Nitric oxide and nitrous oxide emission from Hungarian forest soils; linked with atmospheric N-deposition

Studies of forest nitrogen (N) budgets generally measure inputs from the atmosphere in wet and dry deposition and outputs via hydrologic export. Although denitrification has been shown to be important in many wetland ecosystems, emission of N oxides from forest soils is an important, and often overlooked, component of an ecosystem N budget. During 1 year (2002–03), emissions of nitric oxide (NO) and nitrous oxide (N₂O) were measured from Sessile oak and Norway spruce forest soils in northeast Hungary. Accumulation in small static chambers followed by gas chromatography-mass spectrometry detection was used for the estimation of N₂O emission flux. Because there are rapid chemical reactions of NO and ozone, small dynamic chambers were used for in situ NO flux measurements. Average soil emissions of NO were 1.2 and 2.1 μg N m⁻² h⁻¹, and for N₂O were 15 and 20 μg N m⁻² h⁻¹, for spruce and oak soils, respectively. Due to the relatively high soil water content, and low C/N ratio in soil, denitrification processes dominate, resulting in an order of magnitude greater N₂O emission rate compared to NO. The previously determined N balance between the atmosphere and the forest ecosystem was re-calculated using these soil emission figures. The total (dry+wet) atmospheric N-deposition to the soil was 1.42 and 1.59 g N m⁻² yr⁻¹ for spruce and oak, respectively, while the soil emissions are 0.14 and 0.20 g N m⁻² yr⁻¹. Thus, about 10–13% of N compounds deposited to the soil, mostly as and , were transformed in the soil and emitted back to the atmosphere, mostly as greenhouse gas (N₂O).     

A coupled carbon-water-energy-vegetation model to assess responses of temperate forest ecosystems to changes in climate and atmospheric CO2. Part I. Model concept

Predictions of forest ecosystem response to changes in climate and atmospheric CO(2) concentration require hierarchically structured process models. Present forest simulation models have conceptual limitations that restrict their application to climate-change studies. A major drawback of forest succession models is that they often lack physiological details in the simulation of annual tree growth. On the other hand, aggregated ecosystem models assume spatially homogeneous forests, and do not account for successional changes in forest composition and canopy structure. The concept of a new coupled carbon-water-energy-forest vegetation model is presented which attempts to overcome the main limitations of existing models by implementing a modern view of ecological hierarchy and a robust approach for scaling ecological processes in space and time.     

The formation and fate of chlorinated organic substances in temperate and boreal forest soils

Background, aim and scope  Chlorine is an abundant element, commonly occurring in nature either as chloride ions or as chlorinated organic compounds (OCls). Chlorinated organic substances were long considered purely anthropogenic products; however, they are, in addition, a commonly occurring and important part of natural ecosystems. Formation of OCls may affect the degradation of soil organic matter (SOM) and thus the carbon cycle with implications for the ability of forest soils to sequester carbon, whilst the occurrence of potentially toxic OCls in groundwater aquifers is of concern with regard to water quality. It is thus important to understand the biogeochemical cycle of chlorine, both inorganic and organic, to get information about the relevant processes in the forest ecosystem and the effects on these from human activities, including forestry practices. A survey is given of processes in the soil of temperate and boreal forests, predominantly in Europe, including the participation of chlorine, and gaps in knowledge and the need for further work are discussed. Results  Chlorine is present as chloride ion and/or OCls in all compartments of temperate and boreal forest ecosystems. It contributes to the degradation of SOM, thus also affecting carbon sequestration in the forest soil. The most important source of chloride to coastal forest ecosystems is sea salt deposition, and volcanoes and coal burning can also be important sources. Locally, de-icing salt can be an important chloride input near major roads. In addition, anthropogenic sources of OCls are manifold. However, results also indicate the formation of chlorinated organics by microorganisms as an important source, together with natural abiotic formation. In fact, the soil pool of OCls seems to be a result of the balance between chlorination and degradation processes. Ecologically, organochlorines may function as antibiotics, signal substances and energy equivalents, in descending order of significance. Forest management practices can affect the chlorine cycle, although little is at present known about how. Discussion  The present data on the apparently considerable size of the pool of OCls indicate its importance for the functioning of the forest soil system and its stability, but factors controlling their formation, degradation and transport are not clearly understood. It would be useful to estimate the significance and rates of key processes to be able to judge the importance of OCls in SOM and litter degradation. Effects of forest management processes affecting SOM and chloride deposition are likely to affect OCls as well. Further standardisation and harmonisation of sampling and analytical procedures is necessary. Conclusions and perspectives  More work is necessary in order to understand and, if necessary, develop strategies for mitigating the environmental impact of OCls in temperate and boreal forest soils. This includes both intensified research, especially to understand the key processes of formation and degradation of chlorinated compounds, and monitoring of the substances in question in forest ecosystems. It is also important to understand the effect of various forest management techniques on OCls, as management can be used to produce desired effects.     

Polycyclic aromatic hydrocarbons in Norwegian forest soils: impact of long range atmospheric transport

Levels of nine selected polycyclic aromatic hydrocarbons, PAHs, in surface soils from areas in southern and central Norway are presented. Levels in central Norway are generally low, while the southern Norway soils are about ten-fold higher with respect to 4 and 5 ring PAHs. Comparison with air quality data indicates long-range atmospheric transport to be the major source of the excess 4 and 5 ring PAHs in the south. Analyses of peat cores from ombrotrophic bogs support this assumption, and these provide a potentially useful approach for temporal studies of atmospheric PAH deposition. Analytical data for naphthalene in soils depend very much on the sampling and storage of the samples before analysis.     

Influence of transport and trends in atmospheric CO2 at Lampedusa

The study of the CO₂ 15-year records at Lampedusa (35° 31′N, 12° 37′E) is presented in this work. Short- and long-term CO₂ variability has been investigated. No significant diurnal variations are observable thus remarking the background character and representativeness of the observation site. The CO₂ long-term trend shows a mean linear growth rate (GR) of 1.9 ppm yr^?1. The periodic behaviour of the time series has been analysed and the mean seasonal cycle amplitude has been found to be 8.72 ppm. The seasonal cycle amplitude shows a marked interannual variability. The lowest value of the seasonal cycle amplitude has been detected in 2003, in concomitance with the strong anomalous heat wave recorded in Europe. CO₂ GR behaviour has been related to global processes such as El Niño Southern Oscillation (ENSO) and global temperature (T_g). The influence of ENSO event on GR is remarkable only during 1998. CO₂ GR curve shows peaks in the periods 1995, 2001 and 2005 (1.9, 3.7, 3.2 ppm yr^?1 respectively) that are characterized by high T_g values and by intense biomass burning events. The anomalous decrease in the GR during the warm 2003 has been attributed to changes in the atmospheric circulation regime. Evaluation of the influence of transport on CO₂ variability has been carried out using backward air-mass trajectory analysis and highlights the effect of the regional distribution of sources and sinks. The industrial activities and forests located in the Eastern European and Russian sector strongly affect the CO₂ mixing ratio. The CO₂ content of air-masses originating from this region is influenced in summertime by the high efficiency of the vegetation sink while in the winter period prevails the effect of industrial emissions.     

Eutrophication trends in forest soils in Galicia (NW Spain) caused by the atmospheric deposition of nitrogen compounds

We calculated the sensitivity of Galician forest soils to eutrophication caused by atmospheric deposition of nitrogen compounds, using the Simple Mass Balance (SMB) method as described by [Posch, M., de Vries, W., Hettelingh, J.-P., 1995. Critical loads of sulphur and nitrogen. In: Posch, M., de Smet, P.A.M., Hettelingh, J.-P., Downing, R.J. Calculation and Mapping of Critical Thresholds in Europe. Status Report 1995, Coordination Center for Effects, National Institute for Public Health and the Environment, Bilthoven, The Netherlands, pp. 31-42]. Deposition values were used to calculate critical loads exceedance. Galician natural forest ecosystems can support nitrogen deposition loads of more than 10 kg Nha (-1) yr (-1). The lowest critical loads (approximately 10 kg Nha (-1) yr (-1)) mainly occurred in forest stands in the interior zone, while highest critical load values (approximately 68 kg Nha (-1) yr (-1)) were observed in eucalyptus stands at low altitudes in the littoral area. Exceedances based on N deposition levels, calculated from data recorded in 2001, occurred in 40% of the forest soils, showing the need to control N emissions in these areas to prevent possible eutrophication of soils and waters. Analysis of rainfall bulk composition revealed that ammonium, probably derived from agricultural and cattle activities, was the main compound responsible for N deposition in Galicia.     

Factors influencing nitrogen retention in forest soils

Leaching and agitation experiments with soil organic horizons showed that nitrogen pollutant concentration, temperature, contact time and neutral soluble salts influence the fate of enhanced ammonium and nitrate inputs to the soil and the leaching of inorganic and organic nitrogen. Soils investigated included L, F and H horizons under Sitka spruce, the L and F horizons under Scots pine and Japanese larch and L and O horizons under Calluna. Effects attributable to species were also observed. The results are discussed in the light of their relevance to being incorporated into models of the effects of excess nitrogen inputs to forest soils, and in view of current concern that forest ecosystems in areas of high nitrogen deposition may become nitrogen saturated.     

Stable carbon isotope ratio in atmospheric CO2 collected by new diffusive devices

In this paper, stable carbon isotope ratios (δ ¹³C) were determined in the atmosphere by using a Ca-based sorbent, CaO/Ca₁₂Al₁₄O₃₃ 75:25 w/w, for passively collecting atmospheric CO₂, in both field and laboratory experiments. Field measurements were conducted in three environments characterized by different carbon dioxide sources. In particular, the environments under consideration were a rather heavily trafficked road, where the source of CO₂ is mostly vehicle exhaust, a rural unpolluted area, and a private kitchen where the major source of CO₂ was gas combustion. Samplers were exposed to the free atmosphere for 3 days in order to allow collection of sufficient CO₂ for δ¹³C analysis, then the collected CO₂ was desorbed from the adsorbent with acid treatment, and directly analyzed by nondispersive infrared (NDIR) instrument. δ ¹³C results confirmed that the samplers collected representative CO₂ samples and no fractionation occurred during passive trapping, as also confirmed by an appositely designed experiment conducted in the laboratory. Passive sampling using CaO/Ca₁₂Al₁₄O₃₃ 75:25 w/w proved to be an easy and reliable method to collect atmospheric carbon dioxide for δ ¹³C analysis in both indoor and outdoor places.     

Photosynthetic responses to temperature, light flux-density, CO2 concentration and vapour pressure deficit in Eucalyptus tetrodonta grown under CO2 enrichment

Seeds of Eucalyptus tetrodonta were sown under ambient or CO(2) enriched (700 microl litre(-1)) conditions in tropical Australia. Four sets of measurements were made, the first two after 12 months, on trees growing either in pots or planted in the ground. The third and fourth set were made after 18 and 30 months exposure to CO(2) enrichment, on trees growing in the ground. After 12 months exposure to CO(2) enrichment, the rate of light-saturated assimilation (Amax) of plants growing in the ground was determined. Responses of CO(2) assimilation to variations in leaf temperature, leaf-to-air vapour pressure deficit (LAVPD), light flux density and CO(2) concentration were also measured in the laboratory using plants growing in large pots. There was no significant difference in Amax between pot and ground located plants. Assimilation of E. tetrodonta was relatively insensitive to changes in LAVPD for both ambient and CO(2) enriched plants but the temperature optimum of assimilation was increased in plants grown and measured under CO(2) enrichment. Plants grown with CO(2) enrichment had an increased rate of light-saturated assimilation and apparent quantum yield was significantly increased by CO(2) enrichment. In contrast, carboxylation efficiency was decreased significantly by CO(2) enrichment. After 18 months growth with CO(2) enrichment, there was no sign of a decline in assimilation rate compared to measurements undertaken after 12 months. At low LAVPD values, assimilation rate was not influenced by CO(2) treatment but at moderate to high LAVPD, plants grown under CO(2) enrichment exhibited a larger assimilation rate than control plants. Specific leaf area and chlorophyll contents decreased in response to CO(2) enrichment, whilst foliar soluble protein contents and chlorophyll a/b ratios were unaffected by CO(2) treatment. Changes in soluble protein and chlorophyll contents in response to CO(2) enrichment did not account for changes in assimilation between treatments. After 30 months exposure to CO(2) enrichment, the rate of light-saturated assimilation was approximately 50% larger than controls and this enhancement was larger than that observed after 18 months exposure to CO(2) enrichment.     

Major and trace elements in soils in the Central Pyrenees: high altitude soils as a cumulative record of background atmospheric contamination over SW Europe

Background, aim, and scope  

High mountain soils constitute a long-term cumulative record of atmospherically deposited trace elements from both natural and anthropogenic sources. The main aims of this study were to determine the level of major and trace metals (Al, Ti, Mn, Fe, and Zr) of lithologic origin and airborne contaminating trace elements (Ni, Cu, Zn, As, Cd, and Pb) in soils in the Central Pyrenees as an indication of background contamination over SW Europe, to establish whether there is a spatial pattern of accumulation of trace elements in soils as a function of altitude, and to examine whether altitude-related physicochemical properties of soils affect the accumulation of major metals and trace elements.     

Effects of high atmospheric CO2 concentration on root hydraulic conductivity of conifers depend on species identity and inorganic nitrogen source

We examined root hydraulic conductivity (L_p) responses of one-year-old seedlings of four conifers to the combined effects of elevated CO₂ and inorganic nitrogen (N) sources. We found marked interspecific differences in L_p responses to high CO₂ ranging from a 37% increase in P. abies to a 27% decrease in P. menziesii, but these effects depended on N source. The results indicate that CO₂ effects on root water transport may be coupled to leaf area responses under nitrate (NO₃⁻), but not ammonium (NH₄⁺) dominated soils. To our knowledge, this is the first study that highlights the role of inorganic N source and species identity as critical factors that determine plant hydraulic responses to rising atmospheric CO₂ levels. The results have important implications for understanding root biology in a changing climate and for models designed to predict feedbacks between rising atmospheric CO₂, N deposition, and ecohydrology.     

Biomass consumption and CO2, CO and main hydrocarbon gas emissions in an Amazonian forest clearing fire

Biomass consumption and CO₂, CO and hydrocarbon gas emissions in an Amazonian forest clearing fire are presented and discussed. The experiment was conducted in the arc of deforestation, near the city of Alta Floresta, state of Mato Grosso, Brazil. The average carbon content of dry biomass was 48% and the estimated average moisture content of fresh biomass was 42% on wet weight basis. The fresh biomass and the amount of carbon on the ground before burning were estimated as 528 t ha^?1 and 147 t ha^?1, respectively. The overall biomass consumption for the experiment was estimated as 23.9%. A series of experiment in the same region resulted in average efficiency of 40% for areas of same size and 50% for larger areas. The lower efficiency obtained in the burn reported here occurred possibly due to rain before the experiment. Excess mixing ratios were measured for CO₂, CO, CH₄, C₂–C₃ aliphatic hydrocarbons, and PM_2.5. Excess mixing ratios of CH₄ and C₂–C₃ hydrocarbons were linearly correlated with those of CO. The average emission factors of CO₂, CO, CH₄, NMHC, and PM_2.5 were 1,599, 111.3, 9.2, 5.6, and 4.8 g kg^?1 of burned dry biomass, respectively. One hectare of burned forest released about 117,000 kg of CO₂, 8100 kg of CO, 675 kg of CH₄, 407 kg of NMHC and 354 kg of PM_2.5.     

Elevated atmospheric CO2 concentration and temperature across an urban–rural transect

The heat island effect and the high use of fossil fuels in large city centers are well documented, but by how much fossil fuel consumption is elevating atmospheric CO₂ concentrations and whether elevations in both atmospheric CO₂ and air temperature from rural to urban areas are consistently different from year to year are less well known. Our aim was to record atmospheric CO₂ concentrations, air temperature and other environmental variables in an urban area and compare it to suburban and rural sites to see if urban sites are experiencing climates expected globally in the future with climate change. A transect was established from Baltimore city center (Urban site), to the outer suburbs of Baltimore (suburban site) and out to an organic farm (rural site). At each site a weather station was set-up to monitor environmental variables for 5 years. Atmospheric CO₂ was consistently and significantly increased on average by 66 ppm from the rural to the urban site over the 5 years of the study. Air temperature was also consistently and significantly higher at the urban site (14.8 °C) compared to the suburban (13.6 °C) and rural (12.7 °C) sites. Relative humidity was not different between sites whereas the vapor pressure deficit (VPD) was significantly higher at the urban site compared to the suburban and rural sites. An increase in nitrogen deposition at the rural site of 0.6% and 1.0% compared to the suburban and urban sites was small enough not to affect soil nitrogen content. Dense urban areas with large populations and high vehicular traffic have significantly different microclimates compared to outlying suburban and rural areas. The increases in atmospheric CO₂ and air temperature are similar to changes predicted in the short term with global climate change, therefore providing an environment suitable for studying future effects of climate change on terrestrial ecosystems.     

Long-term residue of DDT compounds in forest soils in Maine

Soils in forests sprayed aerially with DDT in 1958-1967 have been sampled for persistence of residues at intervals since then and most recently in 1993. Results of all the samples are presented and show persistence through 30 years, with evidence of decline of residue only in the third decade. The metabolites of DDT--DDE and DDD--increased over time, each comprising about a third of total residue in 1993. Residue continues to be held in the organic mat with little evidence of movement downward to the inorganic soil horizons.     

Retention and distribution of heavy metals in mangrove soils receiving wastewater

The distribution and chemical fractionation of heavy metals retained in mangrove soils receiving wastewater were examined by soil column leaching experiments. The columns, filled with mangrove soils collected from two swamps in Hong Kong and the People's Republic of China, were irrigated three times a week for 150 days with synthetic wastewater containing 4 mg l(-1) Cu, 20 mg l(-1) Zn, 20 mg l(-1) Mn and 0.4 mg l(-1) Cd. Soil columns leached with artificial seawater (without any heavy metals) were used as the control. At the end of the leaching experiments, soil samples from each column were divided into five layers according to its depth viz. 0-1, 1-3, 3-5, 5-10 and > 10 cm, and analyzed for total and extractable heavy metal content. The fractionation of heavy metals in the surface soil samples (0-1 cm) was investigated by the sequential extraction technique. In both types of mangrove soils, the surface layer (0-1 cm) of the columns receiving wastewater had significantly higher concentrations of total Cu, Cd, Mn and Zn than the control. Concentrations declined significantly with soil depth. The proportion of exchangeable heavy metals in soils receiving wastewater was significantly higher than that found in the control, about 30% of the total heavy metals accumulated in the soil masses of the treated columns were extracted by ammonium acetate at pH 4. The sequential extraction results show that in native mangrove soils (the soils without any treatment), the major portion of Cu, Zn, Mn and Cd was associated with the residual and precipitated fractions with very low concentrations in more labile phases. However, in mangrove soils receiving wastewater, a significantly higher percentage of Mn, Zn and Cd was found in the water-soluble and exchangeable fractions. Copper appeared to be more strongly adsorbed in mangrove soils than the other heavy metals. In general, heavy metal accumulation in the surface mangrove soils collected in Hong Kong was higher than those in the PRC, although the metals in the latter soil type were more strongly bound. These findings suggest that whether the heavy metal retained in managrove soils becomes a secondary source or a permanent sink would depend on the kinds of heavy metals and also the types of mangrove soils.     

A passive sampling method for radiocarbon analysis of atmospheric CO2 using molecular sieve

Radiocarbon (¹⁴C) analysis of atmospheric CO₂ can provide information on CO₂ sources and is potentially valuable for validating inventories of fossil fuel-derived CO₂ emissions to the atmosphere. We tested zeolite molecular sieve cartridges, in both field and laboratory experiments, for passively collecting atmospheric CO₂. Cartridges were exposed to the free atmosphere in two configurations which controlled CO₂ trapping rate, allowing collection of sufficient CO₂ in between 1.5 and 10 months at current levels. ¹⁴C results for passive samples were within measurement uncertainty of samples collected using a pump-based system, showing that the method collected samples with ¹⁴C contents representative of the atmosphere. δ¹³C analysis confirmed that the cartridges collected representative CO₂ samples, however, fractionation during passive trapping means that δ¹³C values need to be adjusted by an amount which we have quantified. Trapping rate was proportional to atmospheric CO₂ concentration, and was not affected by exposure time unless this exceeded a threshold. Passive sampling using molecular sieve cartridges provides an easy and reliable method to collect atmospheric CO₂ for ¹⁴C analysis.     

Plant responses to atmospheric CO2 enrichment with emphasis on roots and the rhizosphere

Empirical records provide incontestable evidence of global changes: foremost among these changes is the rising concentration of CO(2) in the earth's atmosphere. Plant growth is nearly always stimulated by elevation of CO(2). Photosynthesis increases, more plant biomass accumulates per unit of water consumed, and economic yield is enhanced. The profitable use of supplemental CO(2) over years of greenhouse practice points to the value of CO(2) for plant production. Plant responses to CO(2) are known to interact with other environmental factors, e.g. light, temperature, soil water, and humidity. Important stresses including drought, temperature, salinity, and air pollution have been shown to be ameliorated when CO(2) levels are elevated. In the agricultural context, the growing season has been shortened for some crops with the application of more CO(2); less water use has generally, but not always, been observed and is under further study; experimental studies have shown that economic yield for most crops increases by about 33% for a doubling of ambient CO(2) concentration. However, there are some reports of negligible or negative effects. Plant species respond differently to CO(2) enrichment, therefore, clearly competitive shifts within natural communities could occur. Though of less importance in managed agro-ecosystems, competition between crops and weeds could also be altered. Tissue composition can vary as CO(2) increases (e.g. higher C: N ratios) leading to changes in herbivory, but tests of crop products (consumed by man) from elevated CO(2) experiments have generally not revealed significant differences in their quality. However, any CO(2)-induced change in plant chemical or structural make-up could lead to alterations in the plant's interaction with any number of environmental factors-physicochemical or biological. Host-pathogen relationships, defense against physical stressors, and the capacity to overcome resource shortages could be impacted by rises in CO(2). Root biomass is known to increase but, with few exceptions, detailed studies of root growth and function are lacking. Potential enhancement of root growth could translate into greater rhizodeposition, which, in turn, could lead to shifts in the rhizosphere itself. Some of the direct effects of CO(2) on vegetation have been reasonably well-studied, but for others work has been inadequate. Among these neglected areas are plant roots and the rhizosphere. Therefore, experiments on root and rhizosphere response in plants grown in CO(2)-enriched atmospheres will be reviewed and, where possible, collectively integrated. To this will be added data which have recently been collected by us. Having looked at the available data base, we will offer a series of hypotheses which we consider as priority targets for future research.     

Pollution influences on atmospheric composition and chemistry at high northern latitudes: Boreal and California forest fire emissions

We analyze detailed atmospheric gas/aerosol composition data acquired during the 2008 NASA ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) airborne campaign performed at high northern latitudes in spring (ARCTAS-A) and summer (ARCTAS-B) and in California in summer (ARCTAS-CARB). Biomass burning influences were widespread throughout the ARCTAS campaign. MODIS data from 2000 to 2009 indicated that 2008 had the second largest fire counts over Siberia and a more normal Canadian boreal forest fire season. Near surface arctic air in spring contained strong anthropogenic signatures indicated by high sulfate. In both spring and summer most of the pollution plumes transported to the Arctic region were from Europe and Asia and were present in the mid to upper troposphere and contained a mix of forest fire and urban influences. The gas/aerosol composition of the high latitude troposphere was strongly perturbed at all altitudes in both spring and summer. The reactive nitrogen budget was balanced with PAN as the dominant component. Mean ozone concentrations in the high latitude troposphere were only minimally perturbed (<5 ppb), although many individual pollution plumes sampled in the mid to upper troposphere, and mixed with urban influences, contained elevated ozone (ΔO₃/ΔCO = 0.11 ± 0.09 v/v). Emission and optical characteristics of boreal and California wild fires were quantified and found to be broadly comparable. Greenhouse gas emission estimates derived from ARCTAS-CARB data for the South Coast Air Basin of California show good agreement with state inventories for CO₂ and N₂O but indicate substantially larger emissions of CH₄. Simulations by multiple models of transport and chemistry were found to be broadly consistent with observations with a tendency towards under prediction at high latitudes.     

Carbon isotope signature of dissolved inorganic carbon (DIC) in precipitation and atmospheric CO2

This paper describes results of chemical and isotopic analysis of inorganic carbon species in the atmosphere and precipitation for the calendar year 2008 in Wroc?aw (SW Poland). Atmospheric air samples (collected weekly) and rainwater samples (collected after rain episodes) were analysed for CO₂ and dissolved inorganic carbon (DIC) concentrations and for δ¹³C composition. The values obtained varied in the ranges: atmospheric CO₂: 337-448 ppm; δ¹³C_CO2 from −14.4 to −8.4‰; DIC in precipitation: 0.6-5.5 mg dm⁻³; δ¹³C_DIC from −22.2 to +0.2‰. No statistical correlation was observed between the concentration and δ¹³C value of atmospheric CO₂ and DIC in precipitation. These observations contradict the commonly held assumption that atmospheric CO₂ controls the DIC in precipitation. We infer that DIC is generated in ambient air temperatures, but from other sources than the measured atmospheric CO₂. The calculated isotopic composition of a hypothetical CO₂ source for DIC forming ranges from −31.4 to −11.0‰, showing significant seasonal variations accordingly to changing anthropogenic impact and atmospheric mixing processes.