Search

Invasive species frequently need to be controlled as part of efforts to reestablish native species on degraded sites. While the effectiveness of differing control methods are often reported, the impacts these methods have on the establishment of a native plant community are often unknown. To determine methods that effectively reduce spotted knapweed ( Centaurea stoebe ) while enhancing native species establishment, we tested 12 treatment combinations consisting of an initial site preparation (mowing, mowing + clopyralid, or mowing + glyphosate), in factorial combination with annual adult knapweed hand pulling and/or burning. We established 48 plots and applied site preparation treatments during summer 2008, seeded 23 native forbs and grasses during spring 2009, pulled adult knapweed annually from 2009–2012, and burned in the early spring 2012. During July of 2011 and 2012, percent cover of all species was visually estimated. By 2011, seeded species had established in all treatment plots, including plots that retained greater than 50% knapweed cover, indicating that native species successfully established despite knapweed dominance. Mowing alone had no longterm impacts on community development. Clopyralid favored non-native grass establishment, while glyphosate encouraged non-native forbs. Clopyralid had minimal impacts on native forb establishment, but did effectively control knapweed. Pulling reduced knapweed cover, increased non-native grass cover and enhanced native species establishment. Burning had little impact, possibly due to low intensity and unseasonable weather. On the heavily invaded site we studied, combining the use of clopyralid with hand pulling effectively controlled knapweed and favored the establishment of seeded native grasses and forbs.

Extensive areas in the upper Midwest have been invaded by spotted knapweed, and effective management strategies are required to reestablish native plant communities. We examined effects of mowing, mowing plus clopyralid, or mowing plus glyphosate in factorial combination with hand pulling and burning on knapweed abundances on a knapweed-infested site in western Michigan. We applied mowing and herbicide treatments in summer 2008, and seeded all plots with native grasses and forbs in spring 2009. We conducted the knapweed pulling treatment from 2009 to 2012 in July. The prescribed burn was conducted in April 2012. By 2012, hand pulling reduced adult knapweed densities to 0.57 ± 0.12 m−2(0.053 ± 0.011 ft−2) (mean ± SE), which was 5.8% of nonpulled treatments, juvenile densities to 0.29 ± 0.07 m−2(2.1% of nonpulled treatments), and seedling densities to 0.07 ± 0.06 m−2(2.6% of nonpulled treatments). After 3 yr, hand pulling reduced seed bank densities to 68 ± 26 m−2as compared to 524 ± 254 m−2in nonpulled treatments and 369 ± 66 m−2in adjacent untreated areas of the study site. Without hand pulling, effects of mowing or mowing plus glyphosate were short-lived and allowed knapweed to rapidly resurge. In comparison, although a single mowing plus clopyralid treatment maintained significantly reduced densities of knapweed for 4 yr, by 2012 knapweed biomass in the nonpulled clopyralid treatment was approximately 60% of that in the other nonpulled treatments. Burning had minimal impacts on knapweed densities regardless of treatment combination, probably as a result of low fire intensity. Results demonstrated that persistent hand pulling used as a follow-up to single mowing or mowing plus herbicide treatments can be an effective practice for treating isolated spotted knapweed infestations or for removing small numbers of knapweed that survive herbicide applications.

The Pigeon River, a small coolwater stream in western Michigan, has a history of hydrologic, stream habitat, and water quality degradation that led to the loss of its trout population by the late 1980s. After regulatory and watershed management efforts to reduce point- and nonpoint source pollution in the 1990s, the Michigan Department of Natural Resources reinstituted brown trout ( Salmo trutta ) stocking in 2003. As part of these efforts, we monitored water quality in the Pigeon River each fall between 1996 and 2008, and conducted stream surveys in 2006 and 2007 to evaluate the fish community and outcome of trout stocking. Water quality tended to improve and stabilize through time, although point- and nonpoint source pollution still contributed to water quality problems. Hydrologic instability, caused by wetland drainage, agricultural land use, and irrigation withdrawals from the lower mainstream, created periods of environmental stress. As a result, the fish community of the Pigeon River was dominated by common tolerant warmwater species, typical of agricultural watersheds in southern Michigan. Nonetheless, brown trout surviving from initial stockings in 2003 and 2004 had attained lengths of between 18 and 24 inches by 2007, suggesting the thermal regime, water quality, stream habitat, and forage base of macroinvertebrates and small fish were suitable to maintain a stocked brown trout population. Continued efforts to improve water quality, protect in-stream habitat, reduce high stormflows, and maintain adequate summer baseflows are needed to fully restore environmental conditions for the native fish community and stocked brown trout in the Pigeon River.

Municipal solid waste landfill leachate must be removed and treated to maintain landfill cover integrity and to prevent contamination of surface and ground waters. From 2003 to 2007, we studied an onsite disposal system in Ottawa County, Michigan, where leachate was spray irrigated on the vegetated landfill cover. We established six 20-m-diameter circular experimental plots on the landfill; three were spray irrigated as part of the operational system, and three remained as untreated control plots. We quantified the effects of leachate application on soil properties, soil solution chemistry, vegetative growth, and estimated solute leaching. The leachate had high mean levels of electrical conductivity (0.6-0.7 S m(-1)), Cl (760-900 mg L(-1)), and NH(4)-N (290-390 mg L(-1)) but was low in metals and volatile organic compounds. High rates of leachate application in 2003 (32 cm) increased soil electrical conductivity and NO(3)-N leaching, so a sequential rotation of spray areas was implemented to limit total leachate application to

Infestations of the exotic perennial Spotted knapweed (Centaurea maculosa Lam.) hinder the restoration and management of native ecosystems on droughty, infertile sites throughout the Midwestern United States. We studied the effects of annual burning on knapweed persistence on degraded, knapweed-infested gravel mine spoils in western Michigan. Our experiment included 48, 4-m2 plots seeded to native warm-season grasses in 1999 using a factorial arrangement of initial herbicide and fertility treatments. Beginning in 2003, we incorporated fire as an additional factor and burned half of the plots in late April or May for 3 years (2003–2005). Burning increased the dominance of warm-season grasses and decreased both biomass and dominance of knapweed in most years. Burning reduced adult knapweed densities in all 3 years of the study, reduced seedling densities in the first 2 years, and reduced juvenile densities in the last 2 years. Knapweed density and biomass also declined on the unburned plots through time, suggesting that warm-season grasses may effectively compete with knapweed even in the absence of fire. By the end of the study, mean adult knapweed densities on both burned (0.4-m2) and unburned (1.3-m2) plots were reduced to levels where the seeded grasses should persist with normal management, including the use of prescribed fire. These results support the use of carefully timed burning to help establish and maintain fire-adapted native plant communities on knapweed-infested sites in the Midwest by substantially reducing knapweed density, biomass, and seedling recruitment and by further shifting the competitive balance toward native warm-season grasses.

A long-term field experiment was initiated to simulate chronic atmospheric N deposition, a widespread phenomenon in industrial regions of the world. Eight years of experimental nitrate ( $${\text{NO}}_{\text{3}}^-- $$ ) additions (3 g $${\text{NO}}_{\text{3}}^-- $$ -N m−2 per year) to four different northern hardwood forests located along a 500 km geographic gradient dramatically increased leaching losses of $${\text{NO}}_{\text{3}}^-- $$ -N, dissolved organic carbon (DOC), and dissolved organic nitrogen (DON). During the last two water years, the average increase in solution $${\text{NO}}_{\text{3}}^-- $$ -N and DON leaching from the $${\text{NO}}_{\text{3}}^-- $$ -amended plots was 2.2 g N m−2, equivalent to 72% of the annual experimental N addition. Results indicate that atmospheric N deposition may rapidly saturate some northern hardwood ecosystems across an entire biome in the upper Great Lakes Region of the USA. Changes in soil C and N cycling induced by chronic N deposition have the potential in this landscape to significantly alter the flux of DOC and DON from upland to aquatic ecosystems. Michigan Gradient study site characteristics are similar to those of European forests most susceptible to N saturation.

Ecosystem classification identifies interrelationships within and among the geomorphology, soils, and vegetation that converge to form ecosystems across forest landscapes. We developed a multifactor ecosystem classification system for a 13 000 ha southern Appalachian landscape acquired in 1998 by the South Carolina Department of Natural Resources. Using a combination of multivariate analyses, we distinguished five ecosystem types ranging from xeric oak (Quercus spp.) to mesic eastern hemlock (Tsuga canadensis (L.) Carrière) ecosystems. Ecosystems segregated along geomorphic gradients influencing potential moisture availability, with soil properties such as solum thickness distinguishing among ecosystems occupying similar topographic positions. Our results suggest that different combinations of geomorphic and soil factors interact to form similar ecosystems across the landscape, and a given environmental factor can impact ecosystem development at some constituent sites of an ecosystem type but not at other sites. A regional comparison of ecosystem classifications indicates that environmental variables important for distinguishing ecosystems in the southern Appalachians vary, with Jocassee Gorges characterized by unique suites of environmental complexes. Our study supports the contention that the strengths of ecosystem classification are providing (i) comprehensive information on the interrelationships among ecosystem components, (ii) a foundation from which to develop ecologically based forest management plans, and (iii) an ecological framework in which to conduct future research on specific ecosystem components or processes.

Soil temperature and matric potential influence the physiological activity of soil microorganisms. Changes in precipitation and temperature can alter microbial activity in soil, rates of organic matter decomposition, and ecosystem C storage. Our objective was to determine the combined influence of soil temperature and matric potential on the kinetics of microbial respiration and net N mineralization. To accomplish this, we collected surface soil (0-10 cm) from two northern hardwood forests in Michigan and incubated samples at a range of temperatures (5, 10, and 25°C) and matric potentials (-0.01, -0.15, -0.30, -0.90 and -1.85 MPa) that encompass field conditions. Soils were maintained at each temperature-matric potential combination over a 16-wk laboratory incubation, during which we periodically measured the production of CO 2 and inorganic N. First-order kinetic models described the accumulation of CO 2 and inorganic N and accounted for 96 to 99% of the variation in these processes. First-order rate constants (k) for net N mineralization significantly increased with temperature, but the k for microbial respiration did not increase in a consistent manner; it was 0.107 wk -1 at 5°C, 0.123 wk at 10°C, and 0.101 wk -1 at 25°C. Matric potential did not significantly influence k for either process. Substrate pools for microbial respiration and net N mineralization declined between -0.01 and -0.30 MPa, and the decline was greatest at the highest soil temperature; this response produced a significant temperature-matric potential interaction. We conclude that high rates of microbial activity at warm soil temperatures (e.g., 25°C) are limited by the diffusion of substrate to metabolically active cells. This limitation apparently lessens as physiological activity and substrate demand decline at relatively cooler soil temperature (e.g. 5°C).

Elevated soil temperatures may increase C loss from soils by accelerating microbial respiration and dissolved organic C leaching. The authors evaluated the effect of elevated soil temperatures on C losses from a forest Spodosol by incubating soil cores from surface (Oa + A + E) and subsurface (Bhs) horizons at two seasonal temperature regimes. One regime simulated the normal course of soil temperatures in northern lower Michigan, and the other simulated soil temperatures representing an amount of warming the might occur under some global warming theory calculations. The authors measured the amounts of CO{sub 2}-C respired and dissolved organic C leached from the soil cores during a 33-wk period. Microbial respiration rates, after adjustment for variation in initial rates, were significantly increased by soil warming and were greater in surface than in subsurface horizons. Warming significantly increased cumulative C respired, with greater losses from surface soils as compared with subsurface soils. Mean quantities of dissolved organic C leached, ranging from 2.3 to 3.2 mg C g{sup {minus}1} C, did not differ significantly by soil horizon or temperature regime. Increased microbial respiration in surface soil horizons was the process most responsive to soil warming in the Spodosol samples examined. Whether thismore » is a short-term effect that would disappear once pools of labile C are exhausted, or represents a long-term response to soil warming, remains uncertain.« less

Global climate change may impact the cycling of C, N, and S in forest ecosystems because increased soil temperatures could alter rates of microbially mediated processes. We studied the effects of temperature on microbial respiration and net N and S mineralization in surface soils from four northern hardwood forests in the Great Lakes region. Soil samples were incubated in the laboratory at five temperatures (5, 10, 15, 20, and 25°C) for 32 wk. Headspace gas was analyzed for CO2-C at 2-wk intervals, and soils were extracted to determine inorganic N and S. Cumulative respired C and mineralized N and S increased with temperature at all sites and were strongly related (r2 = 0.67 to 0.90, significant at P = 0.001) to an interaction between temperature and soil organic C. Production of respired C and mineralized N was closely fit by first-order kinetic models (r2 > 0.94, P = 0.001), whereas mineralized S was best described by zero-order kinetics. Contrary to common assumptions, rate constants estimated from the first-order models were not consistently related to temperature, but apparent pool sizes of C and N were highly temperature dependent. Temperature effects on microbial respiration could not be accurately predicted using temperature-adjusted rate constants combined with a constant pool size of labile C. Results suggest that rates of microbial respiration and the mineralization of N and S may be related to a temperature-dependent constraint on microbial access to substrate pools. Simulation models should rely on a thorough understanding of the biological basis underlying microbially mediated C, N, and S transformations in soil.

Sulfate adsorption by forest soils modifies the impact of pollutant deposition on cation leaching processes. We examined relationships among SO²⁻₄ adsorption, soil properties, and seasonal variation in soil solution chemistry at 13 sites representing deciduous forest ecosystems common in the Great Lakes region. Objectives of the study were to test the validity of previously proposed SO²⁻₄ adsorption indices, to examine within- and among-site variability in SO²⁻₄ adsorption potential, and to investigate the effects of seasonal changes in soil solution chemistry on SO⁴₂₋ retention. Mineral soils were sampled by horizon at all sites, and soil solutions were sampled at lower E and lower B horizon boundaries at 10 sites. Proposed indices overpredicted sulfate adsorption in certain SO²⁻₄ releasing subsurface horizons, seriously limiting the applicability of the published regression equations. We developed improved regression equations using the sum of initial extractable SO²⁻₄ and additional SO²⁻₄ adsorbed under laboratory conditions as the dependent variable. Sulfate retention indices predicted by the improved equations were independent of existing levels of extractable SO²⁻₄ and past history of atmospheric SO²⁻₄ deposition. Examination of within- and among-site variability in SO²⁻₄ adsorption potentials suggested that soils need to be grouped tightly on a taxonomic basis for modelling purposes. Seasonal variation in soil solution SO²⁻₄ concentration and fluxes was consistent with an annual cycle of SO²⁻₄ retention and release. Although seasonal patterns in SO²⁻₄ concentrations and fluxes appeared to be controlled by hydrologic and S-cycling processes, the magnitude of SO²⁻₄ fluxes was primarily related to atmospheric SO²⁻₄ deposition rates.

The relationship between fine-textured soil bands and forest productivity was studied by comparing three mixed-oak (Quercusrubra L. and Quercusalba L.) stands that had little or no fine-textured banding with three stands that had bands. The degree to which soil factors could account for differences in productivity between banded and unbanded stands was examined using two methods, one based on field observations (banding codes) and the other based on laboratory textural analysis. Because stand ages were not significantly different, overstory biomass was used as an index of productivity. Mean overstory biomass in the banded stands was 312 Mg/ha, significantly greater than 170 Mg/ha measured in the unbanded stands. Mean percent clay + silt and mean banding code also were significantly higher in banded than in unbanded stands. Linear regression analysis indicated that mean percent clay + silt accounted for 57% of the variation in overstory biomass, whereas mean banding code accounted for 40% of the variation. In the oak stands we studied, variation in productivity can be explained largely by differences in soil texture associated with fine-textured bands. We also found a positive relationship between mean banding code and mean percent clay + silt (r2 = 0.90), which suggests that the field method of quantifying banding can produce values that are highly correlated with soil texture and, by extension, forest productivity.

Stressing agents such as defoliation, adverse climatic conditions, and pollutant deposition have the potential to alter forest nutrition. Several recent instances of sugar maple (Acer saccharum Marsh.) decline and dieback have been associated with foliar nutrients deficiencies. This study assessed foliar nutrient status and cycling in five sugar maple dominated northern hardwood forests along a Great Lakes pollution-climatic gradient. Concentrations and contents in mid-July foliage and litterfall were determined at each site for N, P, S, Ca, Mg, K, Al, Fe, Mn, B, Zn, and Cu. Where differences existed among sites in foliar nutrient concentrations, they could be predicted primarily from soil properties. Two notable exceptions were foliar S, which was strongly related to SO4 deposition, and foliar Al, which could be predicted by a combination of soil nutrient cation availability and SO4 deposition. Nutrient content of mid-July foliage and litterfall increased from northwest to southeast along the gradient for N, S, Mg, Al, Fe, B, and Cu. This was the result of an increase in foliage and litterfall biomass, combined in some cases (S, Al, Fe, and B) with increasing foliar nutrient concentrations. Reproductive effort significantly affected total litter return of all nutrients and 43 to 62% of mid-July foliar N, P, K, and S were conserved through retranslocation prior to litterfall. Sugar maple foliar nutrient concentrations for the five sites revealed no obvious nutrient deficiencies or toxicities, and provide a regional baseline against which the effects of long-term pollutant deposition and other stresses can be assessed in the future. I FOLIAR NUTRITION has been associated with several recent instances of sugar maple decline and dieback. Foliar K and P deficiencies were found in declining stands in the Quebec Appalachians (Bernier and Brazeau, 1988a,b; Pare and Bernier, 1989). Magnesium deficiency existed in sugar maple experiencing severe dieback in the Lower Laurentians of southeastern Quebec (Bernier and Brazeau, 1988c). Natural stresses including insect defoliation and adverse climatic conditions appear to be the primary causes of these deficiencies, but acidic deposition has been suggested as a contributing factor (Bernier et al., 1989). Potential mechanisms through which chronic acidic deposition might alter forest nutrient status include: (i) elevated leaching of cations from foliage (Tukey, 1980; Liechty et al., 1993); (ii) long-term soil cation depletion (Johnson et al., 1985; Johnson and Taylor, 1989; MacDonald et al., 1992); and (iii) mobilization of soil Al (Ulrich et al., 1980; Johnson and Taylor, 1989; Foster, 1989) leading to Al toxicity or impaired uptake of Mg and Ca (Thornton et al., 1986; Dewald et al., 1990). The foliar nutrient status of northern hardwood stands has been documented for certain areas of the northeastern USA and southeastern Canada (Likens and A. J. Burton and K.S. Pregitzer, Dep. of Forestry, Michigan State Univ., E. Lansing, MI 48824-1222; and N.W. MacDonald, School of Natural Resources and Environment, Univ. of Michigan, Ann Arbor, MI 48109-1115. Received 30 Oct. 1992. 'Corresponding author. Published in Soil Sci. Soc. Am. J. 57:1619-1628 (1993). Bormann, 1970; Gosz et al., 1972; Lea et al., 1979a,b, 1980; Leaf, 1973; McLaughlin et al., 1985; Morrison, 1985, 1990; Bernier and Brazeau, 1988a,b,c). Detailed information, however, is lacking for the Great Lakes states of the USA. This region experiences a pronounced gradient of pollutant deposition (Schwartz, 1989; MacDonald et al., 1991), and evidence suggests that deposition is influencing regional S cycling (Pregitzer et al., 1992; Ohmann and Grigal, 1990), soil properties (MacDonald et al., 1991, 1993; David et al., 1988), soil cation leaching (MacDonald et al., 1992), foliar cation leaching (Liechty et al., 1993; MacDonald et al., 1993), and plant tissue chemistry (Bockheim et al., 1989; Ohmann and Grigal, 1990). Other stresses, such as insect defoliation, unusual climatic conditions, and disease outbreaks, also have the potential to alter nutrient status of the region's northern hardwood forests (Bernier et al., 1989). As part of a regional study, this work was undertaken to assess foliar nutrient status and cycling in sugar maple dominated northern hardwood forests of the Great Lakes states. Objectives of the study were to: (i) document current foliar nutrient levels for sugar maple across the region; (ii) compare foliar nutrient concentrations with published values for healthy stands and thresholds for deficiency and toxicity; (iii) determine if sugar maple foliar nutrient status can be predicted from soil, pollutant deposition, and climatic variables; and (iv) calculate nutrient contents in midJuly foliage, litterfall, and retranslocation for northern hardwood forests across the region. The results of the study provide a regional baseline against which future evaluations of sugar maple nutrient status can be made and should prove useful to current efforts at modeling nutrient and C flow on a regional scale.

Relationships among pollutant (H+, SO42−, NO3−) deposition, throughfall ionic fluxes, and soil properties were examined at six oak forest sites in the southern Great Lakes region. At each site, precipitation, throughfall, and soil samples were collected and chemically analyzed. Sulfate and NO3− deposition increased from southern Michigan to northern Ohio in both precipitation and throughfall. Throughfall H+ fluxes also increased between these two areas. Throughfall fluxes of H+, SO42−, NO3−, and Ca2+ were significantly related to wet pollutant deposition. Throughfall Ca2+ and Mg2+ fluxes also tended to reflect soil cation abundance, but H+ consumption in the canopy was a major contributor to cation fluxes at certain sites. After accounting for differences in inherent soil properties, additional variation in both surface and subsurface soil properties was statistically related to atmospheric deposition. While topographic position and impeded drainage also may be implicated, elevated extractable SO42−, decreased potential to adsorb SO42−, and lower nutrient cation saturation in the solum of the most polluted site were consistent with pollutant deposition impacts. Results suggest that certain oak ecosystems in the southern Great Lakes region have experienced alteration in throughfall chemistry and soil properties as a result of elevated pollutant deposition.

While a number of studies have investigated adsorption and microbial immobilization as sulfate (SO42−) retention mechanisms, few have investigated these processes under field-like conditions on a regional, ecosystem basis. Adsorption and microbial immobilization of SO42− were studied in four northern hardwood stands that span an atmospheric deposition gradient in the Lake States region (5 to 10 kg S•ha−1•year−1). Soil cores collected in spring, summer, and autumn were labeled with 35SO42− to trace the flux of S between physical and biological sinks, and to investigate seasonal variation in sink strength. Intact soil cores were injected with Na235SO4 and incubated for 8 d in the laboratory at field temperature to study rates of adsorption and microbial immobilization. The amount of 35S recovered within these pools was significantly different between surface and subsurface soil horizons. Microbial immobilization was the dominant S sink in the A + E horizon, whereas adsorption was the most important S sink in the B horizon. During the 8-d incubation, the proportion of 35S that was immobilized in the A horizon (49% of applied 35S) was equivalent to the proportion of 35S adsorbed in the B horizon (47% of applied 35S). Microbial immobilization sequestered an additional 25% of the applied 35S in the B horizon. Adsorption and microbial immobilization were not significantly different among sampling dates. Sulfur retention in forested ecosystems should be viewed as a combination of geochemical and microbially mediated processes. However, given current levels of S deposition at these sites, neither process seems to represent a significant mechanism for long-term S retention.

Emissions of sulfur (S) and nitrogen (N) oxides in the midwestern and northeastern United States result in pronounced regional gradients of acidic deposition. The objective of this study was to determine the extent to which atmospheric deposition alters the uptake and cycling of S and N in five analogous northern hardwood forests located along one of the most pronounced regional gradients of SO42−-S and NO3−-N deposition in the United States. We tested the hypothesis that acidic deposition would alter foliar S and N ratios and nutrient cycling in aboveground litter fall. Sulfate in both wet deposition and throughfall increased by a factor of two across the 800-km deposition gradient. The July concentration of S in sugar maple (Acersaccharum Marsh.) leaves increased from about 1600 μg•g−1 at the northern research sites to 1800–1900 μg•g−1 at the southern sites. Differences in leaf litter S concentration were even more pronounced (872–1356 μg•g−1), and a clear geographic trend was always apparent in litter S concentration. The 3-year average S content of leaf litter was 63% greater at the southern end of the pollution gradient. Nitrate and total N deposition were also significantly greater at the southern end of the gradient. The concentration of N in both summer foliage and leaf litter was not correlated with N deposition, but the content of N in leaf litter was significantly correlated with N deposition. The molar ratios of S:N in mid-July foliage and leaf litter increased as atmospheric deposition of SO42−-S increased. Ratios of S:N were always much greater in leaf litter than in mid-July foliage. The molar ratios of S:N retranslocated from the canopies of these northern hardwood forests were less than those in mid-July foliage or litter fall and showed no geographic trend related to deposition, suggesting that S and N are retranslocated in a relatively fixed proportion. Significant correlations between SO42−-S deposition and foliar S concentration, S cycling, and the molar ratio of S:N in foliage suggest that sulfate deposition has altered the uptake and cycling of S in northern hardwood forests of the Great Lakes region.

A gradient of H + , SO 2-4, and NO - 3 deposition across the Great Lakes region raised concerns over impacts on soil solution chemistry and ion leaching in regional forest ecosystems. Ten study sites representing northern hardwood and oak ecosystems were established across the gradient of increasing deposition from Minnesota to Ohio. Lysimeters were installed at lower E and lower B horizon boundaries at each site and sampled over a 2-yr period.

A pronounced air-pollution gradient exists across the Great Lakes region, with deposition of SO₄, NO₃, and H increasing from northern Minnesota to southern lower Michigan. Soils at six northern hardwood sites along this gradient were examined to characterize soil physical and chemical properties relevant to retention of pollutants, and to investigate the impact of differences in pollutant loading on soil chemical properties. Three randomly located pedons at each site were described and sampled. Soils at all sites were classified in closely related subgroups within the Spodosol order (Entic Haplorthods, Typic Haplorthods, Alfic Haplorthods, and Alfic Fragiorthods). Cation-exchange capacity and SO₄-adsorption potential tended to decrease from north to south along the gradient, largely related to trends in inherent soil properties. In upper B horizons, additional significant positive deposition effects on water-soluble and adsorbed SO₄, and negative deposition effects on SO₄-adsorbing potentials were consistent with hypothesized pollutant impacts on soil SO₄ pools and soil capacities to retain additional atmospheric SO₄. Exchangeable nutrient-cation contents (Ca, Mg, K) in upper B horizons tended to decrease as pollutant deposition increased. Regression analyses suggested that observed trends in nutrient-cation contents were primarily related to inherent soil properties such as clay, organic matter, and cation-exchange capacity. Differential effects of pollutant deposition and natural acidification processes on nutrient-cation reserves could not be separated at this time.

In a previous study, jack pine (Pinus banksiana Lambert) seedlings grown under pH 2.5 simulated rain had larger shoot/root ratios and altered nutrition compared with seedlings grown with pH 4.7 rain. The objective of this study was to determine if these differences in initial seedling characteristics produced long-term effects on survival or growth of outplanted seedlings. Significantly (P < 0.05) greater diameter increment (4.3 vs. 4.0 cm) between ages 6 and 10 of jack pine treated with pH 2.5 rain as seedlings was consistent with a carryover effect from the initial treatments. However, no differences between treatments in jack pine diameter, height, or survival persisted to age 14. Results support recent recommendations that extended measurement periods may be necessary to fully assess the long-term effects of pollutant increases or decreases on growth of immature trees.

Microbial decomposition processes are typically described using first-order kinetics, and the effect of elevated temperature is modeled as an increase in the rate constant. However, there is experimental data to suggest that temperature increases the pool size of substrate C available for microbial respiration with little effect on first-order rate constants. We reasoned that changes in soil temperature alter the composition of microbial communities, wherein dominant populations at higher temperatures have the ability to metabolize substrates that are not used by members of the microbial community at lower temperatures. To gain insight into changes in microbial community composition and function following soil warming, we used molecular techniques of phospholipid fatty acid (PLFA) and lipopolysaccharide fatty acid (LPS-OHFA) analysis and compared the kinetics of microbial respiration for soils incubated from 5 to 25°C. Substrate pools for microbial respiration and the abundance of PLFA and LPS-OHFA biomarkers for Gram-positive and Gram-negative bacteria differed significantly among temperature treatments, providing evidence for a shift in the function and composition of microbial communities related to soil warming. We suggest that shifts in microbial community composition following either large seasonal variation in soil temperature or smaller annual increases associated with global climate change have the potential to alter patterns of soil organic matter decomposition by a mechanism that is not considered by current simulation models.