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Methane Emission from a Tropical Wetland in Ka'au Crater,O'ahu, Hawai'iAuthor(s): Maxime Grand and Eric GaidosSource: Pacific Science, 64(1):57-72. 2010.Published By: University of Hawai'i PressDOI: http://dx.doi.org/10.2984/64.1.057URL: http://www.bioone.org/doi/full/10.2984/64.1.057

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57

Methane Emission from a Tropical Wetland in Ka‘au Crater,O‘ahu, Hawai‘i1

Maxime Grand2,4 and Eric Gaidos3

Abstract: Natural tropical wetlands constitute an important but still poorlystudied source of atmospheric methane, a powerful greenhouse gas. We mea-sured net methane emission, soil profiles of methane generation and oxidation,and related environmental parameters in a tropical wetland occupying the Ka‘auextinct volcanic crater on the Hawaiian island of O‘ahu. The wetland has a fluc-tuating water table with dynamics that can be reproduced using precipitationdata and a simple model. Median net methane flux was 117 mg m�2 day�1 andis consistent with measurements at other tropical sites. Net methane flux in theCommelina diffusa–dominated vegetation pattern (honohono) was significantlyhigher than that of the invasive Psidium cattleianum–dominated pattern (straw-berry guava). Net methane emission in the honohono vegetation pattern wasalso significantly higher during the ‘‘wet’’ season compared with the ‘‘dry’’ sea-son, although we did not find a clear correlation between net methane emission,water table level, or precipitation. We show that the measured fluxes are consis-tent with the integrated potential methane generation over the uppermost 30cm of soil and consumption of @50% of that methane in the soil. Absence ofa correlation between net methane emission and water table level may be due tosuppression of the activity of strictly anaerobic methanogens by dynamic redoxconditions in the upper layers of soil and varying rates of methane oxidation byfacultive methanotrophs.

Atmospheric methane (CH4) is a power-ful greenhouse gas responsible for 20% ofthe anthropogenic change in direct radiativeforcing (Intergovernmental Panel on ClimateChange 2001). Its concentration has morethan doubled since the preindustrial era,although its annual increase of @1% hasslowed in the last two decades (Dlugokencky

et al. 1998). Wetlands constitute an importantnatural source of atmospheric CH4, and bo-real wetlands have been intensely studied inthis context (Chapin et al. 2000, Wuebblesand Hayhoe 2002). Natural tropical wetlands,however, are more poorly studied, despitetheir relatively larger contribution to atmo-spheric methane (Bartlett and Harriss 1993,Milich 1999, Marani and Alvala 2007). Muchof the work at low latitudes has been limitedto the Amazonian floodplain and FloridaEverglades (e.g., Bartlett et al. 1989, Devolet al. 1990) and restricted to area-averaged es-timates of methane flux.

In wetlands, methane is the end product ofthe anaerobic decomposition of organic mat-ter in flooded soils rendered anoxic by thelow diffusivity of oxygen through pore water.Net methane emission is the difference be-tween production by methanogenic archaeain water-logged anoxic zones, and aerobicmethane oxidation by methanotrophic bacte-ria in any aerated zones of soil and in vascularplant tissue. Numerous studies have shown

Pacific Science (2010), vol. 64, no. 1:57–72doi: 10.2984/64.1.057: 2010 by University of Hawai‘i PressAll rights reserved

1 This research was partly supported by an awardfrom the University of Hawai‘i Research Training Re-volving Fund to E.G. and a Global EnvironmentalSciences Fellowship for Senior Thesis Research to M.G.Manuscript accepted 10 February 2009.

2 Department of Oceanography, University of Ha-wai‘i at Manoa, Honolulu, Hawai‘i 96822.

3 Department of Geology and Geophysics, Universityof Hawai‘i at Manoa, Honolulu, Hawai‘i 96822.

4 Corresponding author: (e-mail: [email protected]).

that CH4 flux depends sensitively on whetherthe water table is above or below the soil sur-face (Funk et al. 1994, Roslev and King 1996,Rask et al. 2002) rather than the actual depthof standing water (Harriss et al. 1988, Bart-lett et al. 1989, Bartlett et al. 1990, Devolet al. 1990, Moore et al. 1994, Kettunen et al.2000, Ding et al. 2004). Thus the extent,rather than degree, of flooding may controltotal methane emission. Climate models pre-dict that future global warming will alterpatterns of precipitation, particularly in thecontinental tropics (Wetherald and Manabe2002, Watterson and Dix 2003), affecting thesurface extent and thus total methane emis-sion of wetlands there (Moore 2002).

Vascular plants provide a conduit both forCH4 to the atmosphere and for O2 to the soil,the latter often supporting methanotrophicbacteria in the plant rhizosphere. As such,differences in vegetation may affect net meth-ane emission. For example, C3 plants have ahigher stomatal conductance for the samerate of photosynthesis as C4 plants and thuswould conduct methane to the atmosphere ata higher rate. Conversely, many wetlandplants conduct more oxygen to their root sys-tems; this increased conductance increasesthe potential for in situ methane oxidation(Gerard and Chanton 1993, Le Mer andRoger 2001).

We measured net methane emission, po-tential methane production, and methane ox-idation in a tropical wetland occupying theextinct Ka‘au volcanic crater on the Hawaiianisland of O‘ahu (21� 20 0 00 00 N, 157� 46 0 30 00

W) during a 1-yr study period starting in Au-gust 2003. In this wetland, the water table isalways close to, or slightly above, the surfaceof the soil and is maintained by high annualprecipitation rather than poor drainage, thusgiving the crater a dynamic hydrology. Ourobjectives were to: (1) estimate the methaneflux from this wetland; (2) quantitatively as-sess differences in methane emission withprecipitation and vegetation pattern; and (3)interpret these variations in terms of the dis-tribution of potential methane productionand methane oxidation with depth in the soiland the time-response of these quantities tochanging environmental conditions.

Study Site

Ka‘au Crater is a 450 m diameter explosioncrater (@0.16 km2) that formed during thepostshield-building Honolulu Volcanic Seriesof eruptions on O‘ahu. The crater is locatedimmediately southwest of the crest of theKo‘olau Range (21� 20 0 N), and its floor is atan elevation of 463 m. The underlying unitsof the Ko‘olau shield volcano have a high hy-draulic conductivity (@20 m hr�1) (Oki et al.1999); however the crater itself was initiallyoccupied by a lava lake (Macdonald et al.1983) that created a relatively impermeablefloor. Surface drainage occurs through a sin-gle breach in the crater wall. The estimatedflow rate of the stream approximately equalsthe average input of precipitation into thecrater (@30 liters sec�1 [E.G., unpubl. data]),although subsurface drainage is possible. To-day, peat soil has accumulated in the crater toa depth exceeding 3.6 m (Hotchkiss and Juvik1999). Ka‘au Crater experiences a subtropicalclimate with a weak seasonality and receiveshigh annual precipitation (@415 cm) that ispredominantly orographically controlled.

Kennedy (1975) described five major vege-tation patterns in the crater: The largest isdominated by the native sedge Cladium maris-cus (identified as Cladium leptostachym [‘uki] byKennedy [1975] and Cladium jamaicense by El-liot and Hall [1977]). A second major type is afloristically diverse open scrub that includesdwarfed Metrosideros polymorpha (‘ohi‘a). Athird type consists almost entirely of Cordylineterminalis (ti) with minor incursions of Com-melina diffusa (honohono) and the bulrushShoenoplectus lacustris (hereafter referred to asthe honohono meadow). The final pattern isa dense forest of the invasive Psidium cat-tleianum (strawberry guava) that occupiesthe crater floor in the vicinity of the outletstream and the inner slopes of the craterwall. Although the crater lies within theHonolulu Watershed Reserve it has experi-enced anthropogenic disturbance, includingthe possible construction of a Polynesian fishpond (Elliot and Hall 1977), an attempt toconstruct a reservoir by damming the outletstream (Elliot and Hall 1977), and the intro-duction of feral pigs.

58 PACIFIC SCIENCE . January 2010

materials and methods

Environmental Parameters

Precipitation in Ka‘au Crater was monitoredwith a tipping bucket rain gauge (RG2-M)and data logger (HOBO Micro Station, On-set Computer Corp., Bourne, Massachusetts)placed at 21� 19.68 0 N, 157� 46.30 0 W. Therainfall record of a NOAA cooperative raingauge located 2 km distant and 160 m lower(Palolo Valley, no. 718, 21� 18.73 0 N, 157�

46.83 0 W) was obtained from the NOAA Na-tional Climatic Data Center server (http://www.ncdc.noaa.gov). Water-table level inKa‘au Crater was measured from October toNovember 2003 and February to July 2004using a sensor (miniTROLL, In-Situ, FortCollins, Colorado). The sensor was placed ina pipe that was drilled at 5 cm intervals alongits length and inserted 50 cm into the soilof the honohono vegetation type throughoutthe study period. All water-table level datapresented are daily averages in centimeters.One-meter air temperature was continu-ously recorded at 5-min intervals from Au-gust to November 2003 using a humidity/temperature data logger (SP-2000, VeriteqInstruments, Richmond, British Columbia,Canada).

Net Methane Emission

We made 82 measurements of net methaneemission by the closed flux chamber tech-nique (Schutz and Seiler 1989) using twoidentical chambers during a 1-yr study period(30 August 2003 to 12 August 2004). Eachchamber consisted of a partially translucentpolyethylene cylindrical container 106 cmhigh and 63 cm in diameter, with walls 3mm thick. Large chambers permit the inclu-sion of vascular plants that are an impor-tant conduit for methane from the soil inmany wetlands (Le Mer and Roger 2001).Each chamber was sealed using foam gasketsand clamps to a galvanized aluminum collarthat was previously inserted@10 cm into thesoil, establishing a gas-tight seal. A battery-powered fan maintained circulation withineach chamber. Thirty-milliliter gas sampleswere withdrawn at 30-min intervals through

a butyl rubber stopper in the top of thechambers. These were injected into previ-ously evacuated 10 ml serum bottles, estab-lishing a positive pressure. To verify therepeatability of our chamber-sampling tech-nique, some gas samples were stored by dis-placing water from vials. After 90 to 120 minincubation, each chamber was removed andflushed with air between successive measure-ments. The aluminum collar was left in placeif the next measurement was at the same loca-tion.

Gas samples were taken to the laboratoryand analyzed within 24 hr of collection usinga gas chromatograph (GC) (310C, SRI In-struments, Torrance, California) equippedwith a flame ionization detector and a T 80/100 column (Hayesep, Alltech Assoc., Deer-field, Illinois). The column was maintainedat 35�C, and 30 ml min�1 of grade-6 helium(Gaspro, Honolulu, Hawai‘i) was used as thecarrier gas. The GC was calibrated initiallyand every 10 analysis runs with a 100 ppmCH4 standard (Matheson Tri Gas, Montgom-eryville, Pennsylvania). Atmospheric sam-ples (@1.85 ppm) were also run. Methanefluxes were calculated from the difference be-tween the initial and final concentrations inthe chambers, adjusted for their volumes, sur-face coverage, and incubation time. All fluxesare reported in units of milligrams of meth-ane m�2 day�1. Chamber gas samples col-lected at 30, 60, and 90 min were used todetermine the linearity of methane accumula-tion within each chamber to identify any de-crease in the rate of methane accumulation.The accuracy of individual CH4 measure-ments was determined by withdrawing fourreplicate samples at the final time point ofseveral experiments. Many measurementswere also immediately repeated after flushingthe chamber. To investigate the effect ofwater-table level on net methane emission,one chamber was maintained at a fixed loca-tion in the vicinity of the water-level sensorwithin the honohono meadow vegetationtype. In the meantime, the other chamberwas deployed simultaneously in different lo-cations to evaluate the effect of vegetationon methane flux. During an 8-week period,the chambers were positioned next to each

Methane Emission from a Tropical Wetland . Grand and Gaidos 59

other to determine the repeatability of themeasurements and possible small-scale spatialvariability in methane emission. This allowedassessing the statistical significance of any dif-ference in flux between two different vegeta-tion types.

Methane Generation Potential and MethaneOxidation

Six soil cores 18–32 cm deep were collectednext to the chamber measurement sites withan AMS soil corer fitted with a plastic pipe(length, 46 cm; diameter, 7 cm) and taken tothe laboratory within 3 hr. These were sec-tioned at 4 cm vertical intervals and then sub-divided longitudinally into four subsamples.Methane generation potential was measuredby incubating duplicate subsamples fromeach 4 cm section in 160 ml serum bottleswith 25 ml of sterile deionized water and amagnetic stir bar to turn the soil into slurries.Each bottle was sealed using butyl rubberstoppers and aluminum crimp seals and theirheadspace flushed with nitrogen gas for @5min while the slurry was stirred. Headspacegas samples were extracted immediately afterstirring and again after 24 hr of incubation at22�C. To verify that any headspace methanein these incubations was the product of bio-logical activity (and not the release of previ-ously produced methane), another bottledsoil sample was heated to 121�C for 1 hr us-ing a steam sterilizer (SM510, Yamato Scien-tific America, Orangeburg, New York) andincubated as the others. Headspace volumeswere measured and the soil was dried for 48hr at 60�C and weighed. (We found that nofurther water loss occurred after 24 hr of dry-ing.) Methane generation potential was calcu-lated as the difference between the final andinitial methane headspace concentrations ad-justed for the bottles headspace volumes andnormalized by the dry weight of soil incu-bated. All data are presented in units of milli-grams (g dry soil)�1 day�1.

Methane oxidation was measured by incu-bating the remaining subsamples in duplicate160 ml serum bottles that were sealed withambient headspace to which 25 ml of naturalgas were added to produce an initial meth-

ane concentration between 50 and 200 ppm.Headspace samples were withdrawn and ana-lyzed initially and after 6, 12, and 24 hr of in-cubation at 22�C while the samples werecontinuously agitated at 120 rpm to preventthe onset of anoxia in the soil. Negative con-trols were prepared by autoclaving at 121�Cand incubating them in the same manner. Atthe end of the experiments, the bottle head-space volumes were measured and the slurriesdried and weighed as previously described.Methane oxidation rate constants were cal-culated assuming first-order kinetics anddata were fitted to an exponential function.All fits had a coefficient of determinationR2 > 0:95 and all but two fits had R2 > 0:99.Rate constants were normalized by the dryweight of soil and are reported as hr�1 (g drysoil)�1.

Plant-Associated Methane Oxidation

Our protocol is derived from that of Heilmanand Carlton (2001). Briefly, root and stemsamples (the latter from the soil level andseveral centimeters above) of the species Cla-dium jamaicense and Shoenoplectus lacustris werecollected and immediately transported to thelaboratory. All root and basal stem sampleswere rinsed to remove soil particles and en-sure that any methane oxidation activity oc-curring was from methanotrophs locatedwithin or attached to the plant tissue. Eachsample was then placed in 25 ml serum bot-tles with 2–3 ml water, and 25 ml methanewas added to the headspace to achieve an ini-tial concentration of 50–200 ppm. Changesin the methane concentrations were moni-tored after 0, 1, 2, 3, and 24 hr of incubationas described earlier.

Soil Analyses

Soil pH profiles were obtained in the hono-hono, ‘ohi‘a, and strawberry guava vegetationtypes using a previously described protocol(Van Lierop 1990) where 2.5 g of oven-driedand sieved soil and 5 ml of sterile, deionizedH2O were added to 10 ml Falcon tubes. Eachtube was vortexed for @1 min, and solidswithin the solution were allowed to settle for


1 hr. The pH of the supernatant was mea-sured (Accumet AB 15 Plus meter, Fisher Sci-entific, Pittsburgh, Pennsylvania). pH wasalso measured in several standing bodies ofwater in the wetland. Soil samples were ana-lyzed at the University of Hawai‘i College ofTropical Agriculture and Human ResourcesAgricultural Diagnostic Service Center fornitrogen, organic carbon, and sulfur content(three to nine replicate samples for nitrogenand organic carbon).

Statistical Analyses and Water-Table LevelModel

Differences in methane fluxes between differ-ent vegetation types and between wet and dryseason were tested using the Student t-test as-suming unequal variances when significantheteroskedasticity was detected. All statisticalanalyses were performed using Microsoft Ex-cel 2004. We report median net methaneemissions in addition to average emissions be-cause median values are less subject to out-liers and thus may reveal smaller effects ortrends. Water-table level in Ka‘au Crater wasmodeled using IDL software (ITT Corpora-tion).

results

We measured daily rainfall in Ka‘au Craterduring 178 days in 2003 (9 February–25October). Measurements were terminated bythe failure of the rain gauge’s data logger.The measurements show a linear correlationwith records at the Palolo Valley stationðR2 ¼ 0:95Þ, with the Ka‘au Crater site re-ceiving 25% more rainfall (data not shown).Missing precipitation data in the 1-yr studyinterval was estimated using the Palolo Valleystation and our linear regression results. Theaverage precipitation at the Palolo Valley sta-tion over the past 55 yr is 332 cm, and thedaily average rainfall is 9 mm (Figure 1). Wetherefore estimate that Ka‘au Crater received415 cm of annual precipitation during thesame period. The Palolo Valley historicaldata indicate a weak seasonality of about30% in precipitation, with the wettest seasonextending from 31 October to 20 May (Fig-

ure 1). The historical record also shows arainfall minimum in February of short extent(@20 days [Figure 1]) that we included in the‘‘wet’’ season in the following discussion. Wemeasured temperature on 105 days during the‘‘dry’’ season and 108 days during the ‘‘wet’’season. The average air temperature in thecrater is 21.5�C, the standard deviation indaily-averaged temperature was 1.9�C, andthe wet-dry seasonal difference was 2.7�C.

Water-table level was measured from 29September to 2 November 2003 and from 12February to 13 June 2004 (Figure 1). Thelevel is characterized by fluctuations of up to20 cm on timescales of days. It reached thesoil surface, but surface water was always ofnegligible depth, probably because of effi-cient drainage from the wetland surface. Thewater-table level fluctuations were highly cor-related with precipitation events. We foundthat the water-table level on the nth day hn

can be modeled by the recursive relationship

hn ¼ hn�1 1� 1

t

� �þ Apn�1;

where t is a drainage time constant in days,pn is the precipitation on the nth day, and Ais a factor that accounts for a watershed largerthan the crater rim. Water level is con-strained to be

�Ha ha 0;

where H is the water-table depth at whichdrainage out of the wetland becomes negli-gible. In the absence of precipitation, thewater-table level decays exponentially on atimescale t toward this level. We found thatthe parameter values t ¼ 8:3 days, A ¼ 1:75,and H ¼ 18 cm reproduce two major water-table level excursions in our data, but thatno single set of parameters can reproducethe detail of all excursions (data not shown),possibly because of sinking or tilting of thewater-level sensor over time.

The average soil pH was 4.9 ðn ¼ 19Þ anddid not vary significantly with vegetation pat-tern or depth ðP ¼ 0:21Þ. This is close to theaverage pH (5.5) of open bodies of water inthe wetland. Average organic carbon, nitro-gen, and sulfur contents (by weight) were37% ðn ¼ 16Þ, 2.4% ðn ¼ 16Þ, and 80 ppm


ðn ¼ 3Þ, respectively (Table 1). The differ-ence in the mean nitrogen contents of thesoil in the sedge (2.6%) and strawberry guava(1.9%) vegetation patterns was significantðP ¼ 0:03Þ.

Eighty-two measurements of net methaneemission over the 1-yr study period are tabu-lated in Appendix 1. Dense vegetation limitedthe spatial distribution of chamber measure-ments to the honohono, strawberry guava,

‘ohi‘a and native sedge vegetation patternsnear the crater outlet only. The repeatabilityof our GC measurements was 0.25 ppm atatmospheric (1.85 ppm) CH4 concentra-tions. The standard deviation among replicatechamber samples was 0.65 ppm ðn ¼ 15Þ.The standard deviation among replicate sam-ples stored in water-filled serum vials wasslightly smaller (0.4 ppm), but there was nosignificant difference in fluxes calculated

Figure 1. Daily precipitation in Palolo Valley averaged over the past 55 yr. Data from NOAA National Climatic DataCenter Palolo Valley Station no. 718. Black line shows daily average precipitation (9 mm day�1), which we used todefine the extent of the wet season (thick gray line). Connected open circles are water-table level in Ka‘au Crater mea-sured during two intervals in 2003–2004.

TABLE 1

Soil Organic Carbon, Nitrogen, and Sulfur Content

Vegetation Pattern Organic Carbon [%] Total Nitrogen [%] Sulfur [ppm]

Honohono 35G 3 ðn ¼ 3Þ 2:8G 0:3 ðn ¼ 3Þ 81 ðn ¼ 1ÞStrawberry guava 41G 5 ðn ¼ 5Þ 1:9G 0:3 ðn ¼ 5Þ 53 ðn ¼ 1ÞSedge 34G 6 ðn ¼ 8Þ 2:6G 0:7 ðn ¼ 8Þ 110 ðn ¼ 1Þ


from samples stored in evacuated vials orwater-filled vials. For a typical 90-min exper-iment our minimum detectable (3s) methaneflux was 6.5 mg CH4 m�2 day�1. Our detec-tion threshold is high compared with manyother studies because of our on-column injec-tion system and the large chambers used forthe inclusion of plants. The median percent-age difference between fluxes in 30 pairs ofduplicate experiments in which both cham-bers remained in place was 7.0%. Measure-ments used to determine the accuracy andvariability of our measurements are denotedby a ‘‘V’’ in Appendix 1.

Steady accumulation of methane within achamber should produce a linear trend inCH4 concentration. A reduced chi-squared(w2

u ) was calculated for a linear fit to each ex-periment assuming 0.65 ppm experimentalerror (Appendix 1). In some of our experi-ments the CH4 concentration deviated sig-nificantly from linearity, often reaching anasymptotic value, and it was necessary toeliminate time points or entire experiments(Appendix 1).

The median of all valid flux measurementsis 117 mg CH4 m�2 day�1 ðn ¼ 68Þ, and theaverage is 126G 10 mg CH4 m�2 day�1 (Ta-ble 2). Figure 2 plots the 51 measurements(two nondetections not shown) in the hono-hono vegetation pattern and 1-month aver-aged precipitation in Ka‘au Crater estimatedfrom the Palolo Valley data. We show1-month averaged precipitation rather than ahigher resolution (daily or weekly) to revealseasonal trends. The comparison reveals that,during our study period, the largest methane

emissions in the honohono pattern occurredduring the wet season and that there seemsto be a delay (>30 days) between the highestrainfall events and the highest net methanefluxes recorded. The median values in thehonohono meadow only during the ‘‘dry’’and ‘‘wet’’ seasons were 77 mg CH4 m�2

day�1 ðn ¼ 15Þ and 165 mg CH4 m�2 day�1

ðn ¼ 38Þ, respectively (Figure 2, data indi-cated with an ‘‘S’’ in Appendix 1). The differ-ence in the mean values for the honohonomeadow during the ‘‘dry’’ and ‘‘wet’’ seasons(93 and 174 mg CH4 m�2 day�1, respectively[Table 2]) is significant (t ¼ 7:2, df ¼ 47,P ¼ 2:4� 10�4).

The median values of fluxes measured inthe honohono, ‘ohi‘a, native sedge, and straw-berry guava vegetation patterns are also re-ported in Table 2. The limited sample sizesin the sedge and ‘ohi‘a vegetation patternsprevent us from drawing statistically mean-ingful conclusions, but measurements in thehonohono (mean ¼ 151 mg CH4 m�2 day�1,n ¼ 53) were significantly higher than thosein the strawberry guava (mean ¼ 22 mg CH4

m�2 day�1, n ¼ 8; t ¼ 7:5, df ¼ 24, P ¼ 3:7�10�8).

Profiles of methane-generation potentialare shown in Figure 3 (the autoclaved nega-tive control produced no detectable meth-ane). In all three honohono profiles and oneof the guava profiles there is a peak in themethane-generation potential at a depth of10 cm. However, in the ‘ohi‘a profile themaximum in methane-generation potential isat the surface. We found an exceptionallyhigh rate of methane generation in the core

TABLE 2

Net Methane Emission Statistics (Units are mg CH4 m�2 day�1)

Sample n Median Flux Average Flux Std. Dev Std. Error

All valid 68 117 126 84 10Honohono 53 145 151 72 10‘Ohi‘a 4 43 82 107 53Strawberry guava 8 15 22 28 10Sedge 3 17 17 8 5Honohono (dry season) 15 77 93 56 14Honohono (wet season) 38 165 174 65 11


obtained from the honohono vegetation pat-tern on 28 March 2004.

Profiles of methane oxidation in intactcore samples are shown in Figure 4. Com-pared with methane-generation potential,these profiles do not exhibit order-of-magnitude variations with depth in differentvegetation patterns and sampling times. In allcores but the strawberry guava of 5 May2004, in situ oxidation rate constants show apronounced peak between 14 and 18 cmdepth. We also carried out 86 experiments tomeasure methane oxidation in roots and stemmaterial of the species Cladium jamaicense andShoenoplectus lacustris. In only six experimentswas there a significant decrease in methaneconcentration, all associated with the bulrushShoenoplectus lacustris. However, 57 bulrushsamples did not exhibit this trend, and noclear pattern of plant-associated methane ox-idation emerged from these experiments.

discussion

The precipitation and water-table level rec-ords analyzed here show that the Ka‘auCrater has a dynamic water table, which fluc-tuates with an amplitude of up to 20 cm on atimescale of about 8 days. Using our modelof the water-table level and the rainfall recordat nearby Palolo Valley, we estimate that thewetland was inundated during 45% of the1-yr study period beginning 15 August 2003.Despite the presence of five different vegeta-tion groups, the wetland appears remarkablyuniform in terms of soil chemistry (Table 1).

Chamber methane-accumulation experi-ments are subject to several systematic errors,often manifest by nonlinearity of chamberCH4 concentration versus time. Such behav-ior can be identified using a criterion such asR2 (Khalil et al. 1998) or reduced chi-squared(w2

u ). Linear fits to several of our experiments

Figure 2. Net methane flux in the honohono vegetation pattern (filled circles) and 1-month average precipitation inKa‘au Crater wetland (open triangles) over the 1-yr study period. Ka‘au Crater precipitation data derived from corre-lation with Palolo Valley Station no. 718. Nondetections not shown. Median values during the dry (gray line: 77 mgCH4 m�2 day�1) and wet seasons (dark line: 165 mg CH4 m�2 day�1) are shown.


Figure 3. Profiles of methane-generation potential versus depth in soil cores from the honohono, strawberry guava,and ‘ohi‘a vegetation patterns.

Figure 4. Profiles of intact methane oxidation rate in soil cores from the honohono, strawberry guava, and ‘ohi‘a veg-etation patterns.

have w2u > 1. We compare the distribution

with x ¼ w2u for all linear fits with the ex-

pected distribution

pðxÞ dx ¼ Cx�ðn�2Þ=2e�xn=2;

where n, the number of degrees of freedomof the fit, is equal to n� p, where n is thenumber of measurements and p is the numberof parameters (two). In the majority of ourcases (Appendix 1), n ¼ 4 and the distributionreduces to pðxÞ ¼ expð�xÞ. In that case, thecumulative fraction f of measurements with aw2u less than a particular value should satisfy

w2n ¼ �lnð1� f Þ

The left- and right-hand sides of this equa-tion are plotted in Figure 5 for all ex-periments with n ¼ 4–6. Experiments withreduced w2 < 0:5, about 40% of the total,have the predicted distribution with unitslope. This affirms the estimate of measure-ment error (0.65 ppm) because an incorrect

value would produce a distribution with aslope different than unity.

The other 60% of experiments includesome with significant departures from the ex-pected linear accumulation of CH4 in thechamber. These consist chiefly of abrupt risesin CH4 concentration at the beginning of anexperiment and a decreasing slope in CH4

versus time at the end of the experiment.Replicate samples of chamber gas obtainedusing two different storage techniques (evac-uated and water-filled vials) produced iden-tical fluxes within the measurement errors,demonstrating that our sample acquisitionand analysis methods are not responsible forthe observed deviations from linearity.Rather, these deviations reflect the actualconcentration of CH4 with time. High initialmethane concentrations (up to 70 ppm) wereobserved. Such abrupt increases over the at-mospheric value could result from release ofmethane from shallow soil pools by distur-

Figure 5. Modified cumulative distribution of reduced chi-squared values for all experiments with four to six measure-ments (points) and the theoretical exponential distribution (dashed line).


bance during setup of the experiment. Suchpools can accumulate when the water tablereaches the surface and soil aeration effec-tively ceases.

The lack of a consistent effect in allexperiments or with one chamber arguesagainst a chamber leak or thermal stratifica-tion in the chamber. A decrease in methaneflux from wetland plants because of a reduc-tion in the leaf-to-air concentration gradienthas been previously documented (Knapp andYavitt 1992). However, we consider it un-likely that changes in plant properties (i.e.,stomatal conductance due to reduced solar ra-diation, levels of CO2, or temperature) dur-ing the experiments could be the dominantprocess responsible for the observed non-linearity because some of our experimentsin the guava and ‘ohi‘a vegetation patterns,which were carried out where ground coverwas absent, also exhibited this effect. Rather,as the gradient in methane concentration be-tween the chamber and soil and plant tissuesdecreases, the flux of CH4 into the chamberdecreases. Khalil et al. (1998) found that this‘‘saturation effect’’ appears at concentrationsof 30 ppm and becomes very large by 60ppm. We conservatively exclude from fur-ther consideration 14 measurements wherethe initial concentration exceeded 30 ppm orreached 30 ppm after 60 min incubation time(indicated in Appendix 1). The latter dis-played a decreasing slope in CH4 versus timeafter 30 ppm was reached, indicating the inci-dence of the saturation effect (Khalil et al.1998). In some cases, outlying time pointswere eliminated and the net methane fluxand w2

u were recalculated accordingly (seenotes in Appendix 1).

The median value of our valid Ka‘au Cra-ter net methane-flux measurements (117 mgCH4 m�2 day�1) is comparable with previ-ously published values in natural tropical orsubtropical wetlands (Bartlett and Harriss1993, Marani and Alvala 2007). Althoughmethane production can be temperature-dependent (Segers 1998), the 2.7�C sea-sonal air temperature difference at Ka‘auCrater is expected to produce little variation(e.g., 20% for a Q10 ¼ 2 [Van Hulzen et al.1999]). We computed the cross-correlation

between water-table level (measured or in-ferred from the precipitation record) and netmethane emission measured at a fixed site inthe honohono vegetation pattern ðn ¼ 19Þ.We found a maximum correlation betweenwater-table level the previous day and netmethane emission in the honohono meadow(Pearson correlation coefficient r ¼ 0:45).Monte Carlo calculations with randomlyshuffled data sets show that this result is sig-nificant ðP ¼ 8:0� 10�3Þ. However, a plot ofnet methane flux versus previous day’s water-table level (Figure 6) shows no obvious corre-lation and large scatter, particularly when thewater level is at the surface. This might bedue to a lag (b30 days) between rainfallevents and peaks in net methane flux (Figure2). The absence of a strong correlation be-tween methane flux and water-table level inpermanently flooded wetlands has been previ-ously noted (Harriss et al. 1988, Bartlett et al.1989, Bartlett et al. 1990, Devol et al. 1990,Moore et al. 1994, Kettunen et al. 2000,Ding et al. 2004).

Our data do not indicate any differencein net methane emission between the hono-hono and ‘ohi‘a vegetation patterns. How-ever, net methane emission in the honohonowas significantly higher than that in the inva-sive strawberry guava ðP ¼ 3:7� 10�8Þ. Weattribute this difference to the lower meth-ane generation potential in the strawberryguava soil at 10 cm depth (Figure 3), wherepeaks of potential methane production wererecorded in all other vegetation patternsstudied. Although soil pH and carbon and ni-trogen content appeared relatively uniformacross vegetation patterns, Psidium cattleianumproduces heavy leaf and fruit litter containingbioactive compounds (Pino et al. 2004) thatsuppress the growth of competing plants andperhaps microorganisms such as methano-gens (Segers 1998). Alternatively, both meth-ane emission and vegetation could reflect anindependent variable such as microtopogra-phy. Although the area where the strawberryguava was located was lower than the re-maining wetland, its surface was consistentlybetter drained, perhaps because of highersoil permeability and its proximity to the out-let stream.


Profiles of methane production and con-sumption indicate the presence of activemethanogens and methanotrophs throughoutthe uppermost 30 cm of wetland soil (Figures3 and 4). The increase in methane generationpotential with depth in the honohono coreprobably reflects an increased abundance ofstrictly anaerobic methanogens. The relativevariation in methane oxidation is muchsmaller, probably the result of a greater toler-ance of methane oxidizers for both aerobicand anaerobic conditions (Roslev and King1996).

Given a profile of potential methane gen-eration and assuming that the oxic-anoxic in-terface corresponds to the water-table level,the dependence of methane production onwater-table level can be estimated and com-pared with observed trends. In Figure 6, weplot estimated flux versus water-table depthbased on the integration of two profiles ofmethane-generation potential obtained in the

honohono meadow in April 2004, along withactual net methane-emission observations. Adry-soil specific gravity of 1.2 is assumed.This calculation ignores any latency of mi-crobial activity, microbial growth, substrateacetate limitation, and the existence of anoxicmicroenvironments within soil particles. Thepredicted fluxes generally exceed our field ob-servations and describe an envelope that con-tains nearly all of our measurements (Figure6). Based on this comparison, we estimatethat approximately half of the methane pro-duced in the uppermost 30 cm of soil is oxi-dized in situ, consistent with the detection ofmethanotrophy throughout the soil column.Unfortunately, a quantitative comparison be-tween total oxidation rates and rate constantsrequires unavailable information about therate of CH4 transport (often in bubbles)through soil and soil pore space. A methane-consumption efficiency of 50% is within therange of values in the literature (30–90%)

Figure 6. Net methane flux versus water-table level in the honohono vegetation pattern. The curves are the integratedmethane flux based on the profiles of potential methane production from soil cores in the honohono pattern obtainedon 6 April 2004 (solid curve) and 21 April 2004 (dashed curve). The calculations assume that methane is produced onlyin soil below the water table and ignores methane consumption.


(van der Nat and Middelburg 1998, Dinget al. 2004).

The emergent macrophyte Schoenoplectuslacustris, for which we have tentative evidencefor methane oxidation, has an aerenchymoussystem and has previously been identified asa conduit of methane from a lake (Kankaalaet al. 2003). It is a plausible host for amethane-oxidizing microbial community.However, the lack of consistent activity fromsample to sample or plant to plant suggeststhat such a community is not widely presentor active in this wetland and that methane ox-idation is primarily restricted to the soil.

Our data constitute indirect evidence forthe dynamics of the methanogenic and meth-anotrophic communities in Ka‘au Crater.The high potential methane production inthe 28 March core from the honohono vege-tation pattern (Figure 3) could have been aconsequence of high precipitation and persis-tently elevated water-table levels during theprevious 25 days (Figure 2). The other pro-files, all exhibiting lower potential methaneproduction, were obtained during a relativelydrier period when the water-table level wasfalling (Figure 2). Conversely, the high rateconstants of methane oxidation observed inthe 5 May core from the ‘ohi‘a pattern couldbe a consequence of falling water-table levelsthat have allowed methanotrophs to flourish(Figure 2).

Although we find no significant correlationbetween profile-integrated potential methanegeneration and CH4 oxidation rate constants,we point out that rates of CH4 oxidationshould nonetheless vary with methane pro-duction due to first-order kinetics. This raisesthe possibility that increased methane oxida-tion by methanotrophs may partly compen-sate for variation in methane production bychanges in water-table level. The populationdynamics of both methanogens and methano-trophs subject to rapid water-table fluctua-tions may also obscure any correspondencebetween that water-table level and net meth-ane emission. Another contributing factormay be the inability of strictly anaerobicmethanogens to flourish in the upper 15 cmof soil where the water-table level and oxy-gen concentrations fluctuate greatly. Furtherstudies in wetland systems with a dynamic

and predictable hydrology such as the Ka‘auCrater bog could test these hypotheses.

acknowledgments

We thank Aly El-Kadi and Robert Whittierfor the use of water-level sensors and FredMackenzie, Jane Schoonmaker, and FrankSansone for advice. Access to Ka‘au Craterwas by permit obtained from the HonoluluBoard of Water Supply. We also thank twoanonymous reviewers for their useful sugges-tions on the manuscript.

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Appendix 1

Net Methane-Emission Measurements

Date(ddmmyy) Loc.a

Dt(min) N

C0

(ppm)Cf

(ppm)Flux

(mg m�2 day�1) w2u Notesb

1 30/08/03 G 120 5 1.8 2 N/D 0.02 Nondetection2 7/9/03 H 120 5 3.7 15.3 76.9 0.36 S3 21/09/03 H 120 5 3 13.3 69.2 0.42 S4 21/09/03 S 120 5 2.2 3.5 9 0.045 29/09/03 H 120 5 3 14.7 71.1 1.38 S6 29/09/03 O 90 5 16 36.6 215.2 0.91 Flux excluded7 5/10/03 H 120 5 2.4 10.9 56.6 0.72 S8 5/10/03 O 60 5 12.9 36.9 223.2 19.12 Flux excluded9 12/10/03 H 120 6 3.1 11 54.9 0.47 S10 12/10/03 S 120 6 3.8 7.6 25.5 0.24 15 min pt. excluded11 18/10/03 H 120 6 2.8 16.4 87 0.4 S12 18/10/03 H 120 6 3.9 15.7 80.7 0.38 S13 18/10/03 H 120 6 2 13.8 76.9 0.56 S14 18/10/03 H 120 6 3.8 19.5 93.5 0.45 S15 25/10/03 H 105 6 3.2 25 96.1 0.66 0 min pt. excluded, S16 25/10/03 G 120 6 2.2 4 11.4 0.0717 2/11/03 H 120 6 3.1 17.3 91.9 0.21 S18 2/11/03 S 120 6 2.4 5.1 16.8 0.03 90 min pt. excluded19 11/2/04 H 90 5 7.4 27.3 171.7 0.12 V, S20 11/2/04 H 90 5 4.4 26.4 189.5 0.27 V, S21 11/2/04 H 90 5 4.5 21 145 0.34 30 min pt. excluded, V, S22 11/2/04 H 90 5 3 19.4 145.6 0.54 V, S23 18/02/04 H 90 5 4.9 35 305.6 4.42 90 min pt. excluded, V, S24 18/02/04 H 90 5 4.8 40.9 337.5 2.7 90 min pt. excluded, V, S25 18/02/04 H 90 5 4.1 23.2 166.1 0.49 V, S26 18/02/04 H 90 5 3.5 23.3 174.7 0.28 V, S27 25/02/04 H 90 5 3.2 19 135.5 1.9 V, S28 25/02/04 H 90 5 5 23 146.8 1.79 V, S


Appendix (continued)

Date(ddmmyy) Loc.a

Dt(min) N

C0

(ppm)Cf

(ppm)Flux

(mg m�2 day�1) w2u Notesb

29 25/02/04 H 90 5 3 17.2 123.5 0.99 V, S30 25/02/04 H 90 5 3.5 19.4 136.5 1.36 V, S31 9/3/04 H 90 5 17.6 36.8 162.9 2.14 Flux excluded32 9/3/04 H 90 5 3.6 21.2 156.2 0.25 S33 9/3/04 H 90 5 13.8 25.4 102 0.08 V, S34 9/3/04 H 90 5 4.7 17.1 109.3 0.07 V, S35 17/03/04 H 90 5 6 29.1 200.3 0.69 V, S36 17/03/04 H 90 5 2.8 26.9 209.4 0.18 V, S37 17/03/04 H 90 5 6.7 23.7 145 1.21 V, S38 17/03/04 H 90 5 3.1 21.2 155.5 0.16 V, S39 28/03/04 H 90 4 3.2 23.6 175 0.39 V, S40 28/03/04 H 90 4 4.2 23.9 170.6 0.83 V, S41 28/03/04 H 60 4 7.2 25.4 89.7 0.28 0 min pt. excluded, V, S42 28/03/04 H 90 4 4.7 15.5 94.1 0.04 V, S43 31/03/04 H 90 4 4.8 19.5 125.4 0.68 V, S44 31/03/04 H 90 4 2.8 17.4 126.7 0.09 V, S45 31/03/04 H 90 4 4.5 17.1 108.4 1 V, S46 31/03/04 H 90 4 4.6 15.7 96.3 0.06 V, S47 6/4/04 H 90 4 7.7 40.4 289.2 3.2 90 min pt. excluded, V, S48 6/4/04 H 90 4 4.6 35.7 318.2 0.07 90 min pt. excluded, V, S49 6/4/04 H 90 4 9.7 29.8 174.3 0.16 V, S50 6/4/04 H 90 4 8.1 29.4 187 0.17 30 min pt. excluded, V, S51 13/04/04 H 90 4 14.8 35.7 179.3 5.45 Flux excluded52 13/04/04 H 90 4 2.5 21.4 163.6 0.09 S53 13/04/04 G 60 4 9.8 35.9 115.4 18.42 Flux excluded54 13/04/04 G 90 4 7.2 16.7 86.5 3.03 30 min pt. excluded55 21/04/04 H 90 4 10.1 35.5 254.7 0.07 90 min pt. excluded, S56 21/04/04 H 90 4 2.5 27.8 219.5 0.36 S57 21/04/04 G 90 4 20.8 23 19 2.3758 21/04/04 G 90 4 6.5 6.8 N.D 0.24 Nondetection59 28/04/04 H 90 4 13.4 48.8 313.2 4.39 Flux excluded60 28/04/04 H 90 4 6.2 39 283.6 9.93 Flux excluded61 28/04/04 G 90 4 2.9 7.6 25.8 0.2362 28/04/04 G 90 4 3.3 5.1 15.1 0.2263 5/5/04 H 90 4 15.7 30.2 196.7 1.06 90 min pt. excluded, S64 5/5/04 H 90 4 6.8 24.4 172.9 1.27 S65 5/5/04 O 60 4 24.8 29.8 234.4 0.4 90 min pt. excluded66 5/5/04 O 90 4 9.7 11.5 57.9 8.82 Flux excluded67 11/5/04 H 90 4 15.7 51 312.2 10.91 Flux excluded68 11/5/04 H 90 4 6.8 38.8 295.6 1.34 90 min pt. excluded, S69 11/5/04 O 90 4 24.8 39.3 132.7 2.69 Flux excluded70 11/5/04 O 90 4 9.7 18 77.7 1.271 20/05/04 H 60 4 19.8 66.7 592.3 40.1 Flux excluded72 20/05/04 H 60 4 10.8 56.1 455.8 3.91 Flux excluded73 20/05/04 G 480 7 5.1 16.6 15.1 7.0274 25/05/04 H 90 4 13.1 42.5 258.6 18.05 Flux excluded75 25/05/04 H 90 4 7.5 35.5 280.1 0.11 90 min pt. excluded, S76 25/05/04 O 60 4 18.5 26.8 6.9 0.62 0 min pt. excluded77 25/05/04 O 90 4 3.2 4 8.1 0.0578 13/06/04 H 120 61 71.1 87.7 161.7 47.84 Flux excluded79 21/06/04 H 60 10 2.7 12 110.6 0.34 New location, S80 21/06/04 H 60 10 3.5 13.8 126.2 0.23 Replicate sampling, S81 12/8/04 H 30 4 2 49.8 0.06 Replicate sampling, S82 12/8/04 H 30 4 1.9 64.2 0.01 Replicate sampling, S

a G, strawberry guava; H, honohono; O, ‘ohi‘a; S, sedge.b V, variability data; S, seasonal data.


methane emission from a tropical wetland in ka'au crater, o'ahu, hawai'i ...

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