Nitrous Oxide Emission from Deyeuxia angustifolia ... - Semantic Scholar

Environ Manage (2007) 40:613–622 DOI 10.1007/s00267-006-0349-9

Nitrous Oxide Emission from Deyeuxia angustifolia Freshwater Marsh in Northeast China Junbao Yu Æ Jingshuang Liu Æ Jinda Wang Æ Weidong Sun Æ William H. Patrick Jr. Æ Franz X. Meixner

Received: 28 September 2006 / Accepted: 15 May 2007 Springer Science+Business Media, LLC 2007

Abstract Here we report N2O emission results for freshwater marshes isolated from human activities at the Sanjiang Experimental Station of Marsh Wetland Ecology in northeastern China. These results are important for us to understand N2O emission in natural processes in undisturbed freshwater marsh. Two adjacent plots of Deyeuxia angustifolia freshwater marsh with different water regimes, i.e., seasonally waterlogged (SW) and notwaterlogged (NW), were chosen for gas sampling, and soil and biomass studies. Emissions of N2O from NW plots were obviously higher than from the SW plots. Daily maximum N2O flux was observed at 13 o¢clock and the seasonal maximum occurred in end July to early August. The annual average N2O emissions from the NW marsh were 4.45 lg m–2 h–1 in 2002 and 6.85 lg m–2 h–1 in 2003 during growing sea-

son. The SW marsh was overall a sink for N2O with corresponding annual emissions of –1.00 lg m–2 h–1 for 2002 and –0.76 lg m–2 h–1 for 2003. There were significant correlations between N2O fluxes and temperatures of both air and 5-cm-depth soil. The range of soil redox potential 200–400 mV appeared to be optimum for N2O flux. Besides temperature and plant biomass, the freeze–thaw process is also an important factor for N2O emission burst. Our results show that the freshwater marsh isolated from human activity in northeastern China is not a major source of N2O.

J. Yu (&) J. Liu J. Wang Key Lab of Wetland Ecology and Environment, Northeast Institute of Geography and Agricultural Ecology, Chinese Academy of Sciences, 3195 Weishan Road, Changchun 130012, China e-mail: [email protected]

Introduction

W. Sun Key Lab of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, P.O. Box 1131, Guangzhou 510640, China W. H. Patrick Jr. Wetland Biogeochemistry Institute, Louisiana State University, Baton Rouge, LA 70803, USA F. X. Meixner Biogeochemistry Department, Max Planck Institute for Chemistry, P.O. Box 3060, D-55020 Mainz, Germany

Keywords Nitrous oxide Freshwater marsh Impact factors Diurnal and seasonal variations Freeze–thaw region

Nitrous oxide (N2O) is a radiatively active atmospheric trace gas, currently accounting for 5–6% of the total Global Warming Potential (Dalal and others 2003, Watson and others 1996). On a molar basis, it is about 296 times and 13 times more effective in global warming than CO2 and CH4, respectively (IPCC 2001, Prinn and others 1990). Atmospheric N2O concentration has increased from about 275 ppbv in pre-industrial times to about 314 ppbv in 1998 (IPCC 2001, Khalil and Rasmussen 1992) with significantly accelerated increasing rate, e.g., the atmospheric concentrations of N2O have risen at an average rate of 0.8 ± 0.2 ppbv yr–1 during the period 1980–1998 (Khalil and Rasmussen 1992), indicating an imbalance of 25–30% between global sources and sinks (Vitousek 1994). Because of its long atmospheric lifetime (118 ± 25 years) (Minschwaner and others 1998, Olesen and others 2004)

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and significant effects on stratospheric ozone (Crutzen 1981, Prather 1998), N2O has been a subject of much concern, especially regarding the mechanisms involved in its formation and emission from the soil and other sources, as well as its removal from the atmosphere. About one third of global terrestrial and aquatic N2O emission is considered to be anthropogenic (Seitzinger and others 2000). In aquatic ecosystems, N loading can lead to eutrophication and can affect the exchange of N2O between water and the atmosphere. Significant N2O emissions have been measured from N-enriched rivers, as well as estuarine and coastal water (Seitzinger and Kroeze 1998). Studies showed that the riverine ecosystems are probably ‘‘regional hot spots’’ in N2O production, but their contribution to global N2O release is unknown (Groffman and others 2000). Similar to the streamside ecosystem, wetland ecosystems are also sources of N2O (Huttunen and others 2003, Schiller and Hastie 1994), particularly in wetlands containing a large supply of nitrate, which has been shown to inhibit the reduction of N2O to N2 (Blackmer and Bremner 1976). The microbiological processes that produce N2O, nitrification and denitrification, can be affected by many physical and biochemical factors, such as temperature, pH, redox potential and soil moisture, etc (Bowden 1987, Conrad 1996, Heincke and Kaupenjohann 1999, Liu and others 2003, Moore 2002, Smith and others 2003, Yu and others 2001). The flooding and draining cycle, i.e., dynamics of hydrological condition, which impact above physical and biochemical factors, may accelerate both nitrification and denitrification operating rate and thus has been considered to be an important factor controlling N2O emission in wetland research (Bowden 1987, Liu and others 2003, Martikainen and others 1993). In this paper, we report chamber measurements of N2O in a natural wetland ecosystem over the plant-growing season of 2002–2003. The objectives of this study are to examine the diurnal and seasonal variation of N2O fluxes in a middle latitude region of China, and correlations of temperature, biomass, water status, and redox potential to N2O emission in an undisturbed natural wetland.

Methods Site Description The studied site is located at the Sanjiang Plain (4501¢N to 4828¢N, 13013¢E to 13505¢E) in Heilongjiang province, China (Fig. 1). The Sangjiang Plain wetland is the largest continuous freshwater marsh in China (Zhao 1999). The area of marsh in the Sanjiang Plain was about 53,450 km2 (80.2% of plain area) in the 1950s, which decreased sharply to 14,800 km2 because of large scale reclamation. More

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than 38,000 km2 of the wetland has been changed into cropland within the last 40 years (Liu and Ma 2000). Parts of the marsh receive discharge water or runoff with a high N load from cropland, and the water-holding capacity of the wetland has decreased because the drainage system for the cropland changed water status and increased peat decomposition (Liu and Ma 2000). The average groundwater level in the area was 5.2 m, which declined by 4.0 m on average because of massive extraction of groundwater for paddy fields within the last 10 years (Wang and Tian 2003, Zhao and others 2003). The mean annual precipitation is 500–650 mm, mainly in July and August, and the monthly mean temperature ranges from –20C in January to 22C in July. The altitude is around 56 m above sea level. The marshes in the Sanjiang Plain are mainly divided into four types according to plant species, i.e., Carex lasiocarpa, Carex pseudocuraica, Carex meyeriana, and Deyeuxia angustifolia (Zhao 1999). The D. angustifolia freshwater marsh, at the Sanjiang Experimental Station of Marsh Wetland Ecology, Chinese Academy of Sciences, at approximately 4735¢N, 13331¢E (Fig. 1), was selected for monitoring N2O emissions. Two plots were selected for this study: one is seasonally waterlogged (SW) and the other is not-waterlogged (NW). The studied area has been protected since 1986 when the experimental station was set up, and is separated completely from cropland. Therefore, it is a natural marsh, receiving no discharge water and runoff from the cropland. The plant cover in monitored sites was pure D. angustifolia in the SW marsh and 90% D. angustifolia and 10% shrubs in the NW marsh. In the monitored plots, the soil is classified as a gley soil. The soil profile is composed of three to four layers, from the top to the bottom: standing water layer, root layer, peat layer and gley soil layer. Standing water depth ranges from 0 to 10 cm in the SW marsh. The standing water mainly comes from melted snow from March to June and rainfall in July and August. There is no standing water in the NW marsh during any season of the year. The dead/live root layer depth is about 15–20 cm in the NW marsh and 20–30 cm in the SW marsh. The thickness of the peat layer ranges from 0 to 10 cm in the NW marsh and from 5 to 20 cm in the SW marsh. The gley layer is the bottom of soil profile underneath the peat layer. Average nutrient concentrations in the entire soil profile (0– 120 cm, three sampling layers, three replicates) in four sample sites were total N of 13.8 g kg–1, NO3-N 2.80 mg kg– 1 , NH4-N 47.14 mg kg–1, total P 955 mg kg–1, total S 104 mg kg–1, and total C 169 g kg–1 (Yu and others 2004). Water and soil in marshes are completely frozen from early November to late February with maximum frozen depth of about 90 cm. Thawing starts in March and lasts until middle July. The soil is finally thawed out at soil depths of about 60–65 cm (monitored in 2003).


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N2O Sampling, Analysis, and Calculation Two adjacent plots (10 m · 25 m), one within the SW marsh and the other within the NW marsh, were chosen for this study. Five monitoring sites (replicates) are installed within each plot. Boardwalks were laid to each site in order to minimize disturbance to the marsh during sampling. Gas was collected in a Plexiglas static chamber using equipment similar to that used by Lindau (Lindau 1994). The gas collection chamber consisted of permanently installed metal base units (50 cm · 50 cm · 5 cm) inserted into the ground with four fixed metal pegs, open bottoms, and removable clear Plexiglas tops (50 cm · 50 cm · 70 cm). The connection between the chamber and the base dovetailed, leaving space for the addition of water to seal the enclosed volume from atmosphere. Four pegs were installed about 10 cm into the soil to support the chamber prior to sampling. A thin rubber tubing with clamp (sampling port), thermometer, and battery operated fan were installed in the top of each chamber. Chamber base units were placed in each of the 10 monitoring sites in early May. Nitrous oxide fluxes were measured twice daily on each Saturday from 8:30 h to 9:30 h and from 14:30 h to 15:30 h, and for a day–night cycle at 3-hour intervals on the 18th of each month. The sampling was carried out during the growing seasons (from early May to end October) of 2002

and 2003. At each sampling, the chamber was first sealed and three replicate samples of the chamber air were manually withdrawn through the sampling port with gas-tight 50-mL syringes at 0, 30 and 60 minutes after closure. Each sample was injected into a Tedlar gas sampling bag (produced by Guangming Design and Research Institute of Chemical Industry, Chemical Industry Ministry, China) immediately to prevent exchange or contamination by atmospheric gases. All gas samples were analyzed within 10 hours after collection at the Sanjiang Experimental Station of Marsh Wetland Ecology. The N2O concentration in the gas samples was determined on an Agilent6820 Gas Chromatograph (Agilent Co., USA) equipped with a 63Ni electron capture detector and a 2m Poropak Q (80/100 mesh) column (Lanzhou Atech Technologies Co., LTD., China). The oven, injector, and the detector temperatures were 55C, 250C, and 375C, respectively. The carrier gas was >99.99% N2. The carrier gas (N2) flow rate was 60 mL min–1. Each gas analysis was calibrated by its corresponding certified N2O standards, provided by National Research Institute of Standard Material, China. The flux of N2O emission was calculated from the equation: F ¼qH

DC 273=ð273 þ TÞ Dt

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where F is the flux of N2O (lg N2O m–2 h–1), q is N2O density under standard state (1.96 kg m–3), H is height of static chamber (m), DC Dt is the rate of change in concentration of N2O within the chamber (ppb h–1), and T is the temperature within the chamber (C). The value of DC Dt is determined by using least-squares regression. Biomass and In Situ Soil Measurement Plant material was determined in the middle of each month in four duplicate plots of 50 cm · 50 cm at different sampling locations within the SW and NW plots during the growing season. Live plants and plant litter in each plot were clipped to ground level, then the plots were excavated to a depth of 60 cm and the roots were separated from the soil. Live plant materials and litter were separated manually in the laboratory, oven-dried for 72 hours at 80C to a constant weight. The biomass was calculated on a dryweight basis. Air temperature and soil temperature at 5-cm depth were measured in duplicates using mercury-in-glass thermometer and soil thermometer, respectively, while monitoring N2O emission. Soil pH was measured by pHB-4 portable pH meter (Shanghai Science Instrument Factory of Radium Magnetism, China) in situ at every sampling time. Soil redox potential (Eh) was measured in the field by pHB-4 portable redox meter using in situ Pt electrodes (2 replicates: 5 cm) and an Ag-AgCl reference electrode. Readings were corrected to standard hydrogen half-cell by adding 199 mV (the correction factor for calomel reference electrode at 20C) to field measurements (Hou and others 2000).

Results and Discussion Diurnal and Seasonal Variations of N2O Fluxes N2O emission fluxes in the NW and SW D. angustifolia marsh were low in the early morning, reached a maximum value at 13:00 h, and then decreased (Fig. 2a–e). The average daily maximum N2O flux value was 56.1 lg m–2 h–1 in August in the NW marsh, whereas the average minimum value was –24.4 lg m–2 h–1 in September in the SW marsh. The SW marsh emitted and absorbed N2O alternately during the day from May to October. By contrast, the NW marsh continuously emitted N2O day and night in July and August. The daily fluxes of N2O in the NW marsh are significantly higher than those in the SW marsh (p < 0.001). Emissions of N2O were measured from May 10 to October 12 in 2002 and from May 11 to October 13 in 2003 (Fig. 3a, b). In both years, the N2O fluxes from the NW marsh were higher than that from the SW marsh

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(p < 0.002). The ranges of seasonal N2O fluxes of the NW marsh were –21.78 to 51.93 lg m–2 h–1, and of the SW marsh were –34.51 to 34.44 lg m–2 h–1 in 2002 (Fig. 3a). Similarly, seasonal N2O fluxes varied from –21.96 to 52.53 lg m–2 h–1 for the NW marsh, and –23.87 to 41.78 lg m–2 h–1 for the SW marsh in 2003 (Figure 3b). The annual average N2O emissions from the NW marsh were 4.45 lg m–2 h–1 in 2002 and 6.85 lg m–2 h–1 in 2003, and that from the SW marsh were –1.00 lg m–2 h–1 in 2002 and –0.76 lg m–2 h–1 in 2003 during growing season. The N2O emission rates reached a peak in the end of July (Fig. 3b) to early August (Fig. 3a). The NW plots continuously released N2O gas during the middle of June to mid-to-late September period. Nitrous oxide emission from SW sites fluctuated more, with periods of both emission and absorption. The N2O emissions from all plots mainly occurred in August. Changes of Parameters During Growing Season The average air and soil temperatures throughout the day and plant-growing season are shown in Fig. 2f and Fig. 3c, d, respectively. The daily trends in air temperature and 5cm soil temperature are similar in the NW marsh and the SW marsh (Fig. 2f). Temperatures increase in the morning and generally peaked at 13:00 h. The 5-cm soil temperature in the NW marsh ranges from 8.1 to 18.1C and that in the SW marsh ranges from and 8.12C to 15.3C, and the air temperature ranges from 9.9C to 22.3C. The average daily 5-cm soil temperature in the NW plots is 5C higher than that in the SW marsh. The seasonal air and soil temperature patterns are similar (Fig. 3c, d) in the two marshes. Soil temperature increases gradually from the middle of May and reaches a maximum in August. The average temperature at 5-cm depth in the NW marsh is 2.7C higher than that in the SW marsh during growing season. The correlation between air temperature and soil temperature in both marshes is significant at the 0.01 level (>0.76**). High soil temperature in NW marsh compared to that in SW marsh is due to the high specific heat capacity of waterlogged sediment compared to dry sediment. The air temperatures appear to rise faster than the soil temperatures during the day and over a season. This is also an expected effect of the differences in specific heat between air and water. The average dry weight of total, aboveground, and belowground live plant material reach peak values in August, which is the warm rainy season in both plots (Fig. 4). The total, above- and belowground plant biomasses in the NW marsh are higher than those in the SW marsh in every month. The results are concord with prior reports of the seasonal variations of plant biomass in the region (He 2001, Ni and others 1998). The total biomass is 230– 1570 g m–2 and 440–3680 g m–2 in the SW plots and the


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Fig. 3 Seasonal changes of N2O fluxes (a, b) and average of temperature (c, d) in notwaterlogged (NW) and seasonally waterlogged (SW) plots in 2002 and 2003. Bars represent standard errors. N2O fluxes were measured twice a day every Saturday. Air T and Soil T stand for air temperature and soil temperature, respectively

temperature in f. The fluxes were measured for a day–night cycle at 3hour intervals on the 18th of each month during the growth season in 2002 and 2003. Air T and Soil T stand for air temperature and soil temperature, respectively

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NW plots during the monitored period, respectively. The average ratios of belowground biomass to aboveground biomass are 1.02 and 2.30 in the SW and NW marshes, respectively. The total plant biomass in the research area is higher than that of temperate lakes (0.07–680 g m–2), floating zone (0.56–1191 g m–2) and submerged zone (5.75–71.4 g m–2), New Zealand lakes (50–1000 g m–2) (Sculthorpe 1967, Shardendu and Ambasht 1991), Nabugabo wetland (Uganda) (1638 g m–2) (Okot-Okumu 2004),

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but lower than the values reported for reed swamps in Minnesota (USA) (630–4640 g m–2) (Sculthorpe 1967) and similar to Sarcocarnia fruticosa and Phragmites australis in the Po Delta (Scarton and others 2002) and salt marshes of the Ebre delta (Curco and others 2002). The redox potential ranges from –200 to 500 mV in all plots in the D. angustifolia marsh during the plant growth season. It is 50–500 mV in the NW marsh and –200 to 260 mV in the SW marsh. The difference in Eh between the

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Fig. 4 The variation of average total plant biomass, above- and belowground biomass in notwaterlogged (NW) and seasonally waterlogged (SW) freshwater marshes during different months in (a) 2002 and (b) 2003. Bars represent standard errors

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The daily variation patterns of N2O fluxes from the marsh surface are similar to those of air temperature and 5-cmdepth soil temperature. Both the temperatures and N2O flux reach the maximum values at 13:00 h. The average soil temperature in the NW marsh is higher than that in the SW marsh, reflecting the effect of differences in specific heat of waterlogged sediment and dry sediment. N2O emission rates follow the same pattern, being higher in the NW plots. The correlation coefficients of daily N2O fluxes to air and soil temperatures are 0.838** and 0.726** in the NW plots and, 0.824** and 0.684** in the SW plots (Fig. 6a). The correlation is significant at the 0.01 level between N2O fluxes and air temperature and soil temperature. In both wetland types, N2O emission rates attained maximum values when air and soil temperatures reach a peak (Fig. 3). The correlation coefficients of seasonal N2O fluxes to soil temperature in the NW marsh and in the SW marsh were 0.660* and 0.534* (Fig. 6b). The positive

relation of soil temperature and N2O flux in field measurements in the region has also been reported (Zheng and others 1997). This highly significant correlation is likely due to the fact that an increase in soil temperature positively influences microbiological activity and gas diffusion, whereas it negatively affects the solubility of N2O. In natural ecosystems, most of the N2O is produced from denitrification under moderately anaerobic conditions, although nitrification under aerobic condition is also a con-

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growing season in the marshes is significant (p > 0.05), especially in the SW marsh (Fig. 5). The soil Eh drops sharply when water is ponded on the soil surface. The variation of soil pH is low and relatively stable, ranging from 5.20 to 6.20 (soil/water ratio of 1/5).

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tributor (Williams and others 1992). The numbers of zymogenic bacteria, nitrifiers, and especially denitrifiers, which are responsible for N2O production in soil, increase with increasing temperature (Hou and others 2000). Under certain ambient CO2 conditions, the activities of N-mineralization, urease, and denitrification enzyme activity are higher at elevated temperature (Tscherko and others 2001). An increase in temperature decreases the solubility of N2O in a nonlinear way and increases N2O diffusion to air, and thus restricts the microbial reduction of N2O to N2. This temperature-induced solubility change is relatively strongest at the low temperatures prevalent in soils (Heincke and Kaupenjohann 1999). In a field experiment, an attempt was made to explain the diurnal variability of N2O fluxes in part by a temperature-induced solubility change of N2O dissolved in the soil solution of the soil surface. It was suggested that, because of altering N2O solubility, the rates of N2O emissions were higher than the actual microbial N2O generation when the soil was warming up and were lower when the soil was cooling down (Blackmer and others 1982). Therefore, the N2O emission increasing with temperature is not only the function of temperature-produced N2O by microbial process, but also temperature-induced N2O solubility. The temperature-induced solubility change of N2O is probably the dominant factor in marsh because that much more water in marsh soil and high dissolved N2O concentration (maximal value of 0.73 ppmv) has been observed in floodwater of organic soil (Terry and others 1981). N2O Emission in Relation to Water Status, Biomass, Soil pH and Eh The annual average N2O emissions showed that NW marsh, which had no standing water, were N2O source (4.45–6.85 lg m–2 h–1) and SW marsh, in which standing water depth ranges of 0–10 cm were N2O sink (–1.00 to –0.76 lg m–2 h– 1 ), and in addition the daily and seasonal N2O fluxes from NW marsh were higher than those from SW marsh (Fig. 2a–e, Fig. 3a, b), indicating that the difference of N2O emission from NW and SW marsh may come from the impact of different water status on denitrification process in two spots. N2O is an intermediate in the denitrification pathway and can serve as the sole electron acceptor to support the growth of denitrifying bacteria (Bazylinski and others 1986, Koike and Hattori 1975, Okereke 1993). Interestingly, N2O can also be reduced to N2 by some bacteria that normally reduce nitrate to ammonia, i.e., the so-called DNRA (dissimilatory reduction of nitrate to ammonia) bacteria (Samuelsson 1985, Teraguchi and Hollocher 1989, Tiedje 1988). Thus, both production and consumption of N2O are controlled by this microbial group. Generally, N2O production from wetland soil is a function

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of denitrification under anaerobic conditions (Conrad 1996). Compared to NW soil, high water contents in SW marsh soil may prevent oxygen penetration into soil and restrict the diffusion of N2O, thus causing N2O loss due to reduction to N2 by denitrifiers. Furthermore, the relatively dry condition in NW marsh soil benefits for nitrogen mineralization because decomposition is faster under oxic conditions, and the sufficient supplying of NO3-N can increase N2O production in the denitrification pathway (Martikainen and others 1993). This is consistent with several previous studies of N2O emission rates (Freney and others 1981, Law and others 1992, Lindau and others 1991, Martikainen and others 1993, Samuelsson 1985, Schiller and Hastie 1994, Smith and others 1983) in wetland soils (e.g., paddy fields, fens, and wet tundra). All these studies suggested that the fluxes of N2O from wetland soils are small compared with aerated soils, probably because a larger percentage of N2O is further reduced by denitrifiers to N2. The ratio of N2 to N2O produced increases with the saturation of the soil pore space with water (Weier and others 1993). In a field study, the influence of different water table levels on N2O emission showed that much more N2O outgassed at a water table of 15 cm above the soil than at a water table of 45 cm above the soil. At the higher water table level, almost no N2O emitted, because most of it had been microbiologically converted into N2 during the transport (Kliewer and Gilliam 1995). Another research (Von Arnold and others 2005) found that N2O emissions were significantly lower from the undrained sites than from the drained sites, whereas undrained sites were net sinks and drained sites were a net source of N2O in forest mire because of relatively high nitrogen mineralization and organic matter decomposition under oxic conditions (Clymo 1984). N2O fluxes from both the NW and the SW marshes reached a peak (Fig. 3a, b) when the total and the belowground plant biomass of the marshes also reached a peak in August (Fig. 4). A possible mechanism for this would be that high plant biomass may enhance the interaction of plant processes and soil processes. Plant processes and soil processes interact in various ways, e.g., metabolism of trace gases by the plants, transport of trace gases through the plants, or metabolism of trace gases in the rhizosphere of plants (Thoene and others 1991). The plant metabolism may create a trace gas flux that adds to the flux generated in the soil. Plants can serve as conduits for the transport of trace gases between the soil and the atmosphere. This phenomenon is especially important for wetland soils for a vascular gas transport system to provide the roots with O2 (Armstrong 1979, Conrad 1996). The pH is relatively stable in the marsh, ranging from 5.20 to 6.20. The significant correlation between N2O fluxes and soil pH was not observed in the study maybe because of very little variations in pH.

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High Eh values coupled with high N2O fluxes in NW marsh compared to that in SW (Fig. 5 and Fig. 3a, b) during the study period indicated that much more N2O was further reduced by denitrifiers to N2 in SW marsh under anoxic condition. During periods of saturation and extended saturation, Eh was