JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. D19, PAGES 25,241-25,250, OCTOBER 20, 1998

Emissions of methane, nitrous oxide, and other trace gases from rice fields in China

M. A. K. Khalil, 1 R. A. Rasmussen, 2 M. J. Shearer, 1 Zong-Liang Chen, 3 Heng Yao, 3 and Jun Yang 4

Abstract. We measured the emissions of methane, nitrous oxide, and other environmentally important trace gases from rice fields in China. The experiments were conducted near Beijing, representing temperate latitude rice agriculture, and at Guangzhou, representing tropical and subtropical agriculture. These studies complement our intensive research at Tu Zu in China (subtropical) and the work near Bogor, Indonesia, representing tropical rice agriculture. The experiments described here span 5 years between 1992 and 1996. The seasonally averaged methane emissions from Beijing were about 9 (5-16) mg m -2 h -l. Emissions from the first crop of the year at Guangzhou were about 5 (2-8) mg m -2 h -• and about 30 (18-54) mg m -2 h -• from the second crop. Measurements of nitrous oxide fluxes from the same rice fields show that emissions are confined to events that consist of high fluxes decaying to zero within a few days. Some of the events appear to be triggered by fertilizer applications. It is apparent that rice fields using nitrogen-based fertilizers are a source of N20 , but a robust estimate of whole season average emission rate is still not possible. The composite of all the N20 fluxes we observed gives a geometric mean of about 70 •tg m -2 h -l. These rates are high enough to suggest that the rice fields could be a globally important source of N20. As the agricultural practices change from use of organic fertilizers to nitrogen-based fertilizers, the role of N20 emissions from rice fields may become more important in its global budget. We also observed that chloroform and carbon monoxide were emitted from all fields studied. The flux of CO is not large enough to be important on the global scale, but the emissions of chloroform may be.

1. Introduction

Rice agriculture is one of the largest uses of the world's arable land. Most rice fields are inundated with water, which causes the soil to become anaerobic, creating a habitat for methane-producing bacteria. The large area and favorable conditions make the world's rice fields a major source of atmospheric methane. Present estimates suggest that some 40-80 Tg y-• of methane is emitted from rice fields, out of some 500 Tg y-• from all sources, natural and man-made.

We began a research program on methane emissions from China in 1985. The focus of this research was a detailed study at Tu Zu in the Sichuan Province of China. During the 7 years of the experiment, between 1988 and 1994, some 5000 valid flux measurements were taken. To complement this intensive study at Tu Zu, we carried out similar experiments at other locations, representing different climatic zones where rice is grown. These s•udies were conducted at Beijing and Guangzhou in China and at

• Department of Physics, Portland State University, Portland, Oregon. 2 Department of Environmental Science and Engineering, Oregon

Graduate Institute, Portland. 3 Institute of Engineering and Technology, Beijing, People's Republic

of China.

4 Experimental Central Laboratory of Instrumental Analysis, South China Agricultural University, Guangzhou, People's Republic of China.

Copyright 1998 by the American Geophysical Union.

Paper number 98JD01114. 0148-0227/98/98JD-01114509.00

Sukamandi Station, near Jakarta, in Indonesia. Compared to the site at Tu Zu (29.5øN latitude, 106.7øE longitude), Beijing represents a temperate latitude at 40.5øN, 116.4øE, while Guangzhou is a subtropical site at 23 øN, 113.2øE longitude, and Indonesia was our tropical site at-6.3øS, 106.4øE longitude. These sites span the major climatic and latitudinal zones of the Earth and represent a diversity of agricultural practices for growing rice. The data from Tu Zu and their main features were reported by Khalil et al. [this issue (a)], and the work from Indonesia has been reported by Husin et al. [1995].

In addition to the measurement of methane emissions, we also wanted to explore the emissions of other environmentally important trace gases from the rice fields. Our studies show that the rice fields are a source of carbon monoxide, chloroform, nonmethane hydrocarbons, and nitrous oxide. Nitrous oxide is of particular importance. When nitrogen fertilizers are used, methane emissions may be reduced, compared to fields where organic fertilizers are applied. At the same time, nitrogen fertilizers can result in the emission of nitrous oxide. Nitrous oxide is a extremely potent greenhouse gas. It is about 200-300 times more effective at causing the greenhouse effect than equal amounts of carbon dioxide, partly because of its long atmospheric lifetime of about 150 years; methane, with a shorter lifetime of about 10 years, is about 20-60 times as important as CO2 [Intergovernmental Panel on Climate Change (IPCC, 1995)]. As farmers replace organic fertilizers with nitrogen fertilizers, rice fields may emit more nitrous oxide and less methane. The emissions of nitrous oxide from rice fields are

therefore of considerable importance in understanding the global cycles of greenhouse gases and the effect of human activities on the Earth's climate. Other gases discussed in this paper, such as CO and

25,241

25,242 KHALIL ET AL.: EMISSIONS OF METHANE, N20 , AND OTHER TRACE GASES

nonmethane hydrocarbons, can be local and regional pollutants and eventually, after they are oxidized in the atmosphere, contribute CO2, a greenhouse gas, to the atmosphere. Chloroform is a major contributor to natural gaseous chlorine, especially in the lower atmosphere.

In this paper we present data in two new areas: first, on methane emissions spanning the major climatic zones where rice is grown and, second, on the emissions of other environmentally important trace gases from the rice fields. Specifically, we will report emission of nitrous oxide, carbon monoxide, chloroform, hydrogen, and some man-made chlorocarbons. It should be stated at the onset that these experiments were exploratory in nature and not designed to be definitive. The results will reflect this reality.

2. Experiments

The experiments were conducted in rice fields outside of Beijing (40.5 øN latitude, 116.4 øE longitude) in 1992 and 1993 (see Yao and Chen, 1994). Rice is grown in this region between July and early October. In 1994, 1995, and 1996 the experiments were done at the experimental rice fields of the South China Agricultural University. These fields are in the outskirts of Guangzhou and away from areas of dense population and urbanization. In Guangzhou, two crops of rice are grown each year. For the first 2 years of the experiment we were able to study only the second or late rice crop (August to October), but in the last year (1996) we did extensive experiments on methane emissions from both early and late rice crops. The same rice fields were used each year at both sites.

The fields were managed according to local farming practices. At both sites a combination of organic and nitrogen-based fertilizers was applied. In Beijing, NHnHCO 3 was the commonly used fertilizer. One hundred twenty-five kg N ha '• were applied over three periods, starting just before planting in late June with additional applications in early July and August. At Guangzhou, nitrogenous fertilizers were used along with pig manure or mushroom compost. The rice varieties planted were Huang Jing Guang in Beijing and Shanyou 63 or Jingxian 89 in Guangzhou. At Guangzhou, rice straw from the first crop is incorporated into the soil before the second crop is planted. This can add to the production of methane and nitrous oxide in the second crop.

We describe first the experiments between 1992 and 1995. The experiments in 1996, done only at Guangzhou, were significantly different. Static chambers were used to measure the fluxes of

methane and other trace gases from the rice fields. An aluminum frame with a groove is installed in the field before rice is planted. If the fields are inundated, water levels are usually above the base. Water is added to the groove if there is not enough in the fields to cover the base. This ensures an air tight seal. A rigid polyethylene chamber is placed gently into the groove. The chambers we used were 29 cm on the side of the base and 89 cm high. There is a sampling port at the top of the chamber and an air intake next to it. The ports are stainless steel, and all the lines are Teflon. A battery- operated pump is used to collect the air sample from inside the chambers. We have used this sampling equipment for trace gas measurements for more than 20 years. It has been extensively tested and has proved reliable.

Air from the chamber is pumped into the input of the pump. The output of the pump is connected to the air intake of the chamber. This causes air to circulate inside the chamber until we are ready to collect the sample. A 0.8 L stainless steel, internally electropolished flask was used to collect the air. The flasks are cleaned and evacuated before being sent to the field. The output of

the pump is attached to the inlet of the flask and an air sample is pumped into the flask. In a few minutes the flask is pressurized to about 30 pounds per square inch gauge pressure. The pressure is recorded. Such samples were collected at prescribed intervals over 1 to 2 hours while the chamber was kept on the rice plants. When the air from inside the chambers is being pumped into the stainless steel flasks, it is replaced by ambient air through the air intake. This causes some complications in calculating the flux as we will see later. It also reduces the sensitivity of the sampling system for detecting the fluxes. There are alternatives that avoid mixing ambient air with the air inside the chambers, but we did not use these. Before each set of flux samples is collected, an ambient air sample is taken by the same procedures as described above. It is collected on the walkways between fields. The ambient air sample is necessary to calculate the fluxes. Each flux measurement therefore consisted of one ambient sample and three samples from the chambers.

The methods used in these experiments were similar to those described in our earlier paper [Khalil et al., this issue (a)], but there are some important differences. As in our earlier studies, we used small chambers, which can lead to greater variability in the measured fluxes because of the prevailing spatial variability of the emissions and because of possible artifacts described in our earlier paper. However, these chambers are convenient, easy to use, and have low minimum detection limits for the flux. The chambers

were left on for a much longer time than in the experiments at Tu Zu. This was necessary if we hoped to measure fluxes of gases other than methane. The fluxes of nitrous oxide, for instance, are on average 50-150 times smaller than the fluxes of methane. This measurement alone requires long exposure times to detect the flux. Longer exposure times can cause saturation effects for methane [see Khalil et al., this issue (b)]. To avoid saturation effects, we had a staggered sampling schedule in some years and collected four samples per flux measurement. In this method we obtained one sample at time zero, the next one at about 10 min, the third at 50 min, and the last one at 90 min. The first increment (10 min) was used for calculating the methane flux, and the full 90 min were used for calculating the fluxes of nitrous oxide and other gases. In other years, time between the first two samples was about 30 min. On some occasions, evidence of the saturation effect was observed for methane, and it was corrected by using the first increment. We also tested the linearity of the accumulation rate for methane and nitrous oxide and discarded cases where the r 2 was less than 0.9 [Sass et al., 1992; Khalil et al., this issue (a)]. Less than 3% of the data showed such an effect.

The flasks were kept near the sites and were later sent back to our laboratory. We then analyzed the samples using gas chromatographs with flame ionization, electron capture, and mercuric oxide detectors. The maximum time between sampling and analysis was about 4 months. The gases we are interested in have been shown to be stable in these containers for much longer times.

This procedure of collecting samples in stainless steel flasks and sending them to the laboratory makes it possible to analyze many gases. It would not be practical to do so in the field. However, it adds an important constraint as it limits the number of samples that can be collected. This is both because only a limited number of flasks are available for the experiment and because shipping the containers back and forth is limited by logistical and cost considerations. Consequently, the 160 or so flasks available each year had to be distributed over time and the spatial extent of the rice fields. In each year we chose two plots in the same field or adjacent fields. The sampling frequency was about every 2 days in Beijing and every 4 days or so in Guangzhou, alternating between morning

KHALIL ET AL.: EMISSIONS OF METHANE, N20 , AND OTHER TRACE GASES 25,243

and evening collections. The flux is calculated by using a mass balance of the gas inside the chamber. As the sample is being drawn, the same amount of ambient air flows back into the chamber carrying with it the trace gases in the concentrations determined by the ambient sample. The flux between each increment of sampling is given approximately by

F. = [C.q (1 +« •lA T) - C.(1 -« xlAT)]/AT - xIC• (1)

x I = (V./V•)/AT (2)

Here, Vs is the volume of the air in the sample, and Vc is the volume of air in the flux chamber. C• is the background concentration, Cn is the concentration in the chamber, and AT is the time between samples. The volume of air in the sampling container is calculated by the ideal gas law using the measured pressure in the flask. Usually about 2 L of air were collected. In our case, two such fluxes (F 1 and F2) are calculated when there are three samples (plus

an ambient). The average of the two fluxes is taken to be the flux for that experiment.

In 1996 a different and more intensive sampling protocol was used at Guangzhou (there were no experiments at Beijing). We selected three plots in each of four fields which consist of the fields from the previous years and adjacent fields. We used large chambers of basal area 1.96 m 2 and heights of 0.57 and 1.14 m. Bases were embedded in the rice fields as for the other experiments. Samples were collected in glass syringes, about 2-3 days apart, and analyzed soon afterward using a Gow-Mac Model 69-350 gas chromatograph with a flame ionization detector set up in the laboratory at the Southern China Agricultural University, which was nearby. In this experiment we could measure only methane, but at a higher frequency, with more replicates, and for both growing seasons. A more complete meta data set was also acquired that included water depth, meteorological conditions, and agricultural practices. Details of this methodology and the reasons for choosing it are discussed in detail by Khalil et al., this issue (a). Although the methodology was different, the results for the second crop are very similar to previous years.

45

:• 35

• 15

I I I I

0 20 40 60 80 100

Days Since Transplanting -' 1992 - •- - 1992 --•-- 1993

Plot # 1 Plot #2

0 20 40 60 80 100

Days Since Transplanting --ß-- 1992 --ß-- 1992 --ß-- 1993

Plot #1 Plot #2

Figure 1. (a) Methane emissions from rice fields near Beijing, China, during the entire growing seasons of 1992 and 1993. Beijing represents temperate latitudes. The differences of emissions between the two plots in 1992 and the low emissions in 1993 are likely caused by differences in (b) water levels.

25,244 KHALIL ET AL.: EMISSIONS OF METHANE, N20 , AND OTHER TRACE GASES

30

25 - • Average O Plot A

• 20 -

= 15 -

=• 10

5 -

• 1992, Plot # 2 0 I . , I., I

0 3 6 9 12

Water Level (cm)

Figure 2. The relationship between water level and methane emissions in the rice fields in Beijing. Averages of water level and flux are shown in the figure. The open circles show the average water level and methane flux during the entire growing seasons of 1992 and 1993.

3. Fluxes of Trace Gases

3.1. Methane

Methane emissions at Beijing are plotted in Figure 1 a. In 1992 the two plots gave seasonally averaged fluxes of 4 and 13 mg m -2 h -•. In 1993 in Beijing we started the measurements about a month after the rice had been planted and found that the average emissions were about 1 mg m -2 h -• for the time when samples were collected. The differences of fluxes for these three sets of measurements, two

plots in 1992 and the 1993 data, are explained by the water levels in the fields. In 1992, water levels in the plot with the higher emission rate averaged 5.7 cm and averaged only 3.5 cm in the plot with the smaller flux. Standing water levels were not maintained during the growing season so that the field with lower emissions had no water for several days at a time. There was almost no water in the rice fields during the entire period when we took measurements in 1993. Earlier studies have shown definitively that draining the fields or the lack of water in the fields can result in a cessation of methane flux [Sass et al., 1992; Chen et al., 1993; Husin eta!., 1995]. In Figure lb we have plotted the 3-point running averages of the measured water levels on the same timescale as the fluxes. It is apparent that in these cases water levels seem to control the methane flux. In Figure 2 we show the relationship between water level and methane fluxes for the composite data from these two years of work. Lower water levels, which include periods when there is no water, tend to suppress methane emissions. The figure also contains the averages of the water levels for the entire growing season and the seasonally averaged fluxes (open circles). It is likely that supplies of irrigation water are limited in the area where we did our experiments, and so the methane flux is small compared to other locations. It is common agricultural practice in this region to flood the field for the first month, during active plant growth, then allow the water to evaporate. Only enough irrigation water is supplied to keep the soil moist for the rest of the growing season. This circumstance makes it difficult to assess what the prevailing fluxes are from the Beijing area and whether there might be substantial year-to-year variability of flux depending on the availability of irrigation water or rainfall. In the other locations where we measured methane fluxes, the water supply is more regular and fields were generally flooded for most

or all of the growing season. This causes similar emissions from different fields and from one year to the next. We have adopted the average of 1992 data as representative of the Beijing area as we had more complete data for this year, but we point out that fluxes from this area are likely to be variable. The emission rates of methane we observed are lower than those reported by Yao and Chen [1994] from the same area. Their results were between 15 and 20 mg m -2 h-•, which is compatible with our data for 1992 for the plot with enough water.

The fluxes at Guangzhou are high in contrast to Beijing (Figures 3 and 4). In all 3 years the seasonal pattern is similar for the second crop. High fluxes are observed in the beginning of the second season, and these fall to practically no emissions toward the second half of the growing season. Water level measurements were not reported from Guangzhou in the first 2 years, but records show that the fields were allowed to drain toward the later part of the growing season and only intermittently irrigated afterward. This characteristic is also evident in the water level data from 1996.

The salient points of the results obtained in 1996 are that seasonally averaged emission rates for fields 1,..., 4 were 7.6, 2.4, 5.5, and 6.1 mg m -2 h -• for the first crop and 40, 21, 41, and 24 mg m '2 h '• for the second crop. Organic fertilizer in the form of pig manure was applied to fields 1 and 3 in both crops, and no organic fertilizer was applied to the other two fields. Emissions from the first crop are relatively small, and although the application of organic fertilizer increases emissions, the effect is not important. For the second crop the emissions are higher by factors of about 4-9 compared to the first crop either with or without organic fertilizer use. Some of this effect is no doubt due to the stubble from the

previous crop that is plowed into the fields for the second crop because fields without organic fertilizer applications go from emissions of 2-6 mg m -2 h -• to about 20 mg -2 h -•. In addition to this effect, the use of organic fertilizers increases emissions from 6- 8 mg m -2 h -• to about 40 mg m -2 h -•. The low emissions for the first crop are partly due to lower water levels. The fields sampled are remarkably similar in their responses to the application of organic fertilizers. Finally, the results for the second crop are very similar to the data from the previous 2 years (1994 and 1995) when emissions from only the second crop were measured.

The daily fluxes "F(t)" as shown in Figure 1, for example, can be integrated to estimate the average emissions during the entire

KHALIL ET AL.: EMISSIONS OF METHANE, N20 , AND OTHER TRACE GASES 25,245

160

a

40

o ' ' ::% 0 20 40 60 80 100

Days Since Transplanting (Second Crop)

• 1994 -•- 1995

250 f 200 -

-

- -

•5o - -

- -

100 - _

- _ _

50 - - -

-

_

o

o

12o

! i ,._ _: .......... _"::. , 3O 6O 9O 120

w/Organic Fertilizer

150 180

Days Since Transplanting (First Crop)

- o- - w/o Organic Fertilizer

20

0 30 60 90 120 150 180 210

Days Since Transplanting (First Crop)

Figure 3. (a) Methane emissions from rice fields near Guangzhou, China, during the second growing seasons of 1993 and 1994. (b) Methane emissions for the double crop cycle in 1996. (c) Water depth of fields in 1996. The first crop was drained 35 days after transplanting; no measurements were made until after the first drainage. The second crop was continuously flooded for 35 days, then intermittently irrigated.

growing season: T

F.,, = •- F(t)dt (3) o

where T is the length of the growing season, which is a little over 100 days at the locations we studied. The seasonal average is the most commonly used value for the flux of methane and for the other trace gases as well. It is used to estimate the annual global emission rates from rice fields, which in turn is part of the annual budget of methane. The method for integration is described by Khalil et al.

[this issue (a)], and here we assume that the flux was zero at the time rice is planted (time zero) and after harvest. The seasonally averaged flux is about 9 mg m -2 h -1 for Beijing (not including 1993 data for the reasons discussed earlier) and about 31 mg m -2 h -1 for Guangzhou. These results are shown in Figure 4 along with data from Tu Zu and Indonesia, which were part of the same larger experiment as the present work and used similar field and analytical techniques.

The uncertainties are difficult to evaluate and are caused by a lack of information on spatial variability, infrequent sampling, and

25,246 KHALIL ET AL.' EMISSIONS OF METHANE, N20, AND OTHER TRACE GASES

50

40

30 g

• 20 u 10

:•, N o

I

1.00 0.50 0.00 -0.50

Sin Latitude

Figure 4. Comparison of average methane emissions for the growing season (100-120 days) at Beijing and Guangzhou (second crop). Results from similar experiments at Tu Zu and Indonesia are also shown. The Indonesian site represents tropical rice agriculture, while Tu Zu is subtropical.

lack of other environmental information. The flux measured at a

plot may not be representative of the emissions from the larger areas such as the whole field. Studies have shown that using small chambers with only a few spatial replicates can add substantial uncertainties to the measured fluxes for methane; similar results are expected to hold for other gases as well. In the Tu Zu studies, we found that using only two plots could result in maximum uncertainties of about a factor of 2-2.5 compared to a more extensive sampling coverage using 24 plots, which we take to be representative of the field emission rate [Khalil et al., this issue (b)]. Additional uncertainties are introduced as the sampling frequency is reduced to the rates used in the Beijing and Guangzhou studies. On the basis of the results from the Tu Zu experiments, we estimate that for the Beijing and Guangzhou studies, the maximum uncertainty in the flux, represented by the ratio of the maximum to minimum fluxes, is about 3. These are the maximum errors that could occur given our sampling strategy and assuming that the variability is similar to that observed in the extensive experiments at Tu Zu. These estimates of uncertainties are included in Figure 4. The uncertainties for the Tu Zu data shown in the same figure are the limits of the maximum and minimum seasonally averaged fluxes actually observed at the same fields during the 7-year experiment.

3.2. Nitrous Oxide

The emissions of nitrous oxide from rice fields are sporadic in nature and about 50 to 150 times smaller than methane emissions.

Emissions tend to occur in events that are characterized by high emissions and a rapid decay to no emissions or a slight uptake of N20. Some of these events follow the application of nitrogen-based fertilizers. The results are shown in Figure 5. The variability is so large that we had to attenuate or amplify some of the cases to show them all on the same vertical scale.

The seasonal averages are about the same for the 2 years of the Beijing experiments but very different in the Guangzhou studies. The 1995 data from Guangzhou show the highest fluxes measured in any of the experiments, and the seasonal average emission rate is 10 times larger than at Beijing. By contrast, the 1994 emissions from Guangzhou are the lowest seasonally averaged rates observed. However, this is probably because we have very limited data during 1994. Since the nitrous oxide emissions are dominated by events, a low sampling frequency can miss significant emissions.

Earlier studies of N20 emissions from intermittently flooded rice fields in India by Mosier et al. [1990] show seasonally averaged fluxes of almost 30 •tg m -2 h -l. These are much smaller than the fluxes we measured, particularly from Guangzhou in 1995.

The uncertainties in the seasonal rates are probably very large. Because there is no systematic seasonal cycle of emissions, as for methane, the sampling frequency has to be carefully adjusted to capture the true emissions during the season. Our sampling frequency is regular and low (at once or twice a week). What is needed is to follow each event in detail to quantify the emissions. This requires much more frequent sampling than we were able to obtain. Aside from the issue of whether we have captured the emissions during the growing season, there is also the possibility that emissions would continue after harvest. Unlike methane, there is no reason for the production of nitrous oxide to cease once the field is drained of water and returned to aerobic conditions. These

matters make it difficult to say whether our data give a robust estimate of the annual flux of N20 from rice fields or not.

It is also difficult to estimate the global emissions of N20 from these data, partly for the reasons discussed above and partly because we found a large variability in the emissions between the four experiments reported here. Extrapolating these results to the whole world gives 0.1-1 Tg y-•. While we do not recommend that these numbers should be taken to represent global emissions, we believe they suggest that the rice fields should be studied in more detail to delineate the global cycle of nitrous oxide. It should also be noted that the emissions are episodic, so the average values we have quoted are useful only in determining the emissions of nitrous oxide over the whole growing season and do not represent hourly or daily values as may be construed from the units we have used.

There are relationships between the methane and nitrous oxide emissions that we can only state qualitatively here. When the fields dry, methane emissions fall. When the fields are re-flooded, either by irrigation or by rain, nitrous oxide levels should rise, and in time the methane emissions also recover. These general patterns are seen in our data.

3.3. CO, CHCI3, Ha, and Hydrocarbons

Observations show that several other environmentally important trace gases are emitted from the rice fields. In our studies the most significant are carbon monoxide, chloroform, hydrogen, and

KHALIL ET AL.: EMISSIONS OF METHANE, N20 , AND OTHER TRACE GASES 25,247

11oo

9oo

.c 700 •-.

• 500

¸

•' 300

100

70o

5O0

300

100

-lOO

Beijing 1992

I

0 20 40 60 80 100

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120

1700

1400

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soo

20O

- • Guangzhou 1994

I'-] x10 C

I I I I I I I I I I

0 20 40 60 80 100 120

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Beijing: 1993

1/5

b

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5000

2000

..... i I I I I I I I I I •l

0 20 40 60 80 100 120 0 20 40 60 80

Days Since Transplanting Days Since Transplanting

-lOOO

• Guangzhou 1995 d

['"l x10

.... I I I I I I I I I I I

Figure 5. Nitrous oxide emissions from rice fields during the growing seasons of 1992-1995. The experiments shown are (a) Beijing, 1992, (b) Beijing, 1993, (c) Guangzhou, 1994, and (d) Guangzhou, 1995, in China. The points marked with circles and squares are attenuated or amplified as shown in the legend. This treatment is necessary to show the majority of the data in the presence of large emissions. The emissions of nitrous oxide are described by events during the growing season, sometimes connected with the application of nitrogen fertilizers.

1oo 120

100000

10000

1000

100

10

• I BeOing 1992 - ß ß Bering 1993

$ Guangzhou 1994 • ß & Guangzhou 1995 -

-

-

0.1 --- i i i i i CHa N20 CO H 2 CHC13

Gas

Figure 6. Whole season average fluxes of trace gases from rice fields in China.

25,248 KHALIL ET AL.' EMISSIONS OF METHANE, N20 , AND OTHER TRACE GASES

0.8

0.6

0.2

0.0

-0.2 I I I I I I I I I I I I I

Location

Figure 7. The flux of chlorocarbons from rice fields in China. These measurements ensure that samples were not contaminated. B J, Beijing; GZ, Guangzhou; MCF, methyl chloroform; TET, carbon tetrachloride.

nonmethane hydrocarbons. The seasonally averaged fluxes of these gases are shown in Figure 6, which also includes methane and nitrous oxide for comparison. Positive fluxes were observed in all the studies when these gases were measured but the fluxes observed are generally small.

For chloroform, the fluxes are large enough to be of the order of 10% of the global emissions and warrant further study. In our earlier work, done near Chengdu in Sichuan Province, representing the early phases of the rice crop, chloroform fluxes of 0.1-0.5 gg m -2 h -I were found [Khalil et al., 1990b]. These are smaller by a factor of 2 or more compared with the present work representing the whole growing season. It appears that chloroform emissions are ubiquitous and arise from diverse biological processes on the earth's surface. Sizable emissions have been found from the soils in

Australia near termite mounds, from rice fields in this study, and from biogas pits in the earlier study [Khalil et al., 1990a, b]. Together, these sources may make a sizeable contribution to the global budget of chloroform.

Hydrogen emissions were only measured in 1 year. On the basis of these limited data it is unlikely that the rice fields are a significant fraction of the global emissions. Nonetheless, hydrogen measurements may be important since it is thought to be an intermediary in the methane production process [Boone, 1993].

The CO and hydrocarbon fluxes are too small to be significant in the global budgets but could be important in the local photochemistry of the air over rice fields. The CO emissions observed here are less than in our earlier study when we reported a range of 5-400 gg m -2 h -I [Khalil et al., 1990b]. In our previous study the light hydrocarbons (C2-C4) were measured. The combined emissions were about 1.6 gg m -2 hr -l, which is comparable to the present result for 1994 in Guangzhou.

The present studies demonstrate that CO, CHCI3, H2, and nonmethane hydrocarbons are emitted from the rice fields, since positive fluxes were found in all studies when these gases were

measured. The emission rates are variable but confirm the results

from the earlier even more limited study [Khalil et al., 1990b]. Of the gases discussed here, perhaps chloroform emissions are of the most interest because surface emissions from agricultural and natural ecosystems can be a substantial global source.

3.4. Chlorocarbons

There were two reasons for measuring the chlorocarbons and related compounds. In previous studies, dry soils had been found to be a substantial sink of the chlorofiuorocarbons [Khalil and Rasmussen, 1989]. We are therefore interested in determining whether other soils, such as the rice fields, could also represent a sink. The second motivation for the measurements of

chlorocarbons, especially the chlorofiuorocarbons F- 11 and F- 12, is to test for possible contamination of the system with indoor air or the effects of storage of samples. The results are shown in Figure 7.

The analysis of the data from the experiments at Beijing and Guangzhou shows that while there are some indications of uptake, the evidence is not a convincing case for a sink. A few cases showed a relatively large loss of F-11 and F-12. All but one of these are explained as an artifact of possible residue of indoor air in the chamber when the experiments were done. In the Beijing experiments positive fluxes of methylchloroform and carbon tetrachloride were observed. We believe these may have come from the irrigation water that goes through populated areas. The fluxes observed were small in any case and not likely to be of interest in environmental studies. The fluxes of the chlorofiuorocarbons are

so small and so evenly distributed among positive and negative that we believe they show that our sampling equipment and process were unaffected by contamination from indoor air, local pollution, or storage.

For all the gases measured we looked for effects of diurnal

KHALIL ET AL.: EMISSIONS OF METHANE, N20 , AND OTHER TRACE GASES 25,249

Table 1. Concentrations and Fluxes of Trace Gases Measured at Beijing and Guangzhou, 1992-1995

Beijing Guangzhou

1992 1993 1994 1995 1996 1996

Late Late Early Late

Fluxes

CH4, mg m-2h -• 13. -- 30.6 31.4 N20, gg m -: h -• 89. 82. 25 975 CHC13. gg m-2 h -• 0.6 4.4 -- 1.0 CO, gg m -: h -• 108. 48 .... H•, gg m-: h -• 16. -- -- 6 TO-12, gg m-: h -• .... 1.9 -- F-12, gg m-: h -• -0.03 0.007 -- -0.18 F-11, gg m-: h -• -0.03 0.002 -- -0.19 MCF, gg m -2 h -• 0.03 0.0 -- -0.16 CC14, gg m -2 h -• 0.25 1.5 -- -0.16

5.0 31.6

Ambient Concentrations

CH•, ppbv 3057 2315 2305 2232 N:O, ppbv 322 319 320 322 CHC13, pptv 267 255 -- 61 CO, ppbv 1236 768 .... H:, ppbv 938 .... 785 TO-12, gg m -a .... 269 -- F-12, gg m-: h -• 635 601 -- 584 F-11, gg m-: h -• 320 320 -- 307 MCF, gg m-: h -• 195 181 -- 171 CC14, pptv 163 155 -- 178

1960 2250

Field Conditions

T-soil, øC 24.1 22.6 T-air, øC 25.7 23.0 Water, cm 4.6 0.5

26.5 26.6

28.6 26.9

1.8 1.2

variations and water levels. No such effects could be seen in the

present data sets except for the important relationship between water levels and methane emissions already discussed. The variability of the temperature is small, and the data are too few to show definite relationships. If such relationships exist, more detailed data will have to be acquired to see them.

4. Discussion

We have reported the results of five years of experiments on the rice fields in China• These studies were designed to investigate the effect of climate and agricultural practices on the emissions of methane and to determine if rice fields are a source of other

environmentally important gases. The role of climate is both on soil temperature and the availability of water for irrigation which affects emission rates. Because intensive sampling could not be done, the results are uncertain but compatible with both our previous studies and the published works of other scientists. Table 1 contains a summary of the results.

There has been much concern about methane emissions from rice

fields in recent years. Some of the suggestions to mitigate methane emissions are the use of nitrogen fertilizers and control of irrigation water that can lead to low emissions. Neither of these processes addresses the possible emissions of N20. If organic fertilizers and incorporating rice straw into the fields are replaced by nitrogen- based fertilizers, the emissions of N20 are likely to increase.

Changing water management to control methane emissions would at best leave N20 emissions the same. We must point out that our results on N20 emissions are very variable, and the sampling strategies were not optimal for obtaining robust measures of the seasonal emission rates. Still, we believe the results show, at least qualitatively, that nitrous oxide emissions from rice fields deserve more careful study because they are likely to be large enough to be important in the global budget.

This work puts our intensive research on methane emissions carried out at Tu Zu into a larger perspective of climatological variability of methane emissions from rice fields. The results from Tu Zu are seen as representative of subtropical rice agriculture. And this work points to some new directions that should be explored further to understand the global budgets of man-made greenhouse gases, particularly nitrous oxide.

Acknowledgments. We thank Michael Riches for his many suggestions and his help with the logistics of carrying out the experiments. We thank Wu Shi Zhao (SCAU-Guangzhou), Gui Peng, and Yuan Yi (Beijing), and Bob Dalluge and Don Steams (OGI) for their contributions to this project. Additional support was provided by the Biospherics Corporation and the Andarz Co. The data from the experiments discussed in this paper are archived at CDIAC and may be obtained by writing to: Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6335, USA. This paper should be used as a reference for the data set. This project was supported by the U.S. Department of Energy (DE-FG06-85ER60313).

25,250 KHALIL ET AL.: EMISSIONS OF METHANE, N20 , AND OTHER TRACE GASES

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Z.-L. Chen and H. Yao, Institute of Engineering and Technology, 5-606 Qinghuayuan Building Haidian District, Beijing, 100084, People's Republic of China.

M.A.K. Khalil and M.J. Shearer, Department of Physics, Portland State University, Portland, OR 97207. ( email: aslam•atmos.phy.pdx.edu)

R.A. Rasmussen, Department of Environmental Science and Engineering, Oregon Graduate Institute, Portland, OR 97291.

J. Yang, Experimental Central Laboratory of Instrumental Analysis, South China Agricultural University, Guangzhouy 520642, People's Republic of China.

(Received July 29, 1997; revised February 17, 1998; accepted March 31, 1998.)