Grantee Research Project Results
Final Report: Carbon Exchange Dynamics in a Temperate Forested Watershed: A Laboratory and Field Multidisciplinary Study
EPA Grant Number: R824979Title: Carbon Exchange Dynamics in a Temperate Forested Watershed: A Laboratory and Field Multidisciplinary Study
Investigators: Walter, Lynn M. , Teeri, James A. , Meyers, Philip A. , Budai, Joyce M. , Abriola, Linda M. , Zak, Donald R. , Kling, George W.
Institution: University of Michigan
EPA Project Officer: Packard, Benjamin H
Project Period: October 1, 1996 through September 30, 1999 (Extended to September 30, 2000)
Project Amount: $800,000
RFA: Water and Watersheds Research (1996) RFA Text | Recipients Lists
Research Category: Watersheds , Water
Objective:
The role of temperate forests in modulating buildup of anthropogenic carbon dioxide (CO2) has been of continuing interest. Most recently, a large midlatitude CO2 sink has been inferred by modeling, suggesting that these landscapes already may be active in modulating the anthropogenic CO2 flux. Studies of carbon allocation in forests under enhanced and ambient CO2 and nitrogen (N) fertilization growth conditions show that above and below ground carbon storage, as well as root and microbial respiration, all increase at elevated PCO2 and N2 fertilization. A portion of this additional organic carbon is rapidly recycled via respiration to the atmosphere. However, organic carbon fixed in the rooting zone also may be transported in dissolved form as soil waters migrate to the water table. Our research goal is to track how carbon fixed in temperate forest biomass may be transformed via microbial processing and mineral weathering at landscape and shallow subsurface spatial scales. In forested watersheds established on surficial deposits containing carbonate minerals, CO2-enhanced mineral dissolution may provide an important feedback loop between carbon storage capacity of groundwaters and an additional carbon reservoir to that stored in biomass and soils.Our multidisciplinary approach integrates experimental and natural system measurements on a hydrologically and physiographically constrained catchment within the Cheboygan watershed, located in the uppermost lower peninsula of Michigan. Here, temperate and boreal forest stands have reestablished themselves after logging atop relatively recent glacial deposits (<10,000 years old), which comprise a large unconfined regional aquifer system. Importantly, the glacial drift deposits contain variable amounts of reactive carbonate minerals (calcite, dolomite) and unstable aluminosilicates, making this an ideal site for investigating soil and rooted zone carbon transformations and fluxes to groundwater systems. The CO2 generated within the rooting zone greatly enhances mineral weathering of very soluble carbonates. This, in turn, greatly increases carbon transfer from rooting zones to groundwaters relative to the carbonate-poor landscapes that dominate the eastern and southern United States. The carbon flux also is manifest in streams and rivers that largely are groundwater fed in this watershed.
Summary/Accomplishments (Outputs/Outcomes):
Context of ProjectThe continuing emission of CO2 from fossil-fuel burning in the 21st century is, volumetrically, our most significant environmental pollutant. Of the 5.5 Gt-C (Gt= 1015 gr) emitted/yr, about 2 Gt/yr accumulate in the atmosphere and 2 Gt/yr are absorbed in the oceans. The remaining 1.5 Gt/year, the "missing sink," is widely assigned to terrestrial ecosystems and soils. The predicted response of terrestrial systems to elevated CO2 includes increases in biomass and soil respiration rates. At the same time, changes in land use patterns and vegetative regrowth also are expected. Our EPA-funded work on carbon cycling in a forested watershed in Michigan shows that the rate and magnitude of carbonate mineral dissolution (leaching) from soil profiles is controlled by soil respiration and soil gas PCO2 values. The HCO3- flux from carbonate dissolution is greatest in disturbed soil profiles where carbonates have been reintroduced to organic-rich upper soil zones.
Program Scope and Plan
Our research program addressed how carbon
fixed in temperate forests is: (1) transformed via microbial processing; (2)
solubilized by mineral weathering; and (3) transported to regional groundwater
systems. One of the key features of our research project was the comparison of
carbon transformations in natural forests with those observed in experimental
tree growth chambers. Two well-studied natural forests are established (aspen
and sugar maple) within the confines of the study area. Soil and bedrock
compositional profiles (organic and inorganic carbon contents, carbonate
mineralogy and silicate mineralogy with depth, grain size distribution,
porosity, and permeability) have been determined for typical slope settings from
the aspen versus sugar maple forests. An established array of soil water and gas
samplers and shallow groundwater wells have been sampled repeatedly over two
complete growth seasons. The geochemical, stable isotope (C, H, and O isotopes),
and organic chemical compositions of these samples have been determined.
The experimental component of the project involved the study of aspen and sugar maple growth under variable conditions of N and CO2 fertilization. As part of a larger U.S. Department of Energy study, the response of sapling growth to elevated CO2 was being investigated at the University of Michigan Biological Station, within the confines of the catchment study area. In these experiments, N fertilization variation was accomplished by using different mixes of starting soil, and the CO2 fertilization was accomplished by steady flow of ambient and 2x ambient CO2 gas mixtures into the open-top plastic-shielded chambers. The experimental chambers contained juvenile aspens and sugar maples cultivated under these controlled conditions of CO2 and N fertilization. These experimental open-top chambers utilized natural soils, and the saplings were from trees located within the catchment forest stands (maples, aspens). The support from EPA was used to further characterize the carbon flux from the different chamber treatments, and to determine the proportion of carbonate mineral solubilization and the amount of dissolved organic carbon (DOC) leaving the rooting zone of each chamber treatment type. To accomplish this, each chamber has been outfitted with soil moisture and gas samplers, tensiometers to determine soil moisture, and thermisters to follow temperature changes. The comparison of carbon transformation rates and pathways in natural forests and in these experimental chambers permit a much more realistic forecast to be made of the response of the catchment scale carbon budget to potential changes in fertilization by anthropogenic-forcing factors (e.g., increased temperature, CO2, ecosystem shifts).
Anatomy of the Forested Watershed Field Site
The Cheboygan watershed, located in the northernmost lower peninsula of Michigan, was the main field study site. Recent atmospheric modeling results suggest that large terrestrial carbon sink is located over the northern midcontinental United States, which may be related to the large areas of forest regrowth in this region. Like many other forested northern latitude watersheds, the study catchment is established on unconsolidated glacial drift deposits that host unconfined shallow aquifer systems. Catchment topography is dominated by upland moraine systems, which control drainage through a series of small lakes through the Cheboygan River, out to Lake Huron. Soils and underlying glacial drift in this area are highly permeable, leading to tight chemical linkages between groundwater and surface water systems, a situation common throughout the midwest.
Glacial drift deposits commonly contain carbonate minerals, reworked from
underlying
bedrock formations during the multiple glacial advances and
retreats that have affected the
upper midwest. Soil development over the last
6,000-12,000 years has led to accumulation of
organic carbon at greatly
accelerated rates relative to nonglaciated landscapes. Largely unaltered parent
material is typically encountered within 1-3 m of the surface. Carbonate
dissolution, also termed "leaching" or "weathering," profoundly affects soil pH,
soil profile development, metal hydroxide distributions, and solute fluxes to
groundwaters. Enhanced dissolution of carbonate minerals in glacial drift
deposits may be due to the grinding and stress induced by glacial action. These
geochemical relations contrast sharply with the more intensively studied
carbonate-poor landscapes dominating the eastern and southern United States.
Study sites included two well-characterized forest stands (aspen and sugar maple) and an elevated CO2 open-top chamber experiment established at the University of Michigan Biological Station. The experimental chambers contained juvenile aspens and sugar maples cultivated under controlled conditions of CO2 and N fertilization. We investigated the soil CO2 profiles and soil water fluxes of carbon as dissolved inorganic carbon (DIC) and DOC from the experimental open-top tree growth chambers. We also characterized carbon transformation pathways in natural soils and to quantify carbon fluxes to groundwater and surface water systems.
Carbon Transformations in Natural and Experimental Soil Waters
Geochemical measurements (major elements, DIC, DOC, gas compositions,
stable C, O, and H isotopes of water and DIC) were made for soil waters and
gases, and groundwater and surface waters over three seasons in the catchment.
Comparisons were made between mineral dissolution and carbon fluxes from the
experimental tree growth chamber treatment types (high- vs. low-soil fertility,
ambient vs. elevated [twice ambient] CO2) and those in the
two natural forest systems. These comparisons reveal large differences in
CO2 contents and soil water chemistries (DOC and DIC
contents), which indicate that rapid tree growth rates and soil profile
disturbance both increase carbon fluxes.
Undisturbed soil profiles in forest sites exhibit strong decreases in organic carbon content with depth. Here, the CO2 partial pressure increases from the soil surface to the base of the rooting zone (about 100 cm), then declines with increasing depth. Progressive mineral dissolution and weathering reactions over the last 8,000 years (since glacial retreat) have removed carbonate minerals from the upper 50-100 cm of the soil profile. As a result, soil waters generally transform from dilute, DOC-rich solutions in the upper 50 cm into DIC-rich solutions by about 4 m soil depth.
The CO2 profiles for experimental tree growth chambers vary depending on the fertility of the soil as well as the CO2 treatment, and can attain maximum CO2 values that are about five times greater than those observed in natural forests (about 2,500 vs. about 12,000 ppmv). More data remain to be integrated from the various seasons, treatments, and locations, but it is clear that CO2 partial pressures within soil profiles respond strongly and predictably to changes in fertility and in tree growth rates. Other studies of soil CO2 content and fluxes show that CO2 profiles are generally controlled by depth variation in rates of microbial and root respiration and diffusive transport. The values observed in the experimental chambers are certainly at the high end of observed CO2 partial pressures and likely reflect the very rapid growth phase of these young trees. Very high PCO2 values in soil gases have been observed in golf courses and in other experimental elevated CO2 tree growth experiments, for example. The soil waters sampled from the rooted zone of the experimental tree growth chambers are compositionally more evolved than those in natural forests at equivalent depths. Chemical equilibrium-solute speciation calculations on chamber soil waters reveal that carbonate dissolution proceeds to equilibrium rapidly in these disturbed soil profiles, with tight coupling between soil CO2 and carbonate solubility. Chambers under enhanced N fertilization and under elevated CO2 have even greater soil CO2 partial pressures and carbonate-C dissolution fluxes. This is consistent with findings that N can limit fertility, even in the face of elevated CO2. If the geochemical behavior of the experimental chambers has application to stressed natural systems, our findings suggest that a "greening" of high latitude forests likely would be accompanied by increased DIC and DOC fluxes to regional surface waters and groundwaters.
Carbon Transport in the Surface Water/Shallow Groundwater
System
Surface waters integrate the seasonal and spatial changes in the
balance of respiration/photosynthesis and in mineral weathering. Importantly,
the carbonate alkalinity (dominantly as HCO3-) of surface waters throughout the
watershed remains close to a value of 2.8 mmol/L. At baseflow, alkalinities are
very close to the values of shallow groundwaters in unconfined glacial drift
aquifers. Our data show that there is net export of DOC from the Cheboygan
watershed. However, this DOC can be oxidized or regenerated at other points in
the surface flow system.
There is a significant increase in calcite saturation state of surface waters along the flow path; waters discharging from Burt and Mullet lakes are nearly seven times supersaturated with respect to calcite, largely due to the loss of aqueous CO2 along the surface water flow path. If this supersaturation led to precipitation of CaCO3 during transit, the net uptake from soil zone CO2 would be reduced. Mg+2 is released along with Ca+2 during the weathering of dolomite and calcite; the Mg+2/Ca+2 mole ratio in groundwaters is always about 0.5 in the study area. Only low-magnesium calcite can precipitate if the saturation state increases via CO2 loss because of kinetic inhibition to Mg-bearing dolomite precipitation. Thus, the Mg+2/HCO3- ratio of surface waters relative to groundwaters provides a means of determining whether HCO3- has been lost via CaCO3 precipitation (Mg+2/HCO3- will increase), or whether the water has simply been diluted by rainfall or concentrated by evaporation (Mg+2/HCO3- will be constant).
A constant Mg+2/HCO3- ratio is observed in groundwaters and surface waters throughout the watershed. Thus, calcium carbonate precipitation does not occur despite the considerable supersaturation. The inhibition of carbonate precipitation, even from very supersaturated solutions, has been attributed to sluggish nucleation kinetics at low temperatures and to the inhibitory effects of dissolved humic substances. Thus, the trends in surface water carbonate chemistry suggest that the carbon flux from soil water to groundwater is largely exported to the Great Lakes hydrogeochemical system.
The three watersheds ?Cheboygan, Tahquamenon in the upper peninsula, and Huron in the populous southeastern lower peninsula?have shallow groundwaters of the Ca-Mg-HCO3-type, and exhibit the 1:2 ratio of divalent cations to bicarbonate ion dictated by the stoichiometry of reaction. Groundwater aqueous speciation and degree of saturation relative to calcite show that all groundwaters are at equilibrium with calcite. The tritium (3H) content of these shallow groundwaters is consistent with recharge after bomb testing in the 1950s. Nevertheless, groundwater chemistry is quite distinct among the three watersheds.
Equilibrium thermodynamic relations between carbonate mineral solubility and PCO2 in an open, pure water system at fixed PCO2 and a temperature of 10?C show that large differences in PCO2 readily could produce the observed differences in groundwater chemistry among the watersheds. Similar variations in weathering zone PCO2 at the watershed scale have been reported from Ontario, Canada. The rapid equilibration of soil waters with carbonate minerals in experimental chambers and the apparent control of chemical equilibrium by carbonate minerals in shallow groundwaters are consistent with earlier studies.
Implications
Our findings on dissolved carbon fluxes (primarily as
DIC, with small contributions from DOC), together with the apparently enhanced
carbon fluxes from experimental chambers, have several important implications.
First, carbonate weathering fluxes to regional surface water/groundwater systems
increase with increasing soil zone PCO2, making this a
potentially important feedback on expected increases in atmospheric CO2. Second, disturbance of natural soil profiles (normally
carbonate-depleted in the upper 100 cm) can mix carbonate minerals into upper
soil organic-rich zones where soil PCO2 values are highest.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
Other project views: | All 10 publications | 3 publications in selected types | All 3 journal articles |
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Type | Citation | ||
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Walter LM, Budai JM, Ku TCW, Meyers PA, Baptist K, Abriola LM, Chen Y-M, Zak DR, Kling GW. Carbon exchange dynamics and mineral weathering in a temperate forested watershed (Northern Michigan): links between forest ecosystems and groundwaters. Mineralogical Magazine 1998;62A:1625-1626. |
R824979 (1998) R824979 (1999) R824979 (Final) |
not available |
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Zak DR, Holmes WE, MacDonald NW, Pregitzer KS. Soil temperature, matric potential, and the kinetics of microbial respiration and nitrogen mineralization. Soil Science Society of America Journal 1999;63(3):575-584. |
R824979 (Final) |
not available |
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Zak DR, Pregitzer KS, King JS, Holmes WE. Elevated atmospheric CO2, fine roots and the response of soil microorganisms: a review and hypothesis. New Phytologist, July 2000;147(1):201-222. |
R824979 (1998) R824979 (1999) R824979 (Final) |
not available |
Supplemental Keywords:
CO2, global climate, limestone, isotopes, chemical transport, ecosystem, aquatic, environmental chemistry, engineering, ecology, hydrology, geology, Great Lakes, geochemistry, EPA Region 4., RFA, Scientific Discipline, Water, Waste, Ecosystem Protection/Environmental Exposure & Risk, Water & Watershed, Hydrology, Geochemistry, Fate & Transport, Biochemistry, Ecology and Ecosystems, Watersheds, fate and transport, bioassessment, biogeochemical study, soil water chemistry, predictive model, aquatic ecosystems, carbon exchange, carbon flux, ecology assessment models, forested watershedRelevant Websites:
http://www.bart-wmich.org/program.htmlProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.