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Published online 16 October 2007
Published in J Environ Qual 36:1577-1584 (2007)
DOI: 10.2134/jeq2006.0501
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Methane Oxidation in an Intensively Cropped Tropical Rice Field Soil under Long-Term Application of Organic and Mineral Fertilizers

D. R. Nayaka, Y. Jagadeesh Babub, A. Dattaa and T. K. Adhyaa,*

a Laboratory of Soil Microbiology, Div. of Crop Production, Central Rice Research Institute, Cuttack, Orissa, India
b School of Geography and Geology, McMaster Univ., Ontario, ON L8S4K1, Canada

* Corresponding author (adhyas{at}yahoo.com).

Received for publication November 15, 2006.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Conclusions
 REFERENCES
 
Methane (CH4) oxidation is the only known biological sink process for mitigating atmospheric and terrestrial emissions of CH4, a major greenhouse gas. Methane oxidation in an alluvial soil planted to rice (Oryza sativa L.) under long-term application of organic (compost with a C/N ratio of 21.71), and mineral fertilizers was measured in a field-cum-laboratory incubation study. Oxidation rates were quantified in terms of decrease in the concentration of CH4 in the headspace of incubation vessels and expressed as half-life (t1/2) values. Methane oxidation rates significantly differed among the treatments and growth stages of the rice crop. Methane oxidation rates were high at the maximum tillering and maturity stages, whereas they were low at grain-filling stage. Methane oxidation was low (t1/2 = 15.76 d) when provided with low concentration of CH4. On the contrary, high concentration of CH4 resulted in faster oxidation (t1/2 = 6.67 d), suggesting the predominance of "low affinity oxidation" in rice fields. Methane oxidation was stimulated following the application of mineral fertilizers or compost implicating nutrient limitation as one of the factors affecting the process. Combined application of compost and mineral fertilizer, however, inhibited CH4 oxidation probably due to N immobilization by the added compost. The positive effect of mineral fertilizer on CH4 oxidation rate was evident only at high CH4 concentration (t1/2 = 4.80 d), while at low CH4 concentration their was considerable suppression (t1/2 = 17.60 d). Further research may reveal that long-term application of fertilizers, organic or inorganic, may not inhibit CH4 oxidation.

Abbreviations: cfu, colony forming unit • sMMO, soluble methane monooxygenase • SOM, soil organic matter • SSP, single superphosphate • NRN, Ninhydrin reactive nitrogen • TOC, total organic carbon • TTC, tri-phenyl tetrazolium chloride • WHC, water holding capacity


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results
 Discussion
 Conclusions
 REFERENCES
 
METHANE (CH4) is the second most important greenhouse gas next to water vapor. It currently contributes approximately ~12% to global warming through absorption of infrared radiation and chemical reaction with other atmospheric gases (IPCC, 2001). The current global average atmospheric concentration of CH4 is 1.78 µL L–1, more than double its preindustrial value of 0.8 µL L–1 (Dlugokencky, 2001). The rate of increase in atmospheric CH4 slowed from about ~0.7% per annum in the 1980s to near zero in 1999 (Dlugokencky, 2001). Since 1990, the annual rate of CH4 increase in the atmosphere has been around ~0.5% (IPCC, 2001). Increased anthropogenic activity is generally given as the reason for the increase in the atmospheric concentration of CH4 since about 70% of CH4 production emanates from the anthropogenic sources (IPCC, 2001). Flooded rice fields are considered as the single largest anthropogenic source of atmospheric CH4 with an estimated contribution of about 15% of the 598 Tg global CH4 flux (IPCC, 2001). With intensification of rice cultivation to sustain the estimated increase in the human population, CH4 flux from these economically important but ecologically fragile ecosystems is anticipated to increase.

Methane emission from flooded rice fields is the net effect of CH4 production and its consumption that occur in tandem in the flooded field planted to rice (Neue, 1997). Methane production occurs under highly anaerobic conditions of flooded rice fields. On the contrary CH4 oxidation is mostly an aerobic process (Conrad, 1996), although anaerobic oxidation of CH4 has also been reported (Valentine, 2002). The groups of organisms that may be involved are the methanogenic bacteria, methanotrophic bacteria, and the ammonium oxidizing bacteria (Schimel and Gulledge, 1998). Methane may be oxidized in the interface between anoxic and oxic sites where concentration gradients of CH4 and O2 overlap. Such interfaces are found at the surface of flooded rice soils and in the rhizosphere of rice plants (Leisack et al., 2001). Methane oxidation in rice fields is assumed to consume about one-third of the CH4 production (Bosse and Frenzel, 1997), although values as high as 90% have also been reported (Schutz et al., 1989). Methane oxidation is the only known net biological sink for atmospheric CH4 and terrestrial emissions, where methanotrophic bacteria are able to oxidize CH4 for energy purposes or for building up of microbial biomass (Hanson and Hanson, 1996). Two types of kinetics have been encountered with respect to the consumption of CH4 in soils and sediments (Bender and Conrad, 1992). The first kinetic pattern of CH4 oxidation, known as "low affinity" CH4 oxidation is observed in all CH4–producing soils. This is performed by "conventional" Type I and II methanotrophs that display Km values in the µM range. The second type, known as "high-affinity" CH4 oxidation occurs in soils having CH4 concentrations in the nM range and is supposed to be performed by yet uncultured organisms with novel variants of methane monooxygenase (Bender and Conrad, 1992).

Methane uptake is controlled by the interplay of biotic and abiotic factors providing proximate limitation on CH4 oxidation. There is evidence that agricultural practices have adverse effects on the CH4–oxidizing ability of soils (Arif et al., 1996; Hutsch, 2001; Kessavalou et al., 1998). Cultivation appears to decrease net CH4 consumption as CH4 oxidation potentials of cultivated soils are less than in grasslands (Mosier et al., 1996; Kessavalou et al., 1998). Nitrogen fertilization has also been identified among other factors as an important contributor to this effect. In many cases, NH4 was the most detrimental form of N to CH4 oxidation (Bronson and Mosier, 1994). In a long-term fertilization experiment, CH4 consumption was significantly lowered after application of mineral N (Hutsch et al., 1993). On the contrary, stimulation of CH4 oxidation by NH4–based fertilizers in soil and around rice roots has also been reported—both in microcosm (Bodelier et al., 2000) and under field conditions (Kruger and Frenzel, 2003). It was suggested that elevated CH4/NH4+ ratio in the rooted soil greatly reduces the inhibitory effect of NH4+ (Cai and Mosier, 2000).

The importance of soil organic matter (SOM) for crop production has long been recognized (Paustian et al., 1977). Composted and noncomposted manures have long been used as sources of plant nutrients, especially N and P to supplement or replace synthetic fertilizer. Prominent means to maintain SOM in rice systems in Asia has historically been incorporation of green manures, animal wastes, or crop residues (Olk et al., 2000). Besides its nutrient values, organic matter amendment can affect soil organic C pools, soil nutrients, and microbial environment and activities, which are some of the controlling factors in the emission of CO2, N2O, and CH4 to the atmosphere. Biologically active C promotes microbial activities and CH4 emission from flooded fields planted to rice (Yagi and Minami, 1990). The elevated production of CH4 following organic amendment and putative adverse effect on CH4 oxidation may result in an increased atmospheric concentration of CH4 (Ginting et al., 2003). The aim of the present study was to observe the effect of long-term application of organic (compost) and mineral fertilizers on CH4 oxidation in a flooded alluvial soil under intensive rice cultivation.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
 Conclusions
 REFERENCES
 
Site Description
The field experiment was conducted at the experimental farm of the Central Rice Research Institute, Cuttack, India (85°55' E, 20°25' N; elevation 24 m). Mean annual maximum and minimum temperatures are 39.2 and 22.5°C respectively, and the mean annual temperature is 27.7°C. Annual precipitation is about ~1500 mm, of which 75 to 80% is received during June to September. The difference between mean summer soil temperature and mean winter soil temperature is more than 5°C, thus qualifying for hyperthermic temperature class. The soil of the farm area has been developed from the deltaic sediments of the Mahanadi River in recent times. The soil is an Aeric Endoaquept with sandy clay loam texture (259 g kg–1 clay, 216 g kg–1 silt, 525 g kg–1 sand), bulk density 1.40 Mg m–3, maximum water holding capacity (WHC) 0.437 kg kg–1, percolation rate <10 mm d–1, pH (H2O) 6.16, cation exchange capacity 15 cmolc kg–1, electrical conductivity 0.05 S m–1, total C 6.6 g kg–1 and total N 0.8 g kg–1, exchangeable K 120 kg ha–1.

Experimental Design
The field experiment on intensive rice cropping was established in 1969 to assess the long-term impact of both organic and mineral fertilizers on different soil physicochemical properties and crop yield under intensive rice cultivation in a rice–rice–fallow sequence. Wet season (July–December) rice was grown under rainfed condition followed by the dry season (January–April) rice under irrigated condition. The field was plowed thoroughly and flooded 2 to 3 d before transplanting for puddling and leveling. Rice plants (21-d-old seedlings of cv. Gayatri during the wet season and cv. Lalat during the dry season) were transplanted at a spacing of 20 by 10 cm with two seedlings per hill in the field plots (9 by 5 m) well separated by levees. The experiment was laid out in a randomized block design with three replicates each. There were four treatments: (i) unamended control, (ii) inorganic fertilizer [N–P–K (80–40–40 kg ha–1)], (iii) compost (5 Mg ha–1 yr–1), and (iv) compost (5 Mg ha–1 yr–1) + inorganic fertilizer [N–P–K, 80–40–40 kg ha–1)]. All the field plots remained continuously flooded to a water depth of 12 ± 7 cm during the entire period of crop growth and were drained 10 d before harvest. The crops were grown following normal recommended agronomic practices and harvested at maturity.

Fertilizer Amendments
Nitrogen (80 kg N ha–1 as prilled urea) was broadcast-applied to the field plots in three splits with half of the total N applied at the time of transplantation and the rest divided into two equal halves and applied at maximum tillering and panicle initiation stages, respectively. Potassium (40 kg K2O ha–1) was applied as muriate of potash in two splits with two-thirds of the fertilizer being applied as basal and the remaining one-third at panicle initiation stage. Phosphorus (40 kg P2O5 ha–1) as single superphosphate (SSP) was applied uniformly to the field plots as basal dressing. In select field plots, well-decomposed compost (total organic C 186.5 g kg–1, total N 8.59 g kg–1, cellulose 15.2 g kg–1, lignin 28.3 g kg–1, polyphenols 0.95 g kg–1, P2O5 50 g kg–1, K2O 130 g kg–1) was applied at 5 Mg ha–1 once a year during the last week of May, before the wet season crop.

Soil Sampling and Soil Handling
Soil samples were collected during the cultivation period of both dry (January–May) and wet seasons (July–December) of 2002 at three major growth stages of the rice crop, namely maximum tillering, grain filling, and maturity stages, in the control plots and plots receiving compost and inorganic fertilizer for the last 32 yr. Individual soil cores were taken with a PVC core sampler (at a depth of 0–5 cm) from five different places within individual replicated plots and mixed together to prepare a composite sample for the plot. Thus, {Sigma} = 3 replicated samples (each a composite of 5 core samples) x 4 sampled treatment = 12 individual samples were collected and analyzed for CH4 oxidation. All the samples were collected from in-between the planted rows for better comparison.

Immediately after sampling, excess water was allowed to drain off, visible root fragments and stones removed manually, and the soil was transferred to the laboratory for analyses. Moisture content of individual samples was determined gravimetrically in 10-g portions after drying at 105°C for 48 h.

Methane Oxidation Measurement
Portions of the soil (10 g) were placed in 130-mL sterile serum bottles. The soils were maintained at 60% WHC and allowed to equilibrate with the ambient air for 3 d in the dark in an incubator at 30 ± 2°C. Methane oxidation was initiated by sealing the serum bottles with neoprene septa and injecting the headspace with 5 mL of pure CH4 to provide approximately 47 mL L–1 of CH4 g–1 air-dried soil. Soil samples were incubated in an incubator (30 ± 2°C) in the dark. At select intervals, headspace gas samples (1 mL) of the serum bottles were analyzed for CH4 until the concentration came down to near ambient level. After each sampling, the headspace was replaced with an equivalent amount of high purity N2 to maintain the pressure equilibrium.

To measure the oxidation kinetics at high and low concentrations of CH4, portions (10 g) of soil samples collected from the respective field plots under different treatments were placed in 130-mL serum bottles. High affinity CH4 oxidation was initiated by sealing the serum bottles with neoprene septa and injecting the headspace with 1 mL of pure CH4 to provide approximately ≤2 µL L–1 CH4 (Topp and Hanson, 1991). Low affinity CH4 oxidation was initiated by sealing the serum bottles and injecting the headspace with 10 mL of CH4 to provide approximately ≥40 µL L–1 CH4 (King et al., 1990; Jones and Nedwell, 1993). At selected intervals, headspace gas samples were analyzed for CH4 and oxidation kinetics calculated.

The amount of CH4 remaining in the headspace was plotted on a log scale against the time of incubation (Alexander, 1994). The decomposition of CH4 followed a first-order reaction as the plots yielded straight line based on the equation:

Formula
where C is the concentration of CH4 remaining in the vial after time t, C0 is the initial concentration of CH4, and k is the first-order kinetic constant. The half-life (t1/2) values obtained from these plots indicated the oxidation rate of CH4.

Estimation of Methane
Methane concentration in the headspace samples were estimated by gas chromatography in a Varian 3600 gas chromatograph equipped with FID and Porapak N column (2 m length, 0.32 cm [0.125 inch] o.d., 80/100 mesh, stainless steel column). The injector, column, and detector were maintained at 80, 70, and 120°C, respectively. The carrier gas (N2) flow was maintained at 30 mL min–1. A 1-mL gas sample was injected into the gas chromatograph with a gas-tight syringe. The gas chromatograph was calibrated before and after each set of measurements using 5.38, 9.03, and 10.8 µL CH4 L–1 in N2 (Scotty II analyzed gases, M/s Alltech Assoc., Dearfield, IL) as primary standard and 2.14 µL L–1 in air as secondary standard to provide a standard curve linear over the concentration range used. Under these conditions, the retention time of CH4 was 0.53 min and the minimum detectable limit was 0.5 µL L–1.

Soil Analyses
The soil pH was measured with a portable pH meter (Philips model PW 9424) using a combined calomel glass electrode assembly. Ferrous iron (Fe2+) content in 10-g subsamples was extracted with 50 mL of 1 M sodium-acetate in HCl (pH 2.8) and assayed by reacting with orthophenanthroline (Murti et al., 1966). Ninhydrin reactive nitrogen (NRN) content in further 10-g subsamples was extracted with 0.5 M K2SO4 and estimated colorimetrically (Badalucco et al., 1992). Dehydrogenase activity was determined by reduction of tri-phenyl tetrazolium chloride (TTC) (Chendrayan et al., 1980). The Corg content of the soil samples were determined in a TOC analyzer (Micro N/C model HT 1300, Analytic Jena, Germany). Total N was analyzed by a semi- automated Kjeldahl method (Kjeltech model 2100, Foss Tecator, Sweden).

Microbiological Analyses
The population of total aerobic heterotrophs in the soil samples was estimated by the standard dilution plate technique using tryptone yeast extract medium (Rand et al., 1975) and expressed as colony forming units (cfu) kg–1 dry soil. Methane oxidizers with soluble methane monooxygenase (sMMO) activity were enumerated as described by Graham et al. (1992). Triplicate plates for each dilution were incubated in vacuum desiccators under the atmosphere of CH4 (5%)–air mixture by replenishing the headspace atmosphere with CH4 on every 4 d, for 30 d in an incubator. The colonies that developed a colored complex with naphthalene and O-dianisidine (tetrazotized) were counted for CH4 oxidizers with sMMO. Culturable ammonium-oxidizing bacteria were enumerated by the MPN method (Schmidt and Belser, 1982).

Statistical Analyses
All data were recalculated on the basis of oven-dry soil weight. Individual character datasets were statistically analyzed, and the mean comparison between treatments was established by Duncan's multiple range test using statistical package (IRRISTAT, version 3.1, International Rice Research Institute, Philippines). Simple and multiple correlations between soil chemical and biochemical parameters were analyzed using SYSTAT 5.05 (SPSS, Chicago, IL) to establish possible statistical relationship.


   Results
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
Discussion
 Conclusions
 REFERENCES
 
Methane Oxidation under Intensive Rice Cultivation
Methane oxidation was measured in soil samples collected from unamended control, inorganic fertilizer, compost and compost + inorganic fertilizer–treated field plots at three major growth stages: maximum tillering, grain filling, and maturity during both dry and wet seasons of 2002. Methane oxidation, expressed in terms of decrease in the concentration of CH4 in the headspace of incubation vessels, proceeded rapidly in all the treatments (Fig. 1 ). Methane oxidation varied significantly among the treatments as well as at different growth stages with highest rate at the maximum tillering stage and the lowest at the grain filling stage in soils collected during the wet season of 2002. However, during the dry season of 2002, CH4 oxidation rate was highest in the soils collected at maturity stage. Among the different treatments, mean CH4 oxidation rate in soil samples collected during the wet season of 2002 was lowest in compost + inorganic fertilizer. The decrease in the headspace CH4 concentration and the resultant half-life (t1/2 in days) values were of the following order: compost + inorganic fertilizer (6.60) > control (5.88) > compost (4.22) > inorganic fertilizer (2.84) (Table 1 ). Methane oxidation was faster in compost-amended plots than in control. Similar trends were noticed in the dry season of 2002. Combined application of compost and mineral fertilizers retarded the rate of CH4 oxidation by 2.23 times as compared to mineral fertilizer alone (Table 1).


Figure 1
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  		Fig. 1. Methane oxidation pattern in a flooded alluvial soil planted to rice cv. Gayatri (wet season, 2002) under long-term addition of organic (compost) and inorganic fertilizer. Means of three replicate values plotted. [-{diamondsuit}- control, -x- inorganic fertilizer, -•- compost, -{blacktriangleup}- compost + inorganic fertilizer]

 

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  		Table 1. Effect of long-term addition of organic and inorganic fertilizers on CH4 oxidation potential (t1/2) of an alluvial soil planted to rice at three growth stages (dry and wet season, 2002). In a column, means followed by a common letter are not significantly different at P < 0.05 by Duncan's multiple range test.

 
High and Low Affinity Methane Oxidation
In a subsequent study, experiments were conducted to find CH4 oxidation rates at high and low concentrations of CH4 to correspond with the low and high affinity CH4 oxidation. Results indicate that, high affinity CH4 oxidation (k value) was low and CH4 oxidation rate expressed as t1/2 (d) was high in the treatments exposed to low concentration of CH4 (Table 2 ). However, in experiments with low affinity oxidation, when the treatments were exposed to high concentration of CH4, they showed higher k values and low t1/2. Unlike low affinity oxidation, application of inorganic fertilizer alone resulted in the inhibition of high affinity CH4 oxidation. However, application of compost alone stimulated high affinity CH4 oxidation and was significantly lower than other treatments that were statistically (P < 0.05) at par.


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  		Table 2. Effect of long-term addition of organic (compost) and mineral fertilizers on high and low affinity oxidation of CH4 in a flooded alluvial soil under intensive rice cultivation. In a column, means followed by a common letter are not significantly different at P < 0.05 by Duncan's multiple range test.

 
Methane Oxidation and Related Soil Parameters
Changes in pH of the alluvial soil under different amendments at various sampling stages were monitored in the present study (Table 3 ). A simple correlation analysis between CH4 oxidation and t1/2 showed a significant positive relationship (r = 0.596**, n = 24) indicating the negative association of high pH on CH4 oxidation (Fig. 2 ).


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  		Table 3. Changes in pH of an alluvial soil planted to rice under different amendments at three growth stages (wet season, 2002). In a column, means followed by a common letter are not significantly different at P < 0.05 by Duncan's multiple range test.

 

Figure 2
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  		Fig. 2. Relationship between different soil physicochemical properties and CH4 oxidation kinetics in an intensively cropped alluvial field under long-term fertilization with organic and inorganic fertilizers.

 
Methane oxidation had a strong dependency on changes in soil Fe2+ content (Table 4 ) and on soil dehydrogenase activity (Table 5 ), two key indicators of soil reduction status. Linear regression analysis between CH4 oxidation rate (t1/2) at different growth stages and Fe2+ content revealed a strong positive correlation (r = 0.531**, n = 24), which indicates that higher Fe2+ content is negatively associated with CH4 oxidation. A simple correlation analysis between t1/2 and soil dehydrogenase activity also showed a significant positive correlation (r = 0.471*, n = 24) indicating a negative association with CH4 oxidation (Fig. 2).


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  		Table 4. Changes in Fe2+ content of an alluvial soil planted to rice under different amendments at three growth stages (wet season, 2002). In a column, means followed by a common letter are not significantly different at P < 0.05 by Duncan's multiple range test.

 

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  		Table 5. Changes in dehydrogenase activity of an alluvial soil planted to rice under different amendments at three growth stages (wet season, 2002). In a column, means followed by a common letter are not significantly different at P < 0.05 by Duncan's multiple range test.

 
Soil methanotrophic activity was also related to total organic carbon (TOC) and total N contents. The TOC content, although negative, had no statistically significant effect on CH4 oxidation (r = –0.222, n = 24), but total N content of soil showed a significant negative relationship with t1/2 (r = –0.472**, n = 24) indicating the positive association between total N and CH4 oxidation. Like total N content, NRN content (Table 6 ) exhibited a statistically significant negative correlation with t1/2 (r = –0.410*, n = 24) implying high concentration of NRN was associated with higher CH4 oxidation.


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  		Table 6. Changes in ninhydrin reactive nitrogen (NRN) content of an alluvial soil planted to rice under different amendments at three growth stages (wet season, 2002). In a column, means followed by a common letter are not significantly different at P < 0.05 by Duncan's multiple range test.

 
Total heterotrophic bacterial population was high in inorganic and compost + inorganic fertilizer treatments (Table 7 ). Methanotrophic bacterial population, however, was low in compost + inorganic fertilizer amended plots. Methanotrophic bacterial population was higher in soils from both inorganic fertilizer as well as compost-treated plots. Total culturable nitrifier populations were low in all the treatments excepting compost + inorganic fertilizer amended soils.


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  		Table 7. Population dynamics of select microbial groups at panicle initiation stage of flooded rice planted to an alluvial soil amended with organic or inorganic fertilizers on a long-term basis.{dagger}

 

   Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
Conclusions
 REFERENCES
 
Methane consumption in both upland and wetland soils is affected by nitrogenous inputs, although this generally holds only for NH4+-based fertilizers (Bronson and Mosier, 1994). In the present investigation, high affinity methane oxidation was lower and low affinity CH4 oxidation was higher in inorganic fertilizer–amended soil. Flooded rice soil being a net producer of CH4, low affinity oxidation (King, 1994) is predominant. The above result indicates that at high CH4 concentration, inorganic N was positively associated with CH4 oxidation; at low CH4 concentration CH4 oxidation is inhibited by inorganic N. Visscher and Van Cleemput (2003) divided the process of CH4 oxidation into two phases: first a phase of soil N-dependent methanotrophic activity and then a phase of soil N-independent methanotrophic activity. This is in agreement with known features of Type I and Type II methanotrophic bacteria. Type I bacteria are N-starved methanotrophs and Type II bacteria are able to fix N2. Hence, the stimulatory effect of inorganic N can be attributed to dominant but N-starved Type I methanotrophs and the inhibitory effect on Type II N2–fixing methanotrophs (Bodelier and Laanbroek, 2004). Positive effect of additional N on methanotrophic bacteria in microcosm and rice fields have been reported (Kruger and Frenzel., 2003). Visscher and Van Cleemput (2003) showed a decline in the methanotrophic activity when the inorganic N content reached a steady state, indicating that the activity decline was a response to N shortage. In the present study lower CH4 oxidation in control plots could be due to N limitation, which was represented by lower plant biomass and yield parameters (data not presented). This was further confirmed when CH4 oxidation rates at different plant growth stages were compared. Methane oxidation was lowest at grain filling stage whereas oxidation rates were higher at the other two growth stages. Nitrogen fertilizer was applied at maximum tillering stage, alleviating N limitation, if any. Similarly, release of soil-bound N and its transformation was highest at maturity (Adhya et al., 1996). Nitrogen uptake is highest and the rice rhizosphere can be under N stress at grain-filling stage (Yoshida, 1983). Possibly, depriving methane-oxidizing bacteria of a suitable source of N might have hampered their physiology resulting into reduced activity. Alternately, in the presence of low soil NO3–N content, soil EC affects microbially driven production of greenhouse gases but its direct influence varies according to water content and soluble salt concentrations (Adviento-Borbe et al., 2006). In the present study, although the possibility of high EC cannot be ruled out, statistically significant changes in soil EC was not observed

Since ammonia-oxidizing bacteria also have the potential to consume CH4 (Schimel and Gulledge, 1998), the stimulatory effect of N additions through mineral fertilizers could also be related to enhanced populations of nitrifiers and subsequent enhanced CH4 oxidation by these organisms. However, the phenomena as observed in the present study are possibly not related to ammonia oxidizers as the number of culturable ammonium-oxidizers was not significantly higher in mineral fertilizer–amended soils. Although culturable microbial population represents a minuscule fraction of the total microbial diversity in relation to wider coverage of recent molecular techniques (Hastings et al., 1997), these culturable microbes may represent the most active and hence the most important part of the microbial communities. In a microcosm-cum-field study, population of ammonium-oxidizing bacteria did not increase significantly on ammonium-based fertilization in soils planted to rice (Bodelier et al., 2000a).

In the present study, application of compost stimulated both low affinity and high affinity CH4–oxidation over unamended control. In a long-term fertilization experiment, continuous FYM application to a sandy soil under upland conditions resulted in higher CH4 oxidation rate (Hutsch, 1996). Compost application will lead to higher C, N, and micronutrient availability. The improved availability of C may be important for methanotrophs, which have been demonstrated to profit from additional carbon sources besides CH4 (Jensen et al., 1998). Application of compost in conjunction with mineral fertilizer inhibited CH4 oxidation and this may be related to a higher N-immobilization capacity of the soil depriving CH4–oxidizers from NH4+ availability (Gulledge et al., 1997). In the present study such inhibitory effect of compost + mineral fertilizer was much less during the dry season than the wet season. The lack of inhibitory effect during subsequent dry season may be due to mineralization of major portions of organic C from the compost that was applied earlier before the wet season.

Methanotrophs are more tolerant to pH variation than methanogens and are less sensitive to the acidic environment with the optimum pH for methanotrophy being reported to vary between 5.0 and 6.5 (Dunfield et al., 1993). In the present study, higher pH was inhibitory to CH4 oxidation. Hutsch (1996) reported higher CH4 oxidation in more acid subplots of a long-term fertilization experiment. If methanotrophs operate at the extreme of their pH range, their niches in soil probably get restricted, thereby affecting the overall activity. Lower CH4 oxidation rates in unamended control and compost + inorganic fertilizer amended plots could be as a result of such pH ranges.


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Conclusions
REFERENCES
 
Rice paddies are among the most prominent CH4 sources on earth. Since CH4 oxidation mediated by methanotrophic bacteria can substantially reduce the potential of emitted CH4, factors that limit or even inhibit the activities of methanotrophic bacteria can impact the global CH4 budget in a significant way. Agricultural practices including application of fertilizer, both organic and inorganic, have profound impact on the soil's ability to oxidize CH4. Contribution of rice paddies to global CH4 budget is anticipated to increase in future as the consequence of increased use of fertilizer for crop yield enhancement. Observations of stimulation of CH4 consumption have been explained due to the removal of N limiting conditions for the methanotrophic bacteria as well as improved availability of C in organic matter–amended soils. Our studies indicate that use of fertilizer—inorganic or organic—even on a long-term basis may not inhibit CH4 oxidation.


   ACKNOWLEDGMENTS
 
The work was supported, in part, by the National Agricultural Technology Project entitled, "Greenhouse Gas Emission from Rice-Based Cropping Systems" (Grant 26(4)/97-NATP) by the Indian Council of Agricultural Research, New Delhi and forms a part of the Ph.D. thesis submitted by the senior author to the Utkal University, India. We thank the director, Central Rice Research Institute, Cuttack for permission to publish this article. D.R. Nayak was supported by a fellowship from the Council of Scientific and Industrial Research, New Delhi.


   NOTES
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ABSTRACT
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 Discussion
 Conclusions
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 ABSTRACT
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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Soil Science Society of America Journal Journal of Plant Registrations The Plant Genome