Forest Management Effects on Surface Soil Carbon and Nitrogen Jennifer

Changes in surface soil C and N can resuh from forest management practices and may provide an index of impacts on long-term site productivity. Soil C and N were measured over time for five watersheds in the southern Appalachians: two aggrading hardwood forests, one southand one north-facing, undisturbed since the 1920s; a white pine (Pinus strobus L.) plantation planted in 1956; and two regenerating hardwood forests, a whole-tree harvest in 1980, and a commercial sawlog harvest in 1977. Soils on harvested watersheds were sampled before and for =15 yr after harvest. Surface soil C concentration on the undisturbed watersheds varied significantly among sample years. Concentrations fluctuated on the south-facing and decreased on the north-facing watershed. The pattern for total N was similar. Total N decreased significantly on the north-facing but was stable on the southfacing watershed. In the white pine plantation, C increased while N concentrations decreased during the 20-yr period. Soil C and N concentrations generally declined the first year following whole-tree harvest. Fourteen years after cutting, C remained stable, while N was greater compared with reference watershed soils. The commercial sawlog harvest resulted in large increases in surface soil C and N concentrations immediately after cutting. Carbon levels remained elevated 17 yr following cutting. Our data suggest that the forest management practices examined do not result in long-term decreases in soil C and N. However, the high interannual variation on all watersheds suggests that care must be taken in selecting control sites to determine long-term treatment impacts. V AND SOILS are inextricably linked (Jenny, 1941). As a result, soil properties are partially dependent on forest type (Alban, 1982; David et al., 1988; Grigal and Ohmann, 1992). Forest management practices directly affect vegetation and may also change soil properties such as soil organic matter. Powers (1989) suggested that forest productivity is directly related to soil organic matter content. Soil organic matter content can change due to normal site processes with time such as succession and biomass accumulation (Snyder and Harter, 1984). Changes can also occur through anthropogenic manipulations such as species conversion (Richter et al., 1994; Alban, 1982), or harvesting (MuellerHarvey et al., 1985; Mroz et al., 1985; Kraske and Fernandez, 1993). Soil C or N increases or decreases may correspond to changes in potential site productivity. The objective of this study was to measure the longterm trends in soil C and N on undisturbed mixed hardwood forests and to examine the effects of several forest management practices on soil C and N. Sample periods range from 14 to 20 yr and management regimes included: (i) undisturbed, southand north-facing reference watersheds; (ii) species conversion, a white pine plantation planted after felling all hardwood vegetation, all logged material was left on site; (iii) even-aged management, a commercial whole-tree harvest, all aboveUSD A Forest Service, Coweeta Hydrologic Laboratory, 3160 Coweeta Lab Road, Otto, NC. Received 19 April 1996. ''Corresponding author ([email protected]). Published in Soil Set. Soc. Am. J. 61:928-935 (1997). ground woody material was felled and no logging residue remained; (iv) even-aged management, a commercial sawlog harvest using cable-yarding technique, logging residue was left in place and disturbance to the forest floor was minimized. MATERIALS AND METHODS Site and Treatment Description All study sites are in the Coweeta Hydrologic Laboratory, a 2180-ha USDA Forest Service facility in the southern Appalachians of western North Carolina. The climate is characterized by 1900 mm of precipitation annually with most months receiving at least 100 mm. The growing season extends from early May to early October. Highest mean monthly temperatures are in June through August (20°C) and lowest in December through January (5°C). North-Facing Reference Watershed North-facing reference watershed 18 is 13 ha, with mixedhardwood vegetation. The NREF was selectively logged in the 1920s and subsequently has not been disturbed by humans. Basal area of the forest was =26 nr ha" ' and the overstory is dominated by three Querciisspp., red maple (Acer nibrurn L.), tulip-poplar (Liriodendron tulipifera L.), and pignut hickory [Carya glabra (Miller) Sweet]. Rosebay rhododendron (Rhododendron maximum L.) and mountain laurel (Kalmia latifolia L.), both evergreen species, are the dominant understory species. They make up 7.4 and 5.1% of the total basal area, respectively (Day et al., 1988). Elevation ranges from 726 to 993 m, and slope averages =50%. Two soil series were sampled that represent =90% of the watershed: the Saunook series, a fine-loamy, mixed, mesic Humic Hapludult, found at streamside positions, and CoweeEvard complex soils, fine-loamy, mixed-oxidic, rnesic, Typic Hapludult, found on ridge positions. South-Facing Reference Watershed South-southeast-facing reference watershed 2 is 12 ha. with mixed-hardwood vegetation. The SREF was selectively logged in the 1920s and subsequently has not been disturbed by humans. Vegetation is similar to the NREF described above. Elevation ranges from 709 to 1004 m with slope average of 60%. Soil series on the watershed include the Fannin series (fine-loamy, micaceous, mesic Typic Hapludult) in side-slope position and the Cullasaja-Tuckasegee complex (loamy-skeletal/fine-loamy, mixed, mesic Typic Haplumbrept) at streamside. All plots were on the Fannin soil type, which occupies =60% of the watershed. White Pine Plantation The white pine plantation occupies a north-facing watershed adjacent to NREF. The 13-ha hardwood forest was clearAbbreviations: SREF. south-facing reference watershed: NREF. northfacing reference watershed: WP, white pine planta t ion watershed: CSH, commercial sawlog harvest watershed: WTH, whole-tree harvest watershed: GLM, general l inear models: TKN, tota l Kjeldahl nitrogen: PEN. total N determined using Perkin-Elmer 2400 CRN analyzer: DON, dissolved organic nitrogen.