Organic Matter Decomposition following Harvesting and Site Preparation of a Forested Wetland

Organic matter accumulation is an important process that affects ecosystem function in many northern wetlands. The cotton strip assay (CSA) was used to measure the effect of harvesting and two different site preparation treatments, bedding and trenching, on organic matter decomposition in a forested wetland. A Latin square experimental design was used to determine the effect of harvesting, site preparation, and relative position within the wetland on organic matter decomposition at soil depths of 5, 10, and 20 cm. Repeated measures analysis of variance was used to test for treatment effects on organic matter decomposition, soil temperature, and soil oxidation depth. Cellulose decomposition increased at each soil depth as site disturbance increased, with bedding > trenching > whole-tree harvest > reference. The cellulose decomposition response was correlated with changes in soil temperature; the temperature coefficient Qlo equaled 6.0, which is greater than previously reported values. Position within the wetland relative to an adjoining river affected the decomposition and soil oxidation depth. Because the rate of decomposition is strongly controlled by temperature, higher rates of organic matter decay are expected to continue on harvested and regenerated sites until canopy closure reduces soil temperature. 0 RGANIC MATTER DECOMPOSITION inborealwet lands is controlled primarily by soil temperature, moisture, fertility, and organic matter quality (Heal et al., 1981; Oades, 1988). Nutrients mineralized from that organic matter comprise the primary nutrient supply for plant uptake in most ecosystems (Damman, 1978; Richardson, 1978; Van Cleve et al., 1983; Van Cleve and Yarie, 1986). In undisturbed wetlands, nutrient mineralization is balanced by microbial immobilization and plant uptake, resulting in little leaching loss (Hemond, 1980; Verry and Timmons, 1982). Disturbance of wetland vegetation and soil increases the rate of organic matter decomposition, usually by altering the soil moisture or temperature regime, or by increasing substrate availability to soil microorganisms (Armentano and Menges, 1986; Trettin et al., 1995). Understanding how varying degrees of disturbance affect organic matter decomposition is fundamental to constructing C balances of wetland ecosystems, modeling decomposition processes (Meentemeyer, 1978; Armentano and Menges, 1986; Ineson et al., 1988; Eijsackers and Zehnder, 1990), and evaluating the potential for C and nutrient leaching. Artificial drainage has been used as a basis for determining the effect of disturbance on organic matter decomposition in forested and nonforested wetlands (Armentano and C.C. Trettin, Center for Forested Wetlands Research, USDA Forest Service, 2730 Savannah Hwy., Charleston, SC 29414; M. Davidian, Dep. of Statistics, North Carolina State Univ., Box 8203, Raleigh, NC 27695; M.F. Jurgensen, School of Forestry and Wood Products, Michigan Technological Univ., 1400 Townsend Dr., Houghton, MI; and R. Lea, College of Forest Resources, North Carolina State Univ., Box 8002, Raleigh, NC 27695. Received 10 Aug. 1994. *Corresponding author (trettinc@ cofc.edu). Published in Soil Sci. Sot. Am. J. 60:1994-2003 (1996). Menges, 1986). However, more common, less intensive disturbances of forested wetlands, such as timber harvesting and regeneration practices that involve site preparation, may have larger cumulative effects than drainage due to the higher frequency and broader aerial extent of those silvicultural practices. Silvicultural practices in wetlands can affect organic matter decomposition rate and C pool size by changing soil temperature, moisture, and substrate availability, and by physical displacement of the forest floor. However, few studies have considered the effects of silvicultural practices on decomposition in forested wetlands. In the southeastern USA, Mader (1990) measured increased organic matter decomposition following clear-cutting in bottomland hardwoods. Similarly, Haines et al. (1975) reported that bedding, a site preparation practice that is used to create an elevated planting bed, increased soil organic matter decomposition. Several studies in Europe and Canada have considered the impacts of silviculture in association with forest soil drainage systems. Organic matter decomposition increased as a result of drainage and harvesting (Lahde, 1969; Lieffers, 1988), which usually resulted in a net loss of soil C from the wetland (Trettin et al., 1995). We are aware of no studies on organic matter decomposition response to silvicultural practices in northern wetlands that do not involve drainage. Litter bags are usually used in studies to investigate soil organic matter decomposition. However, due to the important effect of substrate quality on litter decomposition (Co&on and Butterfield, 1978; Swift et al., 1979, p. 118-163), use of a standard substrate is more appropriate for comparing decomposition among different sites (Heal et al., 1981). The CSA provides an effective index of organic matter decomposition by employing a uniform substrate whose degradation is related to environmental factors and soil conditions (French, 1988; Hill et al., 1988). This method cannot be used to quantify the decay of native cellulose because of structural differences in organic matter from native vegetation compared with the cotton substrate (French, 1988). However, results from the CSA and other cellulitic substrates have been positively related to decomposition of native organic matter (Fox and Van Cleve, 1983; Hopkins et al., 1990). The CSA method provides a relative measure of site condition effects on cellulose decomposition (Latter and Harrison, 1988), and it has been used effectively to characterize decomposition potential among different forest soil conditions (Mader, 1990) and sites (Heal et al., 1981; Hill et al., 1985; Maltby, 1988; Bridgham et al., 1991a). Our objective was to test the hypothesis that silvicultural practices in a sub-boreal wetland increases organic matter decomposition, and that the increase in decomposition corresponds to the relative degree of soil distur-