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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)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: GC, gas chromatograph • SW, double cropping of soybean and wheat • UR, single cropping of upland rice • WFPS, water-filled pore space
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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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.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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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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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).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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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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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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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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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
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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.
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ACKNOWLEDGMENTS |
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Received for publication January 27, 2005.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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