Walker Branch Watershed Research Articles

Long‐term hydrological, biogeochemical, and climatological data from Walker Branch Watershed, East Tennessee, USA

AbstractIn 1967, the original Walker Branch Watershed (WBW) project was established to study elemental cycling and mass balances in a relatively unimpacted watershed. Over the next 50+ years, findings from additional experimental studies and long‐term observations on WBW advanced understanding of catchment hydrology, biogeochemistry, and ecology and established WBW as a seminal site for catchment science. The 97.5‐ha WBW is located in East Tennessee, USA, on the U.S. Department of Energy's Oak Ridge Reservation. Vegetation on the watershed is characteristic of an eastern deciduous, second‐growth forest. The watershed is divided into two subcatchments: the West Fork (38.4 ha) and the East Fork (59.1 ha). Headwater streams draining these subcatchments are fed by multiple springs, and thus flow is perennial. Stream water is high in base cations due to weathering of dolomite bedrock and nutrient concentrations are low. Long‐term observations of climate, hydrology, and biogeochemistry include daily (1969–2014) and 15‐min (1994–2014) stream discharge and annual runoff (1969–2014); hourly, daily, and annual rainfall (1969–2012); daily climate and soil temperature (1993–2010); and weekly stream water chemistry (1989–2013). These long‐term datasets are publicly available on the WBW website (https://walkerbranch.ornl.gov/long‐term‐data/). While collection of these data has ceased, related long‐term measurements continue through the National Ecological Observatory Network (NEON), where WBW is the core terrestrial and aquatic site in the Appalachian and Cumberland Plateau region (NEON's Domain 7) of the United States. These long‐term datasets have been and will continue to be important in evaluating the influence of climatic and environmental drivers on catchment processes.

The Atmospheric Chemistry and Canopy Exchange Simulation System (ACCESS): model description and application to a temperate deciduous forest canopy

Abstract. Forest canopies are primary emission sources of biogenic volatile organic compounds (BVOCs) and have the potential to significantly influence the formation and distribution of secondary organic aerosol (SOA) mass. Biogenically-derived SOA formed as a result of emissions from the widespread forests across the globe may affect air quality in populated areas, degrade atmospheric visibility, and affect climate through direct and indirect forcings. In an effort to better understand the formation of SOA mass from forest emissions, a 1-D column model of the multiphase physical and chemical processes occurring within and just above a vegetative canopy is being developed. An initial, gas-phase-only version of this model, the Atmospheric Chemistry and Canopy Exchange Simulation System (ACCESS), includes processes accounting for the emission of BVOCs from the canopy, turbulent vertical transport within and above the canopy and throughout the height of the planetary boundary layer (PBL), near-explicit representation of chemical transformations, mixing with the background atmosphere and bi-directional exchange between the atmosphere and canopy and the atmosphere and forest floor. The model formulation of ACCESS is described in detail and results are presented for an initial application of the modeling system to Walker Branch Watershed, an isoprene-emission-dominated forest canopy in the southeastern United States which has been the focal point for previous chemical and micrometeorological studies. Model results of isoprene profiles and fluxes are found to be consistent with previous measurements made at the simulated site and with other measurements made in and above mixed deciduous forests in the southeastern United States. Sensitivity experiments are presented which explore how canopy concentrations and fluxes of gas-phase precursors of SOA are affected by background anthropogenic nitrogen oxides (NOx). Results from these experiments suggest that the level of ambient NOx influences the pathways by which SOA is formed by affecting the relative magnitudes and fluxes of isoprene oxidation products emitted from the canopy. Future versions of the ACCESS model are planned to be multiphase, including gas- and aerosol-phase chemical and physical processes, to more fully explore these preliminary results.

Effects of throughfall manipulation on soil nutrient status: results of 12 years of sustained wet and dry treatments

AbstractTo investigate the potential effects of changing precipitation on forest ecosystems, the Throughfall Displacement Experiment (TDE) was established on Walker Branch Watershed, Tennessee, in 1993. Three different throughfall amounts were tested: −33% (DRY); ambient (no change, AMB); and +33% (WET). Throughfall manipulations had no statistically significant effects on total C, N, exchangeable Ca2+, Mg2+, bicarbonate‐extractable P, or extractable SO42− in soils after 12 years of sustained treatments. Increased K+ inputs in the WET treatment resulted in relative increases in exchangeable K+compared with the AMB and DRY treatments. Soil C, N, and extractable P declined in all treatments over the 12‐year study, and the declines in N were inexplicably large. Field observations contrasted with earlier simulations from the Nutrient Cycling Model (NuCM), which predicted greater decreases in exchangeable K+, Ca2+, Mg2+, and extractable P in the order WET>AMB>DRY, and no change in C, N, and extractable SO42−. The failure of the NuCM model to accurately predict observed changes is attributed to the lack of mechanisms for deep rooting and the transfer of throughfall K+ from one plot to another in the model. Measurements of element availability using resin membranes during the final years showed higher values in wet and lower values in dry treatments compared with ambient conditions for mineral N, K, Mn, Zn, and Al, but the opposite for B, Ca, and Mg. In the cases of Ca and Mg, the patterns in resin values were similar to those at the soil exchange sites (greatest in the dry treatment) and appeared to reflect pretreatment differences. This study showed that while longer term changes in soil nutrients are likely to occur with changes in precipitation, potential changes over this 12‐year interval were buffered by ecosystem processes such as deep rooting.

Soil Carbon and Nitrogen Changes in Forests of Walker Branch Watershed, 1972 to 2004

Changes in soil C and N concentrations and contents in four samplings during a 32‐yr period on Walker Branch watershed in Tennessee were determined and compared with previously measured C and N fluxes and with changes in ecosystem C and N pools during this period. Soils showed significant increases in C and N concentrations in surface horizons from 1972 to 2004, and most of this increase occurred between 1972 and 1982. A previously observed decline in soil C and N contents between 1982 and 1993 was reversed in 2004 such that the latter increased to near 1982 values. The changes in soil C content could be approximately accounted for by previously measured litterfall and soil CO2–C fluxes. Changes in soil N could not be accounted for by leaching, increments in vegetation, or by laboratory bias, changes during sample storage, or reasonable estimates of field sampling errors. We conclude that, although vegetation C and N pools increased steadily during the sampling period in most cases, changes in soil C and N pools on Walker Branch watershed are highly variable in both space and time, and there has been no unidirectional trend during the time period of this study.

CALCIUM AND FOREST SYSTEMS: DIFFUSION FROM DEEP SOURCES

Input-output budgets of temperate forest systems frequently indicate calcium depletion with time. Anthropic disturbances such as acidic deposition and forest harvesting are presumed to accelerate this loss, yet few studies have shown significant change in ecosystem Ca, including vegetation, surface soil, and forest floor. We propose that diffusion of exchangeable Ca from deep zones of higher concentration associated with less-weathered or unweathered materials to near-surface zones of low Ca concentration can explain some of the enigmatic nutrient budgets. We used data from Walker Branch Watershed, Tennessee (WBW), to explore this hypothesis. Radiotracer data were used to determine a diffusion coefficient for Ca empirically. A numerical model using this coefficient and reasonable estimates of the source concentration and depth yielded both a vertical gradient of soil Ca and a rate of Ca movement to the surface that were consistent with published observations. Diffusion has not previously been recognized as important for ion movement at these larger scales of time (e.g., years) or space (e.g., meters). Although our data were from WBW, the mechanism is universal and we can extend our results to nearly all forest systems with low Ca or other ions in the surface soil and higher concentrations with depth.

The importance of in-stream uptake for regulating stream concentrations and outputs of N and P from a forested watershed: evidence from long-term chemistry records for Walker Branch Watershed

Long-term, weekly measurements of streamwater nitrogen and phosphorus concentra- tions in the West Fork of Walker Branch, a 1st order forested stream in eastern Tennessee, were used to assess the importance of in-stream processes for controlling stream concentrations and watershed exports. Over the period from 1991 to 2002, there was a slight declining trend in watershed export of dissolved inorganic N via streamflow, despite relatively high and constant wet N deposition rates (5 kg/ha/y). The watershed retains >90% of N deposition inputs. Concentra- tions of NO3 � and soluble reactive phosphorus (SRP) showed distinct seasonal patterns with autumn and early spring minima and summer maxima. An end-member mixing analysis indicated that these seasonal concentration patterns were largely a result of seasonal variations in in-stream uptake processes, with net uptake of NO3 � and SRP having the greatest impact on streamwater concentrations in November (reductions of 29 lg N/l and 2.5 lg P/l, respectively). This was likely a result of high rates of uptake by microbes colonizing new inputs of leaf detritus. For NO3 � there was a secondary peak in net uptake in March and April (about 9 lg N/l) resulting from increased uptake by stream algae and bryophytes. Summer was a period of net release of NO3 � to stream- water (peaking at 9 lg N/l in July) and minimal net effects on SRP concentrations. On average, in-stream processes resulted in removal of about 20% of the NO3 � and 30% of the SRP entering the stream from the catchment annually. This study, as well as other recent work, suggests that in-stream processes are important buffers on stream nutrient concentrations and exports reducing the effects of changes in inputs and retention in terrestrial portions of watersheds.

Gap-filling missing data in eddy covariance measurements using multiple imputation (MI) for annual estimations

Missing data is a ubiquitous problem in evaluating long-term experimental measurements, such as those associated with the FluxNet project, due to the equipment failures, system maintenance, power-failure, and lightning strikes among other things. To estimate annual values of net ecosystem carbon exchange (NEE), latent heat flux (LE) and sensible heat flux ( H), such gaps in the measured data must be filled or imputed. So far, no standardized method has been accepted and the imputation methods used are largely dependent on the researchers’ choice. Here, we used multiple imputation (MI) to gap-fill the missing data for annual estimations of NEE, LE and H at three flux sites associated with the FluxNet effort. MI is a Monte Carlo technique in which the missing values are replaced by several simulated values. Each data set imputed is a complete one where the observed values are the same as those in the original data set; only the missing values are different. Thus, the normal statistical analysis (e.g. annual total calculation) can be applied to each data set separately. The results of each analysis can be recombined into one summary. We applied the MI method to eddy covariance measurements collected from Walker Branch Watershed (WBW) site (a deciduous forest), Duke site (a coniferous forest) and Niwot site (a subalpine forest). Results showed that annual estimations of NEE, LE and H by MI were comparable to other imputation methods but MI was much easier to apply because of readily available software and standard algorithms. Besides the normal statistical analyses, MI also provided confidence intervals for each estimated parameter. This confidence interval is most useful when assessing energy, water, and carbon balance closures at a given tower site. Significant differences in annual NEE, LE and H were found among years at the three AmeriFlux sites. NEE at the Niwot Ridge site was lower and LE and H were higher than at the other two sites. With the available software and realistic gap-filling capability, MI has the potential to become a standardized method to gap-fill eddy covariance flux data for annual estimations and to improve the analysis of uncertainties associated with annual estimations of NEE, LE and H from regional and global flux networks.

Modelling the discrimination of13CO2above and within a temperate broad-leaved forest canopy on hourly to seasonal time scales

Fluxes and concentrations of carbon dioxide and 13CO2 provide information about ecosystem physiological processes and their response to environmental variation. The biophysical model, CANOAK, was adapted to compute concentration profiles and fluxes of 13CO2 within and above a temperate deciduous forest (Walker Branch Watershed, Tennessee, USA). Modifications to the model are described and the ability of the new model (CANISOTOPE) to simulate concentration profiles of 13CO2, its flux density across the canopy–atmosphere interface and leaf-level photosynthetic discrimination against 13CO2 is demonstrated by comparison with field measurements. The model was used to investigate several aspects of carbon isotope exchange between a forest ecosystem and the atmosphere. During the 1998 growing season, the mean photosynthetic discrimination against 13CO2, by the deciduous forest canopy (Δcanopy), was computed to be 22·4‰, but it varied between 18 and 27‰. On a diurnal basis, the greatest discrimination occurred during the early morning and late afternoon. On a seasonal time scale, the greatest diurnal range in Δcanopy occurred early and late in the growing season. Diurnal and seasonal variations in Δcanopy resulted from a strong dependence of Δcanopy on photosynthetically active radiation and vapour pressure deficit of air. Model calculations also revealed that the relationship between canopy-scale water use efficiency (CO2 assimilation/transpiration) and Δcanopy was positive due to complex feedbacks among fluxes, leaf temperature and vapour pressure deficit, a finding that is counter to what is predicted for leaves exposed to well-mixed environments.

The Effects of Throughfall Manipulation on Soil Leaching in a Deciduous Forest

The effects of changing precipitation on soil leaching in a deciduous forest were examined by experimentally manipulating throughfall fluxes in the field. In addition to an ambient treatment (AMB), throughfall fluxes were reduced by 33% (DRY treatment) and increased by 33% (WET treatment) using a system of rain gutters and sprinklers on Walker Branch Watershed, Tennessee. Soil leaching was measured with resin lysimeters in the O horizons and with ceramic cup lysimeters in the E (25 cm) and Bt (70 cm) horizons. Large and statistically significant treatment effects on N fluxes were found in the O horizons (lower N fluxes in the DRY and higher N fluxes in the WET treatment). Together with the greater O horizon N content observed in the DRY treatment, this suggested that N was being immobilized at a greater rate in the DRY treatment than in the AMB or WET treatments. No statistically significant treatment effects on soil solution were found in the E horizons with the exception of (Ca2+ + Mg2+) to K+ ratio. Statistically significant treatment effects on electrical conductivity (EC), pH, Ca2+, Mg2+, K+, Na+, SO4(2-), and Cl- were found in the Bt horizons due to differences between the DRY and other treatments. Despite this, calculated fluxes of Ca2+, Mg2+, K+, Na+, SO4(2-), and Cl- were lowest in the DRY treatment. These results suggest that lower precipitation will cause temporary N immobilization in litter and long-term enrichment in soil base cations whereas increased precipitation will cause long-term depletion of soil base cations.

The Effects of Throughfall Manipulation on Soil Leaching in a Deciduous Forest

The effects of changing precipitation on soil leaching in a deciduous forest were examined by experimentally manipulating throughfall fluxes in the field. In addition to an ambient treatment (AMB), throughfall fluxes were reduced by 33% (DRY treatment) and increased by 33% (WET treatment) using a system of rain gutters and sprinklers on Walker Branch Watershed, Tennessee. Soil leaching was measured with resin lysimeters in the O horizons and with ceramic cup lysimeters in the E (25 cm) and Bt (70 cm) horizons. Large and statistically significant treatment effects on N fluxes were found in the O horizons (lower N fluxes in the DRY and higher N fluxes in the WET treatment). Together with the greater O horizon N content observed in the DRY treatment, this suggested that N was being immobilized at a greater rate in the DRY treatment than in the AMB or WET treatments. No statistically significant treatment effects on soil solution were found in the E horizons with the exception of (Ca2+ + Mg2+) to K+ ratio. Statistically significant treatment effects on electrical conductivity (EC), pH, Ca2+, Mg2+, K+, Na+, SO2−4, and Cl− were found in the Bt horizons due to differences between the DRY and other treatments. Despite this, calculated fluxes of Ca2+, Mg2+, K+, Na+, SO2−4, and Cl− were lowest in the DRY treatment. These results suggest that lower precipitation will cause temporary N immobilization in litter and long-term enrichment in soil base cations whereas increased precipitation will cause long-term depletion of soil base cations.

Comparing independent estimates of carbon dioxide exchange over 5 years at a deciduous forest in the southeastern United States

At a deciduous forest in the southeast United States (Walker Branch Watershed, Oak Ridge, Tennessee), as at other sites with tall vegetation and/or complex terrain, it is difficult to temporally integrate eddy covariance data to obtain long‐term estimates of net ecosystem exchange of carbon dioxide (NEE), primarily because of suspected systematic nocturnal errors. Therefore, although eddy covariance data can be invaluable, additional tools such as empirical gap‐filling methods, independent measurements of CO2 flux using chambers, and simulations using canopy process models are often necessary to obtain reliable annual carbon uptake estimates. Two independent approaches for estimating annual NEE using these tools at the Walker Branch site are discussed. One approach is to cumulatively sum the full set of eddy covariance measurements over time. The second approach is to sum the output of NEE from a biophysical canopy exchange model (CANOAK). CANOAK incorporates independent chamber measurements on leaves, soil, and stems and is driven using the observed canopy architecture, meteorology, soil water content, and soil temperatures to predict NEE. Both methods estimate similar trends and magnitudes of daytime (daylight hours) soil respiration and NEE over 5 years. Both methods also suggest similar differences among years (interannual variability). These two estimates of NEE are used to address possible measurement bias errors at this site and to provide plausible estimates of annual NEE. The estimated mean annual NEE at this site is −574 g C m−2 yr−1 between 1995 and 1999, ranging from −470 g C m−2 y−1 (1995) to −629 g C m−2 yr−1 (1999) (negative NEE indicates uptake by forest).

Leaf age affects the seasonal pattern of photosynthetic capacityand net ecosystem exchange of carbon in a deciduous forest

AbstractTemporal trends in photosynthetic capacity are a critical factorin determining the seasonality and magnitude of ecosystem carbonfluxes. At a mixed deciduous forest in the south‐eastern United States (Walker Branch Watershed, Oak Ridge, TN, USA), we independently measured seasonal trends in photosynthetic capacity (using single‐leaf gas exchange techniques) and the whole‐canopycarbon flux (using the eddy covariance method). Soil respiration was also measured using chambers and an eddy covariance system beneath the canopy. These independent chamber and eddy covariance measurements, along with a biophysical model (CANOAK), areused to examine how leaf age affects the seasonal pattern of carbon uptake during the growing season. When the measured seasonality in photosynthetic capacity is representedin the CANOAK simulations, there is good agreement with the eddy covariance data on the seasonal trends in carbon uptake. Removing the temporal trends in the simulations by using the early season maximum value of photosynthetic capacity over the entire growing season over estimates the annual carbon uptake by about 300 g C m−2 year−1– halfthe total estimated annual net ecosystem exchange. Alternatively, use of the mean value of photosynthetic capacity incorrectly simulates the seasonality in carbon uptake by the forest. In addition to changes related to leaf development and senescence, photosynthetic capacitydecreased in the middle and late summer, even when leaf nitrogenwas essentially constant. When only these middle and late summer reductions were neglected in the model simulations, CANOAK still overestimated the carbon uptake by an amount comparable to 25% ofthe total annual net ecosystem exchange.

A six-year study of sapling and large-tree growth and mortality responses to natural and induced variability in precipitation and throughfall.

Global climatic change may cause changes in regional precipitation that have important implications for forest growth in the southern United States. In 1993, a stand-level experiment was initiated on Walker Branch Watershed, Tennessee, to study the sensitivity of forest saplings and large trees to changes in soil water content. Soil water content was manipulated by gravity-driven transfer of precipitation throughfall from a dry treatment plot (-33%) to a wet treatment plot (+33%). A control plot was included. Each plot was 6400 m2. Measurements of stem diameter and observations of mortality were made on large trees and saplings of Acer rubrum L., Cornus florida L., Liriodendron tulipifera L., Nyssa sylvatica Marsh, Quercus alba L. and Quercus prinus L. every 2 weeks during six growing seasons. Saplings of C. florida and A. rubrum grew faster and mortality was less on the wet plot compared with the dry and control plots, through 6 years of soil water manipulation. Conversely, diameter growth of large trees was unaffected by the treatments. However, tree diameter growth averaged across treatments was affected by year-to- year changes in soil water status. Growth in wet years was as much as 2-3 times greater than in dry years. Relationships between tree growth, phenology and soil water potential were consistent among species and quantitative expressions were developed for application in models. These field growth data indicate that differences in seasonal patterns of rainfall within and between years have greater impacts on growth than percentage changes in rainfall applied to all rainfall events.

Partitioning net ecosystem carbon exchange with isotopic fluxes of CO2

SummaryBecause biological and physical processes alter the stable isotopic composition of atmospheric CO2, variations in isotopic content can be used to investigate those processes. Isotopic flux measurements of 13CO2 above terrestrial ecosystems can potentially be used to separate net ecosystem CO2 exchange (NEE) into its component fluxes, net photosynthetic assimilation (FA) and ecosystem respiration (FR). In this paper theory is developed to partition measured NEE into FA and FR, using measurements of fluxes of CO2 and 13CO2, and isotopic composition of respired CO2 and forest air. The theory is then applied to fluxes measured (or estimated, for 13CO2) in a temperate deciduous forest in eastern Tennessee (Walker Branch Watershed). It appears that there is indeed enough additional information in 13CO2 fluxes to partition NEE into its photosynthetic and respiratory components. Diurnal patterns in FA and FR were obtained, which are consistent in magnitude and shape with patterns obtained from NEE measurements and an exponential regression between night‐time NEE and temperature (a standard technique which provides alternate estimates of FR and FA). The light response curve for photosynthesis (FA vs. PAR) was weakly nonlinear, indicating potential for saturation at high light intensities. Assimilation‐weighted discrimination against 13CO2 for this forest during July 1999 was 16.8–17.1‰, depending on canopy conductance. The greatest uncertainties in this approach lie in the evaluation of canopy conductance and its effect on whole‐canopy photosynthetic discrimination, and thus the indirect methods used to estimate isotopic fluxes. Direct eddy covariance measurements of 13CO2 flux are needed to assess the validity of the assumptions used and provide defensible isotope‐based estimates of the component fluxes of net ecosystem exchange.

Stem respiration in a closed-canopy upland oak forest.

Stem respiration was measured throughout 1993 on 56 mature trees of three species (Quercus alba L., Quercus prinus L., and Acer rubrum L.) in Walker Branch Watershed, Oak Ridge, Tennessee. A subset of the trees was remeasured during 1994. Diameter increments, stem temperatures and soil water were also monitored. Respiration rates in the spring and summer of 1993 tracked growth rate increments, except during a drought when growth dropped to zero and respiration increased to its highest rate. During the dormant season, rates of total stem respiration (R(t)) tended to be greater in large trees with thick sapwood but no such trend was observed during the growing season. Before and after the growing season, respiration rates correlated well with stem temperatures. Estimated values of Q(10) were 2.4 for the two oak species and 1.7 for red maple. The Q(10) values were used along with baseline respiration measurements and stem temperatures to predict seasonal changes in maintenance respiration (R(m)). In red maple, annual total R(m) accounted for 56 and 60% of R(t) in 1993 and 1994, respectively. In chestnut oak, R(m) accounted for 65 and 58% of R(t) in 1993 and 1994, respectively. In white oak, R(m) accounted for 47 and 53% of R(t) in 1993 and 1994, respectively. Extrapolating these data to the stand level showed that woody tissue respiration accounted for 149 and 204 g C m(-2) soil surface year(-1) in 1993 and 1994, respectively.

Stable nitrogen isotope ratios in wet and dry nitrate deposition collected with an artificial tree

mounts of dry NO3-N deposition and N isotope ratios in wet and dry NO3-N deposition have been simultaneously determined by examining differences between precipitation collected by open funnels and throughfall collected beneath an artificial Christmas tree. Samples were collected in a forest clearing on Walker Branch Watershed, near Oak Ridge, Tennessee. From mid-summer to early autumn, NO3-N fluxes beneath the artificial tree were always greater than those measured in precipitation indicating the tree’s effectiveness as a passive collector of dry NO3-N deposition. Dry NO3-N deposition averaged 60 ± 9% of total (wet and dry) deposition. The mean (± SD) calculated δ15N value for NO3-N in dry deposition was + 5.6 ± 2.1‰ (n= 6 sampling periods ranging from 4 to 15 days). On average, this was ≈ 6‰ heavier than measured δ15N values for NO3-N in precipitation. The calculated δ15N value for NO3-N in dry deposition was consistent with that expected if NOx precursors to HNO3 vapor (the major constituent of dry deposition at this site) originated principally from coal combustion.

The precise measurement of concentration gradients of mercury in air over soils: A review of past and recent measurements

Measurements of the atmospheric concentration gradients of mercury (Hg) vapor over soils can be used to determine the direction and magnitude of exchange rates of Hg if certain assumptions are met. However, these gradients are quite small and require highly precise sampling to achieve accurate data. We have developed a sampling and analysis procedure which allows quantification of gradients over background soils. With this procedure we can now measure atmospheric Hg at ambient levels with a precision of ∼0.5 to 2% (expressed as relative standard error). This level of precision is well above those published in earlier gradient studies. In our recent studies, gradients measured between 25 and 165 cm above background forest soils at Walker Branch Watershed, Tennessee were quite small, ranging from 0.02 to 0.39 ng/m3 (expressed as concentration differences). These gradients indicated that Hg emission was about 3 times more frequent than dry deposition. Gradients measured over soils at Lake Gardsjon, Sweden were generally smaller but also indicated bidirectional fluxes. By comparison, gradients above Hg-contaminated soils in Tennessee were far larger as expected, ranging from 0.12 to 5.60 ng/m3. These gradients consistently indicated emission of Hg. A number of tests were performed to validate that these gradients were true indications of Hg exchange rates.

Topographic Variation of Soil Nitrogen Dynamics at Walker Branch Watershed, Tennessee

Abstract Understanding the spatial and temporal variability of soil nitrogen (N) transformations is central to quantifying the N dynamics and productivity of ecosystems. The objectives of this work were to examine spatial and temporal variation of soil N dynamics and to identify factors correlated with topographic variation in soil N dynamics in a forest watershed. Net N mineralization and net nitrification potential were measured by aerobic laboratory incubations of surface (0-7 cm) mineral soils. Principal components analysis was used to describe sampling sites across the watershed based on 13 site characterization variables. A topographic index used in hydrologic modeling, In (α/tan β), was calculated for each site as the natural logarithm of the ratio of the upslope drainage area per unit contour length (α) to the local slope angle (tan β). Soils from valley floors had greater total N concentrations, lower carbon-to-nitrogen (C:N) ratios, greater potential net nitrification, and greater microbial activity (as indicated by short-term urease assays) than soils from ridges. Mean net nitrification potential was 0.59 μg N g-1 d-1 in surface soils from valley floors and was < 0.01 on ridges and slopes. The first principal component was related to the N and C properties of soils, leaf litter, and leaf fall at a site. The second principal component was related to forest stand composition. The topographic index was significantly correlated with important variables related to soil N dynamics. Once calibration data are derived, this index may be useful as a first approximation to total soil N concentrations and soil C:N ratios in forest watersheds because In (α/tan β) can be calculated from geographic information systems that contain topographic data. For. Sci. 40(3):497-512.

PREDICTING TEMPORAL AND SPATIAL FLOOD DYNAMICS USING A PRECALIBRATED MODEL1

ABSTRACT: A large storm in December 1990 allowed the evaluation of flood predictions from a hydrologic model (TOPMODEL) that had been previously calibrated on the West Fork of Walker Branch Watershed, a gauged 37.5 ha catchment near Oak Ridge, Tennessee. The model predicts both hydrograph dynamics and the spatial distribution of overland flow using an index based on topography. Maximum extent of overland flow during the storm was determined from patterns of leaf litter removal from valley bottoms. Both the flood hydrograph and the extent of overland flow were accurately predicted using model parameters obtained from a three‐month period of normal flow conditions during 1983.

Emissions of Non‐Methane Organic Compounds and Carbon Dioxide from Forest Floor Cores

AbstractForest floor emissions of CO2 and non‐methane volatile organic compounds (NMOCs) were measured to determine their contribution to C losses from forests. Forest floor cores 19 cm in diameter and approximately 10 cm deep were obtained from beneath loblolly pine (Pinus taeda L.), oak‐maple (Quercus‐Acer), and yellow‐poplar (Liriodendron tulipifera L.) trees on the Walker Branch Watershed, Oak Ridge, TN. The cores were enclosed in an open system gas exchange instrument from which inlet and outlet concentrations of NMOC emissions were sampled. The NMOCs were quantified with a C trap/cryofocus/gas chromatography approach. Total NMOC emissions from the forest floor cores ranged from 15 to 90 nmol C m−2 s−1, and differed beneath each tree species. Cores from beneath yellow‐poplar trees had nearly an order of magnitude greater C equivalent efflux than cores from beneath either the pine or oak‐maple trees. Monoterpenes comprised virtually all of the NMOC emissions from the cores. The C equivalent efflux of NMOCs from the forest floor cores was one to two orders of magnitude less than rates of CO2 efflux (4–8 µmol m−2 s−1), indicating a very minor role for litter and soil NMOC emissions in stand C cycling.