Dry deposition velocity of O3 over a vineyard obtained from models and observations: The 1991 California ozone deposition experiment

Measurements were made of concentrations and flux densities (using the eddy correlation technique) for O₃ over a vineyard for 20 days during July and August 1991. These were compared with modelled dry deposition velocities (V _d), using (a) a module in the air quality model known as ADOM (Acid Deposition and Oxidant Model), which was modified to apply over a specific site, (b) a version of the ADOM module that employs a modified canopy resistance and (c) versions of ADOM which use two new canopy resistance formulations that are referred to as Wesely and Massman, respectively. The Massman parameterization is valid only locally since it was tuned to the site's ozone data. Here it is used as a benchmark for model comparisons. The observed V _dhad an average value of about 0.5 cm s^?1 during the day and about 0.2 cm s^?1 at night. Compared to the modified ADOM, the new parameterizations yielded results that were in better agreement with the observations at night. During the daytime, the original ADOM and the Wesely estimates were much larger than the observations, the Massman values were slightly smaller, and the modified ADOM showed a better agreement. We speculate that the underestimation of the Massman V _dvalues during the day may have been caused by the ADOM aerodynamic resistance rather than the Massman canopy resistance. It is hypothesized that the original ADOM module and its modified version may need a revised aerodynamic or an additional canopy resistance at night in order to bring the estimates closer to the observations.     

Seasonal and diurnal variation in the deposition velocity of ozone over a spruce forest in Denmark

The flux of O₃ was measured by the eddy-correlation method over Norway spruce in periods when the trees had a very low activity, periods with optimum growth, and periods with water stress. The aerodynamic resistance (r _a ), viscous sub-layer resistance (r _b ) and surface resistance (r _c ) to O₃ were calculated from meteorological parameters and the deposition velocity. The canopy stomatal resistance to O₃ was calculated from measurements of the water vapour flux. The deposition velocities showed a diurnal pattern with night-time values of 3.5 mm s^–1 and day-time values of 7 mm s^–1, when the trees had optimal growth conditions. The surface resistance was highly dominating in day-time and the influence of meteorology low. In night-time the surface resistance to 03 was lower than the canopy stomatal resistance. A low surface resistance was also found in winter-time, when the activity of the trees was low. The surface resistance increased when the trees were subject to water stress. It is concluded that stomatal uptake is an important parameter for the deposition of O₃. However, other processes such as destruction of O₃ at surfaces, reaction with NO emitted from the soil, and reactions with radicals produced from VOC's emitted from the forest, should also be taken into consideration.     

Long-term Continuous Measurements of SO2 Dry Deposition over the Speulder Forest

In 1990 a project to develop a dry deposition monitoring method of SO₂, NH₃ and NO₂ to Speulder forest in the Netherlands began. Detailed annual deposition fluxes for these gases were measured throughout November 1992 to December 1995. This paper describes the measurement set-up and the analysis of the deposition parameters for SO₂. The dry deposition velocity was usually smaller than the maximum V_d, showing a resistance to surface uptake, except for periods when the canopy is wet and surface resistance is negligible. Several methods were tested to estimate annual average fluxes from the gradient measurements. Annual fluxes were estimated by selecting the data for periods fulfilling gradient theory and extending the data by using an inferential method for the other periods. The surface resistance parametrisation used in the inferential method was tested using the selected data and was found to yield systematic larger fluxes of the order of 20%. Annual fluxes were 465 mol ha^-1 a^-1 in 1992/1993, 460 mol ha^-1 a^-1 in 1994 and 330 mol ha^-1 a^-1 in 1995. The uncertainty in the annual flux was estimated to be 25%. The annual average dry deposition velocity was 1.5 cm s^-1. No large differences were found in deposition parameters between each of the three years.     

Comparison of dry deposition velocities for SO2, HNO3 and SO4 2− estimated with two inferential models

Dry deposition velocity estimates of SO, HNO₃ and SO₄ ^2? were computed for six locations in eastern North America using two different inferential models; a Big-Leaf model utilized by the U. S. National Dry Deposition Network (NDDN) and, a land-use based model (LUM) that has been used in the past to estimate the relative importance of dry versus wet deposition over selected Canadian regions. There were consistent differences between models that were related to the surface type, chemical species and time of year. Mean monthly dry deposition velocities based upon the 1990–91 time period were compared at two locations. The seasonal cycles in deposition velocity were similar between models, but there were considerable differences in the amplitude of the cycles. The LUM predicted about a 400% increase in S042- deposition velocity from the winter to the summer months, while there was a 50 to 100% increase in the NDDN model estimates, depending upon location. According to the LUM, HN03 deposition to crop land increased by about a factor of 6 from winter to summer, while the big leaf model predicted a 50% increase. Overall, there was better agreement for SO₂. Averaged over 12 months, the differences in deposition velocity between models were smaller and generally within the range of uncertainty associated with inferential models. For all six sites, the mean percent difference between models in deposition velocity for SO₂, HNO₃ and SO₄ ^2? were 13, 35 and 79, respectively. These differences highlight the effect of using different methods for estimating dry deposition and the importance of applying the same model when examining regional patterns in dry/total deposition rates.     

Multiple measurements constrain estimates of net carbon exchange by a Eucalyptus forest

Net ecosystem exchange of carbon (F_NEE) was estimated for a temperate broadleaf, evergreen eucalypt forest ecosystem at Tumbarumba in south-eastern Australia to investigate the processes controlling forest carbon sinks and their response to climate. Measurements at a range of temporal and spatial scales were used to make three different estimates of F_NEE based on: (1) the difference between fluxes of carbon input by photosynthesis and output by autotrophic plus heterotrophic respiration, (2) changes over time in the carbon pools in the above- and below-ground biomass, soil and litter, and (3) micrometeorological flux measurements that provide a continuous estimate of the net exchange. A rigorous comparison of aggregated component fluxes and the net eddy fluxes within a flux tower source area was achieved based on an inventory of the site and a detailed sampling strategy. Measurements replicated in space and time provided mean values, confidence limits and patterns of variation of carbon pools and fluxes that allowed comparisons within known limits of uncertainty. As a result of comparisons between nighttime eddy flux and chamber measurements of respiration, a revised micrometeorological method was developed for estimating nighttime carbon flux using flux tower measurements. Uncertainty in the final estimate of F_NEE was reduced through mutual constraints of each of these measurement approaches. F_NEE for the period October 2001–September 2002, with average rainfall, was an uptake of 6.7 (5.1–8.3) tC ha^?1 yr^?1 estimated from component fluxes, and 5.4 (3.0–7.5) tC ha^?1 yr^?1 estimated from the revised eddy flux method. Biomass increment was 4.5 (3.7–5.4) tC ha^?1 yr^?1 and the remaining 0.9–2.2 tC ha^?1 yr^?1 could represent a carbon sink in the soil and litter pools or lie within the confidence limits of the measured fluxes. F_NEE was reduced to ?0.1 to 2.4 tC ha^?1 yr^?1 during a period of drought and insect disturbance in October 2002–September 2003, with biomass increment being the main component reduced. The forest is a large carbon sink compared with other forest ecosystems, but this is subject to high-annual variability in response to climate variability and disturbance.     

The calculation of transboundary and deposition fluxes of SO2 and sulfate by means of aircraft measurements

Aircraft measurements of the air pollutants SO₂ and sulfate (SO_x is defined as SO₂ plus sulfate), plus data on wind velocity, wind direction and depth of the planetary boundary layer, enable the calculation of transboundary mass fluxes of SO_x. When emission data are available, an emission-deposition balance of SO_x can be determined. The results of a measurement flight, carried out on 12 February, 1986, are presented. Two tracks were flown, along the eastern and along the western border of the Netherlands, respectively. The wind was easterly. The calculated deposition flux of SO_x is converted to an area-averaged dry deposition velocity (ν _d ). The result, ν _d = 1.2 × 10^?2 m s^?1, seems to be in good agreement with literature data.     

Impact of changes in rainfall amounts predicted by climate-change models on decomposition in a deciduous forest

Climate-change models predict a more intense hydrological cycle, with both increased and decreased amounts of rainfall in areas covered with temperate deciduous forests. These changes could alter rates of litter decomposition, with consequences for rates of nutrient cycling in the forest ecosystem. To examine impacts of predicted changes in precipitation on the rate of decay of canopy leaves, we placed litterbags in replicated, fenced 14 m² low-rainfall and high-rainfall plots located under individual rainout shelters. Unfenced, open plots served as an ambient treatment. Litter in the high-rainfall and ambient plots decayed 50% and 78% faster, respectively, than litter in the low-rainfall plots. Litter in the ambient plots disappeared 20% faster than in the high-rainfall treatment, perhaps via greater leaching during heavy rainfall events. Ambient rainfall during the experiment was similar in total amount to the high-rainfall treatment, but was more variable in intensity and timing. We used litterbags of different mesh sizes to examine whether changes in rainfall might alter the impacts of major categories of the fauna on litter decay. However, we found no consistent evidence that excluding arthropods of different sizes affected litter decay rate within any of the three rainfall treatments. This research reveals that changes in rainfall predicted to occur with global climate change will likely strongly alter rates of litter decay in deciduous forests.     

The role of atmospheric deposition in an eastern U.S. deciduous forest

Atmospheric sources contributed significantly to the annual flux of trace metals and sulfate to the forest floor of Walker Branch Watershed, a forested catchment in the southeastern United States. Atmospheric deposition supplied from 14% (Mn) to≈40% (Zn, Cd, So ₄ ⁼ ) to 99% (Pb) of the annual flux to the forest floor; the remainder was attributable to internal element cycling. The measured water solubility of these metals in suspended and deposited particles indicates that they may be readily mobilized following deposition. Dry deposition constituted a major fraction of the total annual atmospheric input of Cd and Zn (≈20%), SO=(≈35%), Pb(≈55%), and Mn (≈90%); however, wet deposition rates for single events exceeded dry deposition rates by one to four orders of magnitude. Interception of rain by the canopy resulted in loss of Cd, Mn, Pb, Zn, and SO⁼ from the canopy, but uptake of H⁺ which increased with increasing free acidity of the incoming rain, and with increasing residence time of the rain on the leaf surface.     

Spatial variation in nitrogen deposition over five adjacent catchments in a larch forest

Abstract

In this report, we propose a new method of evaluating the effect of nitrogen deposition on forest ecosystems, namely the spatial variation in nitrogen deposition enables to detect readily the effect of anthropogenic N deposition on biogeochemical processes in forest ecosystems. We analyzed the nitrogen deposition (throughfall fluxes) and stream water chemistry over five adjacent small catchments in which soil types (Hapludants) and vegetation composition (50 to 60 years old larch plantation) were fairly identical. Thirty-two throughfall collectors were set up in the five catchments (six to eight collectors in each catchment) and throughfall samples were collected after a rain event, while stream water samples were collected once or twice a month. The monitoring was carried out during a period of 6 months (2002 June to 2002 November). Throughfall dissolved inorganic nitrogen (DIN) fluxes were highly variable: the highest N input, 1.32 kg N ha^?1 6 months^?1, was sixty-six times higher than the lowest input, 0.02 kg N ha^?1 6 months^?1. The mean DIN inputs and the mean nitrate concentrations in streams showed a three-time variation across the five catchments. In addition, the DIN inputs showed a high correlation with the stream nitrate concentrations (r = 0.88).     

CO2 efflux from deciduous forest litter and soil in response to simulated acid rain treatment

Using both field and laboratory measurements of CO₂ evolution as an index of decomposer activity, forest microcosms were used to evaluate the impact of simulated acidic precipitation on decomposition. The following pH treatments: 5.7, 4.5, 4.0, and 3.5 annual average were applied for a 30 mo period. No statistically significant effect of treatment on decomposition could be found in the field measurements. When the microcosm was partitioned into 01 and 02 litter, mineral soil (A and B horizons), and roots within the mineral soil horizons for laboratory determination of CO₂ efflux, only the 02 litter exhibited a statistically significant decrease as a function of treatment. The data collected do not allow a complete evaluation of the potential impact of this decrease. However, efflux of CO₂ from the 02 layer was small compared to the other layers, and this may account for the failure to detect a significant response in the field measurements. Although the field data did not exhibit a significant response, there is sufficient question concerning the 02 response to warrant additional investigation, especially since many plants derive a major portion of their nutritional requirements directly from the 02 litter layer.     

Modeling leaf area index from litter collection and tree data in a deciduous broadleaf forest

Leaf area index (LAI) is an important index in ecological and meteorological studies. The litter trap method is commonly used to measure LAI in deciduous forests. To reduce the time consumed in sorting leaf litterfall by species in the litter trap method, we developed four models to predict LAI using litter traps and tree census data. The local dominance model, which estimates the leaf litterfall amount of each species by their local dominance, predicted mean and spatial variability of LAI most accurately compared to the 2 models that did not take into account spatial heterogeneity of species distribution within a forest or the model that estimated litterfall amount from leaf dispersal function. Therefore, this model can be employed instead of sorting leaf litter by species. Furthermore, we found that leaf mass per area (LMA) of at least 10 dominant species are essential for accurate estimation of LAI. Present results suggest that spatial variability of LAI is mainly due to spatial variance of leaf litterfall followed by spatial heterogeneity of species distribution within a forest, and difference in LMA among species.     

Nine years of measurements of atmospheric nitrogen and sulphur deposition to Danish forest

Since 1985 measurements of gasses, aerosols, precipitation and throughfall have been carried out at three forest sites in Denmark with equal aged Norway Spruce plantations. The times series show a downward trend in the concentration of sulphur dioxide. Particulate sulphate, ammonia and particulate ammonium and the total nitrate seem to have a more constant concentration level. The wet deposition measurements show a decreasing trend in the content of acid (protons), sulphate, ammonium and nitrate, though for the nitrogen compounds it is only a slight fall. A decrease in concentrations of protons and sulphate is also seen in the throughfall measurements, in throughfall the nitrogen compounds hardly seem to decrease.     

A comparison of estimated and measured SO2 deposition velocities

A nested-network program for obtaining data on the dry deposition of SO₂ and SO₄ ^? has been initiated at a small array of locations (6 in 1985, presently 13) across North America. The procedures involved rely on the availability of models for deriving dry deposition rates from observations of air concentrations and of meteorological and surface properties known to influence the deposition velocity. At a subset of locations (i.e., 3), the results obtained by this indirect method are tested by comparison against more direct methods. One of the first comparison experiments of this series was conducted at Oak Ridge in July 1985 when the fluxes determined by inferential methods were compared to those measured by eddy correlation. The results obtained suggest that initial computer routines, developed to estimate deposition velocity for SOz on a routine basis, overestimate the deposition velocity by about 20% to a mixed-species deciduous forest. The difference is possibly due to the omission of water stress as a contributing factor in the initial computer routines, but might also be associated with chemical processes at the substomatal level.     

Emission Control of SO2 by Dry Coal-Cleaning and Bio-Briquette Technology

In China, the large amount of sulfur dioxide and dust discharged from the combustion of low-grade raw coal causes severe air pollution and acid rain. Therefore, the need to control the emission of such pollutants is urgent. It is well known that wet coal-cleaning technology is used to prepare clean coal from low-grade raw coal containing large amounts of sulfur and ash. However, this technology is not used in areas where water is scarce or severely polluted, because of the high cost of treating the wastewater. In an attempt to overcome this limitation, we studied an integrated technology, which combines dry coal-cleaning and bio-briquetting technologies, to prepare clean coal from low-grade raw coal. In the dry coal-cleaning method, refined coal was separated from ash and other minerals containing inorganic sulfur as pyrite by means of the differences in their electrostatic character. Most of the sulfur left in the refined coal was organic sulfur. The residual ash was fixed in combustion ashes of bio-briquettes made from coal, biomass, and slaked lime (Ca/S mole ratio =2) under pressure. By combining these two technologies, we were able to decrease the emission of sulfur dioxide and ash by 70≈90% compared to the combustion of raw coal.     

Atmospheric concentrations and deposition of Hg to A deciduous forest atwalker branch watershed,Tennessee, USA

Aerosol and total vapor-phase Hg concentrations in air have been measured at Walker Branch Watershed, Tennessee for ≈ 2 yr. Airborne Hg at this site is dominated by vapor forms which exhibit a strong seasonal cycle, with summer maxima that correspond to elevated air temperature. Concentrations in this forest are near background levels; however, concentrations at a site within 3 km are significantly elevated due to emissions from Hg-contaminated soils. The concentration data have been combined with a recently modified dry deposition model to estimate dry deposition fluxes to the deciduous forest at Walker Branch. Weekly mean modeled V_d values for Hg° ranged from <0.01 (winter) to > 0.1 (summer) cm s¹. Weekly dry deposition fluxes ranged from <0.1 μg m⁻² during winter to > 1.0 μgg m⁻² in the summer. Our dry deposition estimates plus limited measurements of wet deposition in this area indicate that dry deposition may be the dominant input process in this forest, at least during the summer.     

Critical Loads of SO2 Dry Deposition and Their Exceedance in South China

The critical loads of SO₂ dry deposition in South China,which is transferred from critical level, as well the excess ofcritical loads are computed and mapped. The areas with thelowest critical load and the highest excess are, respectively,identified. The research is complementary to the previousresearches on critical loads for soils, and expected to beintegrated with them to make efficient sulfur emission abatement strategy.     

Mass loss measurements and statistical models to predict decomposition of leaf litter in a boreal aspen forest

Abstract

In a southern boreal aspen forest located in Saskatchewan, Canada, we examined decomposition rates of leaf litter from trembling aspen (Populus tremuloides Michx.), hazel (Corylus cornuta March.), and a mixture of different species over a six‐month period. Mass loss was measured in the field using the litter bag method. The greatest mass losses occurred during the first month regardless of litter type. On average, mass loss during the first 28 days was 3.2 g#lbkg^‐1#lbd^‐1 for the aspen leaves, 4.4 for hazel leaves and 4.9 for the mixture. The initial rapid loss of weight is attributed to leaching and decomposition of water soluble material. The decomposition rates of the leaf litter were related to water‐soluble organic carbon and nitrogen content, and C:N ratio. Several models were used to describe mass loss of the aspen, hazel, and mixed leaf litter at the early stages of decomposition. A single model was not found to be appropriate to describe decomposition of all leaf‐litter types. A second order model provided the best fit for the aspen litter decomposition, while the logarithmic model best described the decomposition of hazel and mixed leaf litter.     

Alterations in forest detritus inputs influence soil carbon concentration and soil respiration in a Central-European deciduous forest

In a Quercetum petraeae–cerris forest in northeastern Hungary, we examined effects of litter input alterations on the quantity and quality soil carbon stocks and soil CO₂ emissions. Treatments at the Síkfőkút DIRT (Detritus Input and Removal Treatments) experimental site include adding (by doubling) of either leaf litter (DL) or wood (DW) (including branches, twigs, bark), and removing all aboveground litter (NL), all root inputs by trenching (NR), or removing all litter inputs (NI). Within 4 years we saw a significant decrease in soil carbon (C) concentrations in the upper 15 cm for root exclusion plots. Decreases in C for the litter exclusion treatments appeared later, and were smaller than declines in root exclusion plots, highlighting the role of root detritus in the formation of soil organic matter in this forest. By year 8 of the experiment, surface soil C concentrations were lower than Control plots by 32% in NI, 23% in NR and 19% in NL. Increases in soil C in litter addition treatments were less than C losses from litter exclusion treatments, with surface C increasing by 12% in DL and 6% in DW. Detritus additions and removals had significant effects on soil microclimate, with decreases in seasonal variations in soil temperature (between summer and winter) in Double Litter plots but enhanced seasonal variation in detritus exclusion plots. Carbon dioxide (CO₂) emissions were most influenced by detritus input quantity and soil organic matter concentration when soils were warm and moist. Clearly changes in detritus inputs from altered forest productivity, as well as altered litter impacts on soil microclimate, must be included in models of soil carbon fluxes and pools with expected future changes in climate.     

Acid deposition and ion leaching from a podzolic soil under hardwood forest

The contribution of atmospheric acids to cation leaching from a podzolic soil under mature maple-birch forest in central Ontario was examined during 1983. The movement of base cations was associated largely with NO₃ ^?, SO₄ ^2? and organic acid anions in surface soil horizons, with SO₄ ^2? and NO₃ ^? below the effective rooting zone, and SO₄ ^2? and HCO₃ ^? in streamflow. Mineral soil horizons could adsorb little additional SO₄ ^2? or associated cations at current soil solution SO₄ ^2? concentrations. Therefore it is concluded that the soil in situ lacks a strong affinity for SO₄ ^2?. Current annual inputs to the forest of SO₄ ^2? and NO₃ ^? in bulk precipitation (26.4 and 18.2 kg ha^?1, equivalent to 8.8 kg S and 4.1 kg N ha^?1 , respectively) contributed significantly to cation leaching from the soil. In order to maintain exchangeable cations in soil at current levels, a rate of weathering yielding 29.6, 5.0, 4.4 and 2.2 kg ha^?1 yr^?1 of Ca²⁺, Na⁺, Mg²⁺ and K⁺, respectively, would be required.     

Microbial ethylene production and inhibition of methanotrophic activity in a deciduous forest soil

Production of C₂H₄, but not of CH₄, was observed in anoxically incubated soil samples (cambisol on loamy sand) from a deciduous forest. Ethylene production was prevented by autoclaving, indicating its microbial origin. Ethylene production gradually decreased from 4 to 12 cm soil depth and was not affected by moisture or addition of methionine, a possible precursor of C₂H₄. Oxidation of atmospheric CH₄ in soil samples was inhibited by C₂H₄. Ethylene concentrations of 3, 6 and 10 μl l⁻¹ decreased CH₄ uptake by 21, 63 and 98%, respectively. Methionine and methanethiol, a possible product of methionine degradation, also inhibited CH₄ oxidation. Under oxic conditions, C₂H₄ was consumed in the soil samples. Ethylene oxidation kinetics exhibited two apparent K_m values of 40 μl l⁻¹ and 12,600 μl l⁻¹ suggesting the presence of two different types of C₂H₄-oxidizing microorganisms. Methanotrophic bacteria were most probably not responsible for C₂H₄ oxidation, since the maximum of C₂H₄ oxidation activity was localized in soil layers (2-8 cm depth) above those (8-10 cm depth) of CH₄ oxidation activity. Our observations suggest that C₂H₄ production in the upper soil layers inhibits CH₄ oxidation, thus being one reason for the localization of methanotrophic activity in deeper soil layers.