Methane cycling in soils

Methane is an important greenhouse gas, and it has been estimated that 50% of annual CH^ comes from terrestrial systems. Better and more accurate methods are needed to quantify CH4 flux from terrestrial environments. Two general methods commonly applied to measure trace gas fluxes are soil cover (chamber) techniques, and micrometeorology methods. Both of these methods have advantages and disadvantages, yet little information is available concerning the relative performance of the techniques. This study was conducted to compare CH4 flux measurements obtained by using a closedchamber soil cover technique and a micrometeorological method (Bowen-ratio Energy Balance (BREB)). Methane flux rates obtained by both methods were compared using 9 time points over a 3 day period at a peatland site in north central Minnesota. Mean CH4 fluxes obtained by both methods were of the same magnitude (2.43 to 5.88 mg CH4 m' h* '); however, differences were observed in the magnitudes of temporal variability as well as the detection sensitivities (minimum detectable flux). Spatial variability associated with the closed-chamber flux determinations was high and coefiBcients of variation of 87.3 to 134% were observed (10 replicate chambers). Spatial variability^ of the BREB method could not be assessed because only one Bowen-ratio station was used. Temporal variations 31 for CH4 flux were found to be large using the BREB method (CVs 90.3 to 285%). Temporal variability associated with the chamber method was low (CVs 7.76 to 20.5%). The minimum detectable flux for the closed-chamber method was 9.32 x 10*^ mg CH, m' h'' (for the detection of both consumption & production), while the minimum detectable flux for the BREB method ranged from 2.16 to 25.5 mg CH4 m' h"'. Due to analj^cal uncertainties associated with gas chromatographic determination of CH4 gradients, the BREB is not recommended. INTRODUCTION Methane concentration in the atmosphere has been increasing by about 1% per year (Khalil et al., 1990). Methane's relative global warming potential is 63 times that of carbon dioxide (CO2) (Duxbury et al., 1993). This increased warming potential is partly due to methane's larger absorptive capacity and partly due to methane's participation in other chemical atmospheric processes where other radiatively active gas compounds are formed (Duxbury et al., 1993). Pearce (1989) predicted that CH4 could become the primary greenhouse gas within 50 years. Because of the increase of CH4 in the atmosphere and the potential impact on global warming, assessment of the sources and sinks of CH4 is crucial. Accurate quantitative assessment of CH4 flux rates and of the factors that influence CH4 flux from terrestrial systems is critical. Chamber methods have been widely used for soil cover techniques to measure trace gas fluxes, and nimierous chamber configurations have been developed (Rolston, 1986). Closed-chambers generally employ an open bottom enclosure which is inserted into the 32 soil surface. Flux measurements are determined by estimating the rate of change of gas concentrations mthin the enclosure. Because of their low cost, simplici^ of design and operation, closed chambers are commonly used to determine fluxes of many gaseous compounds from soil (Mosier et al., 1991a; Freijer et al., 1991; Livingston et al., 1995). Despite their design simplici^, closed-chamber methods have limitations. Disruption of the local microenvironment due to modification of atmospheric pressure, wind, and gas concentration gradients can influence measured fluxes (Denmead, 1991; Hutchinson et al.,1993; Rolston, 1986; Wesely et al., 1989; Livingston et al., 1995). Although modifications have been developed to minimize the effect of some of these factors (Mosier, 1990; Hutchinson etal., 1993; Matthias et al., 1980; Rolston, 1986; Livingston et al., 1995) they still remain a source of concern. Another limitation of closed chamber methods is large spatial variability in flux estimates from site to site. Previous studies reported high spatial variability for CH4 flux, with coefBcients of variation ranging from 31 to 168% (Mosier, 1990). In recent years micrometeorological methods have been applied to measure surface/ atmosphere trace gas exchanges (Verma et al., 1992; Wofsy et al., 1993; Wesely et al., 1989). Micrometeorological techniques are non-disruptive to the local environment and enable determination of fluxes without perturbations induced by covering the soil (Denmead, 1991; Wesely et al., 1989). These methods can also allow for continuous measurements (Baldocchi et al.,1988; Woy Wesely et al.. 33 1989). Eddy correlation in conjunction with a tunable diode laser to measure methane concentrations have been used with a high degree of success (Thurtell et al. 1991, Verma et al. 1992, Edwards et al. 1994), Using the eddy correlation technique with a tunable diode laser can result in very accurate estimates of methane flux, however it should be noted that this type of system is expensive ($120,000) and requires considerable ancillary equipment as well expertise in operating the tunable diode laser. The Bowen Ratio Energy Balance (BREB) method is a micrometeorological technique that relies on flux gradient theory to estimate gas flux (Baldocchi et al., 1988). The BREB method has been shown to be reliable for latent heat flux estimation typically yielding errors in flux estimates on the order of 10-20% (Verma and Rosenberg, 1975; Sinclair et al., 1975). This method has also been applied to estimate methane fluxes (Prueger et al., 1995, Denmead, 1991). The primary reason for using the BREB technique was to evaluate a more simple and less costly micrometeorological method in conjunction with gas chromatography (total cost: approx. $25,000) to estimate methane flux from a natural ecosystem and determine the suitability of this type of method in the field. Accurate determination of concentration gradients is critical for applying the BREB method for estimating trace gas flux; however, few guidelines exists concerning the precise degree of accuracy required. Wesely et al, (1989) state that estimation of trace gas flux using the BREB method requires that the concentration gradient of the component of interest be measured with a "high degree of relative accuracy". Subsequently, it was 34 reported that for a rice paddy site CH4 gradients between 10"^ and 10' ppmv were difficult to distinguish from system noise (Denmead, 1991). Other limitations of micromet techniques are the unknown spatial variability (unless more than one tower is used) and the scale of measurement which is not conducive, in many cases, to process oriented studies. In this study the two methods were used to estimate CH4 flux from a peatland system known to display a predominately methanogenic characteristic. The objective of this study was to compare a static closed-chamber method and the BREB method for estimating CH4 fluxes, and to evaluate the utility of using gas chromatographic analysis for determining CH4 fluxes with these methods. The temporal and spatial variability of the estimates is also assessed. MATERIALS AND METHODS Sampling Site This study was conducted between the 21'' and 23"* of September 1993 on a northern peatland within the U.S. Forest Service Marcell Experimental Forest (47° 32'N, 93° 28'W) in Itasca County, Minnesota. The region is a complex of small upland watersheds and kettle-hole bogs. Average annual temperature is 3 "C, and average annual precipitation is 77 cm, of which 30 % falls as snow. This area is know to be highly methanogenic (Dise, 1992,1993; Crill at al., 1988; Veima et al., 1992). The study area (referred to as the Bog Lake Peatland) is an open bog, and was the location of a recent study by Verma et al. (1992). The soils of this site are classified as greenwood peat (USDA-SCS, 1987), and at the time of sampling standing water covered 35 the site. During the 3 day sampling period the average day time air temperatures ranged from 8.5 "C to 12.3 "C. The Bowen-ratio station was erected on a wooden platform which extended approximately 50 meters into the peatland, and chambers were installed around the platform area. Methane flux measurements were performed three times each day (morning, noon, aftemoon) for the closed-chamber method and continuously (morning through the afternoon) for the Bowen-ratio method on three successive days. At each sampling time flux estimates were determined by the BREB method and with soil chambers. Bowen-ratio Energy Balance Method Theory Flux-gradient theory is based on the assumption that turbulent transfer of scalers is analogous to molecular difiusion and can be determined as the product of a mean vertical mixing ratio gradient for any atmospheric constituent and an eddy difiiisivity coefficient (Baldocchi et al., 1988). The flux-gradient profile for an atmospheric constituent can thus be expressed in general form as