JEQ Journal of Natural Resources and Life Sciences Education
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Published online 17 July 2007
Published in J Environ Qual 36:1241-1248 (2007)
DOI: 10.2134/jeq2006.0237
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Li, Z.
Right arrow Articles by Kelliher, F. M.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Li, Z.
Right arrow Articles by Kelliher, F. M.
Agricola
Right arrow Articles by Li, Z.
Right arrow Articles by Kelliher, F. M.
Related Collections
Right arrow Best Management Practices
Right arrow Biogeochemical Processes
Right arrow Global Change

TECHNICAL REPORTS

Methane Oxidation in Freely and Poorly Drained Grassland Soils and Effects of Cattle Urine Application

Zheng Li* and Francis M. Kelliher

Manaaki Whenua-Landcare Research, P.O. Box 40, Lincoln 7640, New Zealand

* Corresponding author (lizh{at}landcareresearch.co.nz)

Received for publication June 20, 2006.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Conclusions
 REFERENCES
 
A sink for atmospheric methane (CH4) is microbial oxidation in soils. We report CH4 oxidation rates in freely and poorly drained soils on an intensively managed dairy farm. Following cattle urine application to half the plots (650 kg of nitrogen [N] ha–1) 31 chamber measurements were made over 100 d during autumn and winter. In the control plots, the freely and poorly drained soils' integrated CH4 oxidation rates averaged 1.8 ± 0.2 and 0.6 ± 0.1 kg CH4 ha–1 yr–1, respectively. In the poorly drained soil, the highest CH4 oxidation rates occurred when water-filled pore space (WFPS) < 56% and CH4 oxidation rate declined by ninefold to near zero as WFPS increased from 56 to 68%. Urine application induced the freely and poorly drained soils' CH4 oxidation rates to decline for up to 2 mo by 0.7 ± 0.2 and 0.4 ± 0.1 kg CH4 ha–1 yr–1, respectively. The two soils' responses were thus not significantly different. After urine application, soil pore space CH4 concentration profiles suggested a simultaneous inhibition of bacteria that were CH4 oxidizers and stimulation of CH4 producers.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results
 Discussion
 Conclusions
 REFERENCES
 
THE global annual soil sink for CH4 was estimated to be 27 Tg, corresponding to 3 to 9% of the atmosphere's removal rate (Smith et al., 2000). It seems likely that this sink would be spatially variant, but data are relatively sparse. Moreover, CH4–oxidizing bacteria in soil can be sensitive to indicators of disturbance, including N application. For example, a single urea fertilizer application of 450 kg N ha–1 to a native, American grassland soil reduced the CH4 oxidation rate by 41% with the effect lasting 1 yr (Mosier et al., 1991). However, for a more fertile soil nearby, a similar N application corresponded with no change of the CH4 oxidation rate. Mosier et al. (1991) suggested that CH4 oxidation rate may be inversely related to N turnover rate, whether induced by fertilizer application or not. Subsequent studies have expanded the range of soils and conditions but results remain mixed. Disturbing a pristine, New Zealand forest soil with a very tight N cycle by N salt application decreased the CH4 oxidation rate, but this depended on the N species applied and the concentration (Price et al., 2004). By contrast, N application caused no change to soil CH4 oxidation rates in a number of field studies (Jarvis et al., 1995; Yamulki et al., 1999; Kammann et al., 2001) yet decreased or increased rates in other studies (Flessa et al., 1996; Kruger and Frenzel, 2003; Reay and Nedwell, 2004).

The largest CH4 oxidation rates have been measured in forest soils (Smith et al., 2000). In New Zealand, the pristine forest soil's CH4 oxidation rate was ten times that of soils under arable and extensive pastoral management (Judd et al., 1999; van der Weerden et al., 1999; Sherlock et al., 2002; Price et al., 2003). The arable and pastoral soils were disturbed by cultivation and N application. However, the degree of soil wetness can also be a rate-limiting variable via diffusion of atmospheric oxygen (O2) and CH4 substrate to the bacteria (Price et al., 2004). On dairy farms, application of N includes excreta deposited by cattle. We estimate N application rates onto dairy farm soils from cattle urine and dung currently average around 370 and 90 kg N ha–1 yr–1, respectively. Under typical intensive management, it is estimated that cattle excreta covered at least 25% of the grazed area by the end of a milking season. Spatial distribution of N application in the urine patch may be complex, depending on the soil's wetness and drainage (Li and Kelliher, 2005).

For the study reported here, our aim was to determine soil CH4 oxidation rate on an intensively managed farm and quantify the effects of N application and wetness. At the end of a milking season, plots were isolated in a dairy farm paddock that contained freely and poorly drained soils. Cattle urine was applied to half the plots and CH4 oxidation rates were measured regularly using chambers for 100 autumn and winter days thereafter. Following Li and Kelliher (2005), CH4 oxidation rates were also estimated using gas samples taken from probes installed horizontally in the soil and concurrent measurements of soil temperature and water content.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
 Conclusions
 REFERENCES
 
Soils and Site
The two soils used (Te Kowhai poorly drained and Horotiu freely drained) were previously described (Li and Kelliher, 2005). Briefly, the site was a dairy farm in the Waikato region of New Zealand's North Island (37.8° S; 175.3° E). The land was flat to gently undulating, and the two soils were 300 m apart with contrasting physical and chemical properties (Table 1). Land management was intensive for the past 30 yr with year round grazing at 4- to 8-wk intervals and average stocking rates of 2 to 3 cows per hectare. The vegetation was dominately ryegrass (Lolium perenne L.) with white clover (Trifolium repens L.). There were drainage ditches along two lower sides of the area where measurements were conducted. Dairy cattle were excluded for 6 mo before commencement of the measurements reported here. Pasture herbage was cut to a height of 1 to 2 cm at regular intervals during the measurements, and the clippings removed.


View this table:
[in this window]
[in a new window]

  		Table 1. Brief soil descriptions and characteristics (0–150-mm depth).

 
Experimental Design
Six plots (1.0 x 0.5 m) were established for each soil during December 2002 with three designated as Urine (application) and three as Controls. This was 4 mo before commencement of the trial. A set of subsurface probes (0.4 m long by 12 mm i.d.) was installed in each plot at 75-, 150-, and 300-mm depths. For installation, slits were cut on one side of a centrally-located trench. Care was taken to minimize soil disturbance with soil replaced in the order of removal. The probe was a perforated stainless steel tube, with the ends sealed by rubber septa. These probes were covered with nylon stocking to prevent soil from blocking the perforations. A connecting stainless steel tube (2.5 mm i.d.), with a plastic three-way stopcock, was attached to one end of the probe for gas sampling.

On 2 Apr. 2003, dairy cattle urine was collected from a local herd. A sample was analyzed immediately for total N content by Kjeldahl digestion, automated steam distillation, and back titration using a Kjeltec Auto Sampler System 1035 Analyzer (Tecator, Sweden). Five liters of urine was applied evenly across each designated 0.5 m2 Urine plot using a watering can (650 kg N ha–1). The application's volume and N loading rate were in the range of a cow's urination according to Haynes and Williams (1993).

Gas Sampling, Analyses, and Calculations
Net CH4 oxidation rate at the soil surface was measured using static chambers (250 mm diam., 130 mm height, stainless steel enclosure insulated by polystyrene foam and covered with aluminum foil). A positive rate indicates the net uptake of atmospheric CH4 into the soil, while a negative rate indicates the net emission of CH4 from soil into the atmosphere. The chamber measurements were taken approximately daily, between 1200 and 1300 h, for the first 2 wk after urine application. Thereafter, the sampling frequency was reduced to 2 to 3 times per week. Details of the gas sampling protocol were provided by de Klein et al. (2003). The chambers were installed at the center of Urine and Control plots. Twenty-five mL of gas was taken, using 50-mL polypropylene gas tight syringes, from each chamber at zero (T0) and forty (T40) minutes after sealing the chamber, and injected into a 12-mL pre-evacuated, septum-sealed glass tube (Exetainer, Labco Ltd, UK). This overpressurized the sample in the Exetainer for gas chromatograph GC analysis.

At the same time as samples were taken from the chambers, gas samples were taken from the subsurface probes at each depth. Twenty-five mL gas samples were extracted, via the stopcock valve, from each probe and injected into 12-mL Exetainers. On each sampling day, three atmosphere samples were taken just above the each soil surface.

Methane concentration in the gas samples was determined using a Shimadzu GC-17A gas chromatograph with a flame ionization detector (FID) and oxygen-free dry dinitrogen (N2) as the carrier gas. Gas samples were always analyzed within 2 to 3 d of collection with respect to a series of CH4 concentrations prepared from a commercial CH4 standard (5.30 µL CH4 L–1, Beta Standard, BOC Gases NZ Ltd). The coefficients of variation associated with the gas chromatograph analysis at background atmosphere CH4 concentrations were ≤1%.

For the chambers, CH4 oxidation rate (CH4_oxidationchamber) was calculated using the linear equation:

Formula 1[1]
where {delta}CH4 is the change in headspace CH4 concentration during the enclosure period (µL CH4 L–1) with an increase denoted negative for methane emission to the atmosphere, {delta}t the enclosure period (h), and M the molar weight of CH4 (g mol–1). Vm is the molar volume of gas at the sampling temperature (L mol–1), V the headspace volume (m3), and A the area covered (m2). For the subsurface probes, CH4 oxidation rate (CH4_oxidationgradient) was calculated using Fick's first Law (Campbell, 1985):

Formula 2[2]
where DCH4,soil (Tsoil) is the soil gas diffusivity coefficient for CH4 (m3 soil air m–1 soil h–1) at soil temperature Tsoil (°K), and {Delta}[CH4]/{Delta}Z the vertical CH4 concentration gradient between the two depths including the surface (mg CH4 m–3 soil air m–1 soil depth). The required soil gas diffusion coefficient for CH4 was estimated by:

Formula 3[3]
where DCH4,soil (Tsoil) is the diffusion coefficient for CH4 in air at soil temperature Tsoil (°K), assuming atmospheric pressure at sea level, {varepsilon} the soil's air-filled porosity (m3 soil air m–3 soil pore space; {varepsilon} = 1– WFPS where WFPS is the fractional water-filled pore space (m3 soil water m–3 soil pore space)), and a and b the dimensionless coefficients that account for the soil's pore tortuosity and size distribution, respectively. The temperature correction of DCH4,air (Tsoil) followed Price et al. (2003) as:

Formula 4[4]
where DCH4,air (Tsoil) was 0.07056 m2 h–1 (Ishizuka et al., 2000).

Soil and Climate Measurements
Soil samples (surface to 75-mm depth, 25-mm diam.) were collected with the gas samples from each Urine and Control plot, and analyzed for nitrate (NO3–), ammonium (NH4+), and water content. The extraction holes were promptly back-filled with similar soil to minimize effects on aeration, and these spots were avoided thereafter. Extraction of NO3–and NH4+ included adding 100 mL of 2 M KCl to 10 g of field-moist soil and then shaking the sample for 1 h. The filtered solution was frozen until analysis using modified hydrazine reduction and a salicylate/dichloroisocyanurate method (Blakemore et al., 1987). Soil water content was determined gravimetrically with subsamples dried at 105°C for 24 h. Gravimetric water contents were converted to a volumetric basis by multiplying by the bulk density, determined separately for each of the two soils (Table 1).

Automated meteorological stations were installed next to the plots of each soil to monitor rainfall, air humidity and temperature (1 m above the soil surface), soil temperature (thermistor, 50-mm depth), and water content (ThetaProbe, mL2x, Delta-T Devices Ltd, UK, at depths of 50, 100, and 300 mm, and 3-m-long Aquaflex belt, Streat Instruments, NZ, over a depth from the surface to 50 mm). Soil water content was expressed as the WFPS, calculated by dividing the measured volumetric water content by the soil's porosity (Linn and Doran, 1984). The porosity was calculated as [1 – (bulk density/particle density)] where the particle density was determined separately for each of the two soils (Table 1). Rainfall was integrated, while other sensors were sampled each minute, with half-hourly averages recorded by dataloggers (CR10X, Campbell Scientific, USA).

Statistics
For each sampling of a set of three plots, an arithmetic mean of the CH4 oxidation rate was calculated and uncertainty expressed as ± 1 SD (1 standard deviation). The CH4 oxidation rate in each plot was integrated over time by the trapezoidal rule. Analysis of variance (ANOVA) was used to determine the significance of differences between the integrated values (GenStat version 6.2, VSN International Ltd).


   Results
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
Discussion
 Conclusions
 REFERENCES
 
Rainfall, Soil Water Content, Soil Temperature, and Soil Mineral Nitrogen
Rainfall during April, May, and June 2003 was 46, 103, and 148 mm, respectively, with a total of 327 mm over the 100 d from 1 April to 10 July (Fig. 1). The maximum daily rainfall was 33 mm, on the fourth day after urine application. Soil WFPS, measured by the ThetaProbe sensor at depth of 100 mm, averaged 61% (ranging 44–68%) and 54% (ranging 45–62%) for the poorly and freely drained soils, respectively (Fig. 1). Changes in WFPS corresponded with rainfall and drainage rates. Soil temperature decreased over the 100 d as the season changed from autumn to winter (Fig. 1). The differences of soil temperature between the two soils were not significant, and the average temperature in both soils was 13°C.


Figure 1
View larger version (42K):
[in this window]
[in a new window]

  		Fig. 1. Methane oxidation rates measured by static chambers, soil temperature, soil water-filled pore space (WFPS), daily rainfall, soil ammonium (NH4+), and nitrate (NO3–) contents. Error bars are ± 1 SD.

 
For the Controls, the NH4+–N content averaged 3.6 and 5.2 mg N kg–1 soil, and the NO3––N content averaged 6.6 and 9.5 mg N kg–1 soil over the 100 d experiment period for the poorly and freely drained soils, respectively. After urine application, the mineral N content of both soils increased significantly (Fig. 1). Immediately after urine application, the NH4+–N content peaked at 630 and 850 mg N kg–1 soil in the poorly and freely drained soils, respectively, but it returned to the Control levels within 25 and 37 d, respectively. About 30 d after urine application, the NO3––N content peaked at approximately 300 and 500 mg N kg–1 soil in the poorly and freely drained soils, respectively. For up to 2 mo after urine application, the NO3––N content in both soils was significantly greater than that of the Controls.

Methane Oxidation Rates Determined by the Chambers
Over the 100 d, the CH4 oxidation rate was measured 31 times (on 31 d). The soils were generally a modest, net CH4 sink. For the Controls, CH4 oxidation rate was –0.1 to 2.1 (averaging 0.6) and –1.2 to 2.3 (averaging 1.8) kg CH4 ha–1 yr–1 in the poorly and freely drained soils, respectively (Fig. 1). Urine application significantly reduced the corresponding rates so they became –1.1 to 1.0 (averaging 0.2) and –1.2 to 1.6 (averaging 1.1) kg CH4 ha–1 yr–1. Following urine application, the poorly drained soil was a net source of CH4 on 2 April and 2, 12, 21, 26, and 78 d later. On the day of urine application (2 April) and 7 and 9 d later, the freely drained soil was also a net source of CH4. There were no significant differences between the Urine and Control plots during most of the study's final 40 d (Fig. 1). Overall, for both soils, urine application significantly reduced CH4 oxidation (p < 0.05, repeated-measures analysis of variance). Over the first 30 d after urine application for the poorly drained soil, the integrated CH4 oxidation rates were 0.3 ± 0.1 and 0.0 ± 0.1 kg CH4 ha–1 yr–1 for the Controls and Urine plots, respectively. For the freely drained soil, the corresponding rates were 0.5 ± 0.1 and 0.2 ± 0.1 kg CH4 ha–1 yr–1. Over 100 d for the poorly drained soils, the integrated CH4 oxidation rates were 0.6 ± 0.1 and 0.2 ± 0.1 kg CH4 ha–1 yr–1 for the Controls and Urine plots, respectively. For the freely drained soil, the corresponding rates were 1.8 ± 0.2 and 1.1 ± 0.2 kg CH4 ha–1 yr–1.

Methane Concentrations in the Atmosphere and Soil Pore Space
Methane concentration in the atmosphere, just above the soil surface, ranged from 1.36 (6 d after urine application) to 2.12 µL CH4 L–1 (on 2 April), averaging 1.77 µL CH4 L–1. Our average agreed with the global average atmospheric concentration of 1.75 µL CH4 L–1 reported by Dlugokencky et al. (2003). Over 100 d after urine application, CH4 concentration in the poorly drained soil's pore space ranged from 0.53 (300-mm depth, Day 76) to 2.75 µL CH4 L–1 (75-mm depth, Day 13), averaging 1.58, 1.35, and 1.09 µL CH4 L–1 at 75-, 150-, and 300-mm depths, respectively (Fig. 2). Corresponding averages in the Control plots were 1.44, 1.06, and 0.82 µL CH4 L–1. In the freely drained soil, the range was 0.08 (300-mm depth, Day 0, Urine plot) to 1.76 µL CH4 L–1 (75-mm depth, Day 9, Urine plot), averaging 1.11, 0.79, and 0.47 and 1.19, 0.90, and 0.59 µL CH4 L–1 at 75-, 150-, and 300-mm depths in the Urine and Control plots, respectively. Pore space CH4 concentration decreased with increasing depth, except for 22 d after urine application in the poorly drained soil. A nonlinear relation, postulated by steady-state gas diffusion theory for soils (Cook and Knight, 2003a, 2003b), was obtained as illustrated by data from the Control plots (Fig. 3). The vertical gradient of pore space CH4 concentration in these plots averaged 4 µL CH4 L–1 m–1.


Figure 2
View larger version (37K):
[in this window]
[in a new window]

  		Fig. 2. Methane concentrations in the soil's pore space. Error bars are ± 1 SD.

 

Figure 3
View larger version (20K):
[in this window]
[in a new window]

  		Fig. 3. Soil pore space methane concentrations in the Control plots on three occasions (days) after urine application. Error bars are ± 1 SD.

 
Methane Oxidation Rates Determined by the Gas Gradient Method
For the two soils, Eq. [3] included the same coefficients (a = 0.9 and b = 2.3 based on Sallam et al., 1984) to account for pore tortuosity and size distribution. Equation [3] also depended on temperature, and this was similar for the two soils. However, on average, {varepsilon} was 20% larger for the freely drained soil as noted earlier. In Eq. [3], this difference was amplified by the power coefficient (= 2.3), so the pore space CH4 diffusion coefficient was, on average, 37% larger for the freely drained soil. When the soils were wettest and driest, the corresponding percentages were 443 and 8%. Additionally, the CH4 oxidation rate determined by Eq. [2] required the vertical, pore space CH4 concentration gradient. After urine application, pore space CH4 concentrations changed differently throughout the two soils (Fig. 2). The gradient, {Delta}[CH4]/{Delta}Z, from measurements at depths of 75 and 150 mm, yielded oxidation rates from Eq. [2] that were most consistently closest to the chamber measurements (data not shown, but see Fig. 4).


Figure 4
View larger version (34K):
[in this window]
[in a new window]

  		Fig. 4. Methane oxidation rates by gas gradient (CH4 concentrations between 75- and 150-mm depths) and chamber methods. Gradient: DCH4, soil (T) is estimated by Eq. [3] with a = 0.9 and b = 2.3; Error bars are ± 1 SD.

 

   Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
Conclusions
 REFERENCES
 
Methane Oxidation Rate in the Control Plots
The poorly and freely drained soils' CH4 oxidation rates were comparable to pastoral agriculture data from short-term studies conducted under cool, well-watered conditions in New Zealand (Judd et al., 1999) and the United Kingdom (Jarvis et al., 1995; Yamulki et al., 1999; Smith et al., 2000). In the poorly drained soil, as WFPS increased from 56 to 68%, CH4 oxidation rate declined from 1 to nearly 0 kg CH4 ha–1 yr–1 and this linear relation accounted for most of the variance (Fig. 5). For WFPS < 56%, no relation was apparent and the average CH4 oxidation rate was around 1.2 kg CH4 ha–1 yr–1. By contrast, the freely drained soil's CH4 oxidation rate was less variable over the experiment period (Fig. 1). Although this soil's maximum WFPS was 62%, such wet conditions were rare and the average WFPS was 54%.


Figure 5
View larger version (28K):
[in this window]
[in a new window]

  		Fig. 5. Relationship of methane oxidation rate and soil water-filled pore space (WFPS, measured at 100-mm depth) in the Control of the poorly drained soil. Error bars are ± 1 SD.

 
Methane Oxidation Rate Following Urine Application
After urine application, the poorly and freely drained soils' CH4 oxidation rates decreased by 67 and 39% on average, respectively. We can express the treatment response as an average difference with respect to the Control plots and uncertainty as a root mean square sum of the two standard deviations. Urine application thus corresponded with the poorly and freely drained soils' CH4 oxidation rate declining by 0.3 ± 0.1 and 0.3 ± 0.1 kg CH4 ha–1 yr–1 for the first 30 d, and by 0.4 ± 0.1 and 0.7 ± 0.2 kg CH4 ha–1 yr–1 for the 100 d, respectively. Thus the two soils' responses to urine application were not significantly different at the 5% level, and the declines of the treatment effect in the first 30 d accounted for 75 and 43% of the 100 d decreases in CH4 oxidation for the poorly and freely drained soils, respectively.

Soils beneath pasture grazed intensively by dairy cattle are regularly subject to heavy N loading from excreta, principally in the form of urea. Our treatment responses suggest that soils can be significantly influenced by a single urination event. Considering our experimental site was located on a dairy farm, these data may also be interpreted to suggest that such responses are repeatable. Repeated urea application to soil from another New Zealand dairy farm induced a 2.6-fold increase of microbial respiration over 9 d (Kelliher et al., 2005). The application of N fertilizer also inhibited CH4 oxidation rate (Mosier et al., 1991, 1996; Castro et al., 1995; Goulding et al., 1996; Christensen et al., 2001; Price et al., 2004). This prohibition has been attributed to competition of CH4 and ammonia (NH3) for the same active site on the CH4– and NH3–monooxygenase enzyme (Knowles, 1993).

Transient, but significant, CH4 emissions were measured from the soil to the atmosphere shortly after urine application in both soils. Coincident with these emission events, the soil's pore space CH4 concentration exceeded that in the atmosphere near the surface (Fig. 1 and 2). In semi-wet soil, above-ambient CH4 concentrations were reported by Kammann et al. (2001) but they also found these were not accompanied by net CH4 emissions from the soil surface to the atmosphere. In contrast, our results appear to be unique in the literature. If the atmosphere is the sole source of CH4 for the CH4 oxidizers, the pore space CH4 concentration should always be less than or equal to the atmospheric concentration. This was generally the case, but 9 to 21 d after urine application (11–23 April), the poorly drained soil's pore space CH4 concentrations exceeded those in the atmosphere (Fig. 2). This indicated the presence of CH4–producing bacteria and their stimulation by the urine application. Isolates of methanotrophs have been shown not to grow when CH4 substrate was supplied at an atmospheric concentration (Conrad, 1996). However, Conrad thought CH4 consumption and production could occur concomitantly in some upland soils. Our results suggest the range of soils may be extended to include the poorly drained soil if subject to urine application by grazing cattle.

The poorly drained soil had a relatively low CH4 oxidation rate, coinciding with a relatively high soil water content that reduced substrate/gas diffusion rate from the atmosphere to the CH4 oxidizers. After urine application, this soil oscillated between net CH4 consumption and emission. If grazing during the wet winter season deposited a large amount of urine N onto this soil, the poorly drained soil should be a small but consistent net CH4 source. The two soils' responses to urine application were not significantly different on an integrated basis, but the freely drained soil had higher CH4 oxidation rates that were ≤0 only rarely and briefly (Fig. 1).

After cattle urine is applied to soil, the urea in it undergoes hydrolysis catalyzed by the enzyme urease to form (NH4)2CO3. Hydrolysis of urea is rapid in New Zealand pastoral soils during summer and autumn with a so-called half-life of only 3 to 5 h according to Sherlock and Goh (1984). The reaction could cause the soil's pH to quickly increase by up to four units, depending on the soil's buffer capacity (Kelliher et al., 2005). In the laboratory, a forest soil's CH4 oxidation peaked at the natural pH of 4.4, and declined by 50% when the pH was increased to 6.4 (Price et al., 2004). This experiment suggested that rapid change to an ‘unnaturally’ high pH limited the activities of CH4 oxidizers in the soil, despite the likely increased availability of soluble carbon substrate (Kelliher et al., 2005). In our soils, urine application immediately induced significant, though brief, increases of pore space CH4 concentration that may, at least in part, be attributed to the changing pH.

Soil CH4 oxidation has sometimes been inhibited by NH4+ accumulation immediately or some time after fertilizer application (Gulledge and Schimel, 1998). Inhibition by ammonia (NH3) may be more effective in neutral to alkaline soils whereas NH4+ salt inhibition effects may be more prevalent in acidic soils (Gulledge and Schimel, 1998). The effect of NH4+ on soil CH4 oxidation after cattle urine deposition has not been studied to our knowledge. After urine application, increased pH and NH4+ accumulation can lead to 15 to 25% of the urine N becoming NH3 with subsequent release into the atmosphere (Sherlock and Goh, 1984). Our poorly and freely drained soils were weakly acidic (pH 5.9) and neutral (pH 6.3), respectively. For urine then, NH4+ and/or NH3 could inhibit the CH4 oxidizers, so reducing CH4 oxidation rate. Immediately after urine application, NH4+ concentration in the top 75 mm of soil peaked at 630 and 850 mg N kg–1 soil for the poorly and freely drained soils, respectively. However, the anticipated NH4+ and NH3 inhibition effects were apparently masked by the stimulation of CH4–producing activity and increased pore space CH4 concentrations. The net effect was CH4 emission into the atmosphere over the period of very high NH4+ concentration, especially in the poorly drained soil.

Methanotrophic bacteria, or methanotrophs, are thought to dominate CH4 oxidation in soil. However, generally, ammonia-oxidizing bacteria (AOB) are also involved (Hanson and Hanson, 1996). We did not distinguish between methanotrophs and AOB but we also think the contribution by AOB was minor because CH4 oxidation rate was not affected by soil NH4+ or NO3– concentration.

Methane Oxidation Rates Determined by the Gas Gradient Method
The method is predicated on gas sampling in the soil and atmosphere. Equation [2] shows the method relies on the CH4 gradient and the CH4 diffusion coefficient. In our study, pore space CH4 concentrations at depths of 75 and 150 mm yielded {Delta}[CH4]/{Delta}Z and soil CH4 oxidation rates most closely correlated with the chamber measurements. For the poorly drained soil's Controls, extrapolation of {Delta}[CH4]/{Delta}Z to the surface yielded reasonable estimates of the measured CH4 concentrations (Fig. 3). Corresponding CH4 oxidation rates by the gas gradient method were generally greater than the chamber measurements (Fig. 4 and 6). The CH4 diffusion coefficient seemed to be reasonable because the relatively high WFPS was compatible with the change of CH4 oxidation rates (Fig. 4 and 5). Applying the same approach to the freely drained soil, however, underestimated the surface CH4 concentrations (Fig. 3), and overstated the influence of WFPS on CH4 oxidation rates in Eq. [2]. Consequently we overestimated the CH4 oxidation rates when the WFPS < 50% and underestimated them when WFPS > 50% (Fig. 4).


Figure 6
View larger version (29K):
[in this window]
[in a new window]

  		Fig. 6. Comparison of methane oxidation rates by gas gradient (CH4 concentrations between 75- and 150-mm depths) and chamber methods in the Control plots.

 
The use of Eq. [2] excluded net consumption or production of CH4 between the concentration measurements, i.e. there were no CH4 sink or source terms. In both soils, for 1 to 2 mo after urine application, Eq. [2] tended to overestimate CH4 oxidation rate by comparison to the chamber measurements (Fig. 4). Two days after urine application, the poorly drained soil's CH4 oxidation rate by Eq. [2] departed most from the chamber measurements. As discussed earlier, this soil's pore space CH4 concentration, at a depth of 75 mm, exceeded that in the atmosphere on four occasions, 9 to 21 d after urine application. While this suggested CH4–producing bacteria were stimulated by the urine application, this activity took time to become apparent in the poorly drained soil. For 2 mo after urine application, the freely drained soil's CH4 oxidation rate by Eq. [2] was generally larger than that of the Controls and it departed significantly from the chamber measurements (Fig. 4). As discussed earlier, this puzzling result reflected the contradiction of relatively constant freely drained soil's CH4 oxidation rates measured by chambers and the strong influence of soil moisture changes using Eq. [3] including the power coefficient b = 2.3.


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Conclusions
REFERENCES
 
Cattle urine application reduced the CH4 oxidation rates of freely and poorly drained soils for up to 2 mo and by 0.7 ± 0.2 and 0.4 ± 0.1 kg CH4 ha–1 yr–1. Overall, the two soils' responses were not significantly different. In Control plots, as the poorly drained soil's water-filled pore space increased from 56 to 68%, CH4 oxidation rate declined by ninefold to near zero and linear regression accounted for most of the variance. This reflected poor drainage and the limited CH4 substrate supply rate from the atmosphere into the soil. When water-filled pore space was <56% in the poorly drained soil, the highest CH4 oxidation rates occurred. In the Control plots of the freely drained soil, average water-filled pore space was significantly lower at 46%, it did not vary widely, and the CH4 oxidation rate averaged 1.8 kg CH4 ha–1 yr–1 or three times that in the poorly drained soil. After urine application, soil pore space CH4 concentration profiles suggested a simultaneous inhibition of CH4–oxidizing bacteria and stimulation of CH4 producers. It was challenging to interpret the CH4 concentration profiles and deduce changes in the CH4 oxidation rate.


   ACKNOWLEDGMENTS
 
This work was supported by funding from the Foundation for Research, Science, and Technology New Zealand (Contract No. C09X0212). The authors thank Alex McGill, Maja Vojvodic-Vukovic, Carolyn Hedley, and Suzanne Lambie for technical support; Tim Clough of Lincoln University for valuable discussion, and two anonymous reviewers for constructive criticisms.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Conclusions
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Conclusions
 REFERENCES
 





This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Li, Z.
Right arrow Articles by Kelliher, F. M.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Li, Z.
Right arrow Articles by Kelliher, F. M.
Agricola
Right arrow Articles by Li, Z.
Right arrow Articles by Kelliher, F. M.
Related Collections
Right arrow Best Management Practices
Right arrow Biogeochemical Processes
Right arrow Global Change

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
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