Atmospheric deposition in forest edges measured by monitoring canopy throughfall

Atmospheric dry deposition in two forest edges was studied by means of monitoring canopy throughfall in Douglas Fir stands. Throughfall fluxes in the first 50 to 100 m of forest edges were found to be substantially higher than fluxes in the interior of forest stands. Sodium and chloride showed the steepest throughfall flux gradients. Ions important for soil acidification and eutrophication showed relatively less steep but still significant gradients. The mean increase of the throughfall flux at 10 m, with respect to the flux at 200 m from the forest edge amounted to 150% for Na⁺, 119% for Cl^?, 54% for S0₄ ^2?, 38% for NO₃ ^? , and 39% for NH₄ ⁺ The enhancement of dry deposition in forest edges strongly depends on wind velocity and wind direction during dry deposition. Particularly trees in forest edges exposed to prevailing wind directions receive relatively large amounts of dry deposition.     

The Behavior of Nitrogen Isotopes in Acidified Forest Soils in the Czech Republic

The effects of atmospheric deposition on N cycling in acidified soils were studied at three spruce and one beech forested sites in the Czech Republic. Nitrogen content and δ¹⁵N were monitored in bulk and throughfall precipitation, needles, leaves, soils and soil solutions. Changes in soil NO₃ ^- production, effect of admixing of atmospheric N in spruce forest and N consumption in deciduous forest are described using changes in ¹⁵N fractionation of mineralized N in soil. Admixing of atmospheric NH4+ can be identified at low concentrations of exchangeable NH₄ ⁺. The δ¹⁵N ratio of atmospheric NO₃ ^- input is on average by 2‰ less negative than the δ5N ratio in soil water; admixing changes the δ¹⁵N of soil NO₃ ^- detected in lysimeters.     

Rainfall,Stemflow, and Throughfall Chemistry at Urban- and Mountain-facing Sites at Mt. Gokurakuji,Hiroshima, Western Japan

Rainfall, stemflow, and throughfall were collected from 1996 to 1999 at two types of forest sites: (1) forests near the traffic roads and urban areas and (2) forests away from the urban areas at Mt. Gokurakuji, Hiroshima, western Japan in order to estimatethe effects of anthropogenic activities on atmospheric deposition. Rainfall deposition for major ions showed small differences between the sites. The NO₃ ^- and SO₄ ^2-concentrations in stemflow were higher at the urban-facing slope than at the mountain-facing slope. Throughfall deposition of NO₃ ^- and SO₄ ^2- was also higher at urban-facing slopes. Net throughfall (NTF) deposition (throughfall minus rainfall) of NO₃ ^- and SO₄ ^2- accounted for 77 and50% of the total throughfall deposition on urban-facing slopes, respectively, while it accounted for 44 and 23% on themountain-facing slopes, respectively. These results indicated a higher contribution from dry deposition on urban-facing slopes compared to mountain-facing slopes. Atmospheric N (NO₃ ^- +NH₄ ⁺) deposition from throughfall was estimated to be around 17–26 kg N ha^-1 yr^-1 on urban-facing slopes, which was greater than the threshold of N deposition that could cause nitrogen leaching in Europe and the United States. The highload of atmospheric N deposition may be one of the factors bringing about the decline of pine forests on urban-facing slopesof Mt. Gokurakuji.     

Elevational trends in the fluxes of sulphur and nitrogen in throughfall in the Southern Appalachian Mountains: Some surprising results

From 1986–1989, a team of scientists measured atmospheric concentrations and fluxes in precipitation and throughfall, and modeled dry and cloudwater deposition in a spruce-fir forest of the Great Smoky Mountains National Park which is located in the Southern Appalachian Region of the United States. The work was part of the Integrated Forest Study (IFS) conducted at 12 forests in N. America and Europe. The spruce-fir forest at 1740 m consistently received the highest total deposition rates (～2200, 1200, and 700 eq ha^?1 yr^?1 for SO₄ ^2?, NO₃ ^?, and NH₄ ⁺). During the summers of 1989 and 1990 we used multiple samplers to measure hydrologie, SO₄ ^2?, and NO₃ ^? fluxes in rain and throughfall events beneath spruce forests above (1940 m) and below (1720 m) cloud base. Throughfall was used to estimate total deposition using relationships determined during the IFS. Although the SO₄ ^2? fluxes increased with elevation by a factor of ～2 due to higher cloudwater interception at 1940 m, the NO₃ ^? fluxes decreased with elevation by ～30%. To investigate further, we began year round measurements of fluxes of all major ions in throughfall below spruce-fir forests at 1740 m and at 1920 m in 1993–1994. The fluxes of most ions showed a 10–50% increase with elevation due to the ～70 cm yr^?1 cloudwater input at 1920 m. However, total inorganic nitrogen exhibited a 40% lower flux in throughfall at 1920 m than at 1740 m suggesting either higher dry deposition to trees at 1740 m or much higher canopy uptake of nitrogen by trees at 1920 m. Differential canopy absorption of N by trees at different elevations would have significant consequences for the use of throughfall N fluxes to estimate deposition. We used artificial trees to understand the foliar interactions of N.     

Deposition of air pollutants in a wind-exposed forest edge

The atmospheric deposition of air pollutants at a forest edge was studied by means of monitoring canopy throughfall at the edge and at five different parallel lines in the forest behind the edge. The investigation was carried out at a pine forest on the Swedish west coast. Throughfall and bulk deposition samples were analyzed for volume, SO ₄ ^2? , NO ₃ ^? , Cl^?, NH ₄ ⁺ , Na⁺, K⁺, Mg²⁺, Ca²⁺, and for pH. The results show that the throughfall flow at the edge was increased substantially for most ions. The ratios in throughfall flows between the edge and the line 50 m into the forest were for SO ₄ ^2? , 1.5, NO ₃ ^? 2.9, NH ₄ ⁺ 2.7, and Na⁺ 3.1. Since this effect is not only valid for forest edges but also for hillsides, hilltops, and edges between stands of different age, etc., there might be substantial areas which get much larger total deposition than the normally considered closed forest.     

Effects of nitrogen load to the forest floor in sitka spruce stands (Picea sitchensis) as affected by difference in deposition and spruce aphid infestations

Studies of biogeochemical cycling and soil acidification have been carried out in even aged stands of Norway spruce, sitka spruce, Douglas fir, beech and oak under the frame of “The Element Cycling Project”. Deposition of excess nitrogen to forests is important as a potential acidifying input. In Denmark, reduced vitality in Norway spruce has promoted extensive planting of sitka spruce. However, several spruce aphid infestations have caused defoliation in many sitka spruce stands. The objectives of this study were to evaluate the effects of deposition and increased litterfall due to spruce aphid infestations on nitrogen transformations in the forest floor in sitka spruce stands on different soil types. The deposition of throughfall nitrogen range from 19 to 35 kg/ha/year. Fluxes of nitrogen in litterfall ranged from 21 to 77 kg/ha/year, whereas nitrogen leaching range from 1 to 57 kg/ha/year. Leaching was lowest at the infertile sites, but increased with magnitude of deposition and aphid infestations. Proton production according to the nitrogen transformations was largest at the fertile site most often affected by infestations. Huge amounts of bird droppings, honey dew and input of easily available nutrients by canopy leaching probably induced litter decomposition and formation of NO ₃ ^? in the soil water.     

Ermittlung der Gesamtsäuredeposition in nordrheinwestfälischen Fichten- und Buchenbeständen

Measurement of total acid deposition into spruce and beech forests in Northrhine-Westfalia During one year the deposition of H⁺, NH₄⁺, Al³⁺, Fe³⁺ and the acidity (BNC_8,2) in bulk precipitation and throughfall of spruce and beech stands was measured in Northrhine-Westfalia. It is shown that the calculation of acid deposition as the sum of the H⁺-equivalents of (H⁺ + NH₄⁺ + Al³⁺ + Fe³⁺ + Mn²⁺) underestimates total deposition of acidity. A simple and useful alternative is the calculation of H⁺-equivalents from (BNC_8,2 + 0.9 NH₄⁺ + Mn²⁺).     

Cloud Water Deposition to Forest Canopies of Cryptomeria Japonica at Mt.Rokko, Kobe, Japan

Cloud water deposition to canopies of Cryptomeria japonica at Mt.Rokko, Kobe, Japan, was estimated from throughfall measurements and fog water collections carried out during a full year. Annual cloud frequency was 11.5–15.5% and liquid water content (LWC) was 0.059 g/m³. Since cloud water deposition on to forest canopies was significantly correlated with the amount of fog water collected, the former parameter could be quantitatively derived from throughfall measurements. Annual cloud water deposition on to Cryptomeria canopies was 1420–2860 mm (Av. 2140 mm), corresponding to 90–180% (Av. 122%) of annual rainfall. The rate of deposition was higher at the mountain ridge and the forest edge than at the mountain side and the forest interior. Annual deposition of SO₄ ^2?, NO₃ ^?, H⁺ and NH₄ ⁺ from cloud water was estimated as 204, 153, 2.5 and 58 kg/ha, respectively, equivalent to 5.8–11.7 times the corresponding deposition via rain. The values are equal to, or exceed, the maximum deposition reported for Appalachian forests in the eastern United States. Multiple regression analyses indicate that cloud water deposition on to Cryptomeria canopies was significantly correlated with the following three parameters: cloud frequency, LWC, and wind speed. Thus, these three factors apparently control cloud water deposition on to forest canopies.     

Above-Ground Sulfur Cycling in Adjacent Coniferous and Deciduous Forests and Watershed Sulfur Retention in the Georgia Piedmont,U.S.A.

Atmospheric deposition and above-ground cycling of sulfur (S) were evaluated in adjacent deciduous and coniferous forests at the Panola Mountain Research Watershed (PMRW), Georgia, U.S.A. Total atmospheric S deposition (wet plus dry) was 12.9 and 12.7 kg ha-¹ yr-¹ for the deciduous and coniferous forests, respectively, from October 1987 through November 1989. Dry deposition contributes more than 40% to the total atmospheric S deposition, and SO₂ is the major source (～55%) of total dry S deposition. Dry deposition to these canopies is similar to regional estimates suggesting that 60-km proximity to emission sources does not noticeably impact dry deposition at PMRW. Below-canopy S fluxes (throughfall plus stemflow) in each forest are 37% higher annually in the deciduous forest than in the coniferous forest. An excess in below-canopy S flux in the deciduous forest is attributed to leaching and higher dry deposition than in the coniferous forest. Total S deposition to the forest floor by throughfall, stemflow and litterfall was 2.4 and 2.8 times higher in the deciduous and coniferous forests, respectively, than annual S growth requirement for foliage and wood. Although S deposition exceeds growth requirement, more than 95% of the total atmospheric S deposition was retained by the watershed in 1988 and 1989. The S retention at PMRW is primarily due to SO₄ ²- adsorption by iron oxides and hydroxides in watershed soils. The S content in white oak and loblolly pine boles have increased more than 200% in the last 20 yr, possibly reflecting increases in emissions.     

Nitrate leaching in forest soils: an analysis of long‐term monitoring sites in Germany

Elevated atmospheric inputs of NH₄⁺ and NO₃^– have caused N saturation of many forest ecosystems in Central Europe, but the fate of deposited N that is not bounded by trees remains largely unknown. It is expected that an increase of NO₃^– leaching from forest soils may harm the quality of groundwater in many regions. The objective of this study was to analyze the input and output of NH₄⁺ and NO₃^– at 57 sites with mature forest stands in Germany. These long‐term study sites are part of the European Level II program and comprise 17 beech, 14 spruce, 17 pine, and 9 oak stands. The chloride balance method was used to calculate seepage fluxes and inorganic N leaching below the rooting zone for the period from 1996 to 2001. Nitrogen input by throughfall was significantly different among most forest types, and was in the order: spruce > beech/oak > pine. These differences can be largely explained by the amount of precipitation and, thus, it mirrors the regional and climatic distribution of these forest types in Germany. Mean long‐term N output with seepage was log‐normal distributed, and ranged between 0 and 26.5 kg N ha^–1 yr^–1, whereby 29 % of the sites released more than 5 kg N ha^–1 yr ^–1. Leaching of inorganic N was only significantly lower in the pine stands (P < 0.05) compared with leaching rates of the spruce stands. Median N output : input ratio ranged between 0.04 and 0.11 for the beech, oak, and pine stands, while the input : output ratio of the spruce stands was 0.24, suggesting a higher risk of NO₃^– leaching in spruce forests. Following log‐transformation of the data, N input explained 38 % of the variance in N output. The stratification of the data by the C : N ratio of the O horizon or the top mineral soil revealed that forests soils with a C : N ratio < 25 released significantly more NO₃^– (median of 4.6 kg N ha^–1 yr^–1) than forests with a C : N ratio > 25 (median of 0.8 kg N ha^–1 yr^–1). The stratification improved the correlation between N input and N output for sites with C : N ratios < 25 (r² = 0.47) while the correlation for sites with C : N ratios > 25 was weaker (r = 0.21) compared with the complete data set. Our results suggest that NO₃^– leaching may increase in soils with wide C : N ratios when N deposition remains on a high level and that the potential to store inorganic N decreases with C : N ratios in the O horizons becoming more narrow.     

Throughfall deposition of ammonium and sulphate during ammonia fumigation of a scots pine forest

The estimation of the dry deposition of sulphur dioxide to forests is confounded by the possibility of co-deposition of SO₂ with NH₃ on leaf surfaces. A sector of Scots pine forest was selectively fumigated with NH₃ to give average concentrations up to 15 ppbV (nL L^–1) above ambient, in order to test the hypothesis that increased air concentrations of NH₃ would enhance the dry deposition of SO₂, and the consequent amounts of SO₄ ^2– measured in throughfall below the forest canopy. Ammonia gas, generated by evaporation of concentrated aqueous solution, was released above the canopy in proportion to wind speed when the wind direction was between south and west. Concentrations of NH₃ at canopy height were measured using passive diffusion tubes; throughfall was preserved with thymol and measured weekly. Meteorological data and SO₂ concentrations were recorded continuously, to permit the estimation of dry deposition input. Deposition of NH₄ ⁺ in throughfall over 8 months was increased by up to 40 meq m^–2 relative to control sites upwind of the NH₃ release point, with largest values closest to the release point. Deposition of SO₄ ^2– in throughfall was also enhanced in the fumigated area, by up to 20 meq m^–2, even though average ambient SO₂ concentrations were 2.3 ppbV. The results are discussed in terms of the factors controlling SO₂ deposition on forest surfaces, the development of appropriate deposition models, and their relevance to using throughfall as an estimate of total S deposition.     

Are Indicators for Critical Load Exceedance Related to Forest Condition?

The aim of this study was to evaluate the suitability of the (Ca?+?Mg?+?K)/Al and the Ca/Al ratios in soil solution as chemical criteria for forest condition in critical load calculations for forest ecosystems. The tree species Norway spruce, Sitka spruce and beech were studied in an area with high deposition of sea salt and nitrogen in the south-western part of Jutland, Denmark. Throughfall and soil water were collected monthly and analysed for pH, NO₃-N, NH₄-N, K, Ca, Mg, DOC and Al_tot. Organic Al was estimated using DOC concentrations. Increment and defoliation were determined annually, and foliar element concentrations were determined every other year. The throughfall deposition was highest in the Sitka spruce stand (maximum of 40 kg N ha^?1yr^?1) and lowest in the beech stand (maximum of 11 kg N ha^?1yr^?1). The Sitka spruce stand leached on average 12 kg N ha^?1yr^?1 during the period 1988–1997 and leaching increased throughout the period. Only small amounts of N were leached from the Norway spruce stand whereas almost no N was leached from the beech stand. For all tree species, both (Ca?+?Mg?+?K)/Al and Ca/Al ratios decreased in soil solution at 90 cm depth between 1989 and 1999, which was mainly caused by a decrease in concentrations of base cations. The toxic inorganic Al species were by far the most abundant Al species at 90 cm depth. At the end of the measurement period, the (Ca?+?Mg?+?K)/Al ratio was approximately 1 for all species while the Ca/Al ratio was approximately 0.2. The lack of a trend in the increment rates, a decrease in defoliation as well as sufficient levels of Mg and Ca in foliage suggested an unchanged or even slightly improved health condition, despite the decreasing and very low (Ca?+?Mg?+?K)/Al and Ca/Al ratios. The suitability of these soil solution element ratios is questioned as the chemical criteria for soil acidification under field conditions in areas with elevated deposition rates of sea salts, in particular Mg.     

Throughfall quality and quantity in polluted and damaged ecosystems in Northern Bohemia

The interaction between high concentrations of polluting gases (SO₂ and NO_x) and damaged forest ecosystems was observed by studying throughfall precipitation in the Erzegebirge Mountains, Northern Bohemia. Qualitative and quantitative data on throughfall for the period November 1989–October 1990 are presented. Weighted averages of SO ₄ ^2? and NO ₃ ^? concentrations in the throughfall were 23.05 mg L^?1 and 13.61 mg L^?1 in a beech and 34.41 mg L^?1 and 11.03 mg L^?1 in a spruce forest respectively. Three variables (the molar ratios of K/Na, N_tot/S and N-NO₃/N-NE₄) were used to compare the spruce throughfall quality to that observed in areas with similar however, less damaged spruce stands. Both K/Na and N-NO₃/N-NH₄ ratios clearly decreased with increasing tree damage, the N_tot/S ratio increased. The results suggest that the throughfall in damaged ecosystems of the Erzegebirge region becomes more like a wet precipitation as the tree canopies get sparser and the trees reduce canopy leaching.     

Sources of acidity in forest-floor percolate from a maple-birch ecosystem

Ion concentrations in water collected within a forest of sugar maple and yellow birch at the Turkey Lakes Watershed near Sault Ste. Marie, Ontario were examined from 1982 to 1984 to determine sources of acidity and the extent of cation leaching from forest floor horizons. Volume-weighted concentrations and ion fluxes in throughfall and forest-floor percolate during the growing and dormant seasons were calculated. Hydrogen ion content of the forest-floor percolate decreased in relation to that of throughfall in the dormant season and increased in the growing season. Hydrogen ion deposition in throughfall could account for 100% of the flux of H⁺ through the forest floor in the dormant period, and 40% of the flux during the growing season. In forest-floor percolate, Ca²⁺ concentrations were positively correlated with those of SO₄ ^2-, NO₃ ^- and organic anions during both dormant and growing seasons. Sources of NO₃ ^- and organic anions within the ecosystem and major external inputs of NO₃ ^- and SO₄ ^2- were critical factors that influenced cation mobility in the forest floor.     

Effects of drying and rewetting on soluble phosphorus and nitrogen in forest floors: An experiment with undisturbed columns

Drying and rewetting (D/W) of soils often resulted in the release of soluble phosphorus (P) and nitrogen (N), thereby changing the availability of both nutrients. Most experiments on D/W have been conducted with disturbed mineral soil samples and with rewetting of the soil samples by abrupt change in the water potential. Here, we studied the effect of D/W on the leaching of P and N from undisturbed forest floors of a European beech and a Norway spruce site under near field conditions of desiccation and rewetting. We hypothesized that even under realistic rewetting of undisturbed forest floors, the leaching of P and N is increased after D/W and that the effects are less pronounced for spruce than for beech because of the larger hydrophobicity of the spruce forest floor. Undisturbed forest floor columns were subjected to desiccation at 20°C until a matrix potential of –100 MPa (pF 6.0) was reached, while controls were kept at moist conditions. Columns were irrigated by 22 mm day^?1 from day 1–3 and by 10 mm day^?1 from day 4–14 given in automated short pulses. Leachates from the soil columns were analyzed for orthophosphate, total P, NH₄, NO₃, and total N. In the spruce forest floor the concentrations of total P in leachates and the leachate fluxes were strongly increased after D/W. The increase of solute P was less for beech than for spruce coinciding with less actual rewetting of the beech forest floor. Leaching of total N from the spruce forest floor was not affected by D/W, however, concentrations and leaching of NH₄ increased, while leaching of NO₃ decreased. For beech the leaching of total N and NH₄ increased after D/W, while NO₃ leaching decreased. The results indicate that also under realistic conditions, D/W of forest floors increases solute P and leads to changes in the ratio of NH₄/NO₃ in solution, thereby altering the availability of the nutrients.     

Canopy and Soil Retention of Nitrogen Deposition in a Mixed Boreal Forest in Eastern Finland

Nitrogen deposition, leaching, and retention were monitored in a mature spruce (Picea abies Karsten) dominated mixed boreal forest in eastern Finland. Bulk precipitation, throughfall, stemflow, and percolation through the podzolic soil profile were monitored from 1993 to 1996. Mean annual bulk deposition of total N was 3.83 kg ha^-1, of which 33% was NH₄ ⁺, 26% was NO₃ ^- , and 41% was organic N. Throughfall+stemflow flux of total N was 2.93 kg ha^-1 yr^-1. Sixty-four % of NH₄ ⁺ and 38% of NO₃ ^- in bulk precipitation was retained by tre three canopy. Organic N was released (0.27 kg ha^-1 yr^-1) from the tree canopy. Nitrate-N was retained and organic N was leached as the water passed through the ground vegetation and soil O-horizon. Ammonium-N and organic N were retained mainly in the E-horizon. The output of total N from the E-horizon was only 5% of the total N deposition in the forest stand during the study period and it was mainly as organic N. The output of inorganic N forms from under B-horizon was seasonal and occurred mainly at spring snowmelt.     

Atmospheric Deposition of Acidifying Components to a Japanese Cedar Forest

One-year field measurements were conducted in a Japanese cedar (Cryptomeria japonica) forest, located in Gunma Prefecture, Japan. On the basis of the meteorological and atmospheric concentration data, the dry deposition of SO₂, HNO₃, NO₂ and HCl was estimated using the inferential method. The annual dry deposition of H⁺ was estimated at 721 eq ha^?1yr^?1, which was 40% larger than the measured annual wet deposition of H⁺ (514 eq ha^?1yr^?1). Therefore, dry deposition is an important pathway for the atmospheric input of H⁺ to the forest in the study site. The contribution of each gas to the dry deposition of H⁺ was as follows: SO₂, 25%; HNO₃, 32%; NO₂, 10%; and HCl, 33%. The extremely high contribution of HCl appeared to be caused by the high emission intensity of HCl due to waste incineration in the site region. The differences between estimated deposition and throughfall and stemflow measurements indicated that about 80% of the total deposition of H⁺ was taken up by the canopy.     

Deposition and Leaching of Sulphate and Base Cations in a Mixed Boreal Forest in Eastern Finland

Monthly fluxes of sulphate (SO₄ ^2-) and base cations(Ca²⁺, Mg²⁺, K⁺) were studied from 1993 to 1996 as precipitation passed through forest vegetation and surfacesoil layers in an area receiving low and declining levels of atmospheric sulphate pollution. The canopy was dominated by mature Norway spruce (Picea abies Karsten) and the soilwas a podzol developed on glacial till material. The mean annual bulk deposition of SO₄ ^2- collected in the open was 136 mol_c ha^-1 and that of Ca²⁺, Mg²⁺ and K⁺ was 44, 11 and 25 mol_c ha^-1, respectively. The annual total throughfall deposition of SO₄ ^2- was 318 mol_c ha^-1 and that of Ca²⁺, Mg²⁺ and K⁺ was 151, 64 and 181 mol_cha^-1, respectively. Sulphate was the dominant anion accompanying the base cations leached from the canopy. More than half (58%) of the annual total throughfall deposition ofSO₄ ^2- was retained by the O-horizon and only 15% leached from below the B-horizon. The annual leaching of Ca²⁺, Mg²⁺ and K⁺ from below the B-horizon was14, 25 and 9% of the annual total throughfall deposition, respectively. The transport of base cations through the soil was predominantely countered by SO₄ ^2- anions.     

Synthesis of Nitrogen Pools and Fluxes from European Forest Ecosystems

To investigate which ecosystem parameters determine the risk and magnitude of nitrate leaching we compiled data from published and unpublished sources on dissolved inorganic nitrogen (DIN: NO₃ ^-) in throughfall, DIN leaching loss in runoff or seepage water, and other ecosystem characteristics from 139 European forests. Not all data were available for all sites: 126 sites had at least one year's data on DIN inputs and DIN leaching loss; 40-50 sites had some data on soil chemistry and/or vegetation pools of N. DIN inputs in throughfall range between <1 and about 70 kg N ha-1 yr-1 and the losses with seepage or runoff range between <1 and 50 kg N ha-1 yr-1. Retention of N within the ecosystem increases with increasing DIN deposition and increasing proportion of NH₄ ⁺ in deposition. The amount of N in needles and litterfall shows a significant linear relationship with throughfall deposition of DIN, whereas the C:N ratio of the organic (OH) horizon is uncorrelated to the level of throughfall-DIN flux. About 50% of the variability in DIN leaching loss can be explained by the flux of DIN in throughfall. Alternatively, about 60% of the variability in DIN leaching loss can be explained in a two-variable multiple regression combining the C:N ratio of the organic soil and the pH of the mineral soil. The survey data suggest that leaching of DIN from forest ecosystems in Europe is related in part to current DIN deposition and in part to the longer-term internal ecosystem N status as reflected in the chemistry of the humus and acidification status of the soil.     

Methane and nitrous oxide fluxes of soils in pure and mixed stands of European beech and Norway spruce

Tree species can affect the sink and source strength of soils for atmospheric methane and nitrous oxide. Here we report soil methane (CH₄) and nitrous oxide (N₂O) fluxes of adjacent pure and mixed stands of beech and spruce at Solling, Germany. Mean CH₄ uptake rates ranged between 18 and 48 μg C m^?2 hour^?1 during 2.5 years and were about twice as great in both mixed and the pure beech stand as in the pure spruce stand. CH₄ uptake was negatively correlated with the dry mass of the O horizon, suggesting that this diminishes the transport of atmospheric CH₄ into the mineral soil. Mean N₂O emission was rather small, ranging between 6 and 16 μg N m^?2 hour^?1 in all stands. Forest type had a significant effect on N₂O emission only in one mixed stand during the growing season. We removed the O horizon in additional plots to study its effect on gas fluxes over 1.5 years, but N₂O emissions were not altered by this treatment. Surprisingly, CH₄ uptake decreased in both mixed and the pure beech stands following the removal of the O horizon. The decrease in CH₄ uptake coincided with an increase in the soil moisture content of the mineral soil. Hence, O horizons may maintain the gas diffusivity within the mineral soil by storing water which cannot penetrate into the mineral soil after rainfall. Our results indicate that conversion of beech forests to beech–spruce and pure spruce forests could decrease soil CH₄ uptake, while the long‐term effect on N₂O emissions is expected to be rather small.