Industrialization has greatly affected the biogeochemistry of northern forests by increasing the atmospheric deposition of acid and nitrogen (N). In 1990, the US Congress amended the Clean Air Act to ...include tighter emissions regulations; this reduced acid deposition (by >50% in this study), but did not effectively lower N deposition. Here, we demonstrate that since this legislation was enacted, there have been marked decreases in sulfur (−16%), calcium (−17%), and aluminum (−42%) concentrations in sugar maple (
Acer saccharum
) foliage across the Upper Great Lakes region of the US, signaling a declining influence of acid deposition. In contrast, N deposition has persistently been over 75% greater than the amount of N needed to offset annual plant N sequestration, creating increases in N availability and soil N leaching. Recent emissions regulations will reduce N deposition somewhat, but further increases in soil N availability and leaching are likely. Policy decisions regarding N deposition will have to weigh increased carbon storage against negative impacts on water quality and species diversity.
Elevated concentrations of atmospheric CO₂ and tropospheric O₃ will profoundly influence future forest productivity, but our understanding of these influences over the long-term is poor. Leaves are ...key indicators of productivity and we measured the mass, area, and nitrogen concentration of leaves collected in litter traps from 2002 to 2008 in three young northern temperate forest communities exposed to elevated CO₂ and/or elevated O₃ since 1998. On average, the overall effect of elevated CO₂ (+CO₂ and +CO₂+O₃ versus ambient and +O₃) was to increase leaf mass by 36% whereas the overall effect of elevated O₃ was to decrease leaf mass by 13%, with similar effects on stand leaf area. However, there were important CO₂ x O₃ x year interactions wherein some treatment effects on leaf mass changed dramatically relative to ambient from 2002 to 2008. For example, stimulation by the +CO₂ treatment decreased (from +52 to +25%). whereas the deleterious effects of the +O₃ treatment increased (from -5 to -18%). In comparison, leaf mass in the +CO₂+O₃ treatment was similar to ambient throughout the study. Forest composition influenced these responses: effects of the +O₃ treatment on community-level leaf mass ranged from +2 to -19%. These findings are evidence that community composition, stand development processes, CO₂, and O₃ strongly interact. Changes in leaf nitrogen concentration were inconsistent, but leaf nitrogen mass (g m¯²) was increased by elevated CO₂(+30%) and reduced by elevated O₃(-16%), consistent with observations that nitrogen cycling is accelerated by elevated CO₃ but retarded by elevated O₃.
Human activity has increased the amount of N entering terrestrial ecosystems from atmospheric NO
3
− deposition. High levels of inorganic N are known to suppress the expression of phenol oxidase, an ...important lignin-degrading enzyme produced by white-rot fungi. We hypothesized that chronic NO
3
− additions would decrease the flow of C through the heterotrophic soil food web by inhibiting phenol oxidase and the depolymerization of lignocellulose. This would likely reduce the availability of C from lignocellulose for metabolism by the microbial community. We tested this hypothesis in a mature northern hardwood forest in northern Michigan, which has received experimental atmospheric N deposition (30
kg
NO
3
−–N
ha
−1
y
−1) for nine years. In a laboratory study, we amended soils with
13C-labeled vanillin, a monophenolic product of lignin depolymerization, and
13C-labeled cellobiose, a disaccharide product of cellulose degradation. We then traced the flow of
13C through the microbial community and into soil organic carbon (SOC), dissolved organic carbon (DOC), and microbial respiration. We simultaneously measured the activity of enzymes responsible for lignin (phenol oxidase and peroxidase) and cellobiose (β-glucosidase) degradation. Nitrogen deposition reduced phenol oxidase activity by 83% and peroxidase activity by 74% when compared to control soils. In addition, soil C increased by 76%, whereas microbial biomass decreased by 68% in NO
3
− amended soils.
13C cellobiose in bacterial or fungal PLFAs was unaffected by NO
3
− deposition; however, the incorporation of
13C vanillin in fungal PLFAs extracted from NO
3
− amended soil was 82% higher than in the control treatment. The recovery of
13C vanillin and
13C cellobiose in SOC, DOC, microbial biomass, and respiration was not different between control and NO
3
− amended treatments. Chronic NO
3
− deposition has stemmed the flow of C through the heterotrophic soil food web by inhibiting the activity of ligninolytic enzymes, but it increased the assimilation of vanillin into fungal PLFAs.
To determine the importance of microorganisms in regulating the retention of anthropogenic NO3
-, we followed the belowground fate and flow of15NO3
-in a mature northern hardwood forest, dominated by ...Acer saccharum Marsh. Total recovery of added15N (29.5 mg15N/m2as NANO3) in inorganic N, microbial immobilization in forest floor and soil microbial biomass, soil organic matter, and root biomass pools (0-10 cm depth) was 93% two hours following application of the15NO3
-but rapidly dropped to ∼ 29% within one month, presumably due to movement of the isotope into other plant tissues or deeper into soil. Microbial immobilization was initially (i.e., at 2 h) the largest sink for15NO3
-(21% in forest floor; 16% in soil microbial biomass). After one month, total15N recovery varied little (24-18%) throughout the remainder of the growing season, suggesting that the major N transfers among pools occurred relatively rapidly. At the end of the four-month experiment, the main fates of the15N label were in soil organic matter (7%), root biomass (6%), and N immobilized in forest floor and soil microbial biomass (6%). Temporal changes in the15N enrichment (atom% excess15N) of plant and soil pools during the first month of the experiment indicated the dynamic nature of NO3
-cycling in this forest. The15N enrichment of soil microbial biomass and the forest floor significantly increased two hours after isotope additions, suggesting rapid microbial immobilization of NO3
-. In contrast, the15N enrichment of soil organic matter did not peak until day 1, presumably because much of the added15N cycled through microorganisms before becoming stabilized in soil organic matter, or it directly entered soil organic matter via physical processes. Furthermore, the15N enrichment of root biomass (<0.5-mm diameter and 0.5-2.0 mm diameter) was greatest between day 7 and day 28, following significant increases in the15N enrichment of soil organic matter (day 1) and, more importantly, NH4
+(day 2). From these data we conclude that microorganisms are immediate, short-term sinks for anthropogenic NO3
-. Although the long-term fate of NO3
-additions to this forest is likely in soil organic matter and plants, the cycling of N through microorganisms appears to be the major short-term factor influencing patterns of NO3
-retention in this ecosystem.
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 degrees 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.
Elevated concentrations of atmospheric carbon dioxide (CO2) and tropospheric ozone (O3) have the potential to affect tree physiology and structure and hence forest water use, which has implications ...for climate feedbacks. We investigated how a 40% increase above ambient values in CO2 and O3, alone and in combination, affect tree water use of pure aspen and mixed aspen-birch forests in the free air CO2-O3 enrichment experiment near Rhinelander, Wisconsin (Aspen FACE). Measurements of sap flux and canopy leaf area index (L) were made during two growing seasons, when steady-state L had been reached after more than 6 years of exposure to elevated CO2 and O3. Maximum stand-level sap flux was not significantly affected by elevated O3, but was increased by 18% by elevated CO2 averaged across years, communities and O3 regimes. Treatment effects were similar in pure aspen and mixed aspen-birch communities. Increased tree water use in response to elevated CO2 was related to positive CO2 treatment effects on tree size and L (+40%). Tree water use was not reduced by elevated O3 despite strong negative O3 treatment effects on tree size and L (-22%). Elevated O3 predisposed pure aspen stands to drought-induced sap flux reductions, whereas increased tree water use in response to elevated CO2 did not result in lower soil water content in the upper soil or decreasing sap flux relative to control values during dry periods. Maintenance of soil water content in the upper soil in the elevated CO2 treatment was at least partly a function of enhanced soil water-holding capacity, probably a result of increased organic matter content from increased litter inputs. Our findings that larger trees growing in elevated CO2 used more water and that tree size, but not maximal water use, was negatively affected by elevated O3 suggest that the long-term cumulative effects on stand structure may be more important than the expected primary stomatal closure responses to elevated CO2 and O3 in determining stand-level water use under possible future atmospheric conditions.
Root respiration may account for as much as 60% of total soil respiration. Therefore, factors that regulate the metabolic activity of roots and associated microbes are an important component of ...terrestrial carbon budgets. Root systems are often sampled by diameter and depth classes to enable researchers to process samples in a systematic and timely fashion. We recently discovered that small, lateral roots at the distal end of the root system have much greater tissue N concentrations than larger roots, and this led to the hypothesis that the smallest roots have significantly higher rates of respiration than larger roots. This study was designed to determine if root respiration is related to root diameter or the location of roots in the soil profile. We examined relationships among root respiration rates and N concentration in four diameter classes from three soil depths in two sugar maple (Acer saccharum Marsh.) forests in Michigan. Root respiration declined as root diameter increased and was lower at deeper soil depths than at the soil surface. Surface roots (0-10 cm depth) respired at rates up to 40% greater than deeper roots, and respiration rates for roots < 0.5 mm in diameter were 2.4 to 3.4 times higher than those or roots in larger diameter classes. Root N concentration explained 70% of the observed variation in respiration across sites and size and depth classes. Differences in respiration among root diameter classes and soil depths appeared to be consistent with hypothesized effects of variation in root function on metabolic activity. Among roots, very fine roots in zones of high nutrient availability had the highest respiration rates. Large roots and roots from depths of low nutrient availability had low respiration rates consistent with structural and transport functions rather than with active nutrient uptake and assimilation. These results suggest that broadly defined root classes, e.g., fine roots are equivalent to all roots < 2.0 mm in diameter, do not accurately reflect the functional categories typically associated with fine roots. Tissue N concentration or N content (mass x concentration N) may be a better indicator of root function than root diameter.
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