Biol Fertil Soils (2012) 48:787–795 DOI 10.1007/s00374-012-0672-7
ORIGINAL PAPER
Nitrous oxide evolved from soil covered with plastic mulch film in horticultural field Seiichi Nishimura & Michio Komada & Masako Takebe & Seiichiro Yonemura & Naoto Kato
Received: 15 October 2011 / Revised: 29 January 2012 / Accepted: 7 February 2012 / Published online: 18 February 2012 # Springer-Verlag 2012
Abstract Soil solarization practice, in which soil is covered with plastic mulch film and exposed to high temperature prior to crop cultivation, is expected to be an effective method for reducing weeds and pathogenic microorganisms without using agricultural chemicals. Although the production of nitrous oxide (N2O), a major greenhouse gas, is enhanced in fertilized soil covered with plastic mulch films, its transport route to the atmosphere has not been sufficiently elucidated to date. In this study, we investigated the N2O evolution from plastic-mulchfilm-covered agricultural soil. In a horticultural field where ridge soil was covered with a plastic mulch film after fertilization, we observed significant N2O flux from the soil surface of the unfertilized furrow between the ridges, indicating the horizontal diffusion of N2O from the ridge soil covered with the mulch film to the adjacent furrow soil surface. On the other hand, the measurement of the permeance (permeation coefficient) of the plastic mulch film for gaseous N2O by laboratory experiment revealed that N2O gradually permeated the mulch film; the permeance increased exponentially with an increase in ambient temperature, indicating possible N2O emission by permeation through the mulch film under field conditions. In winter, the amount of N2O emission by permeation through the mulch film was estimated to be lower than that emitted from the furrow soil surface, and it was lower than that in summer. On the other hand, it was estimated to be much S. Nishimura (*) : M. Komada : M. Takebe : N. Kato National Agricultural Research Center, 3-1-1 Kannondai, Tsukuba, Ibaraki 305-8666, Japan e-mail:
[email protected] S. Yonemura National Institute for Agro-Environmental Sciences, 3-1-3 Kannondai, Tsukuba, Ibaraki 305-8604, Japan
higher in summer owing to the higher permeance of the film at high temperatures. Keywords Diffusion . Furrow . Nitrous oxide . Permeance . Plastic mulch film
Introduction Covering ridge soil with plastic mulch films for planting crops is a major agricultural practice conducted worldwide. The main purpose of mulching soil is to keep the soil temperature and water content high, at levels suitable for cultivating crops. The restriction of arable weed growth is another major purpose of mulching soil. There have been many previous studies in which the effects of mulching soil using plastic films on soil temperature, soil water dynamics, and the growth and yield of various kinds of crops were investigated (e.g., Ban et al. 2009; DíazPérez 2010; Katan and Devay 1991; Zhang et al. 2011). Soil mulching is also conducted prior to crop cultivation, to reduce arable weeds and/or pathogenic microorganisms during the succeeding crop cultivation period. In this practice, often called “soil solarization,” the soil surface is continuously covered tightly with plastic films during the hot season. Thus, the temperature just below the mulch film increases markedly so that seeds and seedlings of weeds and pathogenic microorganisms are expected to be severely damaged. The soil solarization practice has not been widely applied yet. However, it is expected to be an innovative method for reducing weeds and pathogenic microorganisms without using agricultural chemicals, and therefore, many studies of soil solarization have also been conducted previously (e.g., Chase et al. 1998; Chellemi et al. 1997; Egley 1990; Horowitz et al. 1983; Katan and Devay 1991; Katayama et al. 2003; Patterson 1998; Standifer et al. 1984; Vitale et al. 2011).
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Nitrous oxide (N2O) is a major greenhouse gas, which has a 298-fold higher global warming potential than CO2 for a period of 100 years (Forster et al. 2007). Nitrous oxide is produced in soil by various soil microbial species as a byproduct of nitrification and an intermediate product of denitrification (Bouwman 1990; Hutchinson and Davidson 1993); fertilized agricultural soil is one of the major sources of N2O. It is well established that N2O production in soil generally increases under conditions of high inorganic N and organic matter contents and low O2 concentration in the soil (e.g., Akiyama and Tsuruta 2003a,b; Hayakawa et al. 2009; Yanai et al. 2011). Since soil covered with plastic mulch films just after fertilization is generally under conditions of high inorganic N content, high water content, and low O2 concentration, a significantly high amount of N2O may be produced and emitted to the atmosphere. According to a previous study (Ou et al. 2007), the fumigant methyl bromide injected to the soil covered with a polyethylene film was emitted to the atmosphere by both permeation through the film and horizontal diffusion to the adjacent furrow soil surface. In the case of N2O, emissions by both permeation through the film and horizontal diffusion to the adjacent furrow soil surface may also be significant. However, the dynamics of N2O production and transport in soils covered with plastic mulch films have not been sufficiently elucidated to date. Indeed, Arriaga et al. (2011) reported a high amount of N2O emission (0.42–1.31 g N m −2 in 43 days) from a peppercultivated field after removing a mulch film covering the soil for 40 days with fresh or semicomposted manure application, indicating that N2O production is enhanced by the preceding soil mulching. However, they did not measure N2O emission during the period when the soil was covered with the mulch film. In this study, we investigated transport route of N2O produced in fertilized soil covered with a plastic mulch film and emit to the atmosphere. We investigated both transport routes of possible N2O emission to the atmosphere, i.e., permeation through the mulch film, and horizontal diffusion to the
(atmosphere)
adjacent furrow soil surface, as shown in Fig. 1. The N2O emitted from the furrow soil surface was measured by the conventional closed chamber method in a horticultural field with the soil covered with mulch films, whereas that emitted by permeation through the mulch film was estimated by a laboratory experiment on the basis of Fick’s diffusion law.
Materials and methods Outline of experimental field Nitrous oxide flux in the field was measured twice from December 2009 to January 2010 (hereafter, referred to as “winter experiment”) and from June to July 2010 (hereafter, referred to as “summer experiment”) in an experimental field (36°01′ N, 140°06′ E) at the National Agricultural Research Center (NARC), Japan. The soil type in the field was volcanic acid soil (Andisols), and the surface layer (0 to 15 cm) had a clay loam texture. The bulk density of the topsoil at 0–5 cm depths was 0.65 Mg m−3, and the soil pH in H2O was 6.3. The C and N contents of the topsoil (0– 5 cm depths) were 42 mg C/g soil and 3.9 mg N/g soil, respectively. In the winter experiment, a commercial compound organic fertilizer (Agret673, Asahi Industries Group, Japan) of which ingredients are all derived from organic materials, such as poultry litter ash and various industrial byproducts of plant and animal matters, was applied at a rate of 285 kg N ha−1 on December 17, 2009. In the summer experiment, poultry manure (total N, 2.1%) was applied at a rate of 90 kg N ha−1 on June 15, 2010. In both experiments, the fertilizers were applied only within ridges for cultivation and not on the furrow between ridges. Then, plowing, ridging, and mulch film application were conducted simultaneously using a cultivator (TA800SCJ, Kubota, Japan) with an attachment for ridging (SKB-E15C, Kubota, Japan). The depth of the plowing was about 15 cm. The ridge was 12 m in length and 1.2 m in width. The width of the furrow
emission by permeation through mulch film b ridge
mulch film
emission from furrow a
(fertilized, covered with mulch film)
furrow (unfertilized)
N2O
(soil) Fig. 1 The N2O produced in the fertilized soil under the mulch film can be emitted to the atmosphere from the adjacent furrow soil surface (a) or by permeation through the mulch film (b). a Measured by the
conventional gas flux measurement with the closed chamber method. b Estimated on the basis of data on N2O concentration below the film and permeance of the film
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between ridges (distance from the edge of a ridge to that of the adjacent ridge) was 0.6 m. A polyethylene film (0.02-mm thick) for agricultural use was used as the mulch film. In the winter experiment, a black mulch film (GG135FK, Sekisui Chemical, Japan) was used. The main purpose of the mulching was to keep the soil warm and promote N mineralization of the applied organic matter. The black mulch film was maintained as a tight soil surface cover during the entire experiment. After the experiment, the mulch film was perforated on January 14–15, 2010 for the transplantation of lettuce seedlings. In the summer experiment, a transparent mulch film (FT150FK, Sekisui Chemical, Japan) was used. The main purpose of the mulching was soil solarization for controlling weeds, i.e., to minimize weeds during the succeeding carrot cultivation period. Seeds and small seedlings germinated under the mulch film were expected to be severely damaged by extremely high temperatures. The transparent mulch film was maintained as a tight soil surface cover during the entire experiment. After the experiment, the mulch film was removed on July 20, 2010 for the sowing of carrot seeds. Measurement of N2O fluxes in field A chamber consisting of gray polyvinyl chloride plates (5-mm thick) was placed on the soil surface of the furrow between ridges with three replicates. The chamber had a cross-sectional area of 0.18 m2 (0.6×0.3 m) and a height of 0.1 m. The gas sampling for the flux measurement was conducted between 0930 and 1200 hours. It was closed for 30 min. Air inside the chamber was drawn three times at 1, 15, and 30 min after closing the chamber using a syringe and immediately injected into an evacuated glass vial of 25 mL. The N2O concentration of the air in the glass vial was analyzed using a gas chromatograph (GC-14A, Shimadzu, Japan) equipped with an electron capture detector (ECD) and two switching valves for the preliminary separation of N2O from other gases such as water vapor, N2, O2, and CO2 in the sample air. Dinitrogen and CH4 were used as the carrier and quencher gases, respectively. Nitrous oxide flux was calculated on the basis of the increasing rate of the three measured gas concentrations. Measurement of N2O and O2 concentrations in soil A stainless pipe (outer diameter, 3 mm; inner diameter, 1 mm) with small slits was inserted into the ridge soil at 5, 10, 20, and 50 cm depths, each with three replicates. Air in the soil was drawn through the slits of the pipe by a syringe and injected into an evacuated glass vial, and its N2O concentration was later analyzed using a gas chromatograph, as described above. The gas sampling for the measurement of soil air was conducted between 1300 and 1500 hours. The O2 concentration of soil air taken in the glass vial was also analyzed using another gas
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chromatograph (GC-14A, Shimadzu, Japan) equipped with an ECD. Dinitrogen was used as the carrier gas. The air between the soil and the mulch film was also taken by directly pricking the mulch film with a needle connected to a syringe, and N2O and O2 concentrations were measured. Other data measurements in field Ambient air temperature data provided hourly by the Weather Data Acquisition System of the National Institute for AgroEnvironmental Sciences (NIAES), which is located about 700 m from the field, were used for calculating the fluxes. The soil temperature of ridge soil at 5 cm depth was measured with a copper-constantan thermocouple with three replicates and recorded on a data logger (CR1000, Campbell Scientific Instruments, USA) at 10-min intervals. Volumetric water contents at 5- and 20-cm depths of ridge soil were measured with time domain reflectometry moisture sensors (CS615, Campbell Scientific Instruments, USA) and recorded on a data logger (CR1000, Campbell Scientific Instruments, USA). The values were modified for Andisol soils according to the calculation curve proposed by Hatano et al. (1995). Samples of ridge soil at 0–15 cm depths were collected just before and after the field experiments (November 18, 2009 and January 20, 2010 for the winter experiment, and June 15 and July 15, 2010 for the summer experiment) for the analysis of soil inorganic N content. Soil samples were collected randomly from eight locations in each plot and mixed. Fresh soil samples were extracted with potassium chloride solution (10% w/w). Nitrate–nitrogen (NO3−–N) was analyzed by the hydrazine reduction-sulfanilamide, αnaphthylamine absorption spectrophotometry method, and ammonium–nitrogen (NH4+–N) was analyzed by the indophenol blue absorption spectrophotometry method in a continuous flow analyzer (QuAAtro2-HR, BL-TEC, Japan). Measurement of permeance of film The permeance (permeability coefficient; permeation flux per unit difference in the gas concentration between both sides) of the mulch film for gaseous N2O was measured by laboratory experiment, of which the outline is shown in Fig. 2. The mulch film was inserted between two cylindrical glass chambers, with round rubber rings on both sides of the film to connect and seal the chambers tightly. The volume of the chamber was about 300 mL. Two milliliters of a standard gas including N2O of 300 ppmv was injected into one compartment of the system. Then, 0.5 or 1.0 mL of the inner air of both compartments was respectively drawn after 1, 3, and 6 h with a syringe and immediately injected into the gas chromatograph for the analysis of N2O concentration. The measurement was conducted in an incubator at air temperatures of 25, 40, and 55°C. The measurement was repeated
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Biol Fertil Soils (2012) 48:787–795 N2O injection
glass chamber
mulch film
gradual N2O permeation to the other compartment
Fig. 2 Outline of the experiment for measuring permeance of N2O through the plastic mulch film. The closed chamber was divided into two compartments with the mulch film. The N2O injected into one compartment of the chamber gradually moves to the other compartment by permeation through the mulch film. Then the N 2 O
concentration in the compartment with N 2O injection gradually decreases and that without N2O injection increases. On the basis of these decreasing/increasing rates of N2O concentration, the permeance of the mulch film was calculated
twice for each film (black and transparent films) and temperature. The time courses of the N2O concentration in the chambers were fitted to exponential curves, whose parameters were used to calculate the permeance of the film.
gradually increased throughout the experimental period to reach its maximum of 122 μg N m−2 h−1 25 days after fertilization with mulching (January 11), just before the perforation of the mulch film for the transplantation of lettuce seedlings. In the summer experiment, N2O flux increased rapidly to reach its maximum of 205 μg N m−2 h−1 2 days after fertilization with mulching (June 17). Thereafter, N2O flux gradually decreased. By the trapezoidal integration of the flux data during the experiment, the cumulative N2O emission from the furrow soil surface was estimated to be 42±11 and 94±19 mg N m−2, respectively, for the winter and summer experiment.
Simulation of N2O flux by permeation through mulch film in field We assumed that the N2O flux by permeation through the mulch film is the product of the permeance of the film and the difference in N2O concentration. In addition, the permeance of the film was assumed to change exponentially with a change in ambient temperature. The N2O flux to the atmosphere by permeation in the field was then estimated according to Fp ¼ P
dCN2 O ¼ a bT ðCN2 O dx
0 cm
CN2 O
ambi Þ;
where Fp is the N2O flux by permeation through the film, P is the permeance of the film, and dCN2 O /dx is the difference in N2O concentration above and below the film. The permeance of the film (P) was estimated using a·bT, where T is the soil temperature at 5-cm depth. The parameters a and b were obtained by laboratory experiment (shown in Fig. 2). The difference in the N2O concentration (dCN2 O =dx) was estimated using CN2 O 0 cm CN2 O ambi, where CN2 O 0 cm and CN2 O ambi are the N2O concentrations just below the mulch film and the ambient N2O concentration (fixed to 320 ppbv), respectively.
Results Time course of N2O flux from furrow in field As shown in Fig. 3a, a significant amount of N2O was emitted from the soil surface of the furrow between ridges. In the winter experiment, N 2 O flux from the furrow
Nitrous oxide and O2 concentrations in soil The N2O and O2 concentrations in the ridge soil are shown in Fig. 4. In both experiments, the N2O concentration was generally highest at 5 cm depth. The N2O concentration at 0 cm depth (the N2O concentration in the air between the soil and the mulch film) was slightly lower than that at 5 cm depth. In the soil lower than 5 cm depth, the N2O concentration generally decreased with an increase in soil depth. In the winter experiment, the soil N2O concentration gradually increased throughout the experimental period. The maximum N2O concentration at 5 cm depth (10 ppmv) was observed 25 days after fertilization with mulching (January 11). In the summer experiment, the maximum N2O concentration at 5 cm depth (46 ppmv) was observed 2 days after fertilization with mulching (June 17). Then, N2O concentration gradually decreased. The relationships between the N2O flux from the furrow soil surface and the N2O concentrations in the ridge soil at various depths are shown in Fig. 5. Generally, the N2O flux had a significant positive correlation to the soil N2O concentration at any depth. In the winter experiment, the coefficients of determination (R2 value) between the N2O flux
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Fig. 3 Nitrous oxide flux from the furrow soil surface (a) and ridge soil temperature at 5 cm depth and volumetric ridge soil water contents at 5 and 20 cm depths (b) observed in the winter (left) and summer
(right) experiments. Vertical bars show the standard deviation of the three replicates. Vertical arrows show fertilizer application with mulching
and the soil N2O concentration at each soil depth ranged between 0.86 and 0.99. In the summer experiment, they ranged between 0.85 and 0.90 except at 50 cm soil depth (R2 00.56). The O2 concentration in the ridge soil was lower than that in the atmospheric air (ca. 21% in volume) during the experimental periods. In general, the soil O2 concentration was lowest at 5 cm depth. In the winter experiment, the
minimum soil O2 concentration at 5 cm depth (18% in volume) was observed 10 days after fertilization with mulching (December 27). Thereafter, the soil O2 concentration gradually increased. In the summer experiment, the minimum soil O2 concentration at 5 cm depth (12% in volume) was observed 4 days after fertilization with mulching (June 19), and thereafter, the soil O2 concentration gradually increased.
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Fig. 4 Nitrous oxide (a) and O2 (b) concentrations in the ridge soil below the mulch film, observed in the winter (left) and summer (right) experiments. Data at “0 cm depth” show the N2O and O2 concentrations of the air between the mulch film and the soil surface
Biol Fertil Soils (2012) 48:787–795 140 0cm
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2000 1600 1200 800 400 0 0
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Fig. 6 Representative time courses of N2O concentration in the separated compartments of the chamber after injecting standard gas with high N2O concentration. An example of the data of the transparent film with an incubator air temperature of 40°C is shown. For details of the experiment, see the main manuscript and Fig. 2
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Other results in field During the winter experiment, the soil temperature in the ridge soil at 5 cm depth ranged mostly from 0 to 15°C. During the summer experiment, the soil temperature at 5 cm depth showed extensive diurnal fluctuation, mostly from 25 to 50°C. In the daytime on sunny days, the soil temperature ranged mostly from 40 to 50°C (Fig. 3b). The volumetric water content in the ridge soil at 5 cm depth was kept relatively stable, ranging from 24% to 32% and from 26% to 33% during the winter and summer experiments, respectively. At 20 cm depth, the volumetric soil water contents ranged from 47% to 60% and from 53% to 64% during the winter and summer experiments, respectively (Fig. 3b). Although the ridge soil was covered with the mulch film, temporary slight increase in the soil water content was sometimes observed after a rainfall. This was probably caused by the horizontal movement of rain water penetrating into furrow soil. The soil NH4+– and NO3−–N concentrations on November 18, 2009 (before the winter experiment) were 3.9 and 1.7 μg N/g soil, which increased to 199 and 110 μg N/g soil, respectively, after the experiment (January 20, 2010). The soil NH4+– and NO3−–N contents on June 15, 2010 (before the summer experiment) were 7.1 and 67 μg N/g soil, which increased to 15.2 and 171 μg N/g soil, respectively, after the experiment (July 15, 2010). Nitrous oxide permeation flux through mulch film (laboratory experiment) A representative time course of the N2O concentrations in the compartments of the chamber is shown in Fig. 6. After
injecting the standard gas with a high N2O concentration, the N2O concentration of the air in the compartment with the addition of N2O gradually decreased, whereas that in the other compartment gradually increased. This means that some N2O in the compartment with a higher N2O concentration moves to the other compartment by permeation through the film. The decreasing/increasing rates of N2O concentration were higher at higher temperatures; they were also higher for the transparent film than for the black film. The calculated permeance was higher for the transparent film than for the black film. Permeance increased exponentially with an increase in ambient temperature. By fitting exponential regression curves, permeance (P; [μg N–N2O m−2 h−1 ppbv–ΔN2O−1]) was estimated as a function of ambient temperature (T; °C) to be P ¼ 0:241 1:39T and P ¼ 0:330 1:56T for the black and transparent films, respectively (Fig. 7). Simulation of N2O flux by permeation in field The estimated N2O fluxes emitted by permeation through the mulch films are shown in Fig. 8. The estimated N2O fluxes showed extensive diurnal fluctuations according to the diurnal change in soil temperature. The simulated N2O flux through -1
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Fig. 7 Relationships between the calculated permeance of N2O through the plastic mulch film and ambient air temperature. Vertical bars show the standard deviation
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Fig. 8 Estimated N2O fluxes by permeation through the mulch film in the winter (a) and summer (b) experiments. The observed N2O flux from the furrow soil surface (shown in Fig. 3a) was plotted here again for comparison with that by permeation. Vertical arrows show fertilizer application with mulching
the black mulch film was lower than the flux from the furrow soil surface during the winter experiment, whose maximum was 36 μg N m−2 h−1. In contrast, the N2O flux through the transparent mulch film was much higher than that from the furrow soil surface during the summer experiment, whose maximum value reached 1,230 μg N m−2 h−1. By the trapezoidal integration of the estimated flux data during the experiment, the cumulative N2O emission by permeation through the mulch film was estimated to be 9 and 191 mg N m−2, respectively, for the winter and summer experiment.
Discussion In the winter experiment, the N2O flux from the furrow soil surface was still increasing at the end of the experiment (January 11; Fig. 3a). In addition, both the amounts of residual NH4+– and NO3−–N in the soil were still higher than 100 μg N/g soil−1 at the end of the experiment. These results indicate that both N mineralization and nitrification proceeded moderately and were still ongoing at the end of the experiment. The soil temperature at 5 cm depth was mostly from 0 to 15°C. The soil water content remained relatively stable, ranging from 24% to 32%, throughout the experimental period (Fig. 3b) probably because both the evaporation of soil water and the penetration of rain water into the soil were restricted by the mulch film. These
conditions may have been suitable for the moderate progresses of N mineralization and nitrification. During the summer experiment, the soil temperature at 5 cm depth generally ranged from 40°C to 50°C in the daytime. Under the high-temperature condition, the soil NO3−–N concentration was significantly increased from 67 to 171 μg N/g soil, whereas increase in the NH4+–N concentration was small, from 7.1 to 15.2 μg N/g soil−1. Nitrogen mineralization rate generally increases with the increase in temperature and is not significantly suppressed at least up to 50°C (Curtin et al. 2011). In addition, the soil water content during the summer experiment (26–33% in volume) was generally thought to be suitable for the progress of nitrification (Linn and Doran 1984). Therefore, the organic N provided by the poultry manure was probably once mineralized to NH4+–N and then rapidly nitrified to NO3−–N during the summer experiment. Although nitrification activity of autotrophic nitrifying bacteria may be significantly suppressed under temperature higher than 40°C, some previous studies have found thermophilic nitrifiers in composted manures which keep high nitrification activities under temperature higher than 50°C (Shimaya and Hashimoto 2011; Yamamoto et al. 2010). In this study, it is not certain whether significant suppression of nitrification occurred or not by the high soil temperature of 40–50°C in summer daytime. Future studies with investigation of soil microbial community are expected to elucidate possible suppression of nitrification under high temperature condition. On the other hand, denitrification is generally enhanced under conditions of higher soil water content than nitrification. According to the previous studies (Amha and Bohne 2011; Linn and Doran 1984), denitrification was significantly enhanced when the soil water-filled pore space became higher than 60%. In this study, the soil NO3−–N concentration increased to be higher than 100 μg N/g soil−1 after both experiments. This indicates that NO3−–N produced by nitrification accumulated in the soil and that the amounts of decrease in NO3−–N concentration caused by denitrification and leaching were low in the soil covered with the mulch film. The water-filled pore space was estimated to be consistently lower than 60% by the data on the soil bulk density (0.65 Mg m−3) of this field and the volumetric soil water content during the entire experimental periods (24–33% in volume), which also indicates small denitrification activity in this study. The decrease in the O2 concentration in the ridge soil (Fig. 4) was caused by the O2 consumption by the respiration of soil microorganisms under the condition of low O2 diffusivity. The minimum O2 concentrations observed in the experiment in this study were 18% and 12% in volume in the winter and summer experiments, respectively. Although a decrease in O2 concentration often enhances N2O production derived from denitrification, no significant enhancement of N2O production by denitrification occurred except under
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highly anoxic condition in the soil. For example, Khalil et al. (2004) reported that the contribution of denitrification to N2O production is low at O2 concentrations higher than 0.76 kPa, corresponding to 0.75% in volume in ambient air. The observed minimum O2 concentrations in this study were much higher than this value, which also indicates that N2O production by denitrification was not enhanced significantly. We have observed that N2O flux from the soil surface in this field was generally lower than 10 μg N m−2 h−1 under unfertilized and bare conditions (data not shown). The significant enhancement of N2O flux from the furrow soil surface after fertilization with mulching of the ridge soil (Fig. 3a) was clearly caused by the horizontal diffusion of N2O produced in the adjacent fertilized ridge soil. Diffusive gas flux is described by Fick’s diffusion law, as the product of the gas diffusion coefficient of the medium and the gradient of the gas concentration. The coefficient of gas diffusion in the soil depends mainly on the temperature and air porosity of the soil, changing proportionally to the changes in the square of absolute temperature and in air porosity (Millington and Quirk 1961; Suwa et al. 2004). Although it is slightly changed by ambient temperature, the difference in the diffusion coefficient in the soil is not large for a temperature difference of 10°C or 20°C. In addition, soil mulching restricted the evaporation of soil water and the penetration of rain water into the soil so that the soil water content in the topsoil was kept comparatively stable throughout the experimental periods. This means that the change in the air porosity in the soil was also small during the experimental periods. Therefore, the diffusion coefficient of the ridge soil under the mulch film did not markedly change throughout the experimental periods in this study. As a result, N2O concentration in the ridge soil became the most responsible factor for determining N2O flux from the adjacent furrow soil surface; a significant positive correlation was found between these data (Fig. 5). As for N2O emission by permeation through the mulch film, we estimated the flux using two parameters: soil N2O concentration and ambient temperature. Contrary to the gas diffusion in the soil, the effect of ambient temperature on the permeance of the mulch film should be indispensably taken into account since the permeance of thin plastic films for gases remarkably changes exponentially with a change in ambient temperature, as shown in Fig. 7. In the summer, the N2O flux by permeation through the mulch film was higher than that by horizontal diffusion to the furrow. In particular, the N2O flux by permeation showed extensive diurnal fluctuation and became extremely high in the daytime owing to the higher permeance of the mulch film under high-temperature condition (Fig. 8b). In the winter, on the other hand, the N2O flux by permeation through the mulch film was lower than that by horizontal diffusion to the furrow (Fig. 8a). It was also lower than that
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in the summer. The low N2O flux by permeation in the winter was caused by the low temperature (Fig. 3b), low N2O concentration under the mulch film (Fig. 4a), and lower permeance of the black mulch film than that of the transparent one (Fig. 7). These results mean that the relative contribution of the N2O flux by permeation increases markedly with an increase in ambient temperature. Since we did not measure air temperature at the soil surface (just below the mulch film) in this study, we used soil temperature data at 5 cm depth to calculate the permeance of the film in the field. The actual air temperature at the soil surface may have been slightly higher than the soil temperature at 5 cm depth on sunny daytime. In addition, in the outside field, the film may be gradually degraded by high temperatures and ultraviolet solar rays, or it may develop small holes or cracks accidentally. Taking these various factors into account, the N2O flux by permeation through the mulch film may be in many cases higher than the estimated values shown in this study.
Conclusion To the best of our knowledge, studies of N2O emission from soil covered with mulch films have been very limited to date. In this study, we showed for the first time detailed data on N2O emission to the atmosphere. Nitrous oxide emission from soil covered with mulch films can also be estimated in other fields by a similar procedure to that used in this study, i.e., measurement of gas flux from furrow soil surface, measurement of N2O concentration in the soil, and investigation of permeance of the mulch film and its temperature dependence. Since the amounts of N2O production and transport should be significantly changed by various factors including soil type, environmental conditions, material of the mulch film, and agricultural practices, further study is required to elucidate in more detail the mechanisms of N2O production and transport. In addition, the relative contribution of N2O emission during the periods when the soil is covered with mulch films to the annual total emission should also be determined in future studies. Acknowledgments The authors thank Sumio Itoh (NARC) for his valuable advice on the experiment. The authors also thank Shigeto Sudo, Hiroko Akiyama, Akiko Yoshizawa, Sachiko Banzawa (NIAES), Yumiko Nemoto, and the staff of the experimental field management section in the NARC, and Fumio Sato (National Institute of Vegetable and Tea Science) for their assistance with the experiment and management of the experimental field. The authors also thank the editor and the anonymous reviewers for providing many valuable suggestions and comments for improving the manuscript. This study was supported by a project entitled “Development of innovative cropping system with utilization of organic fertilizer” funded by the Ministry of Agriculture, Forestry and Fisheries of Japan.
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