Dearomatization drives complexity generation in freshwater organic matter.

Dissolved organic matter (DOM) is one of the most complex, dynamic and abundant sources of organic carbon, but its chemical reactivity remains uncertain1-3. Greater insights into DOM structural features could facilitate understanding its synthesis, turnover and processing in the global carbon cycle4,5. Here we use complementary multiplicity-edited 13C nuclear magnetic resonance (NMR) spectra to quantify key substructures assembling the carbon skeletons of DOM from four main Amazon rivers and two mid-size Swedish boreal lakes. We find that one type of reaction mechanism, oxidative dearomatization (ODA), widely used in organic synthetic chemistry to create natural product scaffolds6-10, is probably a key driver for generating structural diversity during processing of DOM that are rich in suitable polyphenolic precursor molecules. Our data suggest a high abundance of tetrahedral quaternary carbons bound to one oxygen and three carbon atoms (OCqC3 units). These units are rare in common biomolecules but could be readily produced by ODA of lignin-derived and tannin-derived polyphenols. Tautomerization of (poly)phenols by ODA creates non-planar cyclohexadienones, which are subject to immediate and parallel cycloadditions. This combination leads to a proliferation of structural diversity of DOM compounds from early stages of DOM processing, with an increase in oxygenated aliphatic structures. Overall, we propose that ODA is a key reaction mechanism for complexity acceleration in the processing of DOM molecules, creation of new oxygenated aliphatic molecules and that it could be prevalent in nature.

Measurements of Methane Emissions from a Biofertilizer Storage Tank Using Ground-Based Hyperspectral Imaging and Flux Chambers.

Open storages of organic material represent potentially large sources of the greenhouse gas methane (CH4), an emissions source that will likely become more common as a part of societal efforts toward sustainability. Hence, monitoring and minimizing CH4 emissions from such facilities are key, but effective assessment of emissions without disturbing the flux is challenging. We demonstrate the capacity of using a novel high-resolution hyperspectral camera to perform sensitive CH4 flux assessments at such facilities, using as a test case a biofertilizer storage tank for residual material from a biogas plant. The camera and simultaneous conventional flux chamber measurements showed emissions of 6.0 ± 1.3 and 13 ± 5.7 kg of CH4 h-1, respectively. The camera measurements covered the whole tank surface of 1104 m2, and the chamber results were extrapolated from measurements over 5 m2. This corresponds to 0.7-1.4% of the total CH4 production at the biogas plant (1330 N m3 h-1 corresponding to 950 kg h-1). The camera could assess the entire tank emission in minutes without disturbing normal operations at the plant and revealed additional unknown emissions from the inlet to the tank (17 g of CH4 h-1) and during the loading of the biofertilizer into trucks (3.1 kg of CH4 h-1 during loading events). This study illustrates the importance of adequate measurement capacity to map methane fluxes and to verify that methane emission mitigation efforts are effective. Given the high methane emissions observed, it is important to reduce methane emissions from open storage of organic material, for example by improved digestion in the biogas reactor, precooling of sludge before storage, or building gastight storage tanks with sealed covers. We conclude that hyperspectral, ground-based remote sensing is a promising approach for greenhouse gas monitoring and mitigation.

Electronic Nose for Improved Environmental Methane Monitoring.

Reducing emissions of the key greenhouse gas methane (CH4) is increasingly highlighted as being important to mitigate climate change. Effective emission reductions require cost-effective ways to measure CH4 to detect sources and verify that mitigation efforts work. We present here a novel approach to measure methane at atmospheric concentrations by means of a low-cost electronic nose strategy where the readings of a few sensors are combined, leading to errors down to 33 ppb and coefficients of determination, R2, up to 0.91 for in situ measurements. Data from methane, temperature, humidity, and atmospheric pressure sensors were used in customized machine learning models to account for environmental cross-effects and quantify methane in the ppm-ppb range both in indoor and outdoor conditions. The electronic nose strategy was confirmed to be versatile with improved accuracy when more reference data were supplied to the quantification model. Our results pave the way toward the use of networks of low-cost sensor systems for the monitoring of greenhouse gases.

Erratum: Large emissions from floodplain trees close the Amazon methane budget.

This corrects the article DOI: 10.1038/nature24639.

Chlorine Distribution in Soil and Vegetation in Boreal Habitats along a Moisture Gradient from Upland Forest to Lake Margin Wetlands.

The assumed dominance of chloride (Cl-) in terrestrial ecosystems is challenged by observations of extensive formation of organically bound Cl (Clorg), resulting in large soil Cl storage and internal cycling. Yet, little is known about the spatial distribution of Cl in ecosystems. We quantified patterns of Cl distribution in different habitats along a boreal hillslope moisture gradient ranging from relatively dry upland coniferous forests to wet discharge areas dominated by alder. We confirmed that dry habitats are important for Cl storage but found that Cl pools tended to be larger in moist and wet habitats. The storage of Clorg was less important in wet habitats, suggesting a shift in the balance between soil chlorination and dechlorination rates. Cl concentrations in the herb layer vegetation were high in wet and moist sites attributed to a shift in plant species composition, indicating plant community-dependent ecosystem Cl cycling. Mass-balance calculations showed that internal Cl cycling increased overall ecosystem Cl residence times at all sites and that plant uptake rates of Cl- were particularly high at wet sites. Our results indicate that habitat characteristics including plant communities and hydrology are key for understanding Cl cycling in the environment.

Higher Apparent Gas Transfer Velocities for CO2 Compared to CH4 in Small Lakes.

Large greenhouse gas emissions occur via the release of carbon dioxide (CO2) and methane (CH4) from the surface layer of lakes. Such emissions are modeled from the air-water gas concentration gradient and the gas transfer velocity (k). The links between k and the physical properties of the gas and water have led to the development of methods to convert k between gases through Schmidt number normalization. However, recent observations have found that such normalization of apparent k estimates from field measurements can yield different results for CH4 and CO2. We estimated k for CO2 and CH4 from measurements of concentration gradients and fluxes in four contrasting lakes and found consistently higher (on an average 1.7 times) normalized apparent k values for CO2 than CH4. From these results, we infer that several gas-specific factors, including chemical and biological processes within the water surface microlayer, can influence apparent k estimates. We highlight the importance of accurately measuring relevant air-water gas concentration gradients and considering gas-specific processes when estimating k.

Large emissions from floodplain trees close the Amazon methane budget.

Wetlands are the largest global source of atmospheric methane (CH4), a potent greenhouse gas. However, methane emission inventories from the Amazon floodplain, the largest natural geographic source of CH4 in the tropics, consistently underestimate the atmospheric burden of CH4 determined via remote sensing and inversion modelling, pointing to a major gap in our understanding of the contribution of these ecosystems to CH4 emissions. Here we report CH4 fluxes from the stems of 2,357 individual Amazonian floodplain trees from 13 locations across the central Amazon basin. We find that escape of soil gas through wetland trees is the dominant source of regional CH4 emissions. Methane fluxes from Amazon tree stems were up to 200 times larger than emissions reported for temperate wet forests and tropical peat swamp forests, representing the largest non-ebullitive wetland fluxes observed. Emissions from trees had an average stable carbon isotope value (δ13C) of -66.2 ± 6.4 per mil, consistent with a soil biogenic origin. We estimate that floodplain trees emit 15.1 ± 1.8 to 21.2 ± 2.5 teragrams of CH4 a year, in addition to the 20.5 ± 5.3 teragrams a year emitted regionally from other sources. Furthermore, we provide a 'top-down' regional estimate of CH4 emissions of 42.7 ± 5.6 teragrams of CH4 a year for the Amazon basin, based on regular vertical lower-troposphere CH4 profiles covering the period 2010-2013. We find close agreement between our 'top-down' and combined 'bottom-up' estimates, indicating that large CH4 emissions from trees adapted to permanent or seasonal inundation can account for the emission source that is required to close the Amazon CH4 budget. Our findings demonstrate the importance of tree stem surfaces in mediating approximately half of all wetland CH4 emissions in the Amazon floodplain, a region that represents up to one-third of the global wetland CH4 source when trees are combined with other emission sources.

Diel variability of methane emissions from lakes.

Lakes are considered the second largest natural source of atmospheric methane (CH4). However, current estimates are still uncertain and do not account for diel variability of CH4 emissions. In this study, we performed high-resolution measurements of CH4 flux from several lakes, using an automated and sensor-based flux measurement approach (in total 4,580 measurements), and demonstrated a clear and consistent diel lake CH4 flux pattern during stratification and mixing periods. The maximum of CH4 flux were always noted between 10:00 and 16:00, whereas lower CH4 fluxes typically occurred during the nighttime (00:00-04:00). Regardless of the lake, CH4 emissions were on an average 2.4 higher during the day compared to the nighttime. Fluxes were higher during daytime on nearly 80% of the days. Accordingly, estimates and extrapolations based on daytime measurements only most likely result in overestimated fluxes, and consideration of diel variability is critical to properly assess the total lake CH4 flux, representing a key component of the global CH4 budget. Hence, based on a combination of our data and additional literature information considering diel variability across latitudes, we discuss ways to derive a diel variability correction factor for previous measurements made during daytime only.

Practical Guide to Measuring Wetland Carbon Pools and Fluxes.

Wetlands cover a small portion of the world, but have disproportionate influence on global carbon (C) sequestration, carbon dioxide and methane emissions, and aquatic C fluxes. However, the underlying biogeochemical processes that affect wetland C pools and fluxes are complex and dynamic, making measurements of wetland C challenging. Over decades of research, many observational, experimental, and analytical approaches have been developed to understand and quantify pools and fluxes of wetland C. Sampling approaches range in their representation of wetland C from short to long timeframes and local to landscape spatial scales. This review summarizes common and cutting-edge methodological approaches for quantifying wetland C pools and fluxes. We first define each of the major C pools and fluxes and provide rationale for their importance to wetland C dynamics. For each approach, we clarify what component of wetland C is measured and its spatial and temporal representativeness and constraints. We describe practical considerations for each approach, such as where and when an approach is typically used, who can conduct the measurements (expertise, training requirements), and how approaches are conducted, including considerations on equipment complexity and costs. Finally, we review key covariates and ancillary measurements that enhance the interpretation of findings and facilitate model development. The protocols that we describe to measure soil, water, vegetation, and gases are also relevant for related disciplines such as ecology. Improved quality and consistency of data collection and reporting across studies will help reduce global uncertainties and develop management strategies to use wetlands as nature-based climate solutions. Supplementary Information: The online version contains supplementary material available at 10.1007/s13157-023-01722-2.

Global increase in methane production under future warming of lake bottom waters.

Lakes are significant emitters of methane to the atmosphere, and thus are important components of the global methane budget. Methane is typically produced in lake sediments, with the rate of methane production being strongly temperature dependent. Local and regional studies highlight the risk of increasing methane production under future climate change, but a global estimate is not currently available. Here, we project changes in global lake bottom temperatures and sediment methane production rates from 1901 to 2099. By the end of the 21st century, lake bottom temperatures are projected to increase globally, by an average of 0.86-2.60°C under Representative Concentration Pathways (RCPs) 2.6-8.5, with greater warming projected at lower latitudes. This future warming of bottom waters will likely result in an increase in methane production rates of 13%-40% by the end of the century, with many low-latitude lakes experiencing an increase of up to 17 times the historical (1970-1999) global average under RCP 8.5. The projected increase in methane production will likely lead to higher emissions from lakes, although the exact magnitude of the emission increase requires more detailed regional studies.

Ground-based remote sensing of CH4 and N2O fluxes from a wastewater treatment plant and nearby biogas production with discoveries of unexpected sources.

This study is an attempt to assess CH4 and N2O emissions from all the treatment steps of a wastewater treatment plant (WWTP) in Sweden, serving 145 000 persons, and an adjacent biogas production facility. We have used novel mid-IR ground-based remote sensing with a hyperspectral camera to visualize and quantify the emissions on 21 days during a year, with resulting yearly fluxes of 90.4 ± 4.3 tonne CH4/yr and 10.9 ± 1.3 tonne N2O/yr for the entire plant. The most highly emitting CH4 source was found to be sludge storage, which is seldom included in literature as in-situ methods are not suitable for measuring emissions extended over large surfaces, still contributing 90 % to the total CH4 emission in our case. The dominating N2O source was found to be a Stable High rate Ammonia Removal Over Nitrite reactor, contributing 89 % to the total N2O emissions. We also discovered several unexpected CH4 sources. Incomplete flaring of CH4 gave fluxes of at least 30 kg CH4/min, corresponding to plume concentrations of 2.5 %. Such highly episodic fluxes could double the plant-wide yearly emissions if they occur 2 days per year. From a distance of 250 m we found a leak in the biogas production facility, corresponding to 1.1 % of the CH4 produced, and that loading of organic material onto trucks from a biofertilizer storage tank contributed with high emissions during loading events. These results indicate that WWTP emissions globally may have been grossly underestimated and that it is essential to have effective methods that can measure all types of fluxes, and discover new potential sources, in order to make adequate priorities and to take effective actions to mitigate greenhouse gas emissions from WWTPs.

Turbulence in a small boreal lake: Consequences for air-water gas exchange.

The hydrodynamics within small boreal lakes have rarely been studied, yet knowing whether turbulence at the air-water interface and in the water column scales with metrics developed elsewhere is essential for computing metabolism and fluxes of climate-forcing trace gases. We instrumented a humic, 4.7 ha, boreal lake with two meteorological stations, three thermistor arrays, an infrared (IR) camera to quantify surface divergence, obtained turbulence as dissipation rate of turbulent kinetic energy (ε) using an acoustic Doppler velocimeter and a temperature-gradient microstructure profiler, and conducted chamber measurements for short periods to obtain fluxes and gas transfer velocities (k). Near-surface ε varied from 10-8 to 10-6 m2 s-3 for the 0-4 m s-1 winds and followed predictions from Monin-Obukhov similarity theory. The coefficient of eddy diffusivity in the mixed layer was up to 10-3 m2 s-1 on the windiest afternoons, an order of magnitude less other afternoons, and near molecular at deeper depths. The upper thermocline upwelled when Lake numbers (L N ) dropped below four facilitating vertical and horizontal exchange. k computed from a surface renewal model using ε agreed with values from chambers and surface divergence and increased linearly with wind speed. Diurnal thermoclines formed on sunny days when winds were < 3 m s-1, a condition that can lead to elevated near-surface ε and k. Results extend scaling approaches developed in the laboratory and for larger water bodies, illustrate turbulence and k are greater than expected in small wind-sheltered lakes, and provide new equations to quantify fluxes.

Forest streams are important sources for nitrous oxide emissions.

Streams and river networks are increasingly recognized as significant sources for the greenhouse gas nitrous oxide (N2 O). N2 O is a transformation product of nitrogenous compounds in soil, sediment and water. Agricultural areas are considered a particular hotspot for emissions because of the large input of nitrogen (N) fertilizers applied on arable land. However, there is little information on N2 O emissions from forest streams although they constitute a major part of the total stream network globally. Here, we compiled N2 O concentration data from low-order streams (~1,000 observations from 172 stream sites) covering a large geographical gradient in Sweden from the temperate to the boreal zone and representing catchments with various degrees of agriculture and forest coverage. Our results showed that agricultural and forest streams had comparable N2 O concentrations of 1.6 ± 2.1 and 1.3 ± 1.8 µg N/L, respectively (mean ± SD) despite higher total N (TN) concentrations in agricultural streams (1,520 ± 1,640 vs. 780 ± 600 µg N/L). Although clear patterns linking N2 O concentrations and environmental variables were difficult to discern, the percent saturation of N2 O in the streams was positively correlated with stream concentration of TN and negatively correlated with pH. We speculate that the apparent contradiction between lower TN concentration but similar N2 O concentrations in forest streams than in agricultural streams is due to the low pH (<6) in forest soils and streams which affects denitrification and yields higher N2 O emissions. An estimate of the N2 O emission from low-order streams at the national scale revealed that ~1.8 × 109  g N2 O-N are emitted annually in Sweden, with forest streams contributing about 80% of the total stream emission. Hence, our results provide evidence that forest streams can act as substantial N2 O sources in the landscape with 800 × 109  g CO2 -eq emitted annually in Sweden, equivalent to 25% of the total N2 O emissions from the Swedish agricultural sector.

Methane fluxes show consistent temperature dependence across microbial to ecosystem scales.

Methane (CH4) is an important greenhouse gas because it has 25 times the global warming potential of carbon dioxide (CO2) by mass over a century. Recent calculations suggest that atmospheric CH4 emissions have been responsible for approximately 20% of Earth's warming since pre-industrial times. Understanding how CH4 emissions from ecosystems will respond to expected increases in global temperature is therefore fundamental to predicting whether the carbon cycle will mitigate or accelerate climate change. Methanogenesis is the terminal step in the remineralization of organic matter and is carried out by strictly anaerobic Archaea. Like most other forms of metabolism, methanogenesis is temperature-dependent. However, it is not yet known how this physiological response combines with other biotic processes (for example, methanotrophy, substrate supply, microbial community composition) and abiotic processes (for example, water-table depth) to determine the temperature dependence of ecosystem-level CH4 emissions. It is also not known whether CH4 emissions at the ecosystem level have a fundamentally different temperature dependence than other key fluxes in the carbon cycle, such as photosynthesis and respiration. Here we use meta-analyses to show that seasonal variations in CH4 emissions from a wide range of ecosystems exhibit an average temperature dependence similar to that of CH4 production derived from pure cultures of methanogens and anaerobic microbial communities. This average temperature dependence (0.96 electron volts (eV)), which corresponds to a 57-fold increase between 0 and 30°C, is considerably higher than previously observed for respiration (approximately 0.65 eV) and photosynthesis (approximately 0.3 eV). As a result, we show that both the emission of CH4 and the ratio of CH4 to CO2 emissions increase markedly with seasonal increases in temperature. Our findings suggest that global warming may have a large impact on the relative contributions of CO2 and CH4 to total greenhouse gas emissions from aquatic ecosystems, terrestrial wetlands and rice paddies.

Structure, function and resilience to desiccation of methanogenic microbial communities in temporarily inundated soils of the Amazon rainforest (Cunia Reserve, Rondonia).

The floodplain of the Amazon River is a large source for the greenhouse gas methane, but the soil microbial communities and processes involved are little known. We studied the structure and function of the methanogenic microbial communities in soils across different inundation regimes in the Cunia Reserve, encompassing nonflooded forest soil (dry forest), occasionally flooded Igapo soils (dry Igapo), long time flooded Igapo soils (wet Igapo) and sediments from Igarape streams (Igarape). We also investigated a Transect (four sites) from the water shoreline into the dry forest. The potential and resilience of the CH4 production process were studied in the original soil samples upon anaerobic incubation and again after artificial desiccation and rewetting. Bacterial and archaeal 16S rRNA genes and methanogenic mcrA were always present in the soils, except in dry forest soils where mcrA increased only upon anaerobic incubation. NMDS analysis showed a clear effect of desiccation and rewetting treatments on both bacterial and archaeal communities. However, the effects of the different sites were less pronounced, with the exception of Igarape. After anaerobic incubation, methanogenic taxa became more abundant among the Archaea, while there was only little change among the Bacteria. Contribution of hydrogenotrophic methanogenesis was usually around 40%. After desiccation and rewetting, we found that Firmicutes, Methanocellales and Methanosarcinaceae became the dominant taxa, but rates and pathways of CH4 production stayed similar. Such change was also observed in soils from the Transects. The results indicate that microbial community structures of Amazonian soils will in general be strongly affected by flooding and drainage events, while differences between specific field sites will be comparatively minor.

Large but variable methane production in anoxic freshwater sediment upon addition of allochthonous and autochthonous organic matter.

An important question in the context of climate change is to understand how CH4 production is regulated in anoxic sediments of lakes and reservoirs. The type of organic carbon (OC) present in lakes is a key factor controlling CH4 production at anoxic conditions, but the studies investigating the methanogenic potential of the main OC types are fragmented. We incubated different types of allochthonous OC (alloOC; terrestrial plant leaves) and autochthonous OC (autoOC; phytoplankton and two aquatic plants species) in an anoxic sediment during 130 d. We tested if (1) the supply of fresh alloOC and autoOC to an anoxic refractory sediment would fuel CH4 production and if (2) autoOC would decompose faster than alloOC. The addition of fresh OC greatly increased CH4 production and the δ13C-CH4 partitioning indicated that CH4 originated exclusively from the fresh OC. The large CH4 production in an anoxic sediment fueled by alloOC is a new finding which indicates that all systems with anoxic conditions and high sedimentation rates have the potential to be CH4 emitters. The autoOC decomposed faster than alloOC, but the total CH4 production was not higher for all autoOC types, one aquatic plant species having values as low as the terrestrial leaves, and the other one having values as high as phytoplankton. Our study is the first to report such variability, suggesting that the extent to which C fixed by aquatic plants is emitted as greenhouse gases or buried as OC in sediment could more generally differ between aquatic vegetation types.

Influence of Multiple Environmental Factors on Organic Matter Chlorination in Podsol Soil.

Natural chlorination of organic matter is common in soils. The abundance of chlorinated organic compounds frequently exceeds chloride in surface soils, and the ability to chlorinate soil organic matter (SOM) appears widespread among microorganisms. Yet, the environmental control of chlorination is unclear. Laboratory incubations with 36Cl as a Cl tracer were performed to test how combinations of environmental factors, including levels of soil moisture, nitrate, chloride, and labile organic carbon, influenced chlorination of SOM from a boreal forest. Total chlorination was hampered by addition of nitrate or by nitrate in combination with water but enhanced by addition of chloride or most additions including labile organic matter (glucose and maltose). The greatest chlorination was observed after 15 days when nitrate and water were added together with labile organic matter. The effect that labile organic matter strongly stimulated the chlorination rates was confirmed by a second independent experiment showing higher stimulation at increased availability of labile organic matter. Our results highlight cause-effect links between chlorination and the studied environmental variables in podsol soil-with consistent stimulation by labile organic matter that did overrule the negative effects of nitrate.

Oxidative mitigation of aquatic methane emissions in large Amazonian rivers.

The flux of methane (CH4 ) from inland waters to the atmosphere has a profound impact on global atmospheric greenhouse gas (GHG) levels, and yet, strikingly little is known about the dynamics controlling sources and sinks of CH4 in the aquatic setting. Here, we examine the cycling and flux of CH4 in six large rivers in the Amazon basin, including the Amazon River. Based on stable isotopic mass balances of CH4 , inputs and outputs to the water column were estimated. We determined that ecosystem methane oxidation (MOX) reduced the diffusive flux of CH4 by approximately 28-96% and varied depending on hydrologic regime and general geochemical characteristics of tributaries of the Amazon River. For example, the relative amount of MOX was maximal during high water in black and white water rivers and minimal in clear water rivers during low water. The abundance of genetic markers for methane-oxidizing bacteria (pmoA) was positively correlated with enhanced signals of oxidation, providing independent support for the detected MOX patterns. The results indicate that MOX in large Amazonian rivers can consume from 0.45 to 2.07 Tg CH4 yr(-1) , representing up to 7% of the estimated global soil sink. Nevertheless, climate change and changes in hydrology, for example, due to construction of dams, can alter this balance, influencing CH4 emissions to atmosphere.

Temperature-controlled organic carbon mineralization in lake sediments.

Peatlands, soils and the ocean floor are well-recognized as sites of organic carbon accumulation and represent important global carbon sinks. Although the annual burial of organic carbon in lakes and reservoirs exceeds that of ocean sediments, these inland waters are components of the global carbon cycle that receive only limited attention. Of the organic carbon that is being deposited onto the sediments, a certain proportion will be mineralized and the remainder will be buried over geological timescales. Here we assess the relationship between sediment organic carbon mineralization and temperature in a cross-system survey of boreal lakes in Sweden, and with input from a compilation of published data from a wide range of lakes that differ with respect to climate, productivity and organic carbon source. We find that the mineralization of organic carbon in lake sediments exhibits a strongly positive relationship with temperature, which suggests that warmer water temperatures lead to more mineralization and less organic carbon burial. Assuming that future organic carbon delivery to the lake sediments will be similar to that under present-day conditions, we estimate that temperature increases following the latest scenarios presented by the Intergovernmental Panel on Climate Change could result in a 4-27 per cent (0.9-6.4 Tg C yr(-1)) decrease in annual organic carbon burial in boreal lakes.

Experimental evidence of large changes in terrestrial chlorine cycling following altered tree species composition.

Organochlorine molecules (Clorg) are surprisingly abundant in soils and frequently exceed chloride (Cl(-)) levels. Despite the widespread abundance of Clorg and the common ability of microorganisms to produce Clorg, we lack fundamental knowledge about how overall chlorine cycling is regulated in forested ecosystems. Here we present data from a long-term reforestation experiment where native forest was cleared and replaced with five different tree species. Our results show that the abundance and residence times of Cl(-) and Clorg after 30 years were highly dependent on which tree species were planted on the nearby plots. Average Cl(-) and Clorg content in soil humus were higher, at experimental plots with coniferous trees than in those with deciduous trees. Plots with Norway spruce had the highest net accumulation of Cl(-) and Clorg over the experiment period, and showed a 10 and 4 times higher Cl(-) and Clorg storage (kg ha(-1)) in the biomass, respectively, and 7 and 9 times higher storage of Cl(-) and Clorg in the soil humus layer, compared to plots with oak. The results can explain why local soil chlorine levels are frequently independent of atmospheric deposition, and provide opportunities for improved modeling of chlorine distribution and cycling in terrestrial ecosystems.