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Phosphomonoesterases play an important role in the soil phosphorus (P) cycle since they hydrolyze P monoester to phosphate. Their activity is generally measured in soil extracts, and thus, it remains uncertain how mobile these enzymes are and to which extent they can be translocated within the soil profile. The presence of phosphomonoesterases in soil solutions potentially affects the share of labile dissolved organic P (DOP), which in turn would affect P leaching. Our study aimed at assessing the production and leaching of phosphomonoesterases from organic layers and topsoil horizons in forest soils and its potential effect on dissolved P forms in leachates obtained from zero-tension lysimeters. We measured phosphomonoesterase activities in leached soil solutions and compared it with those in water extracts from litter, Oe/Oa, and A horizons of two beech forests with a contrasting nitrogen (N) and P availability, subjected to experimental N × P fertilization. In addition, we determined phosphate and DOP. In soil solutions leached from litter, Oe/Oa, and A horizons, phosphomonoesterase activities ranged from 2 to 8 μmol L–1 h–1 during summer, but remained below detection limits in winter. The summer values represent 0.1–1% of the phosphomonoesterase activity in soil extracts, indicating that enzymes can be translocated from organic layers and topsoils to greater soil depths. Activities of phosphomonoesterases obtained by water extracts were greater in the organic layer of the P-poor site, while activities of those in soil solutions were similar at the two sites. Nitrogen addition increased phosphomonoesterase activities in leached soil solutions of the organic layer of the N- and P-poor soil. Using a modeling approach, we estimated that approx. 76% of the initial labile DOP was hydrolyzed to dissolved inorganic P within the first 24 h. Back calculations from measured labile DOP revealed an underestimation of approx. 15% of total dissolved P, or 0.03 mg L–1. The observed leaching of phosphomonoesterases implies that labile organic P could be hydrolyzed in deeper soil horizons and that extended sample storage leads to an underestimation of the contribution of DOP to total dissolved P leaching. This has been neglected in the few field studies measuring DOP leaching.

Soil organic matter (SOM) is sensitive to land use and to physico-chemical soil properties as well soil microbial communities controlling SOM stabilization. Our study aimed at exploring how carbonates of Mediterranean soils – known to stabilize SOM by enhancing soil aggregation and binding SOM by calcium – are affecting SOM and microbial communities under different land uses. In soils with and without carbonates sampled in forests, cultivated and abandoned fields, we have fractionated SOM-pools according to particle sizes and determined C mineralization rates and polyvalent cations in these fractions. In agreement with larger scale assessments in the Mediterranean, our results show that SOM contents are greater in forest than in agricultural soils, but that the differences among land use types depend on carbonate contents. While SOM contents are greater in carbonated soils of forests, cultivated soils have low SOM levels independent of the carbonate content, presumably due to a reduced protection of particulate SOM by century-long intense tillage. Forest soils contain particularly large amounts of SOM associated with the coarser soil fraction. In this fraction, C mineralization was smallest in the carbonated forest soil, possibly due to a low SOM quality indicated by high C/N ratios and low δ15N values. Greater contents of Ca, Fe and Al in carbonated soils also suggests that stabilization through Ca and phyllosilicates was important. Mineralization of sand-associated SOM was highest in cultivated and abandoned soils, which is indicative for a small SOM stabilization, which in turn might restrict SOM accumulation after land abandonment. Microbial communities assessed by phospho-lipid fatty acid signatures and microbial carbon use efficiency were more closely associated with land use than with carbonate content – a pattern that reflects the distribution of C pools among size fractions which differed more strongly among land use types than between soils with and without carbonates.

Phosphorus (P) is a key nutrient but still we have a limited knowledge on the controls of mobilization and fluxes of P in forest soils. Our study explored the linkages between P mobilization in organic horizons and mineral soils and the P status of soils, as affected by two consecutive drying and rewetting (D/W) cycles. We sampled litter layers (Oi), mixed Oe-Oa horizons, and A horizons in three beech forests along a P availability gradient in Germany. Carbon mineralization and release of dissolved organic matter (DOC, DOP) and dissolved inorganic P (DIP) were studied in microcosms exposed to an initial harsh drying (40 °C for 72 h) and a moderate dry spell (1 month at 20 °C). In Oi horizons, net P mineralization decreased with decreasing P status despite a similar C mineralization at all sites. This supports the general concept that the stoichiometric difference between substrate and microbial biomass primarily drives P release from decomposing organic matter. Counterintuitively, P mobilization per unit soil P increased towards P-poor sites in the mineral soil, likely due to decreasing contents of reactive secondary minerals and the consequently smaller P sorption. Drying and rewetting caused stronger mobilization of DIP and DOP (+108% on average) than of DOC (+51%). The parallel decline in specific UV absorptivity of DOM suggests that lysis of microbial cells drove the drought-induced P release. The D/W effects on P mobilization were particularly strong in P-poor soils, where greater portions of P are bound to microbial biomass, which are prone to become released upon rewetting. Since mobilized P can potentially be leached from soils, our findings indicate, that drought-induced P mobilization fosters the progressive P depletion of already P-poor soils. The possible P leaching losses from mineral soils seem rather be driven by soil mineralogy than by P status.

Understanding controls on the persistence of soil organic matter (SOM) is essential to constrain its role in the carbon cycle and inform climate-carbon cycle model predictions. Emerging concepts regarding formation and turnover of SOM imply that it is mainly comprised of mineral-stabilized microbial products and residues, however, direct evidence in support of this concept remains limited. Here, we introduce and test a method for isolation of isoprenoid and branched glycerol dialkyl glycerol tetraethers (GDGTs) – diagnostic membrane lipids of archaea and bacteria, respectively – for subsequent natural abundance radiocarbon analysis. The method is applied to depth profiles from two Swiss pre-alpine forested soils. We find that the ∆14C values of these microbial markers markedly decrease with increasing soil depth, indicating turnover times of millennia in mineral subsoils. The contrasting metabolisms of the GDGT-producing microorganisms indicates it is unlikely that the low ∆14C values of these membrane lipids reflect heterotrophic acquisition of 14C-depleted carbon. We therefore attribute the 14C-depleted signatures of GDGTs to their physical protection through association with mineral surfaces. These findings thus provide strong evidence for the presence of stabilized microbial necromass in forested mineral soils.

Soil nitrogen (N) availability is critical for grassland functioning. However, human activities have increased the supply of biologically limiting nutrients, and changed the density and identity of mammalian herbivores. These anthropogenic changes may alter net soil N mineralization (soil net Nmin), that is, the net balance between N mineralization and immobilization, which could severely impact grassland structure and functioning. Yet, to date, little is known about how fertilization and herbivore removal individually, or jointly, affect soil net Nmin across a wide range of grasslands that vary in soil and climatic properties. Here we collected data from 22 grasslands on five continents, all part of a globally replicated experiment, to assess how fertilization and herbivore removal affected potential (laboratory‐based) and realized (field‐based) soil net Nmin. Herbivore removal in the absence of fertilization did not alter potential and realized soil net Nmin. However, fertilization alone and in combination with herbivore removal consistently increased potential soil net Nmin. Realized soil net Nmin, in contrast, significantly decreased in fertilized plots where herbivores were removed. Treatment effects on potential and realized soil net Nmin were contingent on site‐specific soil and climatic properties. Fertilization effects on potential soil net Nmin were larger at sites with higher mean annual precipitation (MAP) and temperature of the wettest quarter (T.q.wet). Reciprocally, realized soil net Nmin declined most strongly with fertilization and herbivore removal at sites with lower MAP and higher T.q.wet. In summary, our findings show that anthropogenic nutrient enrichment, herbivore exclusion and alterations in future climatic conditions can negatively impact soil net Nmin across global grasslands under realistic field conditions. This is an important context‐dependent knowledge for grassland management worldwide.

Wildfires are a key component in the global carbon (C) cycle, releasing significant amount of CO2 to the atmosphere, but also producing one of the most persistent terrestrial organic C forms by the incomplete combustion of natural vegetation, namely fire-derived or pyrogenic carbon (PyC). Post-fire, PyC is deposited on the soil surface, but can then be laterally redistributed by wind or water erosion. Due to the lack of vegetation cover after a fire, primary factors that govern rate of PyC distribution post-fire will be changes in soil surface properties and physical characteristics of PyC. However, the drivers and quantities of transported PyC (and non-fire derived organic matter) by water erosion from its site of production remain largely unknown, which limits our understanding of PyC movement in the landscape.In this study, we tracked PyC erosion (and movement in soil) in a controlled experiment using a gravity-type rainfall simulator (at an intensity of 50mmh-1). We studied the quantity of PyC eroded from the soil surface (runoff and splash sediment) as well as its vertical movement within the soil (soil cores) for two Swiss temperate forest soils (initial soil moisture of 12-16%). We calculated the distribution of PyC using the isotopic 13C signature differences of grass-PyC (Miscanthus grass), wood-PyC (spruce wood grown under FACE conditions) and natural forest soil C in soil flume of 0.25m2. We studied the combination of the following factors: soil texture (sandy silt and clay loam), slope (10° and 25°), PyC particle size (63µm and 63µm-2mm) and PyC feedstock (grass and wood) to identify the major drivers of PyC redistribution.We hypothesize that: i) higher quantities of PyC can be found in the sediment of the sandy silt soil, with higher slopes and with bigger PyC particles, ii) higher quantities of PyC move vertically in the clay loam soil and with smaller PyC particles and iii) PyC is preferentially eroded compared to bulk SOC.Results suggest that we find 100 times more runoff sediment on sandy silt soil compared to clay loam soil, and two times more runoff sediment on 25° slope compared to 10°. We also find 1.5 times more splash erosion on sandy silt soil than on the clay loam soil. Regarding PyC erosion, observations suggest that > 50% of initial PyC is eroded on the sandy silt soil, whereas a majority of the PyC particles moves vertically through the soil for the clay loam.

Glycerol dialkyl glycerol tetraethers (GDGTs), membrane lipids synthesized by archaea (isoprenoid GDGTs) and bacteria (branched GDGTs), form the basis of a suite of molecular proxies used in terrestrial as well as marine environments. Compound-specific radiocarbon analysis has provided valuable insights into the sources and yielded constraints on transport dynamics of different biomarkers in the context of carbon cycle processes. To complement the existing biomarker radiocarbon toolbox, and to shed new light on the sources and fate of GDGTs, we developed a new method to measure GDGT radiocarbon compositions in natural samples.Isoprenoid and branched GDGTs are isolated using two UHPLC silica columns in series coupled to a fraction collector set to eluent recovery at different time intervals. The accuracy of the method was tested using a modern and a radiocarbon-dead reference material. Procedural blanks show that the separation procedure adds less than 3 µg carbon with a Fm of 0.64.The method is first applied to determine the Δ14C composition of isoprenoid and branched GDGTs in two soil core profiles from a temperate and subalpine forest ecosystem in order to explore the range of typical values encountered in natural systems. The cores, which reach a depth of 80 cm and 40 cm respectively, have previously been analyzed with respect to radiocarbon characteristics of long-chain n-alkanes and fatty acids as well as bulk particulate and dissolved organic carbon (OC) [1]. For each core, GDGTs were separated and analyzed from 3 different depth intervals. The Δ14C of both isoprenoid and branched GDGTs decreases, at a similar rate as the bulk, by -350‰ and -200‰ along the temperate and the subalpine core respectively, hence confirming their potential for constraining transport-dynamics of soil-derived matter in rivers.The radiocarbon age of GDGTs in a suite of fluvial sediments is older than expected under the assumption that topsoil-derived organic matter is the main source of the compounds. Potentially, this offset could be caused by rapid degradation of the compounds during transport and therefore alter the proxy signal on the way to sedimentary archives. [1] van der Voort, T. S., et al., 2017 - Geophysical Research Letters 44, 23

Enhanced frequency and intensity of freeze-thaw cycle (FTC) owing to global climate change may influence soil carbon (C) and phosphorus (P) cycling in terrestrial ecosystems. However, a comprehensive understanding of soil C and P cycling in response to FTC is still lacking. Here, we compiled data of 2471 observations from 75 publications and conducted a meta-analysis on the responses of soil C and P cycling and the stoichiometry of C, N and P cycling to FTC. Results showed that experimental FTC significantly increased soil dissolved organic C (+38%), instant and cumulative CH4 (+41% and +59%, respectively), dissolved organic C leaching (+62%), total salt-extractable P (+27%), dissolved organic P (+9.4%), leaching of dissolved total P (+312%), dissolved organic P (+30%), and dissolved inorganic P (+115%), and the ratio of available N to P (+21%). In contrast, soil microbial biomass C (−10%), cellulase activity (−16%), microbial biomass P (−10%), and the ratio of microbial biomass C to nitrogen (−8.1%) significantly decreased under FTC treatments. The likely reason for the increases in soluble soil C and P after FTC is the C and P release from dead soil microorganisms and changes in soil structure enhancing organic matter availability. The mean effect size of FTC generally increased with increasing FTC intensity, which was probably also the main reason for higher responses of soil C and P pools and fluxes to FTC observed in laboratory than in field experiments. However, mean effect sizes of FTC generally decreased with increasing duration and frequency of FTC, very likely due to substrate depletion through microbial uptake and leaching. The results of this meta-analysis contribute to a better understanding of the overall responses of soil C and P pools and fluxes to FTC, providing the basis for more accurate prediction of the impacts of future global climate change on biogeochemical cycles.

Climate warming and elevated CO2 can modify nutrient cycling mediated by enzymes in soils, especially in cold-limited ecosystems with a low availability of nutrients and a high temperature sensitivity of decomposition and mineralization. We estimated responses of soil extracellular enzyme activities (EEAs) to 6 years of soil warming and 9 years of CO2 enrichment at an Alpine treeline site. EEAs were measured in the litter (L), fermentation (F) and humified (H) horizons under Larix decidua and Pinus uncinata trees. Soil warming indirectly affected EEAs through altered soil moisture, fine root biomass, and C:N ratio of the organic horizons. Warming increased β-glucosidase and β-xylosidase activities in the F horizon but led to reduced laccase activity in the L horizon, probably caused by drying of the litter horizon associated with the treatment. In the H horizon, previous CO2 enrichment altered the activity of leucine amino peptidase, N-acetylglucosaminidase, and phosphatase. No interactive effects between warming and CO2 enrichment were detected. Warming affected the temperature sensitivity of β-xylosidase but not of the other enzymes. Altered EEAs after six years of soil warming indicate a sustained stimulation of carbon, nitrogen and nutrient cycling under climatic warming at the alpine treeline.

Carbon (C), nitrogen (N), and phosphorus (P) become released in inorganic or organic forms during decomposition of soil organic matter (SOM). Environmental perturbations, such as drying and rewetting, alter the cycling of C, N, and P. Our study aimed at identifying the patterns and controls of C, N, and P release in soils under beech forests. We exposed organic and mineral horizons from a nutrient availability gradient in Germany to permanent moist conditions or dry spells in microcosms and quantified releases of inorganic and organic C, N, and P. Under moist conditions, mobilization of DOC, DON, and DOP were interrelated and depended on the C:N:P ratio of SOM, whilst net mineralization rates of C, N, and P correlated poorly. Mineralization of C decreased with soil depth from Oi to A horizons, reflecting the increasing SOM stability. Net mineralization of N and P showed divergent depth patterns. In the Oi horizon, net mineralization was smaller for N than for C and P, indicating more pronounced microbial immobilization for N than for P. In A horizons, net mineralization of P was less than of N, very likely because of strong sorption of released phosphate by mineral phases. Counterintuitively, net P mineralization in A horizons increased toward P-poor sites, probably due to decreasing contents of clay and pedogenic oxides, and thus, declining P sorption. Drying and rewetting caused stronger release of inorganic and organic P, and organic N than of inorganic C and inorganic N, most likely by lysis of microbial biomass with tight C:N:P ratios. Due to the divergent patterns in N and P cycling, the organic layer seems more crucial for net mineralization of P than the mineral soil; for N the mineral soil appears more important. Consequently, the loss of the organic layer would deteriorate P nutrition, in particular at nutrient-poor sites. Overall, our results indicate that the cycling of C, N, and P in soil is not directly coupled because of the different microbial immobilization and, in the mineral soil, differential sorption of inorganic N and P. This may ultimately cause imbalances in N and P nutrition of forests.

Understanding and quantification of phosphorus (P) fluxes are key requirements for predictions of future forest ecosystems changes as well as for transferring lessons learned from natural ecosystems to croplands and plantations. This review summarizes and evaluates the recent knowledge on mechanisms, magnitude, and relevance by which dissolved and colloidal inorganic and organic P forms can be translocated within or exported from forest ecosystems. Attention is paid to hydrological pathways of P losses at the soil profile and landscape scales, and the subsequent influence of P on aquatic ecosystems. New (unpublished) data from the German Priority Program 1685 ‘‘Ecosystem Nutrition: Forest Strategies for limited Phosphorus Resources’’ were added to provide up-to-date flux-based information. Nitrogen (N) additions increase the release of water-transportable P forms. Most P found in percolates and pore waters belongs to the so-called dissolved organic P (DOP) fractions, rich in orthophosphate- monoesters and also containing some orthophosphate-diesters. Total solution P concentrations range from ca. 1 to 400 mg P L–1, with large variations among forest stands. Recent sophisticated analyses revealed that large portions of the DOP in forest stream water can comprise natural nanoparticles and fine colloids which under extreme conditions may account for 40–100% of the P losses. Their translocation within preferential flow passes may be rapid, mediated by storm events. The potential total P loss through leaching into subsoils and with streamswas found to be less than 50mgPm–2 a–1, suggesting effects on ecosystems at centennial to millennium scale. All current data are based on selected snapshots only. Quantitative measurements of P fluxes in temperate forest systems are nearly absent in the literature, probably due to main research focus on the C and N cycles. Therefore, we lack complete ecosystem-based assessments of dissolved and colloidalP fluxeswithin and fromtemperate forest systems. peerReviewed

Altered drying-rewetting patterns due to climate change may affect soil nitrogen (N) and phosphorus (P) cycling in terrestrial ecosystems. The responses of soil N and P cycling to drying and rewetting cycles can vary with drying-rewetting patterns, experimental methods, ecosystems, and soil types, thus making a synthesis of these studies necessary for understanding mechanisms and predicting future responses to climate change. Here, we compiled data of 1882 observations from 79 studies for a meta-analysis of the responses of soil N and P pools and fluxes to drying and rewetting and how these responses are modified by experimental conditions. Results showed that 1) experimental drying increased NH4+, extractable organic nitrogen (EON), and available P in the soil significantly by 22, 27, and 72%, respectively. In contrast, soil NO3−, enzymatic activities, microbial biomass, net nitrification, and N2O emissions significantly decreased by 37, 13–21, 21–28, 39, and 93%, respectively. 2) Upon rewetting of dried soil, soil EON, extractable organic phosphorus (EOP), net N mineralization, nitrification, phosphatase activity, dissolved organic N leaching, dissolved inorganic P leaching, and N2O emissions significantly increased by 59, 27, 19, 15, 12, 60, 116, and 218%, respectively, while soil NO3− and NO3− leaching significantly decreased by 9 and 74%, respectively. Soil microbial N and P as well as enzymatic activities recovered from drought during the rewetting phase. The mean effect sizes of drying and rewetting generally increased with drying intensity, which was probably also the main reason for greater effect sizes observed in laboratory than in field experiments. Our meta-analysis showed stronger positive responses of available P to drying and rewetting than mineral N, which agreed with greater effect sizes on P than on N leaching. This suggests that drying and rewetting induce an imbalance between N and P, which was more pronounced in soils from forests than from agricultural systems. Overall, these results imply that the expected increase in the frequency and intensity of droughts potentially decouples the cycling of P and N, with consequences for nutrient leaching and the supply of plants and microorganisms with these nutrients.

Fine roots support the water and nutrient demands of plants and supply carbon to soils. Quantifying turnover times of fine roots is crucial for modeling soil organic matter dynamics and constraining carbon cycle–climate feedbacks. Here we challenge widely used isotope-based estimates suggesting the turnover of fine roots of trees to be as slow as a decade. By recording annual growth rings of roots from woody plant species, we show that mean chronological ages of fine roots vary from, Fine-root lifetimes and carbon inputs from roots into soil impact carbon cycle-climate feedbacks yet remain poorly constrained. Here, using annual-growth rings and radiocarbon dating, the authors show that the chronological age of fine roots is substantially younger than that of the carbon used for their growth.

The cycling of dissolved organic matter (DOM) in soils is controversial. While DOM is believed to be a C source for soil microorganisms, DOM sorption to the mineral phase is regarded as a key stabilization mechanism of soil organic matter (SOM). In this study, we added 14 C-labelled DOM derived from Leucanthemopsis alpina to undisturbed soil columns of a chronosequence ranging from initial unweathered soils of a glacier forefield to alpine soils with thick organic layers. We traced the 14 C label in mineralized and leached DOM and quantified the spatial distribution of DO 14 C retained in soils using a new autoradiographic technique. Leaching of DO 14 C through the 10 cm-long soil columns amounted up to 28% of the added DO 14 C in the initial soils, but to less than 5% in the developed soils. Biodegradation hardly contributed to the removal of litter-DO 14 C as only 2–9% were mineralized, with the highest rates in mature soils. In line with the mass balance of 14 C fluxes, measured 14 C activities in soils indicated that the major part of litter DO 14 C was retained in soils (>80% on average). Autoradiographic images showed an effective retention of almost all DO 14 C in the upper 3 cm of the soil columns. In the deeper soil, the 14 C label was concentrated along soil pores and textural discontinuities with similarly high 14 C activities than in the uppermost soil. These findings indicate DOM transport via preferential flow, although this was quantitatively less important than DOM retention in soils. The leaching of DO 14 C correlated negatively with oxalate-extractable Al, Fe, and Mn. In conjunction with the rapidity of DO 14 C immobilization, this strongly suggests that sorptive retention DOM was the dominating pathway of litter-derived DOM in topsoils, thereby contributing to SOM stabilization.

The impact of a warmer climate on CH 4 fluxes from soils is highly uncertain, because soil warming may affect methanotrophic bacteria in two opposed ways: CH 4 assimilation in soils might be increased by the decreasing soil moisture often associated with soil warming. In contrast, CH 4 oxidation might be suppressed by higher NH 4 + concentrations in warmed soils resulting from an accelerated nitrogen mineralization. We investigated effects of soil warming on soil-atmosphere CH 4 fluxes in the last two years of a six-year long field experiment at a Swiss alpine treeline. Specifically, we measured CH 4 fluxes using static chambers, and characterized N cycling by quantifying soil N 2 O emissions and NH 4 + and NO 3 − concentrations. We further labeled intact soil cores with C 14 H 4 and traced the labeled bacteria using an auto-radiographic technique to study the potential warming-related changes in the micro-distribution of methanotrophic bacteria within the soils. Our results did not show a significant effect of soil warming on net CH 4 fluxes after five and six years of soil warming. In general, soils were a net sink for CH 4 but CH 4 emissions were observed occasionally. One reason for the unaltered CH 4 fluxes might be the negligible warming effects on soil water contents in the treeline environment with frequent rainfalls. In the warmed soils, soil moisture was lower in the litter layer, but not deeper in the soils. Therefore, soil warming did not affect gas transport rates into deeper soil layers where methanotrophic bacteria were located. Another reason might be the general absence of substantial warming effects on mineral N, with NH 4 + concentrations being marginally significantly higher in warmed soils only in ion exchange resin bags (P 14 C distribution and a warming-induced shift of methanotrophic bacteria toward the soil surface. The absence of responses of CH 4 fluxes to warming in this alpine treeline ecosystem is likely related to the rather minimal changes in the putative drivers soil moisture and NH 4 + concentration.

Different development stages of algae-dominated and moss-dominated biological soil crusts (BSCs) were sampled on a natural sand dune (

Atmospheric carbon dioxide (CO2 ) and reactive nitrogen (N) concentrations have been increasing due to human activities and impact the global carbon (C) cycle by affecting plant photosynthesis and decomposition processes in soil. Large amounts of C are stored in plants and soils but the mechanisms behind the stabilization of plant and microbial derived organic matter (OM) in soils are still under debate and it is not clear how N deposition affects soil OM dynamics. Here we studied the effects of 4 years of elevated ((13) C depleted) CO2 and N deposition in forest ecosystems established in open top chambers on composition and turnover of fatty acids (FAs) in plants and soils. FAs served as biomarkers for plant and microbial derived OM in soil density fractions. We analyzed above and belowground plant biomass of beech and spruce trees as well as soil density fractions for the total organic C and FA molecular and isotope (delta(13) C) composition. FAs did not accumulate relative to total organic C in fine mineral fractions showing that FAs are not effectively stabilized by association with soil minerals. The delta(13) C values of FAs in plant biomass increased under high N deposition. However the N effect was only apparent under elevated CO2 suggesting a N limitation of the system. In soil fractions only isotope compositions of short chain FAs (C16+18 ) were affected. Fractions of 'new' (experimental derived) FAs were calculated using isotope depletion in elevated CO2 plots and decreased from free light to fine mineral fractions. 'New' FAs were higher in short chain compared to long chain FAs (C20 30 ) indicating a faster turnover of short chain compared to long chain FAs. Increased N deposition did not significantly affect the quantity of 'new' FAs in soil fractions but showed a tendency of increased amounts of 'old' (pre experimental) C suggesting that decomposition of 'old' C is retarded by high N inputs.

Nitrous oxide (N2O) emissions from snow-covered soils represent a significant fraction of the annual flux from alpine, subalpine or cold-temperate regions. In winter 2010-2011, we investigated the temporal variability of N2O emissions and source processes from a subalpine valley in the Swiss Alps. The study included regular measurements of N2O snow profiles at a fixed location and an intensive sampling campaign along a transversal cut through the valley with grassland at the bottom and coniferous forest at the slopes. During the intensive campaign, recently developed laser spectroscopy was employed for high-precision N2O isotopomer analysis. Maximum N2O fluxes (0.77±0.64 nmol m(-2) h(-1)) were found for periods with elevated air temperature and, in contrast to our expectations, were higher from forest than from grassland in mid-February. At maximum snow height (63 cm) the main N2O source processes were heterotrophic denitrification and nitrifier denitrification. The reduction of N2O by heterotrophic denitrifiers was much more pronounced for the grassland compared with the forest soil, as indicated by the (15)N site preferences of 16.4±11.5 ‰ (grassland) and-1.6±2.1 ‰ (forest). This illustrates the potential of laser spectroscopic N2O isotopomer analysis for the identification of source processes even at low emission rates in nutrient poor ecosystems.

The stability and potential vulnerability of soil organic matter (SOM) to global change remains incompletely understood due to the complex processes involved in its formation and turnover. Here we combine compound-specific radiocarbon analysis with fraction-specific and bulk-level radiocarbon measurements in order to further elucidate controls on SOM dynamics in a temperate and sub-alpine forested ecosystem. Radiocarbon contents of individual organic compounds isolated from the same soil interval generally exhibit greater variation than those among corresponding operationally-defined fractions. Notably, markedly older ages of long-chain plant leaf wax lipids (n-alkanoic acids) imply that they reflect a highly stable carbon pool. Furthermore, marked 14C variations among shorter- and longer-chain n-alkanoic acid homologues suggest that they track different SOM pools. Extremes in SOM dynamics thus manifest themselves within a single compound class. This exploratory study highlights the potential of compound-specific radiocarbon analysis for understanding SOM dynamics in ecosystems potentially vulnerable to global change.

The dynamics of dissolved organic carbon (DOC) have been intensively studied in mature ecosystems, but little is known about DOC dynamics and the significance of DOC as a substrate for microbial activity in early-successional catchments. We determined the concentration, chemical composition, source, radiocarbon age, and bioavailability of DOC along the hydrological flow path from soil solution to a downstream pond in a recently constructed catchment (Chicken Creek Catchment, Germany). Soil solution, upwelling ground water, stream water, subsurface water in an alluvial fan, and pond water all had high DOC concentrations (averages: 6.0–11.6 mg DOC L–1), despite small carbon stocks in both vegetation and soil of the catchment. Solid-state CPMAS 13C NMR of DOC in upwelling ground water revealed a higher proportion of aromatic compounds (32%) and a lower proportion of carbohydrates (33%) than in pond water (18% and 45%, respectively). The average 14C age of DOC in upwelling ground water was 2600 to 2900 yr, while organic matter of the Quaternary substrate of the catchment had a 14C age of 3000 to 16 000 yr. Both the 14C age data and 13C NMR spectra suggest that DOC partly derived from organic matter of the Quaternary substrate (about 40 to 90% of the C in the DOC), indicating that both recent and old C of the DOC can support microbial activity during early ecosystem succession. However, in a 70 day incubation experiment, only about 11% of the total DOC was found to be bioavailable. This proportion was irrespective of the water type. Origin of the microbial communities within the catchment (enriched from soil, stream sediment or pond water) also had only a marginal effect on overall DOC utilization., Biogeosciences, 10 (7), ISSN:1726-4170

Although greenhouse gas emissions during winter contribute significantly to annual balances, their quantification is still highly uncertain in snow-covered ecosystems. Here, carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) fluxes were measured at a subalpine managed grassland in Switzerland using concentration gradients within the snowpack (CO 2 , CH 4 , N 2 O) and the eddy covariance method (CO 2 ) during the winter 2010/2011. Our objectives were (1) to identify the temporal and spatial variation of greenhouse gases (GHGs) and their drivers, and (2) to estimate the GHG budget of the site during this specific season (1 December–31 March, 121 days). Mean winter fluxes (December–March) based on the gradient method were 0.77 ± 0.54 μmol m −2 s −1 for CO 2 (1.19 ± 1.05 μmol m −2 s −1 measured by eddy covariance), −0.14 ± 0.09 nmol m −2 s −1 for CH 4 and 0.23 ± 0.23 nmol m −2 s −1 for N 2 O, respectively. In comparison with the CO 2 fluxes measured by eddy covariance, the gradient technique underestimated the effluxes by 50%. While CO 2 and CH 4 fluxes decreased with the progressing winter season, N 2 O fluxes did not follow a seasonal pattern. The major variables correlating with the fluxes of CO 2 and CH 4 were soil temperature and snow water equivalent, which is based on snow height and snow density. N 2 O fluxes were only explained poorly by any of the measured environmental variables. Spatial variability across the valley floor was smallest for CO 2 and largest for N 2 O. During the winter season 2010/2011, greenhouse gas fluxes ranged between 550 ± 540 g CO 2 m −2 estimated by the eddy covariance approach and 543 ± 247 g CO 2 m −2 , −0.4 ± 0.01 g CH 4 m −2 and 0.11 ± 0.1 g N 2 O m −2 derived by the gradient technique. Total seasonal greenhouse gas emissions from the grassland were between 574 ± 276 and 581 ± 569 g CO 2 eq. m −2 , with N 2 O contributing 5% to the overall budget and CH 4 reducing the budget by 0.1%. Cumulative budgets of CO 2 were smaller than emissions reported for other subalpine meadows in the Swiss Alps and the Rocky Mountains. Further investigations on the GHG exchange of grasslands in winter are needed in order to (1) deepen our currently limited knowledge on the environmental drivers of each GHG, (2) to thoroughly constrain annual balances, and (3) to project possible changes in GHG flux magnitude with expected shorter and warmer winter periods.

Soils are accumulating C during their initial development but little is known on the evolution of soil C fluxes and their components. In this study, we investigated soil CO 2 effluxes and leaching of dissolved organic carbon (DOC) during initial stages of soil formation along the granitic Damma glacier forefield in the Swiss Alps. We added isotopic labelled litter ( Leucanthemopsis alpina , δ 13 C = +88‰) to soil columns in three successional stages (10 yr, 70 yr, 120 yr) and traced the ‘new’ litter-derived C in soil CO 2 effluxes, DOC leaching, and in phospholipid fatty acids (PLFA), biomarkers for soil microbial communities. The results showed increasing total soil C fluxes with progressing soil development due to increasing C stocks in plants and soils. Throughout three summer months, 15–63% of the added litter C were lost via mineralisation and leaching. Along the initial soil chronosequence, litter-derived CO 2 C effluxes did not change systematically with soil age. The distribution of δ 13 C signatures in PLFAs was very similar at all site ages, suggesting that decomposer communities did almost not change with succession. Instead, moisture conditions of surface soils varying at the local scale seemed to primarily control litter mineralisation with the driest topsoils and lowest mineralisation at the oldest site. Only a small fraction of the added litter, less than 1% was leached as DOC at 10 cm depth. This indicates a strong removal of litter-derived DOC with passage through the mineral soils. The highest litter-derived DOC leaching occurred in the initial soils with the smallest contents of secondary Fe and Al oxides as potential sorbents for DOC. Litter addition had a negligible priming effect on soil respiration and DOC leaching in all three successional stages.

Elevated atmospheric CO2 was often shown to stimulate belowground C allocation, but it is uncertain if this increase also alters the structure of soil microbial communities. Here, we assessed the effects of nine years of CO2 enrichment on soil microbial communities of an alpine treeline ecosystem with 35-year-old Larix decidua and Pinus mugo ssp. uncinata trees. We also tracked the 13C signal of supplemental CO2 in soil-respired CO2, microbial biomass, and phospholipid fatty acids (PLFA) in undisturbed mor-type organic layers. We found a persistently increased soil CO2 efflux (+24% on average), but negligible effects of elevated CO2 on the biomass and community structure of soil microorganisms under both tree species determined with PLFA and T-RFLP (terminal restriction fragment length polymorphism). The 13C tracing over 9 years revealed that 24–40% of the soil microbial biomass was composed of ‘new’ plant-derived C. PLFA from gram-negative biomarkers did not significant shift in 13C by the CO2 addition, while those of gram-negative bacteria were significantly altered. The highest 13C signals in individual PLFA was found in the fatty acid 18:2ω6,9 with 65–80% new C, indicating that new plant-derived C was primarily incorporated by soil fungi. However, CO2 enrichment did not affect the production of mycelia biomass and the structure and composition of the fungal communities analysed by high-throughput 454-sequencing of genetic markers. Collectively, our results suggest that C flux through the plant–soil system will be accelerated but that the biomass and composition of microbial communities will be little affected by rising atmospheric CO2 in organic matter rich treeline soils.

Summary How will carbon source–sink relations of 35-yr-old larch trees (Larix decidua) at the alpine treeline respond to changes in atmospheric CO2 and climate? We evaluated the effects of previously elevated CO2 concentrations (9 yr, 580 ppm, ended the previous season) and ongoing soil warming (4 yr, + 4°C). Larch branches were pulse labeled (50 at% 13CO2) in July 2010 to trace fresh assimilates through tissues (buds, needles, bark and wood) and non-structural carbon compounds (NCC; starch, lipids, individual sugars) using compound-specific isotope analysis. Nine years of elevated CO2 did not lead to increased NCC concentrations, nor did soil warming increase NCC transfer velocities. By contrast, we found slower transfer velocities and higher NCC concentrations than reported in the literature for lowland larch. As a result of low dilution with older carbon, sucrose and glucose showed the highest maximum 13C labels, whereas labels were lower for starch, lipids and pinitol. Label residence times in needles were shorter for sucrose and starch (c. 2 d) than for glucose (c. 6 d). Although our treatments showed no persistent effect on larch carbon relations, low temperature at high altitudes clearly induced a limitation of sink activities (growth, respiration, root exudation), expressed in slower carbon transfer and higher NCC concentrations.

Recent reviews indicate that N deposition increases soil organic matter (SOM) storage in forests but the undelying processes are poorly understood. Our aim was to quantify the impacts of increased N inputs on soil C fluxes such as C mineralization and leaching of dissolved organic carbon (DOC) from different litter materials and native SOM. We added 5.5 g N m−2 yr−1 as NH4NO3 over 1 year to two beech forest stands on calcareous soils in the Swiss Jura. We replaced the native litter layer with 13C-depleted twigs and leaves (δ13C: −38.4 and −40.8‰) in late fall and measured N effects on litter- and SOM-derived C fluxes. Nitrogen addition did not significantly affect annual C losses through mineralization, but altered the temporal dynamics in litter mineralization: increased N inputs stimulated initial mineralization during winter (leaves: +25%; twigs: +22%), but suppressed rates in the subsequent summer. The switch from a positive to a negative response occurred earlier and more strongly for leaves than for twigs (−21% vs. 0%). Nitrogen addition did not influence microbial respiration from the nonlabeled calcareous mineral soil below the litter which contrasts with recent meta-analysis primarily based on acidic soils. Leaching of DOC from the litter layer was not affected by NH4NO3 additions, but DOC fluxes from the mineral soils at 5 and 10 cm depth were significantly reduced by 17%. The 13C tracking indicated that litter-derived C contributed less than 15% of the DOC flux from the mineral soil, with N additions not affecting this fraction. Hence, the suppressed DOC fluxes from the mineral soil at higher N inputs can be attributed to reduced mobilization of nonlitter derived ‘older’ DOC. We relate this decline to an altered solute chemistry by NH4NO3 additions, an increased ionic strength and acidification resulting from nitrification, rather than to a change in microbial decomposition.

Very few field studies have quantified the different pathways of C loss from decomposing litter even though the partitioning of C fluxes is essential to understand soil C dynamics. Using 0.75 kg m−2 of 13C-depleted leaf (δ13C = −40.8 ‰) and 2 kg m−2 of twig litter (δ13C = −38.4 ‰), we tracked the litter-derived C in soil CO2 effluxes, dissolved organic C (DOC), and soil organic matter of a beech forest in the Swiss Jura. Autotrophic respiration was reduced by trenching. Our results show that mineralisation was the main pathway of C loss from decomposing litter over 1 yr, amounting to 24 and 31 % of the added twig and leaf litter. Contrary to our expectations, the leaf litter C was mineralised only slightly (1.2 times) more rapidly than the twig litter C. The leaching of DOC from twigs amounted to half of that from leaves throughout the experiment (2 vs. 4 % of added litter C). Tracing the litter-derived DOC in the soil showed that DOC from both litter types was mostly removed (88–96 %) with passage through the top centimetres of the mineral soil (0–5 cm) where it might have been stabilised. In the soil organic C at 0–2 cm depth, we indeed recovered 4 % of the initial twig C and 8 % of the leaf C after 1 yr. Much of the 13C-depleted litter remained on the soil surface throughout the experiment: 60 % of the twig litter C and 25 % of the leaf litter C. From the gap in the 13C-mass balance based on C mineralisation, DOC leaching, C input into top soils, and remaining litter, we inferred that another 30 % of the leaf C but only 10 % of twig C could have been transported via soil fauna to soil depths below 2 cm. In summary, over 1 yr, twig litter was mineralised more rapidly relative to leaf litter than expected, and much less of the twig-derived C was transported to the mineral soil than of the leaf-derived C. Both findings provide some evidence that twig litter could contribute less to the C storage in these base-rich forest soils than leaf litter.

Laser spectroscopy is an emerging technique to analyze the stable isotopic composition of soil-respired CO 2 (δ 13 C resp , δ 18 O resp ) in situ and at high temporal resolution. Here we present the first application of a quantum cascade laser-based spectrometer (QCLS) in a closed soil-chamber system to determine simultaneously δ 13 C resp and δ 18 O resp . In a Swiss beech forest, a total of 90 chamber measurements with 20 min sampling time each were performed. The instrument measured the δ 13 C and δ 18 O of the CO 2 in the chamber headspace at every second with a precision of 0.25‰, resulting in Keeling plots with 1200 data points. In addition, we calculated δ 13 C resp directly from the flux ratio of 13 CO 2 and 12 CO 2 . The flux-ratio values were 0.8‰ lower than the Keeling plot intercepts when the flux rates were derived from quadratic curve fits of the CO 2 increase. The δ 18 O-Keeling plots showed a significant bending very likely due to the equilibration of chamber CO 2 with the 18 O of surface soil water. Therefore, we used a quadratic curve fit of the Keeling plots to estimate δ 18 O resp . Our results also revealed that δ 13 C resp was not constant throughout the CO 2 accumulation in the closed soil chambers: there were significant but non-systematic variations in δ 13 C resp for the first 10 min, and systematic shifts in δ 13 C resp of on average 1.9 ‰ in the second part of the 20-min measurements. These biases were probably caused by non-steady-state conditions in the soil-chamber system. Our study illustrates that the high temporal resolution of QCLS measurements allows the detection of non-linearities in the isotopic effluxes of CO 2 from the soil due to soil-chamber feedbacks. This information can be used to improve the estimates for δ 13 C resp and δ 18 O resp .

Cryptopodzols are black soils that occur under forests dominated by chestnut trees (Castanea sativa) in Southern Switzerland. Their soil organic carbon (SOC) stocks reach an average of 150 t C ha−1 and are thus among the highest of European forest soils. We investigated the processes leading to the accumulation and stabilisation of SOC in these soils by analysing three Cryptopodzols and one Cambisol for charred organic matter content (macrocharcoal and BPCA), the amounts of Fe and Al, and the colour and SOC content in bulk soil and density fractions. The results showed that charred organic matter produced by frequent fires in the area for more than 10,000 years is highly abundant in Cryptopodzols: the stocks of macrocharcoal and BPCA-C amount to up to 31 t ha−1 and 17 t ha−1, respectively. These high amounts of charred organic matter are responsible for the dark soil colour and high SOC concentrations that are, however, also closely related to Fep and Alp concentrations. We concluded that the occurrence of charcoal across the whole profiles of Cryptopodzols seems to be the dominating factor, although both the formation of organo-metallic or organo-mineral complexes in the subsoil and the high abundance and stability of charred organic matter are responsible for the high SOC stocks in Cryptopodzols.

In carbonate-containing soils a reliable determination of organic C requires a method that effectively separates organic and inorganic C without altering the organic matter. This study was conducted to determine whether HCl vapor completely removes carbonates even in dolomite-rich soils and to what extent a widely used acid-fumigation method has to be modified for humus-rich soils. Furthermore, it was tested whether HCl fumigation alters organic-C content. Since C and N parameters are often analyzed simultaneously we also tested the influence of acid-vapor treatment on N content and on δ 13 C of soil organic matter. We applied fumigation with 37% HCl for 8 and 32 h using 9 carbonate-containing soil samples. Inorganic C ranged from 7 to 124 and organic C from 9 to 267 g kg -1 . The maximum contents of dolomite and calcite were 940 and 640 g kg -1 , respectively. A time of 8 h was enough to completely remove all carbonates. Neither the content nor the δ 13 C of organic C were significantly affected by fumigation. In contrast, N contents were altered by acid treatment. Based on these results and on our experience in analyzing more than 1000 soil samples, a recommended procedure for acid fumigation of carbonate-containing soils with a wide range of organic- and inorganic-C contents was derived. Samples pretreated in this way can be analyzed reliably for their organic-C content and δ 13 C. Furthermore, N and inorganic-C contents can be determined with a quality sufficient for many purposes.

Historical photographs document that during the last century, forests have expanded upwards by 60-80 m into former tundra of the pristine Ural mountains. We assessed how the shift of the high-altitude treeline ecotone might affect soil organic matter (SOM) dynamics. On the gentle slopes of Mali Iremel in the Southern Urals, we (1) determined the differences in SOM stocks and properties from the tundra at 1360m above sea level (a.s.l.) to the subalpine forest at 1260 m a.s.l., and (2) measured carbon (C) and nitrogen (N) mineralization from tundra and forest soils at 7 and 20°C in a 6-month incubation experiment. C stocks of organic layers were 3.6±0.3 kg C m-2 in the tundra and 1.9±0.2 kg C m-2 in the forest. Mineral soils down to the bedrock stored significantly more C in the forest, and thus, total soil C stocks were slightly but insignificantly greater in the forest (+3 kg C m-2). Assuming a space for time approach based on tree ages suggests that the soil C sink due to the forest expansion during the last century was at most 30 g C m-2 yr-1. Diffuse reflective infrared spectroscopy and scanning calorimetry revealed that SOM under forest was less humified in both organic and mineral horizons and, therefore, contained more available substrate. Consistent with this result, C mineralization rates of organic layers and A horizons of the forest were two to four times greater than those of tundra soils. This difference was similar in magnitude to the effect of increasing the incubation temperature from 7 to 20°C. Hence, indirect climate change effects through an upward expansion of forests can be much larger than direct warming effects (Δ0.3K across the treeline). Net N mineralization was 2.5 to six times greater in forest than in tundra soils, suggesting that an advancing treeline likely increases N availability. This may provide a nutritional basis for the fivefold increase in plant biomass and a tripling in productivity from the tundra to the forest. In summary, our results suggest that an upward expansion of forest has small net effects on C storage in soils but leads to changes in SOM quality, accelerates C cycling and increases net N mineralization, which in turn might stimulate plant growth and thus C sequestration in tree biomass. © 2009 Blackwell Publishing.

This study aimed to gain insight into the generation and fate of dissolved organic carbon (DOC) in organic layers. In a Free Air CO2 Enrichment Experiment at the alpine treeline, we estimated the contribution of C-13-depleted recent plant C to DOC of mor-type organic layers. In an additional laboratory soil column study with 40 leaching cycles, we traced the fate of C-13-labelled litter-DOC (22 and 45 mg l(-1)) in intact Oa horizons at 2 and 15 degrees C. Results of the field study showed that DOC in the Oa horizon at 5 cm depth contained only 20 +/- 3% of less than six-year-old C, indicating minor contributions of throughfall, root exudates, and fresh litter to leached DOC. In the soil column experiment, there was a sustained DOC leaching from native soil organic matter. Less than 10% of totally added litter-DOC was leached despite a rapid breakthrough of a bromide tracer (50 +/- 7% within two days). Biodegradation contributed only partly to the DOC removal with 18-30% of added litter-DOC being mineralized in the Oa horizons at 2 and 15 degrees C, respectively. This was substantially less than the potential 70%-biodegradability of the litter-DOC itself, which indicates a stabilization of litter-DOC in the Oa horizon. In summary, our results give evidence on an apparent 'exchange' of DOC in thick organic layers with litter-DOC being retained and 'replaced' by 'older' DOC leached from the large pool of indigenous soil organic matter. (C) 2009 Elsevier Ltd. All rights reserved.

A common method to estimate the carbon isotopic composition of soil-respired air is to use Keeling plots (delta(13)C versus 1/CO2 concentration). This approach requires the precise determination of both CO2 concentration ([CO2]), usually measured with an infrared gas analyser (IRGA) in the field, and the analysis of delta(13)C by isotope ratio mass spectrometry (IRMS) in the laboratory. We measured [CO2] with an IRGA in the field (n = 637) and simultaneously collected air samples in 12 mL vials for analysis of the 13C values and the [CO2] using a continuous-flow isotope ratio mass spectrometer. In this study we tested if measurements by the IRGA and IRMS yielded the same results for [CO2], and also investigated the effects of different sample vial preparation methods on the [CO2] measurement and the thereby obtained Keeling plot results. Our results show that IRMS measurements of the [CO2] (during the isotope analysis) were lower than when the [CO2] was measured in the field with the IRGA. This is especially evident when the sample vials were not treated in the same way as the standard vials. From the three different vial preparation methods, the one using N2-filled and overpressurised vials resulted in the best agreement between the IRGA and IRMS [CO2] values. There was no effect on the (13)C-values from the different methods. The Keeling plot results confirmed that the overpressurised vials performed best. We conclude that in the cases where the ranges of [CO2] are large (>300 ppm; in our case it ranged between 70 and 1500 ppm) reliable estimation of the [CO2] with small samples using IRMS is possible for Keeling plot application. We also suggest some guidelines for sample handling in order to achieve proper results.

The aim of our study was to identify controls on initial dissolved organic matter (DOM) leaching from decomposing forest litter and to estimate how it is affected by increasing atmospheric CO 2 . Using microcosms, we measured initial C mineralization and leaching rates of DOC, DON and biodegradable DOC from litter of eight tree species from CO 2 enrichment experiments in a 100 year-old broadleaf forest and a 30 year-old treeline ecosystem. Over 11 weekly leaching cycles, between 2.5% (Pinus uncinata, Fagus sylvatica) and 15% (Carpinus betulus) of litter C were leached as DOC, corresponding to 9–36% of the total mass loss. Significantly less, 0.9% (Pinus) to 4.5% (Tilia platyphyllos) of litter N was leached as DON. Leaching of DOC was not correlated to C mineralization, which ranged between 12% (Fagus) and 32% (Tilia) of litter C. While C mineralization increased with decreasing litter C/N ratios and lignin contents, DOC leaching particularly the initially leached DOC was significantly related to concentrations of non-structural carbohydrates (NSC) and phenolics. DOC leached after the third leaching cycle did not correlate with any of the measures of litter quality, but with the molar UV absorptivity of DOC, suggesting that DOC production is linked to lignin degradation. Previous CO 2 enrichment increased NSC and phenolics in the litter and decreased lignin contents, which resulted in significantly greater initial C mineralization ( + 5 % ) and DOC leaching rates ( + 16 % ) . However, these CO 2 effects were only significant during the initial leaching phase and much smaller than the differences between tree species. Initially leached DOC was less biodegradable when its parent litter was grown under elevated than under ambient CO 2 (38% vs. 42% of DOC across all species, P 0.05). Therefore, leaching of ‘refractory’ DOC was increased under elevated CO 2 , which will rather accelerate DOC inputs into mineral soils than further stimulate microbial activity. In summary, our study shows (1) that initial DOM leaching is controlled by other factors than C mineralization; and (2) that CO 2 enrichment of forests can stimulate initial mineralization and leaching of C from litter by altering its quality, but these effects will be short-term and much smaller than any change in species composition.

Summary The relative contributions of litter and humified organic matter as the source of dissolved organic carbon (DOC) leached from organic layers of forest soils are poorly understood. In the present investigation, 13C labelled spruce litter was used to study the role of recent litter in the leaching of DOC from a coniferous forest floor in southern Sweden, while litterbags were used to quantify the total loss of C from the labelled litter. The labelled litter applied on bare lysimeters released considerable amounts of DOC during the first weeks, but the concentration of DOC originating from labelled litter decreased gradually from 176 mg litre−1 during the first sampling period in May to 5 mg litre−1 in the last sampling period in October. Only a moderate flush of DOC from the labelled litter occurred under the Oe and Oa horizons, with concentrations of 20 and 6 mg litre−1 from labelled litter, equal to 19 and 9% of the total DOC flux, respectively, during the first sampling period. Total flux of DOC from labelled litter from May to September was 16 g m−2, whereas only 2.2 and 0.9 g m−2 were captured under the Oe and Oa horizons, respectively. The almost complete loss of new DOC implies that DOC leached from the Oe and Oa horizons consists not of recent litter-derived carbon, but of DOC produced in these two horizons themselves. Water-extractable organic carbon from labelled litter left in litterbags in the field for 4 months consisted of about one-third native carbon from external sources at the experimental site and two-thirds of the labelled litter. In contrast, the 13C content of the bulk litter from the litterbags was not changed by the incubation in the field. We suggest that the soluble native carbon in water extracts originated from throughfall DOC that had been assimilated by microorganisms in the litterbags.

Rhizosphere respiration is a significant component of the total carbon dioxide (CO2) efflux from forest soils. Since the root-derived CO2 is not part of soil C loss, it is worth estimating this fraction and determining its driving forces. In order to determine the dependency of the root respiration on recent photosynthates in a European chestnut (Castanea sativa Mill.) stand in Southern Switzerland, we girdled 104 tree stems on an area of 625 m2 at the end of August. The response of forest girdling on the below-ground C cycle was assessed by measuring soil respiration, soil microbial carbon biomass and root composition during the 37 days of experiment. Recent photosynthates contributed significantly to the soil respiration, which declined by 22 and up to 36% on average after girdling. The difference in soil respiration between girdled and control plots was significant only during a short time period (9–20 days after girdling). A significant decline in soil respiration was observed up to a distance of 4 m from the stems. Fine roots from girdled trees were strongly depleted in starch (−90%), although they were functional as shown by the dehydrogenase test. We assume that a large fraction of the starch loss was most likely respired and contributed to the soil respiration or there was a decline in new starch supply due to reduced carbon allocation to the roots.

The fate of immobilized N in soils is one of the great uncertainties in predicting C sequestration at increased CO2 and N deposition. In a dual isotope tracer experiment ( 13 C, 15 N) within a 4-year CO2 enrichment ( 1200ppmv) study with forest model ecosystems, we (i) quantified the effects of elevated CO2 on the partitioning of N; (ii) traced immobilized N into physically separated pools of soil organic matter (SOM) with turnover rates known from their 13 C signals; and (iii) estimated the remobilization and thus, the bio-availability of newly sequestered C and N. (1) CO2 enrichment significantly decreased NO3 concentrations in soil waters and export from 1.5m deep lysimeters by 30‐80%. Consequently, elevated CO2 increased the overall retention of N in the model ecosystems. (2) About 60‐80% of added 15 NH4 NO3 were retained in soils. The clay fraction was the greatest sink for the immobilized 15 N sequestering 50‐60% of the total new soil N. SOM associated with clay contained only 25% of the total new soil C pool and had small C/N ratios (o13), indicating that it consists of humified organic matter with a relatively slow turn over rate. This implies that added 15 N was mainly immobilized in stable mineral-bound SOM pools. (3) Incubation of soils for 1 year showed that the remobilization of newly sequestered N was three to nine times smaller than that of newly sequestered C. Thus, inorganic inputs of N were stabilized more effectively in soils than C. Significantly less newly sequestered N was remobilized from soils previously exposed to elevated CO2. In summary, our results show firstly that a large fraction of inorganic N inputs becomes effectively immobilized in relative stable SOM pools and secondly that elevated CO2 can increase N retention in soils and hence it may tighten N cycling and diminish the risk of nitrate leaching to groundwater.

The aim of this study was to investigate the effects of increased N deposition on new and old pools of soil organic matter (SOM). We made use of a 4-yr experiment, where spruce and beech growing on an acidic loam and a calcareous sand were exposed to increased N deposition (7 vs. 70 kg N ha −1 yr −1 ) and to elevated atmospheric CO 2 . The added CO 2 was depleted in 13 C, which enabled us to distinguish between old and new C in SOM-pools fractionated into particle sizes. Elevated N deposition for 4 yr increased significantly the contents of total SOM in 0–10 cm depth of the acidic loam (+9%), but not in the calcareous sand. Down to 25 cm soil depth, C storage in the acidic loam was between 100 and 300 g C m −2 larger under high than under low N additions. However, this increase was small as compared with the SOM losses of 600–700 g C g C 0.25 m −1 m −2 from the calcareous sand resulting from the disturbance of soils during setting up of the experiment. The amounts of new, less than 4 yr old SOM in the sand fractions of both soils were greater under high N deposition, showing that C inputs from trees into soils increased. Root biomass in the acidic loam was larger under N additions (+25%). Contents of old, more than 4 yr old C in the clay and silt fractions of both soils were significantly greater under high than under low N deposition. Since clay- and silt-bound SOM consists of humified compounds, this indicates that N additions retarded mineralization of old and humified SOM. The retardation of C mineralization in the clay and silt fraction accounted for 60–80 g C m −2 4 yr −1 , which corresponds to about 40% of the old SOM mineralized in these fraction. As a consequence, preservation of old and humified SOM under elevated N deposition might be a process that could lead to an increased soil C storage in the long-term.

The relative contributions of sources of carbon in soils, such as throughfall, litter, roots, microbial decay products and stable organic fractions, to dissolved organic C are controversial. To identify the origin of dissolved organic C, we made use of a 4-year experiment where spruce and beech, growing on an acidic loam and on a calcareous sand, were exposed to increased CO 2 that was depleted in 13 C. We traced the new C inputs from trees into dissolved organic C, into water-extractable organic C, and into several particle-size fractions. In addition, we incubated the labelled soils for 1 year and measured the production of dissolved organic C and CO 2 from new and old soil C. In the soil solutions of the topsoil, the dissolved organic C contained only 5-10% new C from the trees. The δ 13 C values of dissolved organic C resembled those of C pools smaller than 50 μm, which strongly suggests that the major source of dissolved organic C was humified old C. Apparently, throughfall, fresh litter and roots made only minor contributions to dissolved organic C. Water-extractable organic C contained significantly larger fractions of new C than did the natural dissolved organic C (25-30%). The δ 13 C values of the water-extractable organic C were closely correlated with those of sand fractions, which consisted of little decomposed organic carbon. The different origin of dissolved and water-extractable organic C was also reflected in a significantly larger molar UV absorptivity and a smaller natural 13 C abundance of dissolved organic C. This implies that the sampling method strongly influences the characteristics and sources of dissolved organic C. Incubation of soils showed that new soil C was preferentially respired as CO 2 and only a small fraction of new C was leached as dissolved organic C. Our results suggest that dissolved organic C is produced during incomplete decomposition of recalcitrant native C in the soils, whereas easily degradable new components are rapidly consumed by microbes and thus make only a minor contribution to the dissolved C fraction.

The aim of this study was to estimate (i) the influence of different soil types on the net input of new C into soils under CO2 enrichment and (ii) the stability and fate of these new C inputs in soils. We exposed young beech–spruce model ecosystems on an acidic loam and calcareous sand for 4 years to elevated CO2. The added CO2 was depleted in 13C, allowing to trace new C inputs in the plant–soil system. We measured CO2-derived new C in soil C pools fractionated into particle sizes and monitored respiration as well as leaching of this new C during incubation for 1 year. Soil type played a crucial role in the partitioning of C. The net input of new C into soils under elevated CO2 was about 75% greater in the acidic loam than in the calcareous sand, despite a 100% and a 45% greater above- and below-ground biomass on the calcareous sand. This was most likely caused by a higher turnover of C in the calcareous sand as indicated by 30% higher losses of new C from the calcareous sand than from the acidic loam during incubation. Therefore, soil properties determining stabilization of soil C were apparently more important for the accumulation of C in soils than tree productivity. Soil fractionation revealed that about 60% of the CO2-derived new soil C was incorporated into sand fractions. Low natural 13C abundance and wide C/N ratios show that sand fractions comprise little decomposed organic matter. Consistently, incubation indicated that new soil C was preferentially respired as CO2. During the first month, evolved CO2 consisted to 40–55% of new C, whereas the fraction of new C in bulk soil C was 15–23% only. Leaching of DOC accounted for 8–23% of the total losses of new soil C. The overall effects of CO2 enrichment on soil C were small in both soils, although tree growth increased significantly on the calcareous sand. Our results suggest that the potential of soils for C sequestration is limited, because only a small fraction of new C inputs into soils will become long-term soil C.

In the last growing season of a 4-yr experiment, the effects of elevated atmospheric CO 2 (370 vs. 570 μmol CO 2 mol −1 ) and increased N deposition (0.7 vs. 7 g N m −2 yr −1 ) on concentrations, fluxes, and properties of dissolved organic carbon (DOC) were investigated in two different forest soils. Model ecosystems with spruce and beech were established in large open-top chambers with lysimeters containing an acidic loam and calcareous sand. The added CO 2 was depleted in 13 C and thus, the net input of new C into soil organic matter (SOM) and DOC could be quantified. The effects of elevated CO 2 and increased N deposition on concentrations and properties of DOC collected with suction cups at 5–10 cm depth were negligible. Export of DOC by drainage was reduced under elevated CO 2 in the calcareous sand (−22%), but as concentrations were unaffected this was a secondary effect of a reduced drainage. The differences in DOC between the two soils were large, indicating that soil properties had a greater influence on DOC than the CO 2 and N induced changes in above- and below-ground biomass. This was probably caused by the low input of ‘new’, less than 4-yr-old C into DOC. At 5–10 cm depth, the fraction of new C was 5±1% of total DOC in the acidic loam and 8±1% in the calcareous sand under elevated CO 2 . This indicates that potential sources of new C such as throughfall, litter leaching, and rhizodeposits made minor direct contributions to the DOC of the soil solution during the four experimental years. The fractions of new C in DOC at 5–10 cm depth were significantly smaller than those in SOM at 0–10 cm depth, which accounted for 22±2% of total SOM in the acidic loam and for 17±2% in the calcareous sand. The fractions of new C in water-soluble organic C (WSOC) of the bulk soil at 0–10 cm (25–29%) were larger than in DOC, showing that the origins of WSOC and DOC were different. The losses of new C through drainage (30 mg C m −2 yr −1 ) were small in comparison to the net input of new C into SOM (240–400 g C m −2 4 yr −1 at 0–10 cm depth). This strongly suggests that exports of DOC are not a major part of the ‘missing carbon sink’.

Atmospheric nitrogen (N) deposition has frequently been observed to increase soil carbon (C) storage in forests, but the underlying mechanisms still remain unclear. Changes in microbial community composition and substrate use are hypothesized to be one of the key mechanisms affected by N inputs. Here, we investigated the effects of N deposition on amino sugars, which are used as biomarkers for fungal- and bacterial-derived microbial residues in soil. We made use of a 4-year combined CO2 enrichment and N deposition experiment in model forest ecosystems, providing a distinct (13) C signal for 'new' and 'old' C in soil organic matter and microbial residues measured in density and particle-size fractions of soils. Our hypothesis was that N deposition decreases the amount of fungal residues in soils, with the new microbial residues being more strongly affected than old residues. The soil fractionation showed that organic matter and microbial residues are mainly stabilized by association with soil minerals in the heavy and fine fractions. Moreover, the bacterial residues are relatively enriched at mineral surfaces compared to fungal residues. The (13) C tracing indicated a greater formation of fungal residues compared to bacterial residues after 4 years of experiment. In contradiction to our hypotheses, N deposition significantly increased the amount of new fungal residues in bulk soil and decreased the decomposition of old microbial residues associated with soil minerals. The preservation of old microbial residues could be due to decreased N limitation of microorganisms and therefore a reduced dependence on organic N sources. This mechanism might be especially important in fine heavy fractions with low C/N ratios, where microbial residues are effectively protected from decomposition by association with soil minerals.

Soil contains the major part of carbon in terrestrial ecosystems, but the response of this carbon to enriching the atmosphere in CO 2 and to increased N deposition is not completely understood. We studied the effects of CO 2 concentrations at 370 and 570 μmol CO 2 mol -1 air and increased N deposition (7 against 0.7 g N m -2 year -1 ) on the dynamics of soil organic C in two types of forest soil in model ecosystems with spruce and beech established in large open-top chambers containing an acidic loam and a calcareous sand. The added CO 2 was depleted in 13 C and thus the net input of new C into soil organic carbon and the mineralization of native C could be quantified. Soil type was the greatest determining factor in carbon dynamics. After 4 years, the net input of new C in the acidic loam (670 ± 30 g C m -2 ) exceeded that in the calcareous sand (340 ± 40 g C m -2 ) although the soil produced less biomass. The mineralization of native organic C accounted for 700 ± 90g C m -2 in the acidic loam and for 2800 ± 170 g C m -2 in the calcareous sand. Unfavourable conditions for mineralization and a greater physico-chemical protection of C by clay and oxides in the acidic loam are probably the main reasons for these differences. The organic C content of the acidic loam was 230 g C m -2 more under the large than under the small N treatment. As suggested by a negligible impact of N inputs on the fraction of new C in the acidic loam, this increase resulted mainly from a suppressed mineralization of native C. In the calcareous sand, N deposition did not influence C concentrations. The impacts of CO 2 enrichment on C concentrations were small. In the uppermost 10 cm of the acidic loam, larger CO 2 concentrations increased C contents by 50-170 g C m -2 . Below 10cm depth in the acidic loam and at all soil depths in the calcareous sand, CO 2 concentrations had no significant impact on soil C concentrations. Up to 40% of the 'new' carbon of the acidic loam was found in the coarse sand fraction, which accounted for only 7% of the total soil volume. This suggests that a large part of the CO 2 -derived 'new' C was incorporated into the labile and easily mineralizable pool in the soil.

Natural abundance (δ) of the stable isotopes 13 C and 15 N has gained acceptance for studying C and N cycling in forests. In most studies, bulk soil samples are collected to determine isotope abundance. Such sampling overlooks the potential impact of preferential flow on isolope distribution. The objective of this study was to investigate the effects of preferential flow on the distribution of soil organic carbon (SOC), total N, δ 13 C, and δ 15 N in a forest soil in Central Switzerland. Preferential flow paths in the soil were identified with a dye tracer, Brilliant Blue (Pluss-Staufer AG, Oftringen, Switzerland), that was homogeneously applied to the soil surface. In the stained preferential flow paths, concentrations of SOC and total N were 15 to 75% higher than in the soil matrix. The total increase of SOC in preferential flow paths ranged from 740 to 960 g C m -2 in four individual soil plots. Values of δ 13 C and δ 15 N were lowest in tree leaves and in the forest floor, and inereased with soil depth, thus with the degree of decomposition of SOC. In the mineral soil, preferential flow paths were significantly depleted in 13 C by 0.15 to 0.4‰ as compared with the soil matrix. The δ 15 N values increased with soil depth from 0.9 to 4.7‰ in the preferential flow paths and from 0.5 to 6‰ in the soil matrix. Adding a highly enriched 15 N-tracer homogeneously to the soil surface showed a higher recovery of 15 N in the soil and in the fine roots sampled from preferential flow paths than in those sampled from the soil matrix. Our results suggest that in preferential flow paths, SOC is younger and N cycling is more rapid than in the soil matrix.

Summary As a consequence of heterogeneous transport in soils, only a small part of the soil might be responsible for sorbing incoming elements. After staining preferential flow paths in forested Dystric Cambisol with a colour dye, we sampled soil material from the flow paths and from the soil matrix. We measured chemical properties and sorption isotherms of these two flow regions and estimated the significance of preferential flow paths for the transport of solutes leached from wood ash applied at the surface. In the A horizon (0–9 cm depth), the cation exchange capacity of the flow paths was 83.8 mmolc kg−1, while that of the soil matrix was only 74.6 mmolc kg−1. The base saturation was 42% and soil organic matter content was 41% larger in flow paths than in the soil matrix. The sorption capacity for Cu was also larger than in the matrix, whereas the sorption capacity for Sr was similar in both flow regions. The impact of the addition of 8 t wood ash ha−1 on soil chemical properties was restricted mainly to the flow paths in the uppermost 20 cm of the soil; it was negligible in the matrix and at greater depths. Concentrations of exchangeable Ca in the flow paths increased nearly 10-fold during the 6 months following the addition of the wood ash, and those of organically bound Pb by 50%. The opposite effect was found for exchangeable Al. Our results show that only part of the whole soil volume, approximately 50% of 0–20 cm in our study, is involved in transporting and sorbing the elements applied with the wood ash or as tracers. Such differences must be considered when calculating the maximal impact of any addition of fertilizer, wood ash, or liming agent.

In this study, we investigated the dynamics of dissolved inorganic and organic nitrogen (N) in throughfall, in soil solutions of Gleysols, and in outflows from experimental sub-catchments as well as from a headwater catchment in Switzerland. Additionally, we studied the effect of increased N deposition on dissolved N by applying 3 g NH 4 NO 3 –N m −2 year −1 to soil plots and to one of the sub-catchments. Dissolved organic N (DON) was the dominant form of total dissolved N (TDN) in the soil and surface waters. The proportion of DON in TDN increased from the throughfall down to the subsoil, which indicates that the retention of DON was lower than that of inorganic N. Concentrations of DON in the subsoil were higher under reducing than under oxidising conditions. In the soil solution and in the runoff from all catchments, nitrate and DON displayed inverse seasonal patterns with concentrations of NO 3 − being highest in late winter and those of DON being maximal in summer and fall. This difference shows that during periods of increased biological activity, NO 3 − was retained in the forest ecosystem while the production of DON was stimulated. Concentrations of both NO 3 − and DON were negatively correlated with the amount of throughfall. In the outflow of the headwater catchment, however, the concentration–discharge relationship was negative for NO 3 − , but positive for DON. The reason for this difference appeared to be the larger contribution of topsoil water at high flow, which was poor in NO 3 − and rich in DON. Experimentally increasing the N deposition increased NO 3 − leaching significantly, but had no effects on DON leaching and on DON export from the catchments. In conclusion, our results show that the dynamics of dissolved organic and inorganic N are controlled by different factors.

The responses of nitrogen transformations and nitrate (NO_3 -) leaching to experimentally increased N deposition were studied in forested sub-catchments (1500 m2) with Gleysols in Central Switzerland. The aim was toinvestigate whether the increase in NO3 - leaching,due to elevated N deposition, was hydrologically driven orresulted from N saturation of the forest ecosystem.Three years of continuous N addition at a rate of 30 kgNH4NO3-N ha-1 yr-1 had no effects on bulksoil N, on microbial biomass N, on K2SO4-extractableN concentrations in the soil, and on net nitrification rates.In contrast, N losses from the ecosystem through denitrification and NO3 - leaching increased significantly. Nitrate leaching was 4 kg N ha-1yr-1at an ambient N deposition of 18 kg N ha-1 yr-1.Leaching of NO3 - at elevated N deposition was 8 kg Nha-1 yr-1. Highest NO3 - leaching occurredduring snowmelt. Ammonium was effectively retained within theuppermost centimetres of the soil as shown by the absence ofNH4 + in the soil solution collected with microsuction cups. Quantifying the N fluxes indicated that 80% ofthe added N were retained in the forest ecosystem.Discharge and NO3 - concentrations of the outflow from the sub-catchments responded to rainfall within 30 min. The water chemistry of the sub-catchment outflow showed thatduring storms, a large part of the runoff from this Gleysol derived from precipitation and from water which had interactedonly with the topsoil. This suggests a dominance of near-surface flow and/or preferential transport through this soil. The contact time of the water with the soil matrix wassufficient to retain NH4 +, but insufficient for a complete retention of NO3 -. At this site with soilsclose to water saturation, the increase in NO3 - leaching by 4 kg N ha-1 yr-1 through elevated N inputsappeared to be due to the bypassing of the soil and the rootsystem rather than to a soil-internal N surplus.

The effect of increased nitrogen deposition (30 kg N ha −1 per year as NH 4 NO 3 ) on nitrogen losses by denitrification was studied in a Norway spruce ( Picea abies ) forest in Central Switzerland. Denitrification and potential controlling variables were measured on five replicate areas, treated with a solution of NH 4 NO 3 (N-treated plots), or only moistened with rain water for comparison (control plots). Maximum nitrogen loss by denitrification was observed with increasing soil temperature and soil pH and decreasing soil redox potential. The spatial variation of the denitrifying activity was driven by the micro-topography of the forest. On drained mounds with raw humus layers (redox potential: 659 ± 7 to 738 ± 3 mV) the rate of denitrification was low throughout the year. In waterlogged depressions (redox potential: 259 ± 15 to 515 ± 7 mV) the denitrifying activities were highest in spring and summer and reached up to 6.49 mg N m −2 per day. Stimulation of the denitrification rate by increasing temperature was most pronounced on the plots having a low redox potential. Elevated N deposition increased the annual denitrification rate from 1.7 at ambient deposition to 2.9 kg N ha −1 per year (May 1996–October 1997). Increased denitrifying activity due to elevated N inputs occurred particularly after rainfall events with nitrogen additions and reached up to 13.58 mg N m −2 per day. This suggests that part of the N input is immediately denitrified and partly compensates the elevated nitrogen inputs.

In this study, we estimated whether changes in hydrological pathwaysduring storms could explain the large temporal variations of dissolvedorganic carbon (DOC) and nitrogen (DON) in the runoff of threecatchments: a forest and a grassland sub-catchment of 1600m2 delineated by trenches, and a headwater catchment of 0.7km2. The average annual DOC export from the sub-catchments was 185 kg DOCha−1 y−1 for the forest, 108 kg DOCha−1 y−1 for the grassland and 84 kgDOC ha−1 y−1 for the headwatercatchment. DON was the major form of the dissolved N in soil and streamwater. DON export from all catchments was approximately 6 kg Nha−1 y−1, which corresponded to 60% ofthe total N export and to 50% of the ambient wet N deposition. DOC andDON concentrations in weekly samples of stream water were positivelycorrelated with discharge. During individual storms, concentrations andproperties of DOC and DON changed drastically. In all catchments, DOCconcentrations increased by 6 to 7 mg DOC l−1 comparedto base flow, with the largest relative increment in the headwatercatchment (+350%). Concentrations of DON, hydrolysable amino acids, andphenolics showed comparable increases, whereas the proportion ofcarbohydrates in DOC decreased at peak flow. Prediction of DOC and DONconcentrations by an end-member mixing analysis (EMMA) on the base ofinorganic water chemistry showed that changes in water flow pathslargely explained these temporal variability. According to the EMMA, thecontribution of throughfall to the runoff peaked in the initial phase ofthe storm, while water from the subsoil dominated during base flow only.EMMA indicated that the contribution of the DOC and DON-rich topsoil washighest in the later stages of the storm, which explained the highestDOC and DON concentrations as the hydrograph receded. Discrepanciesbetween observed and predicted concentrations were largest for thereactive DOC compounds such as carbohydrates and phenolics. Theyoccurred at base flow and in the initial phase of storms. This suggeststhat other mechanisms such as in-stream processes or a time-variantrelease of DOC also played an important role.

The objectives of this study were to estimate how soil type, elevated N deposition (0.7 vs. 7 g N m−2y−1) and tree species influence the potential effects of elevated CO2 (370 vs. 570 μmol CO2 mol−1) on N pools and fluxes in forest soils. Model spruce-beech forest ecosystems were established on a nutrient-rich calcareous sand and on a nutrient-poor acidic loam in large open-top chambers. In the fourth year of treatment, we measured N concentrations in the soil solution at different depths, estimated N accumulation by ion exchange resin (IER) bags, and quantified N export in drainage water, denitrification, and net N uptake by trees. Under elevated CO2, concentrations of N in the soil solution were significantly reduced. In the nutrient-rich calcareous sand, CO2 enrichment decreased N concentrations in the soil solution at all depths (−45 to −100%). In the nutrient-poor acidic loam, the negative CO2 effect was restricted to the uppermost 5 cm of the soil. Increasing the N deposition stimulated the negative impact of CO2 enrichment on soil solution N in the acidic loam at 5 cm depth from −20% at low N inputs to −70% at high N inputs. In the nutrient-rich calcareous sand, N additions did not influence the CO2 effect on soil solution N. Accumulation of N by IER bags, which were installed under individual trees, was decreased at high CO2 levels under spruce in both soil types. Under beech, this decrease occurred only in the calcareous sand. N accumulation by IER bags was negatively correlated with current-years foliage biomass, suggesting that the reduction of soil N availability indices was related to a CO2-induced growth enhancement. However, the net N uptake by trees was not significantly increased by elevated CO2. Thus, we suppose that the reduced N concentrations in the soil solution at elevated CO2 concentrations were rather caused by an increased N immobilisation in the soil. Denitrification was not influenced by atmospheric CO2 concentrations. CO2 enrichment decreased nitrate leaching in drainage by 65%, which suggests that rising atmospheric CO2 potentially increases the N retention capacity of forest ecosystems.