Sensitivity of soil methane ¯uxes to reduced precipitation in
boreal forest soils
S.A. Billings
a,*, D.D. Richter
b, J. Yarie
ca
Biological Sciences Department, University of Arkansas, Fayetteville, AR 72701, USA
b
Nicholas School of the Environment, Duke University, Durham, NC 22708, USA
c
University of Alaska Fairbanks, School of Agriculture and Land Resource Management, Fairbanks, AK 99775, USA Received 4 August 1999; received in revised form 25 January 2000; accepted 14 February 2000
Abstract
In order to better predict soil sinks of methane, we need to examine soil methane ¯ux patterns and responses to altered soil moisture regimes. Estimates of the global atmospheric CH4 budget must also account for ¯uxes in the vast boreal region. We measured methane ¯uxes into the soil surface, methane concentrations, water content, and temperature in the soil pro®le in two interior Alaskan forests, over two growing seasons. At each site, a 0.10 ha rain-shelter limited summer precipitation from entering the soil. Limiting summer precipitation at the upland site generally increased that site's soil uptake of methane. Average rates of soil methane uptake among upland plots ranged from 0.10 to 0.95 mg mÿ2 dayÿ1. At the ¯oodplain site, limiting precipitation decreased the soil methane uptake of that site, and the rates here ranged fromÿ0.02 to 0.57 mg mÿ2dayÿ1. Using soil pro®le methane concentrations, we calculated CH4 ¯uxes using Fick's Law. Our inability to precisely measure the concentration gradient across the soil surface resulted in calculated ¯ux estimates that more likely represent ¯uxes within the soil pro®le. Methane sources and sinks in the soil pro®le also confounded the comparison of measured and calculated ¯uxes.72000 Elsevier Science Ltd. All rights reserved.
Keywords:Soil methane consumption; Boreal forest soils; Fick's Law; Soil moisture; Methanotrophy; Methanogenesis
1. Introduction
The atmospheric concentration of methane is cur-rently increasing at a rate of about 1% per year (Torn and Harte, 1996). Since methane is a greenhouse gas, studies on the role of soil as a sink for atmospheric methane have been conducted in temperate forests (Steudler et al., 1989; Yavitt et al., 1990; King and Adamsen, 1992; Dorr et al., 1993; Castro et al., 1994; Hutsch et al., 1994; Schnell and King, 1994; Bender and Conrad 1995; Ambus and Christensen, 1995; Sitaula et al., 1995; Castro et al., 1995; Yavitt et al., 1995), boreal forests (Whalen et al., 1991; Whalen et
al., 1992; Castro et al., 1993; Whalen and Reeburgh, 1996; Gulledge et al., 1997; Gulledge and Schimel, 1998a, 1998b), arctic tundra (Whalen and Reeburgh, 1988; Whalen and Reeburgh, 1990a, 1990b; Schimel, 1995), montane soils (Torn and Harte, 1996), tropical soils (Keller et al., 1983, Keller et al., 1986; Dorr et al., 1993), and deserts (Striegl et al., 1992). About 10 1012g yearÿ1 of methane are consumed by soil mi-crobes globally (Schlesinger, 1991).
Some evidence suggests that physical and not bio-logical controls govern the oxidation capacity of soils: methane ¯uxes into soils often show only a slight tem-perature response (Born et al., 1990). Increased soil moisture often decreases soil methane uptake (Mosier et al., 1991), which could indicate limited microbial access to CH4. In addition, soil methane uptake rates
are similar in many dierent ecosystems, with daily values ranging from 1 to 2 mg mÿ2dayÿ1(Born et al.,
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* Corresponding author. Tel.: 2227; fax: +1-501-575-4010.
1990; Steudler et al., 1990; Yavitt et al., 1990; Mosier et al., 1991). As Schimel et al. (1993) stated, we would expect a wider range of soil oxidation rates if biotic factors control methane consumption. Biotic limi-tations do occur with physiological water stress (Striegl et al., 1992; Torn and Harte, 1996; Gulledge and Schi-mel, 1998b), but most data suggest that physical limi-tations on soil oxidation of methane dominate methane oxidation.
Soil moisture is a factor that aects both physical and biotic limitations of methane oxidation, and yet few soil methane studies have experimentally reduced soil moisture content in the ®eld to examine eects of both water-®lled pore space and water bioavailability on methane ¯uxes. This study examines the eects of reduced soil moisture content on soil methane ¯uxes in two forested sites. To evaluate eects of soil moisture on the methane oxidation capacity of boreal forest soils, we examined soil CH4 concentrations and CH4
uptake under altered and control precipitation regimes. We assume soil CH4 uptake to be the net dierence
between CH4production and consumption by the soil.
2. Materials and methods
Soil atmosphere samples, CH4 ¯ux measurements,
and soil temperature and moisture data were collected at two boreal sites with contrasting moisture regimes. Data were collected throughout the 1996 and 1997 growing seasons at both an upland and a ¯oodplain site. Both sites are located in the Bonanza Creek Long Term Ecological Research site, 20 km southwest of Fairbanks, Alaska (648N, 1488N). The mean annual temperature is ÿ3.58C; the growing season is 90±100 days (van Cleve and Yarie, 1986). About 37% of mean annual precipitation, 269 mm, is snow (Viereck et al., 1993). Potential evapotranspiration is 466 mm (Patric and Black, 1968). Snow cover generally is pre-sent from mid-October through April. Permafrost is not present at either site.
The well-drained upland site is located on a ridge of wind-deposited loess, 308 m above mean sea level. The site is on a 258 slope, facing east±southeast. The soils are Al®c Cryoquepts. Approximately 7 cm of decom-posing litter overlies a rock-free, silt loam and silt sub-soil. Paper birch (Betula papyrifera Marsh.), white spruce (Picea glauca (Moench) Voss), and balsam poplar (Populus balsamifera L.) are in the overstory and total 2767 stems/ha. Dominant tree height is about 18 m. It has been approximately 80 years since the last forest ®re, the primary disturbance factor in forests of this type.
The ¯oodplain site is about 10 km from the upland forest, and is on poorly drained, alluvial Entisols, classi®ed as Typic Cryo¯uvents. The top 25±30 cm of
the soil pro®le is silt loam, and from approximately 30 to 100 cm the pro®le is coarse sand. Several soil pits contained 10±15 organic horizons from previous ¯oods, all within the top 100 cm. Litter depth on the soil surface is about 9 cm. The site supports 780 stems/ha, comprising 40- to 50-year-old white spruce reaching into an upper canopy of 100-year-old balsam poplar. Dominant tree height is 20 m. A ¯ood was the last major disturbance in this ecosystem, approxi-mately 50 years ago. River terrace height is 3±3.5 m above river water, and the site is 139 m above mean sea level (Bonanza Creek Experimental Forest web page, 1999). When the river level rises, the water table can reach to 1 m below the soil surface.
At each site, a 0.10 ha, corrugated ®berglass shelter is installed each May to limit summer precipitation in®ltrating into the soil. These shelters have been installed since 1989, and are removed each September to allow a snowpack to accumulate. Soils receive moisture input with snowmelt in the spring. All litter accumulation from the shelter surface is spread over the treatment area at the end of each growing season. A moisture barrier of plastic sheeting is buried verti-cally 0.6 m deep around the perimeter of each shelter to limit lateral moisture ¯ow into the treatment area. Three replicate chambers and three replicate pro®les existed both inside and outside of the shelters at both sites.
Soil atmosphere CH4 concentrations were sampled
from tubing inserted into the ground at 20, 40 and 100 cm. Sampling locations were randomly picked inside and outside the shelter. Nalgene plastic tubing, perfo-rated at the ends to allow air ¯ow, was used at the 20 and 40 cm depths. The 100 cm depths have copper tubing installed down to a 10 cm diameter, 30 cm long PVC soil atmosphere reservoir. Flexible rubber tubing was sealed to the tops of the sampling tubes with sili-con. During the 1996 growing season, 10 ml syringe samples were taken weekly at each 20, 40 and 100 cm depth. Samples were taken at each depth every 2 weeks during 1997. At the upland site, samples were also taken at 200 cm during the 1997 growing season. Syringes were glass BDPack syringes with stopcocks epoxied to the tips. Data from the 20 cm depth were used to calculate methane ¯uxes into the soil from the atmosphere. Shallower sampling wells were not installed because of the 6±9 cm of organic horizon overlying the mineral soil at both sites; we assumed the zone of maximum oxidation to be within the top 10±20 cm of mineral soil (Whalen et al., 1992). Data from depths deeper than 20 cm were used to examine treatment eects on pro®le concentrations, and to see if any methanotrophic or methanogenic activity at these depths occurred that could aect the methane concentration at 20 cm.
sampling locations as the CH4 pro®le concentrations,
each time pro®le samples were taken, using chamber techniques. Plastic, 5.5 l containers with air-tight lids were cut into the soil approximately 10 cm, adjacent to the pro®le tubing. Chambers were 272.3 cm2 in area. Syringe samples were taken from septa installed in the lids, using the same type of syringe as for below-ground atmosphere sampling. A sample of above-ground atmosphere was taken immediately adjacent to the chamber to compare with the initial, time zero sample inside the chamber. Subsequent samples were taken at 15 and 30 min.
Samples were transported to a lab at the University of Alaska Fairbanks and analyzed within 24±48 h. No dierence was found between samples immediately analyzed and those analyzed after 3 days. Samples were analyzed by a ¯ame ionization detector in a Shi-madzu GC-14A gas chromatograph (Shimadzu, Columbia, MD, USA), with a 2 m stainless steel col-umn packed with Porapak Q. Standards were Scotty gas standards (Scotty Gases, San Bernadino, Califor-nia). We used the time-linear rate of decrease of CH4
in chambers in conjunction with chamber volume and area to determine the rate of soil CH4uptake.
Soil temperature was continuously recorded by ther-mistors (MK820, Siemens, Erlangen, Germany) con-nected to a datalogger (DL-2, Delta-T, Burwell, UK) at 20, 40 and 100 cm depths, at locations inside and outside the shelter corresponding to gas sampling lo-cations. Soil volumetric moisture content (VMC) was measured weekly using time domain re¯ectometry (TDR, Tektronix, Beaverton, OR, USA) at two lo-cations inside and two outside the shelters at 4, 10, 20 and 50 cm. VMC re¯ects the percentage water content of the total soil volume. Soil water holding capacity (WHC) was measured by saturating replicates of soil samples from each gas sampling depth. Soil was then allowed to drain for 24 h. Samples were weighed, dried, and re-weighed for water content.
Attempts to predict soil methane uptake were made using Fick's Law. This assumes that methane ¯ux is primarily driven by diusion gradients and not by tem-perature gradients. Fick's Law states that gas ¯uxJ is a function of the gas concentration gradient and the gas diusion coecient in free aboveground air:
J ÿDa
tration gradient inml lÿ1across depths.
The value of D in a soil medium is an estimated fraction of the known diusion coecient in air, due to tortuous path lengths and the moisture through which soil gas must travel. Estimates of ¯ux calculated
from Fick's Law are thus limited in accuracy by this estimated value of D (Penman, 1940; Marshall, 1959; Millington and Quirk, 1961; Currie, 1965; Rolston et al., 1978), which is governed by constants particular to the soil medium and the air-®lled porosity. Air-®lled porosity was estimated from VMC and soil bulk den-sity, which was determined using intact core samples, according to Vomocil (1965). Fick's Law estimates are also limited in accuracy by howdC=dzis calculated.
We used a form of Fick's Law used by Whalen et al. (1992) at nearby, upland loessal sites in the Bonanza Creek Experimental Forest, where soil prop-erties were similar: included to account for the tortuosity of the soil med-ium (Whalen et al., 1992; Campbell, 1985).
Statistical tests on the eects of the shelters were performed using t-tests for equal means. When the data were normally distributed, we used the non-parametric Wilcoxon rank-sum test. Signi®cance was determined at a0:05: Since our data were collected from one rain-sheltered area at each site, these ana-lyses compare a single set of treatment and control soils at each ¯oodplain and upland locations. Results, therefore, must be interpreted carefully, with an ap-preciation of the problems of pseudo-replication. We assume for all tests that sheltered and unsheltered soils were similar before rain-shelter installation.
3. Results
3.1. Soil moisture and temperature
The eect of the upland rainout shelters is most vis-ible after rain events (Fig. 1). At the ¯oodplain, rain-out shelters appreciably limited available soil moisture throughout the summer (Fig. 1). Volumetric moisture content (VMC) decreased with availability of water from snowmelt under the shelter at each site. At the upland site, VMC at 20 cm inside the shelters ranged from 24 to 8% in 1996 and from 39 to 8% in 1997, with higher values occurring immediately after snow-melt. Outside the shelters, the range was approximately the same, but higher values occurred sporadically throughout the growing season with precipitation events.
inside and from 65 to 61% outside the shelter in 1996. Values ranged from 50 to 28% inside and from 50 to 46% outside the shelter in 1997. During August 1997, a ¯ooding event caused both sheltered and unsheltered soil to increase in water content after minimum values in early July.
Soil temperatures increased until August (July at the ¯oodplain in 1996) during both growing seasons (Fig. 2). Small temperature dierences were found between locations inside and outside the rain-shelters at each site. At the upland site, soil temperatures underneath the shelter were consistently higher than those outside in 1996. The dierence was generally less than 1.58C. The same trend was noted at the beginning of the season in 1997. Temperatures under the shelter were not recorded for part of this season. Soil tem-peratures at 20 cm ranged from 4 to 128C; in 1997 the range was from 2 to 208C.
At the ¯oodplain site the shelter had the opposite eect on soil temperature as that observed at the upland; soil temperatures inside the shelter were con-sistently lower at 20 cm in 1996, for much of 1997 (Fig. 2) and throughout both growing seasons at 40
cm. The temperature dierence was usually less than 28C. Temperatures at 20 cm in 1996 ranged from 0.5 to 78C; in 1997 they ranged from 1 to 7.88C. Soil tem-peratures were lower in 1996 at both sites; mean monthly air temperatures during the growing season of 1996 ranged from 1 to 4.3, below those of 1997.
3.2. Soil pro®le CH4concentrations
At the upland site, average methane concentrations in the soil pro®le ranged from values close to atmos-pheric levels (at 20 cm) to a low of 0.29 ml lÿ1 at the upland site at 100 cm. Pro®le methane concentrations inside the shelter were signi®cantly lower than those outside only at the 40 cm depth during 1996 and 1997 (PR0.05). A signi®cant dierence also was recorded at the 200 cm depth in 1997 (P< 0.05).
At the ¯oodplain site, pro®le concentrations ranged from close to atmospheric levels at 20 cm to a low of 0.40 ml lÿ1at 100 cm. Concentrations inside the shelter were signi®cantly higher than those outside at 20 and 40 cm depths in 1996 (P< 0.05).
3.3. Rates of soil methane uptake
Average rates of soil uptake of methane at the upland site ranged from 0.10 to 0.95 mg mÿ2 dayÿ1 (Fig. 3a). During both years, a signi®cant dierence was found between those rates measured inside and those outside the shelter, with higher rates of methane consumption occurring inside the shelter (P< 0.05).
At the ¯oodplain site, average rates of soil methane uptake ranged from ÿ0.02 to 0.57 mg mÿ2dayÿ1 (Fig. 3b). Rates outside the shelter increased over the summer of 1996. No such trend is visible during 1997, when the rates were more varied both between sampling locations and over the season. During 1996 rates of methane uptake were signi®cantly higher out-side the shelter than inout-side (P< 0.05).
4. Discussion
4.1. Methane ¯uxes and soil pro®le methane concentrations
Soil methane uptake rates at the upland site ranged from 0.10 to 0.95 mg mÿ2dayÿ1 and were within the range of those found at similar sites by Whalen et al.
(1991) (see Fig. 3a). Floodplain rates had generally lower values, ranging from ÿ0.02 to 0.57 mg mÿ2dayÿ1(Fig. 3b). Periodic methane release occurred at this site. These uptake rates are low compared to most rates recorded in temperate forests, but are within the range of ¯ux rates reported in several stu-dies (Steudler et al., 1989; Yavitt et al., 1990; King and Adamsen, 1992; Dorr et al., 1993, Castro et al., 1994; Hutsch et al., 1994; Schnell and King, 1994; Bender and Conrad, 1995; Ambus and Christensen, 1995; Sitaula et al., 1995; Castro et al., 1995; Yavitt et al., 1995). We expected lower rates of methane uptake at the ¯oodplain site because of generally high soil moisture there. For both summers, mean uptake rates of methane in both sheltered and unsheltered areas at the ¯oodplain were 0.1620.02 mg mÿ2dayÿ1. At the upland site, analogous mean rates were 0.4120.03 mg mÿ2dayÿ1. The lower mean rates at the ¯oodplain site suggest that high soil moisture may limit methane uptake.
Some research indicates that limited soil water can inhibit methane uptake. Torn and Harte (1996) reported an optimum %WHC of 50% in temperate montane soils, with oxidation becoming inhibited at soil moisture levels below 20% moisture by weight. Striegl et al. (1992) reported a positive relationship
between soil moisture and methane uptake in desert soils. Many other studies report a negative relationship between soil water content and methane consumption, presumably because of diusion limitations on methane supply (King and Adamsen, 1992; Castro et al., 1994; Castro et al., 1995; Sitaula et al., 1995; Wha-len and Reeburgh, 1996). In a study on upland soils supporting a white spruce forest in interior Alaska, Gulledge and Schimel (1998b) found an optimum
moisture level of 30±40% of soil water holding ca-pacity (%WHC) for soil methane oxidation rates in the lab.
Our ®eld studies also suggest optimum moisture levels for soil methane uptake. At the upland site, where soil moisture is less than 20% by volume for most of the growing season (Fig. 1), reducing water availability signi®cantly increased methane uptake rates (Fig. 3a). This suggests that the microbes were
limited in oxidation capacity by gas diusion rates. The soil at this site naturally was at or less than 28% WHC for most of the two growing seasons of this study, below the values suggested by Gulledge and Schimel's (1998b) work for optimum methane uptake. The increase of CH4 uptake rates under the shelter
suggests that being well-adapted to dry conditions allowed the upland soil microbes to respond positively to increased supplies of atmospheric methane, without being negatively aected by the lack of moisture.
In comparison to the upland site, ¯oodplain methane uptake data show the opposite response to limited precipitation (Fig. 3b). Soil methane uptake under the rain shelter, in spite of the reduction in water-®lled pore space, was lower than at the unshel-tered plots. The data suggest that the methane-con-suming bacteria at this ¯oodplain site are so well-adapted to volumetric soil moisture values of 60% and higher (103% WHC) that reducing the water avail-ability to 45% or lower (Fig. 1, 78% WHC) reduced their ability to function, in spite of a presumed greater ability of atmospheric methane to diuse into the soil. Soil moisture values recorded at the ¯oodplain were often far above the optimum of 30±40% WHC reported by Gulledge and Schimel (1998b) for upland boreal forest soils and yet many recorded uptake rates at the ¯oodplain site were similar to those of the upland soils.
The decrease in methane uptake under the ¯ood-plain rain-shelter could also result from chemical dierences between precipitation and groundwater. The treatment soils could be qualitatively dierent from the control soils in a way that is critical for methanotrophic activity; soil pH, soil solution conduc-tivity, or nutrient availability may have been altered in a manner that limited methanotrophic activity, or that fostered methanogenic activity. Another possibility is that evaporative movement of soil water underneath the shelter created a soil environment with too high a salt concentration for methanotrophic activity. This high salt environment as a result of soil water evapor-ation has been documented on ¯oodplain soils near this site (Dyrness and van Cleve, 1993), though pri-marily at early succession sites.
A comparison of methane uptake by soils at both upland and ¯oodplain sites suggests several possible scenarios. Because we measured methane uptake and not activity levels of methanotrophs and methanogens, we need to consider possible responses of both popu-lations. Populations of methanotrophic and methano-genic microbes in both ecosystems may be similar, but may function dierently because of the dierent physi-cal and chemiphysi-cal soil environment at the sites. Mi-crobial population size and activity levels may also dier between sites as a result of dierences in the soil environment. Our study could also suggest that the
soil microbial populations responsible for soil methane oxidation and methanogenesis at the ¯oodplain site may be composed of dierent microbes than that at the upland site. This scenario seems reasonable given the suggestion in Gulledge et al. (1997) of physiologi-cally distinct methane oxidizing populations at upland sites of dierent successional stages.
In spite of the assumed increased availability of at-mospheric methane to the soil pro®le under the shelter, there was no signi®cant dierence between sheltered and unsheltered methane pro®les at the upland site depths of 20 cm. We presume this is due to either increased methane consumption in the sur®cial layers of the pro®le, or decreased methanogenesis. Data from both summers also suggest that microbial methane consumption increased, or methanogenesis decreased, at 40 cm underneath the shelter; at this depth, the shel-tered plots exhibited lowered methane concentrations. We also found lower sheltered concentrations at 200 cm during the 1997 growing season. One possible ex-planation is spatially random sources of methane at depths greater than 100 cm, providing substrate for methanotrophic activity at these depths. On several oc-casions methane concentrations at 200 cm were higher than those at 100 cm, ranging up to 1.62 ml lÿ1.
At the ¯oodplain site, the higher concentrations at 20 and 40 cm under the shelter correspond with the lower methane uptake rates at these plots (Fig. 3b). The increase in air-®lled pore space in the sheltered soil allows more atmospheric methane to diuse into the soil, but lowered consumption rates result in elev-ated concentrations. Although the 1997 data fall short of signi®cance, a similar trend is evident as that in 1996.
4.2. Soil moisture, temperature and ¯ux relationships
Multivariate analysis of methane uptake rates with soil moisture and temperature left much variability in rates unexplained. Only one signi®cant model was found, at the upland unsheltered sites in 1997 (P = 0.05, R2 = 69), as a result of the strong relationship with soil temperature (P< 0.05, R2= 0.64). Because multivariate models in all other instances did not suc-cessfully predict our methane uptake rates, we exam-ined soil moisture and temperature separately to try to determine if and when they did aect ¯ux rates.
Although there were some signi®cant relationships between soil moisture and methane ¯ux rates, at both sites much of the variability in ¯ux was left unexplained by soil water content. At the upland site in 1996, there was a negative relationship between soil moisture at 20 cm and the CH4surface
24% from June through September. In 1997, we were able to begin measurements sooner after snowmelt, when volumetric soil moisture values were as high as 39%. Partially as a result of the low methane uptake rates recorded at these high soil moisture values, both the sheltered and the unsheltered plots have signi®cant, negative relationships between soil water and methane consumption in 1997, explaining 64 and 60%, respect-ively of the variation in uptake rates.
At the ¯oodplain site, the sheltered sites show lit-tle relationship between soil moisture at 20 cm and methane uptake rates. Outside the shelter in 1996, a negative relationship was established, with soil moisture explaining 51% of the variation in uptake rates, all at moisture values above 60% (P< 0.05). Outside the shelter at the ¯oodplain site in 1997, two points representing the lowest rates were leverage points in a signi®cant, positive relationship between moisture and methane uptake rates. One of these points most likely represents a pulse of methane pro-duction, as there was net eux inside a chamber. The other point was the ®rst data point of the season, when methane uptake was negligible and moisture availability was low because of freezing soil tempera-tures.
Soil temperature was also not a good predictor of methane ¯ux. This corresponds to a study by Castro et al. (1993). At both sites, lower uptake rates generally occurred at the beginning of the sea-son when soil moisture was high and soil tempera-tures were low. However, there was only one positive, signi®cant relationship between methane consumption and soil temperature at 20 cm, at the upland unsheltered plots in 1997 (P< 0.05, R2 = 0.64). The increase in methane uptake rates for this one signi®cant relationship occurred at soil tempera-tures above 108C; Castro et al. (1995) suggested that temperature had a positive eect on methane con-sumption betweenÿ5 and 108C and no eect between 10 and 208C. Also contrasting with our results, Crill (1991) reported a strong temperature response in early spring and a lessened eect during the summer. Because of the generally non-signi®cant relationships between methane uptake and soil temperature at our sites, we assume that the small soil temperature dier-ences resulting from the rain-shelters had a negligible eect on uptake rates; soil temperature dierences may have been more pronounced nearer the soil surface, however.
The lack of a signi®cant relationship between soil temperature and methane ¯ux at most plots suggests that soil physical parameters may be more in¯uential determinants of methane ¯ux. This is consistent with several other studies of soils functioning as a CH4sink
(Torn and Harte, 1996; Whalen and Reeburgh, 1996; Born et al., 1990). To more closely examine concepts
of physical limitations on soil CH4 uptake, we used
Fick's Law to predict soil CH4¯uxes.
4.3. Fick's Law predictions
Many studies have used Fick's Law to calculate ¯uxes of various gases into and out of the soil surface (Table 1). Fick's Law represents gaseous mass ¯ux across a horizontal plane. The ability of Fick's Law to estimate gas ¯uxes at the soil surface plane depends on several conditions:
1. dC=dz measurements at depth z must be close enough to the soil surface for the concentration gra-dient to re¯ect the concentration gragra-dient immedi-ately below the soil surface. This is violated at depths commonly used for measurements of dC=dz such as 10 or 20 cm (Yavitt et al., 1990).
2. Alterations to Daccounting for soil tortuosity must account for heterogeneous soil conditions (Rolston et al., 1978). Heterogeneous quantities of roots, root channels, moisture variability and organic matter all limit the accuracy of tortuosity coecients.
3. Gas consumption or production within the pro®le depth from which ¯uxes are calculated must be neg-ligible. This is true in the case of biologically inert gases such as radon (Dorr and Munnich, 1990); CH4 can be both produced and consumed by
bio-logical processes in a given soil pro®le, especially near the ground surface.
Because of these conditions, most calculations of soil gas ¯ux represent ¯uxes within the soil pro®le, qz,
rather than the surface ¯ux, q0: If the concentration of
CH4 decreased linearly with depth through the top 20
cm soil, we assume that the concentration gradient we measured from 20 cm to the atmosphere immediately above the soil surface re¯ects the gradient across a plane with a midpoint in this soil compartment atz= 10 cm. Thus our Fick's Law calculations more likely represent the ¯ux of gas across this plane qz and not
the soil surface ¯ux q0). The calculations must be
con-sidered estimations because of studies suggesting that CH4 concentrations do not decrease linearly with
depth (Whalen et al., 1992; Yavitt et al., 1990).
Calculations ofqzcannot be expected to corroborate
with measurements of q0 because of microbial
con-sumption and production of CH4 at these shallow
depths in the soil pro®le. The conservation of mass (de Jong and Schappert, 1972; Striegl 1993) explains how biological sources or sinks of CH4 aect the relation
of qz to q0. The one-dimensional continuity equation
states that the change in gas concentration over time @C
@t at any point is the change in ¯ux
@q
@z, plus any
@C
@t
@q
@zS
Integrating over the depth pro®le [0, z], the depth-averaged equation becomes:
z@C
@t q0ÿqzzS
whereq0is ¯ux at the soil surface plane and qz is ¯ux
across the plane at depth z. Thus the only way q0
(measured surface ¯ux) can equal qz (calculated
Fick-ian ¯ux) is if zSandz@C
@t are the same, meaning
that the source or sink of CH4in the soil pro®le
com-partment must exactly equal the change in depth-aver-aged concentration with time. Since q0 was measured
over 30 min intervals, we assume that @C
@t10;
conse-quently, any dierence between q0 and qz is only due
to S-. Because signi®cant CH4 oxidation capacity can
be concentrated near 10 cm in the soil pro®le (Whalen et al., 1992; Gulledge et al., 1996), S- can be a signi®-cant factor in the equation.
Table 1
Studies using Fick's Law to calculate ¯ux rates of various gases at the soil surface, estimations of diusion coecient (if available), estimation of dC/dz, and results of any comparison to measured ¯uxes. Soil taxa given when provided in study
Soil system Estimate ofD Estimate ofdC/dz Comparison with measurements Authors
Global D/D0from Millington
and Shearer (1971)
0.04 ppmv cmÿ1 No Potter et al.
(1996) Agricultural soil, Ottawa, Canada (a) Flux measured in
cores to deriveD (b) 0:9Df2:3,D
cm2sÿ1
Slope of gradient to 20 cm Yes; both methods within 3.8 mmol mÿ2dayÿ1of measured CH
4
¯uxes, 19.1±23.9mmol mÿ2dayÿ1
Dun®eld et al. (1995)
New York state hardwood forest 0:9Df2:3,Dcm2sÿ1 Unspeci®ed for calculations,
measured at various depths
Yes; Fick's law overestimated except in early spring (see reference)
Yavitt et al. (1995)
Agricultural loamy sand (Psammentic Hapludalf) Ontario, Canada
1:3Df1:7,Dm2sÿ1 Unspeci®ed for calculations,
measured at various depths
No Burton and
Beauchamp (1994) Alpine, sub-alpine Wyoming 0.139 cm2sÿ1CO
2,
N2O; 0.22 cm 2
sÿ1
CH4
Average gradient from 100 cm in snowpack
No Sommerfeld et
al. (1993)
Boreal forest, Alaska 0:9Df2:3,Dcm2
sÿ1
Slope of gradient to 20 cm Yes; calculations were higher than static chamber measured ¯uxes
Whalen et al. (1992) Appalachian mountains, West
Virginia
0.186 cm2sÿ1, Marrero and Mason (1972)
Unspeci®ed for calculations, measured at various depths
Yes; similar to ¯uxes in cores Yavitt et al. (1990)
Fig. 4. Calculated qzvs. measured (qo) soil CH4¯ux rates at upland and ¯oodplain sites, 1996 and 1997. Perfect correspondence between
Fick's Law predictions of qz were signi®cantly
re-lated to the observed CH4 ¯ux (q0) at the sheltered
upland plots during 1997 (Fig. 4). At all other lo-cations during 1997 and at both the upland and ¯ood-plain sites during 1996, both sinks and sources of CH4
in the top 10 cm of the soil pro®le prevented qz from
being signi®cantly related to measured surface ¯ux,q0.
The varying eects of soil moisture on both methano-trophy and methanogenesis were particularly evident at the ¯oodplain plots, whereqz often was lower than
q0: Our work with Fick's Law indicates that the sink
term for methane in the top layers of these soil pro®les can overwhelm calculations of ¯ux estimates.
5. Conclusions
If we assume treatment and control soils were simi-lar before installation of the rain-shelters, the contrast-ing responses of soil CH4 uptake to summer rainfall
exclusion at these sites suggest several possible scen-arios. Forests with some tree species in common, but dierent soil chemical and physical environments, may support dierent population sizes of methanogenic and methanotrophic microbes, dierent species of mi-crobes, or varied levels of soil microbial activity. This knowledge is critical for predicting the forest soil sink for atmospheric methane on a landscape level, particu-larly for modelers who may use broad classi®cation schemes to de®ne forest types.
Acknowledgements
This work was supported by a NASA Earth System Science Global Change Fellowship. Many thanks go to the sta of the Bonanza Creek Experimental Forest and the University of Alaska Fairbanks. Dr. Richard Boone generously provided lab space and equipment. Dr. Steve Whalen was generous with his time, equip-ment and helpful comequip-mentary. Drs. Jay Gulledge and Gabriel Katul provided insightful discussions.
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