Anthony C. Delany

Winter forest soil respiration controlled by climate and microbial community composition.

Most terrestrial carbon sequestration at mid-latitudes in the Northern Hemisphere occurs in seasonal, montane forest ecosystems. Winter respiratory carbon dioxide losses from these ecosystems are high, and over half of the carbon assimilated by photosynthesis in the summer can be lost the following winter. The amount of winter carbon dioxide loss is potentially susceptible to changes in the depth of the snowpack; a shallower snowpack has less insulation potential, causing colder soil temperatures and potentially lower soil respiration rates. Recent climate analyses have shown widespread declines in the winter snowpack of mountain ecosystems in the western USA and Europe that are coupled to positive temperature anomalies. Here we study the effect of changes in snow cover on soil carbon cycling within the context of natural climate variation. We use a six-year record of net ecosystem carbon dioxide exchange in a subalpine forest to show that years with a reduced winter snowpack are accompanied by significantly lower rates of soil respiration. Furthermore, we show that the cause of the high sensitivity of soil respiration rate to changes in snow depth is a unique soil microbial community that exhibits exponential growth and high rates of substrate utilization at the cold temperatures that exist beneath the snow. Our observations suggest that a warmer climate may change soil carbon sequestration rates in forest ecosystems owing to changes in the depth of the insulating snow cover.

The use of relaxed eddy accumulation to measure biosphere-atmosphere exchange of isoprene and other biological trace gases

Abstract The micrometeorological flux measurement technique known as relaxed eddy accumulation (REA) holds promise as a powerful new tool for ecologists. The more popular eddy covariance (eddy correlation) technique requires the use of sensors that can respond at fast rates (10u2009Hz), and these are unavailable for many ecologically relevant compounds. In contrast, the use of REA allows flux measurement with sensors that have much slower response time, such as gas chromatography and mass spectrometry. In this review, relevant micrometeorological details underlying REA are presented, and critical analytical and system design details are discussed, with the goal of introducing the technique and its potential applications to ecologists. The validity of REA for measuring fluxes of isoprene, a photochemically reactive hydrocarbon emitted by several plant species, was tested with measurements over an oak-hickory forest in the Walker Branch Watershed in eastern Tennessee. Concurrent eddy covariance measurements of isoprene flux were made using a newly available chemiluminesence instrument. Excellent agreement was obtained between the two techniques (r2u2009=u20090.974, nu2009=u200962), providing the first direct comparison between REA and eddy covariance for measuring the flux rate of a reactive compound. The influence of a bias in vertical wind velocity on the accuracy of REA was examined. This bias has been thought to be a source of significant error in the past. Measurements of normalized bias () alone would lead us to think that a large potential error exists at this site. However, with our isoprene data and through simulations of REA with fast-response H2O and CO2 data, we conclude that accurate REA flux measurements can be made even in the presence of a bias in w.

The contribution of beneath-snow soil respiration to total ecosystem respiration in a high-elevation, subalpine forest

[1] The respiratory loss of CO 2 from soil microbes beneath winter snow in forests from cold climates can significantly influence the annual carbon budget. We explored the magnitude of winter soil respiration using continuous measurements of beneath-snow CO 2 concentration within the footprint of a flux tower in a subalpine forest in the Rocky Mountains. We used eddy covariance measurements from the tower to obtain estimates of total wintertime ecosystem respiration and compared them to the calculated beneath-snow CO 2 flux. Soil respiration in the winter was estimated to contribute 35-48% of the total wintertime ecosystem respiration, and 7-10% of the total annual ecosystem respiration. The largest increase in soil respiration occurred in the late winter following an earlier-than-normal initiation of snowmelt and increase in snow density. Following this melt event, respiration rates increased approximately sixfold, despite an increase in soil temperature of only 0.3°-0.5°C. We interpret the late-winter surge in soil respiration to be triggered by a strong response of beneath-snow microbes to the pulse of meltwater coupled with extremely high sensitivity of the microbial biomass to increases in soil temperature.

Verification of flux measurement using relaxed eddy accumulation

Abstract Businger and Oncley (1990; Flux measurement with conditional sampling, J. Atmos. Ocean Technol. 7 , 349–352) proposed a method to measure fluxes of scalar quantities by determining the mean concentration difference between air samples collected selectively during updrafts. This method has been tested for carbon dioxide over a growing cotton field where the fluxes were large. Simultaneous direct flux measurements were made using the eddy-correlation technique. These two techniques gave fluxes which were mostly within 20% on one of the two experiment days; however, the other day had cases with large differences. These differences are suspected to have been caused by a failure of the infra-red absorption sensor used to make the fast carbon dioxide measurements for the eddy-correlation technique.

Deployment and Evaluation of a System for Ground-Based Measurement of Cloud Liquid Water Turbulent Fluxes

Abstract Direct interception of windblown cloud water by forests has been dubbed “occult deposition” because it represents a hydrological input that is hidden from rain gauges. Eddy correlation studies of this phenomenon have estimated cloud water fluxes to vegetation yet have lacked estimates of error bounds. This paper presents an evaluation of instrumental and methodological errors for cloud liquid water fluxes to put such eddy correlation measurements in context. Procedures for data acquisition, processing (including correction factors), and calibration testing of the particulate volume monitor (PVM) and forward-scattering spectrometer probe (FSSP) are detailed. Nearly 200 h of in-cloud data are analyzed for intercomparison of these instruments. Three methods of coordinate system rotation are investigated; the flux shows little sensitivity to the method used, and the difference between fluxes at different stations is even less sensitive to this choice. Side-by-side intercomparison of two PVMs and one ...

Heat Balance in the Nocturnal Boundary Layer during CASES-99

Abstract A unique set of nocturnal longwave radiative and sensible heat flux divergences was obtained during the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99). These divergences are based on upward and downward longwave radiation measurements at two levels and turbulent eddy correlation measurements at eight levels. In contrast to previous radiation divergence measurements obtained within 10 m above the ground, radiative flux divergence was measured within a deeper layer—between 2 and 48 m. Within the layer, the radiative flux divergence is, on average, comparable to or smaller than the sensible heat flux divergence. The horizontal and vertical temperature advection, derived as the residual in the heat balance using observed sensible heat and radiative fluxes, are found to be significant terms in the heat balance at night. The observations also indicate that the radiative flux divergence between 2 and 48 m was typically largest in the early evening. Its magnitude depends on how fast the gr...

An Evaluation of Calibration Techniques for In Situ Carbon Dioxide Measurements Using a Programmable Portable Trace-Gas Measuring System

Abstract The construction and deployment of a portable trace-gas measurement system (TGaMS) is described. The air-collection system (dubbed HYDRA) collects air samples from 18 different locations and was connected to either one or two LI-COR LI-7000 gas analyzers to measure CO2. An in situ “field calibration” method, that uses four calibration gases with an uncertainty on the order of ±0.1 μmol mol−1 relative to the WMO CO2 mole fraction scale, revealed CO2 output from the LI-7000 had a slightly nonlinear relationship relative to the CO2 concentration of the calibration gases. The sensitivity of the field-calibrated CO2 to different forms of the field-calibration equation is investigated. To evaluate TGaMS performance, CO2 from collocated inlets, portable gas cylinders, and nearby independent CO2 instruments are compared. Results are as follows: 1) CO2 measurements from HYDRA multiple inlets are feasible with a reproducibility of ±0.4 μmol mol−1 (based on the standard deviation of the CO2 difference betwe...

Structure of Carbon Dioxide Exchange Processes Above a Spruce Forest

Several micrometeorological techniques,’such as the flux-gradient method or the eddy covariance technique, offer the potential to measure net fluxes of water vapor, CO2 and other trace gases exchanged between ecosystems and the atmosphere (e.g., Baldocchi and Meyers 1998). Subsequent data analyses allow the calculation of net ecosystem CO2 exchange. These net fluxes, however, reflect the balance between different component fluxes. In the case of CO2, two opposing fluxes contribute to this net flux: CO2 uptake during photosynthesis and CO2 release during respiration from above- and belowground organisms. Distinguishing among these components is critical to obtain insights into the processes underlying ecosystem responses to climate forcing (Buchmann 2002). This is because environmental parameters, such as temperature and soil moisture, differentially affect biological activities (e.g., Baldocchi et al. 2001).

The Role of Quantitative Infrared Imagery in Investigations of the Nocturnal Boundary Layer

Abstract During the Cooperative Atmospheric–Surface Exchange Study in 1999 (CASES99), an intensive investigation of the stable nocturnal boundary layer, a versatile and sensitive cryogenically cooled thermal imaging radiometer, the Inframetrics PM 380 Thermal Camera, was deployed. The 60-m-high instrumented tower at the central CASES99 site provided the perch from which to survey the instrumented research field. The field of view of 16° (0.28 sr) and an angular resolution of 1/16° (0.0011 sr) enabled a segment of landscape 500 m distant, of approximately 150-m width, to be viewed with a resolution of approximately 0.5 m. Or, looking down from the 50-m level of the tower, a section of the ground surface 15 m on a side could be viewed with a resolution of 5 cm. The surface brightness temperature of any area could be surveyed with a temperature resolution of 0.1°C and a time resolution of 30 Hz. The information obtained from analysis of these thermal images uniquely complemented the data acquired by the more...