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a Institute of Soil Science and Forest Nutrition, Univ. of Göttingen, Büsgenweg 2, 37077 Göttingen, Germany
b Dep. of Environmental Chemistry, Univ. of Kassel, Nordbahnhofstr. 1a, 37213 Witzenhausen, Germany
* Corresponding author (rteepe{at}gwdg.de).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: GWP, global warming potential • TSM, compacted soil in the middle of the trafficked soil • TSB, compacted soil at the border of the trafficked soil • USM, uncompacted soil between the two wheel tracks (middle of the skid trail) • USB, uncompacted soil at the border of the trafficked soil • WFPS, water-filled pore space
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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The use of a forwarder and harvester machines with a total weight of up to 20 Mg for wood harvesting may cause serious soil compaction in forests. However, these machines are commonly used because of their economic efficiency. For an optimum and efficient use of harvester machines a skid trail distance of 20 m is required, because the grip arms of harvester machines have mostly a 10-m radius of action to both sides of the skid trail. This means that skid trails make up approximately 12 to 16% of the total operational area. In Germany a permanent skid trail system with a skid trial distance of 20 m is recommended by the Forestry Commissions (Anonymous, 1991) to ensure efficient wood harvesting but also to avoid the risk of unrestricted soil trafficking during harvesting.
Soil compaction by harvesting machines alters many important soil properties, such as bulk density, aeration porosity (McNabb et al., 2001), hydraulic conductivity (Gent et al., 1983), and even the root density (Wronski, 1984; Wasterlund, 1985). The effect of soil compaction on the fluxes of N2O and CH4 in forest soil has not received as much attention as in agricultural systems (Bakken et al., 1987; Hansen et al., 1993; Ruser et al., 1998). Therefore, the objectives of this study were (i) to quantify the change of N2O and CH4 fluxes across skid trails on three soils of different texture, (ii) to evaluate the changes in these trace gases with regard to their overall contribution to the greenhouse effect, and (iii) to determine whether soil temperature and moisture control the flux rates at differently disturbed soils.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Trace Gas Measurements
The fluxes of N2O and CH4 at the silt and silty clay loam sites were measured at four different locations across the skid trail: (i) compacted soil in the middle of the trafficked soil (TSM); (ii) compacted soil at the border of the trafficked soil (TSB); (iii) uncompacted soil between the two wheel tracks (middle of the skid trail) (USM), and (iv) uncompacted soil at the border directly adjacent to the trafficked soil (USB) (Fig. 1)
. At the sandy loam site trace gases were measured only in the TSM. These fluxes were compared with the flux on the uncompacted forest soil (control) located about 10 m away from the skid trail. Three plots (each about 10 x 25 m) were established at each site (Fig. 1). The gas fluxes at the sandy loam and silty clay loam sites were measured for 1 yr from April 1999 to March 2000 at 2- to 3-wk intervals using the closed chamber method (Matthias et al., 1980). At the silt site, gas measurements were conducted from April 1999 to the beginning of November 1999. Annual fluxes of this site were estimated by assuming that lowest fluxes measured in October/November and April also occurred during the period from November to March. This assumption was based on the low values of trace gas flux measured at the sandy loam and silty clay loam sites in October/November and April, and on a similar level of trace gas fluxes during winter (November–March).
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Calculation of the Global Warming Effect
To assess the climatic impact of N2O and CH4 resulting from trafficking in forests, the annual fluxes of N2O and CH4 were calculated on a field scale with and without the contribution of the N2O and CH4 fluxes derived from the differently affected areas (Table 2) of the skid trail. The difference between these two calculated values provided the increase of N2O emission and the decrease of CH4 consumption due to soil trafficking. The fractional area of the skid trails to total area calculated in our study was 13.2% (Table 2), assuming a distance of 20 m between skid trails. The annual N2O emissions and CH4 consumption derived from soil compaction were converted into CO2 equivalents using the appropriate factors of global warming potential (GWP). A GWP of 296 was determined for N2O and a GWP of 23 for CH4 (IPCC, 2001).
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Statistical Analysis
The cumulative gas fluxes for each site and treatment were expressed as arithmetic means with standard deviation (n = 6 for the control and TSM and n = 3 for USB, USM, and TSB). The cumulative gas fluxes, bulk densities, pore spaces, and macropores on different sections of the skid trail were compared using analysis of variance. A nonparametric test was selected because trace gas fluxes are not normally distributed. The Mann-Whitney U-Test was used with the factors treatment (control, USB, USM, TSM, and TSB) and spatial replication. The factor spatial replication was considered to be a random factor.
The relations between soil temperature (at a depth of 5 cm), WFPS (at a depth of 5–10 cm) and the trace gas fluxes were tested by applying a stepwise multiple linear regression. Before these analyses were performed, the frequency distributions were tested for normality using the Kolmogrof-Smirnov test. The CH4 fluxes were normally distributed except for the skid trails of the clayey site where they were lognormally distributed (p < 0.05). The N2O fluxes were normally distributed except for the skid trails of the sandy site where a lognormal distribution was observed. The soil temperature and WFPS were normally distributed. Lognormally distributed data were log-transformed before the statistical analyses were run.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Soil trafficking and the slipping of the wheels mixed the humus layer with the mineral soil and created a new soil layer, which had a different organic matter content than that found on the forest floor and in the surface mineral soil (Table 5).
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Soil compaction increased N2O emissions significantly. The high N2O emissions occurring after compaction were restricted to short periods at the sandy loam (Fig. 2) and silty clay loam sites (Fig. 4) whereas emissions at the silt site (Fig. 3) remained high throughout the entire growing season. During the growing season the highest cumulative emissions at the TSM microsites were observed at the silt site (272 mg N2O-N m–2), followed by the sandy loam (216 mg N2O-N m–2) and silty clay loam (116 mg N2O-N m–2) sites (Table 4).
Higher N2O emissions after compaction are probably due to a change in the pore-size distribution (Table 3) affecting the gas exchange between soils and the atmosphere. A macropore volume of <10% is usually considered to be too low to maintain a sufficient supply of oxygen into the soil (Startsev and McNabb, 2001; Xu et al., 1992). This effects a reduction in aerobic processes and therefore may change the gaseous N emissions (Granli and Bøckman, 1994). The reduction in macropore volume will also increase the WFPS. The average WFPS of the compacted soil (TSM) was much higher at the silt and silty clay loam sites than at the sandy loam site (Table 4) and may explain the lower N2O emissions on the sandy loam site. Similarly, the WFPS can be related to the high N2O emissions from the compacted silt site, which showed a constant decrease of the WFPS during the growing season (Fig. 3). However, on the silty clay loam site low N2O emissions (Fig. 4) were observed despite the high WFPS values. The slipping of the wheels probably sealed the surface of the silty clay loam to a greater extent than at the other sites and thus reduced the O2 diffusivity with the result that a higher proportion of N2O was reduced to N2 (Davidson, 1991).
The N2O emissions from the compacted soil were not similar on all parts of the skid trail. Emission values increased from TSM to TSB (Table 4). Compared with TSM the emissions at the border area, TSB, were about 25% (silt site) and 50% (silty clay loam site) higher. However, these differences were not significant. Nevertheless, it can be speculated that lower emission values in the middle of the track were probably related to an increased level of reduction of N2O to N2. Nitrous oxide emissions do not provide an evaluation of the degree of compaction, because under extremely anaerobic conditions the N2/N2O ratio will rapidly alter with small changes in oxygen concentration (Davidson, 1991).
The increase in N2O emissions following soil trafficking was not confined to the compacted area, because higher N2O emissions were observed in the areas adjacent to the impacted ones (USB and USM, Fig. 1) than in the nearby control plot (Table 4). The enhanced N2O emissions from the soils bordering to the trafficked soil may partly be explained by a lateral diffusion of N2O from the compacted soil to the undisturbed soil forced by the high N2O concentration in the compacted soil.
Soil compaction did not change the N2O emissions in the autumn and winter months. It has been observed that soil frost plays a significant role in N2O emissions due to high N2O emission rates during soil thawing (Papen and Butterbach-Bahl, 1999; Teepe et al., 2000). In the present study the effect of frost combined with that of soil compaction was not observed due to the absence of soil freezing at these sites during the measuring period. Low soil temperatures (0–5°C) from November to March resulted in emission values of <10 µg N2O-N m–2 h–1 for both the trafficked and the undisturbed soil (control).
A small fraction of the temporal variability in N2O emissions released from the undisturbed soils (control) was related to soil temperature and soil moisture conditions. The variance in N2O emissions of the trafficked soil explained by soil temperature was 13% at the silty clay loam site, 25% at the silt site and 39% at the sandy loam site. The variance explained in relation to WFPS in the trafficked soils was 50% at the silt site and <10% at the other two sites.
Effect of Soil Compaction on CH4 Flux
The CH4 consumption values on the sandy loam site (Fig. 2) and silt site (Fig. 3) are some of the highest in northern European soils (Smith et al., 2000). In contrast, the CH4 consumption at the silty clay loam site was much lower during the entire year (Fig. 4) resulting in a cumulative CH4 consumption of 1.5 kg CH4–C ha–1 yr–1. The high CH4 consumption on the sandy loam and silt sites showed a distinct seasonal pattern with maximum rates of 160 µg CH4–C m–2 h–1 (Fig. 2 and 3). The temporal pattern of CH4 consumption in soils depends primarily on the soil water status. Soil temperature plays a minor role because the substrate limitation of the methanotrophic bacteria is primarily controlled by soil water content (Castro et al., 1994; Crill et al., 1994; King and Adamsen, 1992). However, approximately 35% (sandy loam and silty clay loam sites) to 44% (silt site) of the temporal variation of the CH4 consumption could be explained by soil temperature in our study.
The correlation between CH4 consumption and soil moisture provided different results for the three sites. For the silt site the moisture content of the surface mineral soil could explain 55% of the temporal variation in CH4 consumption with low values for the silty clay loam site (r2 = 34%) and very low for the sandy loam site (r2 = 9%). The degrees of explanation of the variability in CH4 consumption explained by both soil temperature and moisture parameters, as obtained using multiple regression equations, were 87% (silt), 49% (silty clay loam), and 41% (sandy loam). The importance of the WFPS as a regulator for CH4 consumption was evident from field scale data, in which cumulative values of CH4 consumption increased with decreasing mean values of WFPS during the growing season; whereas the WFPS of the various sites conformed to the following order: silty clay loam > silt > sandy loam (Table 4).
Soil compaction by trafficking, for example, at TSM, caused a large reduction in the CH4 consumption. The extent of reduction in the cumulative value of CH4 consumption during the growing season at different sites was as follows: –92% (sandy loam), –77% (silt), and greater than –100% (silty clay loam). The silty clay loam became a source of CH4 emissions in June when WFPS increased to approximately 100% (Fig. 4). A similar large reduction in CH4 consumption after compaction has been found in agricultural soils (Hansen et al., 1993; Ruser et al., 1998; Sitaula et al., 2000).
The decrease in the CH4 consumption in compacted soil is probably directly related to a reduction in macropore volume (Table 3). Both parameters, CH4 consumption and macropore volume, were significantly different for the trafficked and untrafficked soil (Table 3 and 4) at each site. The reduction in macropore volume caused an increase in WFPS (Fig. 2–4), which, in turn, reduced CH4 consumption due to retardation of CH4 diffusion from the atmosphere into the soil. However, at the sandy loam site even a small increase in the WFPS significantly reduced the CH4 consumption, indicating that factors associated with the soil surface are probably more important than those that could be determined by bulk density measurements. For example, the soil surface may be sealed due to soil compaction and due to soil surface disturbance associated with the wheel slip, which significantly reduces diffusion processes.
In the trafficked area, the reduction of CH4 consumption differed in the TSM and the TSB areas of the wheel tracks without any clear trend (Table 4). At the silty clay loam site the cumulative CH4 flux for TSM calculated for the growing season indicated CH4 emissions; whereas in the TSB the cumulative value was zero. The cumulative value of CH4 consumption at the border and between the trafficked soil (USB and USM) was reduced by approximately 50% (44–60%) compared with the values at the control site.
Effect of Wood Harvesting on N2O and CH4 Flux
The impact of wood harvesting on the fluxes of N2O and CH4 were calculated for the harvested area by considering the different fractional areas of the skid trail and by assuming a distance of 20 m between the skid trails (Table 2). The decrease in the CH4 consumption on the skid trails reduced the cumulative CH4 consumption of the total area only by <10%, whereas the N2O emissions doubled. The cumulative values of increase due to compaction were approximately 0.1 N2O-N kg ha–1 yr–1 for the sandy loam and silty clay loam sites and 0.3 kg N2O-N kg ha–1 yr–1 for the silt site.
The estimate of the trace gas flux due to trafficking was based on a small compacted area of approximately 6% of the total area. Soil compaction on a higher fraction of the harvested area is often observed on forests harvested by clear cutting and will change the trace gas flux considerably when calculated on a whole field scale. Therefore, we recommend that the fraction of trafficked area should be restricted to <10%. This can be achieved by establishing a permanent skid trail system with a distance of 20 m between the trails used for all harvest operations.
The greenhouse effect of CH4 and N2O fluxes were calculated as CO2 equivalents using GWP (IPCC, 2001). For the undisturbed area (control) (Table 3) the CO2 equivalents calculated from the N2O emissions were as follows: 49 (silty clay loam), 60 (sandy loam), and 158 kg CO2 ha–1 yr–1 (silt); and the CO2 equivalents calculated from the CH4 consumption were: 47 (silty clay loam), 157 (silt) and 249 kg CO2 ha–1 yr–1 (sandy loam). Assuming that soil compaction was restricted to 6% of the harvested area, the CH4 consumptions expressed in CO2 equivalents would be reduced by 4, 11, and 13 kg CO2 ha–1 yr–1 and the CO2 equivalents of the N2O emissions would be enhanced by 57 kg (silty clay loam), 60 kg (sandy loam), and 134 kg (silt) CO2 ha–1 yr–1. Thus, the total impact of soil trafficking on the greenhouse effect was <150 kg CO2 ha–1 ha–1 yr–1. This indicates that the impact of harvesting on the greenhouse gas emissions remains small, but this needs to be viewed with respect to other C fluxes on the harvested site, for example, the effects of woody detritus left on the site (Harmon et al., 1996) or the amount of harvested wood. In Germany the latter amounts to approximately 3.7 m3 ha–1 yr–1 (BMELF, 1996). The value of approximately 150 kg CO2 equivalents ha–1 yr–1 can also be viewed in relation to 12 Mg CO2 emissions ha–1 yr–1 from forest soils measured in Lower Saxony (Brumme and Beese, 1992).
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ACKNOWLEDGMENTS |
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Received for publication October 22, 2002.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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