal crops, are cultivated in drained paddy fields; e.g., 56% of the wheat cultivation area and 84% of the soybean cultivation area in Japan were drained paddy fields in 2002, respectively (Ministry of Agriculture, Forestry and Fisheries of Japan, 2003). About 50% of Japanese agricultural fields for consecutive upland crop cultivation consist of volcanic ash soils. Volcanic ash soil tends to maintain its aerobic conditions due to its high porosity and thus nitrification is considered to be the major process for N2O production in the soil. On the other hand, most types of Japanese paddy soil are of alluvial origin, and Gray lowland soil (Fluvisols) is the most common (Japanese Society of Pedology, 2002). In contrast to volcanic ash soil, Gray lowland soil is characterized by a heavy texture and tends to develop anaerobic conditions after precipitation, which presumably provides suitable conditions for denitrification (Linn and Doran, 1984). Besides soil classification, soils in drained paddy fields have some other physicochemical properties. Soil clods tend to be kept larger (Takahashi et al., 1999), which is presumably suitable for developing anaerobic microsites inside. In addition, although organic matter content in paddy soil is kept higher than that in upland soil due to the restricted decomposition rate during the submerged periods (Kyuma, 2004), temporal enhancements of the decomposition rate and N mineralization rate are expected by the drainage of paddy fields for upland crop cultivations (Takahashi et al., 2003). These characteristics of the drained paddy soil may also influence N2O emission, and therefore it is possible that the dynamics of N2O in the drained paddy fields are significantly different from those in the fields with consecutive upland crop cultivation. Field experiments on N2O emission have also been conducted in Japan, in arable fields with some horticultural upland crop cultivations (e.g., Akiyama et al., 2000; Akiyama and Tsuruta, 2003; Hou and Tsuruta, 2003; Koga et al., 2004), and the effects of various environmental factors (temperature, rainfall, spring thawing, etc.) and agricultural practices (application of slow-release chemical fertilizer, incorporation of residue or organic matter, etc.) to N2O flux have been clarified. However, as far as our knowledge, there have been no studies on N2O emission from drained paddy fields for upland crop cultivations to date.

In the present study, measurement of N2O flux from drained paddy fields with two kinds of upland crop cultivation; that is, single cropping of upland rice and double cropping of soybean and wheat has been conducted with an automated continuous monitoring system. The monitoring system in the present study enables continuous measurement of N2O flux at an interval of six times per day. By the continuous frequent measurement, detailed time courses, including many temporal enhancements within short periods and diel fluctuations, of the N2O flux can be evaluated. Factors influencing the N2O flux were discussed with emphases on chemical fertilizer application, rainfall, labile organic matter supply, and crop growth stage.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Outline of Upland Crop Cultivation
The experiment was conducted at an experimental field in the National Institute for Agro-Environmental Sciences (NIAES) (36°01' N, 140°07' E), Japan. The field includes six lysimeters, each with a 9 m2 (3 by 3 m) cross-sectional area and a 1.0-m depth. The soil type was Gray lowland soil (Fluvisols). The bulk density of the top soil (0- to 5-cm depth) is 0.84 g cm–3. The soil pH in H2O is 5.7. The carbon and N contents in the top soil (0- to 5-cm depth) are 18.9 mg C g soil–1 and 1.6 mg N g soil–1, respectively. Single cropping of paddy rice cultivation was conducted in the field for approximately 10 consecutive years before the experiment. The field was finally drained in September 2001 for the harvest of rice, and then kept drained throughout the experiment period.

Two kinds of upland crop cultivation, single cropping of upland rice (cv. Toyohatamochi) (hereafter; UR plot) and double cropping of soybean (cv. Enrei) and wheat (cv. Norin-61) (hereafter; SW plot), were conducted from 2002 to 2003, each in two of the lysimeters. The single cropping of paddy rice was consecutively conducted in the two rest plots (Nishimura et al., 2004). Urea, superphosphate-fused phosphate mixture, and KCl are applied as N, PO43–, and K fertilizers, respectively, in all the upland crop cultivations. Basal fertilizer was incorporated before sowing to the top soil layer of 0 to 10 cm with a portable rotary tiller, whereas supplemental fertilizer was applied by top-dressing just beside the crops. The specific dates of the agricultural practices are shown in Table 1.


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  		Table 1. Specific dates of agricultural practices from 1 Jan. 2002 to 19 June 2003 in experimental plots with single cropping of upland rice (UR) and double cropping of soybean and wheat (SW). Basal fertilizer was incorporated with plowing. Supplemental fertilizer was applied by top-dressing.

 
From the autumn of 2001 to April 2002, drainpipes equipped at the bottom of the lysimeters were kept open so that all the free water in the soil was drained. Then, the drainpipes at the bottom were closed and other drainpipes equipped at the 90-cm depth were opened so that groundwater levels were kept to the 90-cm depth.

Measurement of Nitrous Oxide Flux
Nitrous oxide flux has been monitored in the field from 1 Jan. 2002 by a closed chamber system with an automated gas sampling and analyzing equipment. Six chambers made of transparent polycarbonate and acrylic plates were placed, each at the center of a lysimeter plot. The cross-sectional area of each chamber was 0.81 m2 (0.9 by 0.9 m). The height of each chamber was 0.6 m during the fallow period or when the crop height was lower than 0.6 m, and it was changed to 1.2 m by connecting additional sidewalls when the crop became higher. About every 40 min, the lids of one of the chambers were closed with pressure cylinders, kept closed for about 30 min, and then opened again. Each chamber closed every 4 h (six times per day). During the closed period, the air inside of the chamber was circulated with a pump at a flow rate of 5 to 7 L min–1. The air inlet and outlet were located 20 cm below the top of the chamber. Part of the circulated air was led to a gas chromatograph (GC) four times at an interval of 8.5 min. N2O flux was calculated based on the increasing rate of the four measured gas concentrations.

A GC (GC-14B, Shimadzu, Japan) equipped with an electron capture detector and with switching valves was used for the analysis of N2O concentration. The columns were packed with Porapak Q (Waters Corp., Milford, MA). Argon with 5% CH4 mixture gas was supplied to the carrier gas lines, by which Ar works as the carrier and the mixed CH4 works as the quencher (Mosier and Mack, 1980). Water in the sample air was preliminarily separated in the precolumn by changing the 10 port switching valve and exhausted (without going to the detector) through the choke column, and then N2, O2, and CO2 in the sample air were subsequently separated in the main column by changing the six port switching valve and exhausted through another choke column. The GC and switching valves were controlled with an integrator (C-R7A plus, Shimadzu, Japan) and a relay controller (PRG 102A, Shimadzu, Japan). Some other details of the flux monitoring system are described in our previous report (Nishimura et al., 2005).

Cumulative N2O emissions were calculated by integrating the daily means of six consecutive measurement data per day. There were 31 to 32 d (different among the plots) with a data deficit (wholly or partly) in the whole monitoring period, due to some problems or system maintenance. The daily mean N2O fluxes for the dates with a data deficit were estimated by linear interpolation using the flux data of adjacent dates without a data deficit.

Other Data Measurements
Air temperatures inside the chambers were measured for the calculation of the gas fluxes, with platinum resistance thermometers placed at approximately 30 cm above the soil surface and recorded on a data logger (HR2400, Yokogawa, Japan). Ambient air temperature and precipitation data were provided hourly from the climate data acquisition station in the NIAES.

Volumetric water contents at 10-cm depths of the soil were monitored using time domain reflectrometry moisture sensors (CS615, Campbell Scientific Instruments, Logan, UT) and recorded on a data logger (CR10X, Campbell Scientific Instruments, Logan, UT). The water-filled pore space (WFPS) of the soil was calculated based on the measured volumetric water content and the porosity of the soil (0.61 ± 0.03).

Soil core samples of 0- to 5-cm depth were collected occasionally during the study period for the analyses of soil inorganic N contents. Soil samples were collected randomly from five locations in each plot, except after the supplemental fertilizer application for the upland rice or wheat, and then mixed. After the top-dressing of supplemental fertilizer for upland rice or wheat, the five soil samples were collected just beside the rows where the supplemental fertilizer was applied. The 15-g samples of the collected fresh soil were extracted with 100 mL KCl solution (100 g KCl L–1). Nitrate–nitrogen (NO3–N) was analyzed by the Cu–Cd reduction method, and NH4–N was analyzed by the indophenol blue method in a continuous flow analyzer (TRRACS, Bran + Luebbe, Nordersterdt, Germany).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Crop Biomass
Planting densities, fertilization rates, harvested aboveground biomasses, and the amounts of straw incorporation are summarized in Table 2. The harvested aboveground biomass of the upland rice plants in the chambers was 549 g m–2, which was 25% lower than that outside the chambers, whereas those of the soybean and wheat plants in the chambers were 524 and 1237 g m–2, respectively, which were 11 and 20% higher than those outside the chambers, possibly as a result of the so-called "chamber effect" (Nishimura et al., 2005). Part of the harvested straw of the upland rice was dried and incorporated in the autumn of 2002.


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  		Table 2. Planting densities, fertilizer application rates, harvested aboveground biomasses (mean ± standard deviation) and amounts of straw incorporation (mean ± standard deviation).

 
Seasonal Course of Nitrous Oxide Flux
The seasonal courses of N2O fluxes, the WFPS of the soil, soil NH4–N and NO3–N contents, and ambient air temperature and precipitation in the UR and SW plots are shown in Fig. 1 and 2 . The N2O fluxes and WFPS are shown for each replicate. Increases in N2O flux after fertilizer application were not distinctive in this study, in either crop cultivation. Distinctive temporal peaks of N2O flux were observed both in the UR and SW plots after heavy rainfalls during mid-June and mid-July 2002, which lasted for several days. In the UR-2 plot (only at one plot of the duplication), a temporal increase in N2O flux was also observed from March to early April 2003. This peak of N2O flux was synchronized with a significant temporal increase in the soil WFPS up to more than 0.6. During this period, the underground drainpipes at the UR-2 plot suffered from unexpected choking, which caused an unexpected increase in water content in the top soil. In the UR plots, a significant increase in N2O flux was also observed after the harvest of upland rice in September 2002, which lasted for about 2 wk. A slight increase in N2O flux was also observed after the harvest of wheat in June 2003, in the SW plots. On the other hand, a distinct increase in N2O flux was not observed after the harvest of soybean in October 2002. The most significant high peaks of N2O flux were observed from late July 2002 both in the UR and SW plots, and the markedly high N2O flux lasted for about 1 mo during the summer season. The maximum N2O flux was observed on 1 Aug. 2002 in the SW-2 plot (3894 µg N m–2 h–1, Fig. 3b) . Nitrous oxide emissions from the beginning of flowering to the harvest of upland rice (UR plots) and soybean (SW plots) were 145 and 239 mg N m–2, which corresponded to 60 and 75% of the cumulative N2O emissions, respectively. In the SW plots, an increase in N2O flux was also observed in the spring of 2003, around the flowering and ripening stages of the wheat.



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  		Fig. 1. Seasonal courses of (a) daily N2O emission, (b) water-filled pore space (WFPS) of the soil at 10-cm depth, (c) soil NH4–N and NO3–N contents, and (d) ambient air temperature and precipitation in the fields with single cropping of upland rice (UR-1, 2) from 1 Jan. 2002 to 19 Jun. 2003. Daily N2O emissions are the daily means of six consecutive measurement data per day in each plot. WFPS and ambient air temperature are also the daily averages. Soil NH4–N and NO3–N contents are mean values of the duplicated plots. Periods without lines in (a), (b), and (d) indicate failure of data acquisition due to the problems or maintenances of the monitoring system. Horizontal arrows at the bottom of (c) show the period that the soil samples were taken just beside the rows where the supplemental fertilizer was applied. Abbreviations with vertical arrows in the figures show the following practices. S, seed sowing; BF, SF, applications of basal and supplemental fertilizer; F, beginning of flowering; H, crop harvest; SI, straw incorporation.

 


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  		Fig. 2. Seasonal courses of (a) daily N2O emission, (b) water-filled pore space (WFPS) of the soil at 10-cm depth, (c) soil NH4–N and NO3–N contents, and (d) ambient air temperature and precipitation in the fields with double cropping of soybean and wheat (SW-1, 2) from 1 Jan. 2002 to 19 June 2003. The compositions of subfigures, details of the data, and abbreviations in the figure are the same as those in Fig. 1.

 


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  		Fig. 3. Detailed time courses of N2O fluxes during some peak periods with magnified time scales in the fields with single cropping of upland rice (UR-1, 2) and with double cropping of soybean and wheat (SW-1, 2). All measurement data of N2O fluxes are plotted. Abbreviations with arrows in the figures are the same as those in Fig. 1.

 
Diel Course of Nitrous Oxide Flux During Peak Periods
Detailed time courses of N2O flux during some peak periods are shown in Fig. 3, with magnified time scales. As mentioned above, markedly high peaks of N2O flux were observed both in the UR and SW plots during the summer of 2002. As shown in Fig. 3b, the diel fluctuations of N2O flux, with a high N2O flux at daytime and a low N2O flux at nighttime, were distinctive during this peak period. In the SW plots, similar distinctive diel fluctuations of the N2O flux were also observed in the spring of 2003, although the magnitude of the flux was less than those in the summer season (Fig. 3d). In contrast, the diel fluctuations of the N2O flux were less distinctive during the peak period after the harvest of upland rice (Fig. 3c).

Soil Water Content
On the whole, the WFPS of the soil was higher in winter and lower in summer (Fig. 1, 2). Although there was a slight increase in WFPS due to the temporal heavy rainfall on Aug. 1, 2002, the lowest level of WFPS was from late July to August 2002, during which the markedly high peak of N2O flux was observed. Weak positive correlations between N2O fluxes and soil WFPS were found during this period both in the UR and SW plots (Fig. 4) . Significant increase in WFPS observed during March and early April 2003 in the UR-2 plot was due to the unexpected choking of the soil water at the bottom of the lysimeter, which may be closely related to the simultaneous enhancement of N2O flux (Fig. 1).



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  		Fig. 4. Relationships between daily N2O emission and water-filled pore space (WFPS) of the soil at 10-cm depth in the fields with single cropping of upland rice (UR) and with double cropping of soybean and wheat (SW) from 28 July to 12 Aug. 2002 (during the period shown in Fig. 3b). Daily N2O emissions are the daily means of six consecutive measurement data per day in each plot. WFPS are also the daily averages. Data in the two replicates were plotted together.

 
Inorganic Nitrogen Contents of Soil
As shown in Fig. 1c, slight and temporal increases in both soil NO3–N and NH4–N contents were observed just after the basal fertilizer application (on 19 Apr. 2002) in the UR plots, although the maximum values at the beginning of May were below 20 µg g soil–1. Soil NO3–N content then increased after late May and reached a maximum (98 µg g soil–1) on 13 June 2002. The increase in soil NH4–N content after supplemental fertilizer application (on 28 June 2002) was distinctive and it reached a maximum of 93 µg g soil–1 on 4 July 2002, whereas the increase in soil NO3–N content was less distinctive. After August 2002, both soil NO3–N and NH4–N contents were kept lower than 20 µg g soil–1 until the basal fertilizer application in the following year.

As shown in Fig. 2c, the immediate increase in soil NH4–N content after basal fertilizer application for soybean cultivation (on 24 May 2002) was followed by the subsequent increase in soil NO3–N content in the SW plots, suggesting that the immediate hydrolysis of the applied urea and subsequent nitrification proceeded during this period. Both soil NO3–N and NH4–N contents then decreased to lower than 10 µg g soil–1 at the end of June 2002 and were kept at low levels during the summer season. After the basal fertilizer application for winter wheat (on 6 Nov. 2002), soil NH4–N content increased again, and the subsequent decrease during the winter season was so slow that it remained higher than 20 µg g soil–1 until the supplemental fertilizer application in the following spring (on 12 Mar. 2003). The increase in soil NH4–N content according to this supplemental fertilization lasted more than 1 mo but ended before the end of April 2003.

As discussed in the following, it should be noted that N2O fluxes were maintained at low levels both in the UR and SW plots during late May and mid-July 2002 except just after the heavy rainfalls and were not significantly enhanced according to the increase in soil NO3–N content (Fig. 3a). This suggests that the contribution of nitrification according to the fertilizer application is relatively low for the enhancement of N2O flux.

Cumulative Nitrous Oxide Emission
Annual cumulative N2O emission in 2002 was 2.41 kg N ha–1 in the UR plots, whereas cumulative N2O emission during the whole cycle of the double cropping (total: 392 d, from 19 May 2002 to 19 June 2003) was 3.19 kg N ha–1 in the SW plots. The ratio of the cumulative N2O–N emitted to applied fertilizer N was 4.02% in the UR plots (annual total), whereas they were 13.25 and 0.54% in the SW plots (during the soybean and wheat cultivation periods, respectively) (Table 3). The amount of N2O emission in the SW plots during the soybean cultivation period was higher than that in the UR plots in spite of the low N fertilization level, which resulted in a markedly high ratio in the SW plots during the soybean cultivation period. In the rest two experimental plots with single cropping of paddy rice cultivation, annual N2O emission in 2002 was 0.602 kg N ha–1 (Nishimura et al., 2004). Compared to this value, cumulative N2O emissions in the UR and SW plots were 4.0 and 5.3 times higher, respectively.


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  		Table 3. Cumulative N2O emissions (mean ± standard deviation) and the ratios of cumulative N2O–N emitted to the applied fertilizer N from the drained paddy fields with single cropping of upland rice (UR) and double cropping of soybean and wheat (SW).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Seasonal Course of Nitrous Oxide Flux
On the whole, the peaks of N2O flux observed in the present study can be classified into the following three patterns; that is, the increase in N2O flux related to the increase in the soil water content, the increase in N2O flux after the crop harvest, and the increase in N2O flux during the flowering and ripening stages of the cultivated crops.

At first, the response of N2O flux depending on the fertilizer (urea) application and the subsequent increase in soil NO3–N content is discussed here. The significant increase in N2O flux after fertilizer application has been reported in many previous studies, and nitrification has been indicated to be the main process of N2O production (e.g., Conrad et al., 1983; Lessard et al., 1996; Dobbie et al., 1999; Akiyama et al., 2000; Dobbie and Smith, 2003, Hou and Tsuruta, 2003; Sehy et al., 2003). In the present study, however, the response of N2O flux to fertilizer application was different from these previous studies. During late May and early June 2002, N2O fluxes were maintained at low levels (<20 µg N m–2 h–1) both in the UR and SW plots, whereas soil NO3–N content increased during this period. Also, in the UR plots, N2O flux did not significantly increase just after supplemental fertilizer application on 28 June 2002 (Fig. 3a). These suggest that the contribution of nitrification depending on the fertilizer applications to the enhancement of N2O flux is relatively low in this field. This may not be consistent with the results of many of the above-mentioned reports, although some other previous studies (Sawamoto and Hatano, 2000; Kusa et al., 2002; Koga et al., 2004) have also found less distinctive increase in N2O flux after fertilizer application, which is similar to the result of this study. The low N2O flux during the progress of nitrification may be partly attributed to the low soil water content (Lipschultz et al., 1981; Linn and Doran, 1984). In addition, the soil microbial community in drained paddy fields may be quite different from that in the fields with consecutive upland crop cultivations. However, detailed mechanisms remain still uncertain and further investigation will be required to clarify in which cases N2O flux can be significantly enhanced according to the progress of the nitrification of the applied fertilizer.

Some temporal enhancements of N2O flux were observed around heavy rainfalls on 18 June and 10 July 2002 both in the UR and SW plots (Fig. 3a). In spite of the different timings and the rates of fertilizer application, the timings and the magnitudes of the enhanced N2O flux in the UR and SW plots were similar. This suggests that not only the N of the applied urea but also mineralized soil organic N became the N source of the N2O produced during this period. Decomposition of soil organic matter may have been stimulated by the heavy rainfalls and the increase in ambient temperature during this period (Fig. 1d, 2d). Enhancements of soil NO3–N contents observed both in the UR and SW plots around mid-June 2002 might also have been partly attributed to decomposition of soil organic matter. Although enhancements of both nitrification and denitrification are assumed as the processes of the N2O production, enhancement of nitrification is likely most responsible during this period with soil WFPS lower than 0.6, as reported by Linn and Doran (1984).

Significant effect of temporal increase in soil water content was considered to be the main cause of the temporal increase in N2O flux observed from March to early April 2003 in the UR-2 plot, which were synchronized with the unexpected enhancement in WFPS up to higher than 0.6 due to the choking of the drainage (Fig. 1). In this case, significant enhancement of denitrification was thought to have occurred in the high soil WFPS condition (Linn and Doran, 1984). This kind of temporal but significant increases in soil water content, which often causes temporal waterlogging, is often observed in drained paddy fields in Japan, due to insufficient drainage after heavy rainfalls. This may in some cases significantly contribute to cumulative N2O emission.

Effect of organic matter derived from dead root to N2O production was indicated by enhanced N2O flux observed after crop harvest. In the UR plots, a temporal increase in N2O flux was observed after the harvest of upland rice (Fig. 1, 3c). A slight temporal increase in N2O flux was also observed in the SW plots after the harvest of wheat (Fig. 2). In the adjacent plots in this experimental field, we also found a similar temporal increase in N2O flux after the harvest of paddy rice (Nishimura et al., 2004). A distinct increase in N2O flux, however, was not found after the harvest of soybean. In the present study, the amounts of root and stubble remaining after the harvest were the highest for the upland rice and the lowest for the soybean (data not shown), which were on the same order of magnitude of N2O flux just after the harvest. This indicates that the amount of biomass remaining after the harvest is closely related to the increase in N2O flux just after the harvest. In a pot experiment of soybean cultivation, Yang and Cai (2005) also showed significant enhancement of N2O flux immediately after cutting the aboveground plant body and discussed that the senescence and decomposition of the soybean root was closely related. They also discussed that the amount of root biomass significantly influenced the magnitude of the N2O flux.

The most characteristic result of the experiment in the present study may be the markedly high peaks of N2O flux from late July to mid-August 2002 both in the UR and the SW plots. Some of the previous studies have also shown markedly high peaks of N2O flux during the summer season (Cates and Keeney, 1987; Sawamoto and Hatano, 2000; Kusa et al., 2002; Sehy et al., 2003), which are apparently similar to the result of the present study. All of the high peaks of N2O flux in these previous studies were apparently synchronized with the high amount of rainfall during the summer season. Another previous report with high peaks of N2O flux during the summer in the field with tea cultivation (Tokuda and Hayatsu, 2004) may be closely related to the extremely high amount of fertilizer application (1200 kg N ha–1). However, the mechanisms that caused the high peaks of N2O flux during the summer season in the present study may be different from those observed in these previous studies. Distinctive rainfall did not occur except for a temporal rainfall on 1 Aug. 2002, and therefore soil water content was maintained at a low level during most of this period. In addition, more than about 30 d (in the UR plots) and 60 d (in the SW plots) had passed since the last fertilizer applications so that the peaks of soil inorganic N contents had already ended in this period (Fig. 1c, 2c). A similar significant peak of N2O flux was found in the SW plots from late April to mid-May 2003 (Fig. 3d). This peak of the N2O flux has some similar characteristics to those found in the summer of 2002, described as follows. First, the increase in N2O flux began more than about 40 d after the last fertilization (supplemental fertilizer application for wheat on 12 Mar. 2003). Second, the significant peaks of soil inorganic N contents depending on the fertilization have already ended so that soil NH4–N and NO3–N contents were lower than those in the adjacent UR plots. Third, this peak of N2O flux lasted more than 20 d and was apparently not synchronized with the rainfalls. Fourth, distinctive diel courses of N2O flux, with high N2O fluxes at daytime and low at nighttime, have been found in the both peak periods (Fig. 3b, 3d). It should be noted that the diel course of N2O flux was less distinctive during the peak period just after the harvest of upland rice (Fig. 3c) or paddy rice (Nishimura et al., 2004).

In general, N2O flux remains at a low level under the conditions of low soil inorganic N content and low soil water content. Therefore, the mechanisms of these peaks of N2O flux may not be well accounted by factors such as the addition of N fertilizer or temporal enhancement of soil water content. However, another explanation for these peaks of N2O flux may be possible from the viewpoint of the cultivated crops in the chambers. The influences of the crops on N2O flux can be assumed both for N2O production and transport. A recent report (Smart and Bloom, 2001) showed possible N2O production via photoassimilation of nitrite in the leaves of wheat seedlings, although the amounts of the N2O emission from the shoots were small (ca. 2.6 µg N m–2 h–1). Other recent report (Hakata et al., 2003) also showed N2O production in the seedlings of 16 plant species, although the amounts of the produced N2O were also small (maximum of 0.45 ng N2O g fresh weight–1 wk–1). According to these reports, the contribution of N2O produced in plants to the total N2O emission in the field condition is, if any, thought to be small. Significant N2O production in the rhizosphere according to the senescence and decomposition of the crop root, as suggested by Yang and Cai (2005), may be more responsible for the enhancement of N2O flux from the flowering to ripening stages of the crops. Some other recent studies have shown the possibility of the plant body acting as an effective pathway for N2O transport. Li and Chen (1993) conducted flux measurement with separated chambers so as to discriminate N2O flux from the soil surface and that through the crops, and detected a distinct amount of N2O flux through the crops. In addition, they also showed that the amount of N2O flux through the crops significantly changed during the crop growth stages, with the highest peaks observed around the flowering stages in all of the investigated crop species [soybean, wheat, and millet (Panicum miliaceum L.)]. Chang et al. (1998) also showed possible N2O transport through the plant body in the seedlings of canola (Brassica napus L.) and barley (Hordeum vulgare L.). In particular, the present results that distinctive diel fluctuations of N2O flux were found only during the peak periods with crops in the flowering to ripening stages (Fig. 3) may further support the possible influence of the crops on N2O flux, since this suggests that the physiology of the crops such as the photoassimilation of inorganic N or enhanced root respiration rate or decomposition rate during the daytime is closely related to the N2O production and/or transport. Reports on N2O flux, which focus on the influences of crops, are limited to date. The details of the mechanisms of N2O production and transport, including the amount of N2O produced in the soil and emitted into the atmosphere through the crops, require further investigation in future studies.

Cumulative Nitrous Oxide Emission
Bouwman (1996) has proposed an equation for the estimation of cumulative N2O emission as

where E and F are the annual N2O emission and the amount of applied N fertilizer in kg N ha–1, respectively. Both the cumulative N2O emission in the UR plots in 2002 (2.41 kg N ha–1) and that in the SW plots during one cycle of the double cropping (3.19 kg N ha–1) obtained in the present study were slightly higher than that obtained with this estimation. The ratio of the emitted N2O–N to applied-fertilizer N was quite different between the cropping systems. It was extremely high (13.25%) in the SW plots during the soybean cultivation period (Table 3). This was due to the cumulative N2O emission during the soybean cultivation period being higher than that in the adjacent UR plots in spite of the low N application rate for the soybean cultivation (20 kg N ha–1). Bouwman (1996) has also suggested the possibility of higher cumulative N2O emissions than those estimated with the above-mentioned equation in the case of leguminous crop cultivations, due to some influences of the bacteria in the roots resulting in symbiotic N uptake. The result in the present study supports this suggestion. On the other hand, both cumulative N2O emission and the ratio of the emitted N2O–N were comparatively low during the wheat cultivation period (0.54%; Table 3). Cultivations of winter crops may produce less N2O emission due to the lower ambient temperature.

In comparison with the paddy rice cultivation (Nishimura et al., 2004), cumulative N2O emissions in the drained upland crop fields were respectively 4.0 times (UR plots) and 5.3 times (SW plots) higher. It should be noted that the same soil was filled and the same cropping system (single cropping of paddy rice cultivation) had been continuously conducted until 2001 in all the six experimental plots. Significant increase in net global warming potential by the increased N2O emission was therefore indicated according to the land use change from paddy rice cultivation to upland crop cultivations. However, absence of methane emission is also expected by the land use change from paddy rice cultivation to upland crop cultivations, which means significant decrease in net global warming potential. Dynamics of CO2 emission from the soil is also thought to change significantly. Comprehensive influence of land use change from paddy rice cultivation to upland crop cultivations to net global warming potential should be investigated in future studies.


   ACKNOWLEDGMENTS
 
The authors thank Drs. Yasuhiro Nakajima, Seiichiro Yonemura, and Tomoyasu Yamada, NIAES, for their valuable discussion and technical support, and Messrs. Fumio Suzuki, Tadao Suzuki, Takahiro Ara, and Hiroshi Kamimura, NIAES, for their help in executing the experiment and in managing the experimental field. The authors also thank the associate editor and the anonymous reviewers for providing many valuable suggestions and comments for improving the manuscript. This study was partly supported by the project named "Elucidation of Vulnerability in Agriculture, Forestry and Fishery to Global Warming and Development of Mitigation Techniques" conducted by the "Research Initiatives" in the field of agro-environmental studies of the Ministry of Agriculture, Forestry and Fisheries from 2002 to 2006.

Received for publication January 27, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
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