Nitrogen concentration and δ15N signature of ombrotrophic Sphagnum mosses at different N deposition levels in Europe

Alteration of the global nitrogen (N) cycle because of human‐enhanced N fixation is a major concern particularly for those ecosystems that are nutrient poor by nature. Because Sphagnum‐dominated mires are exclusively fed by wet and dry atmospheric deposition, they are assumed to be very sensitive to increased atmospheric N input. We assessed the consequences of increased atmospheric N deposition on total N concentration, N retention ability, and δ¹⁵N isotopic signature of Sphagnum plants collected in 16 ombrotrophic mires across 11 European countries. The mires spanned a gradient of atmospheric N deposition from about 0.1 up to about 2 g m^?2 yr^?1. Mean N concentration in Sphagnum capitula was about 6 mg g^?1 in less polluted mires and about 13 mg g^?1 in highly N‐polluted mires. The relative difference in N concentration between capitulum and stem decreased with increasing atmospheric N deposition, suggesting a possible metabolic mechanism that reduces excessive N accumulation in the capitulum. Sphagnum plants showed lower rates of N absorption under increasing atmospheric N deposition, indicating N saturation in Sphagnum tissues. The latter probably is related to a shift from N‐limited conditions to limitation by other nutrients. The capacity of the Sphagnum layer to filter atmospheric N deposition decreased exponentially along the depositional gradient resulting in enrichment of the mire pore water with inorganic N forms (i.e., NO₃^?+NH₄⁺). Sphagnum plants had δ¹⁵N signatures ranging from about ?8‰ to about ?3‰. The isotopic signatures were rather related to the ratio of reduced to oxidized N forms in atmospheric deposition than to total amount of atmospheric N deposition, indicating that δ¹⁵N signature of Sphagnum plants can be used as an integrated measure of δ¹⁵N signature of atmospheric precipitation. Indeed, mires located in areas characterized by greater emissions of NH₃ (i.e., mainly affected by agricultural activities) had Sphagnum plants with a lower δ¹⁵N signature compared with mires located in areas dominated by NO_x emissions (i.e., mainly affected by industrial activities).     

Variations in nitrogen-15 natural abundance of plant and soil systems in four remote tropical rainforests, southern China

The foliar stable N isotope ratio (δ¹⁵N) can provide integrated information on ecosystem N cycling. Here we present the δ¹⁵N of plant and soil in four remote typical tropical rainforests (one primary and three secondary) of southern China. We aimed to examine if (1) foliar δ¹⁵N in the study forests is negative, as observed in other tropical and subtropical sites in eastern Asia; (2) variation in δ¹⁵N among different species is smaller compared to that in many N-limited temperate and boreal ecosystems; and (3) the primary forest is more N rich than the younger secondary forests and therefore is more ¹⁵N enriched. Our results show that foliar δ¹⁵N ranged from ?5.1 to 1.3 ‰ for 39 collected plant species with different growth strategies and mycorrhizal types, and that for 35 species it was negative. Soil NO₃ ^? had low δ¹⁵N (?11.4 to ?3.2 ‰) and plant NO₃ ^? uptake could not explain the negative foliar δ¹⁵N values (NH₄ ⁺ was dominant in the soil inorganic-N fraction). We suggest that negative values might be caused by isotope fractionation during soil NH₄ ⁺ uptake and mycorrhizal N transfer, and by direct uptake of atmospheric NH₃/NH₄ ⁺. The variation in foliar δ¹⁵N among species (by about 6 ‰) was smaller than in many N-limited ecosystems, which is typically about or over 10 ‰. The primary forest had a larger N capital in plants than the secondary forests. Foliar δ¹⁵N and the enrichment factor (foliar δ¹⁵N minus soil δ¹⁵N) were higher in the primary forest than in the secondary forests, albeit differences were small, while there was no consistent pattern in soil δ¹⁵N between primary and secondary forests.     

Nitrogen input together with ecosystem nitrogen enrichment predict nitrate leaching from European forests

The IFEF database (Indicators of Forest Ecosystem Functioning), consisting of nitrogen deposition, nitrate leaching fluxes, and soil and ecosystem characteristics, is analysed to evaluate the C/N ratio in the organic horizon as an indicator of nitrate leaching. One hundred and eighty one forests are examined, from countries across Europe ranging from boreal to Mediterranean regions, encompassing broadleaf and coniferous sites and plot and catchment studies. N input in throughfall ranges from less than 1 kg N ha^?1 y^?1 in northern Norway and Finland to greater than 60 kg N ha^?1 y^?1 in the Netherlands and Czech Republic. The amount of NO₃^– leached covers a smaller range, between 1 and 40 kg N ha^?1 y^?1. Nitrate leaching is strongly dependent on the amount of nitrogen deposited in throughfall (N input) and simply adding the C/N ratio in the organic horizon to a regression equation does not improve this relationship. However, when the data are stratified based on C/N ratios less than or equal to 25 and greater than 25, highly significant relationships (P < 0.05) are observed between N input and NO₃^– leached. The slope of the relationship for those sites where C/N ratio is ≤ 25 (′nitrogen enriched′ sites) is twice that for those sites where C/N ratio is > 25. These empirical relationships may be used to identify which forested ecosystems are likely to show elevated rates of nitrate leaching under predicted future nitrogen deposition scenarios. Elevated NO₃^– leaching also shows a relationship with soil pH, with high rates of NO₃^– leaching only observed at sites with a pH < 4.5 and N inputs > 30 kg N ha^?1 y^?1. Tree age and species have no significant impact on the ecosystem response to N input at a regional scale.     

Multiyear dual nitrate isotope signatures suggest that N‐saturated subtropical forested catchments can act as robust N sinks

In forests of the humid subtropics of China, chronically elevated nitrogen (N) deposition, predominantly as ammonium (NH₄⁺), causes significant nitrate (NO₃^?) leaching from well‐drained acid forest soils on hill slopes (HS), whereas significant retention of NO₃^? occurs in near‐stream environments (groundwater discharge zones, GDZ). To aid our understanding of N transformations on the catchment level, we studied spatial and temporal variabilities of concentration and natural abundance (δ¹⁵N and δ¹⁸O) of nitrate (NO₃^?) in soil pore water along a hydrological continuum in the N‐saturated Tieshanping (TSP) catchment, southwest China. Our data show that effective removal of atmogenic NH₄⁺ and production of NO₃^? in soils on HS were associated with a significant decrease in δ¹⁵N‐NO₃^?, suggesting efficient nitrification despite low soil pH. The concentration of NO₃^? declined sharply along the hydrological flow path in the GDZ. This decline was associated with a significant increase in both δ¹⁵N and δ¹⁸O of residual NO₃^?, providing evidence that the GDZ acts as an N sink due to denitrification. The observed apparent ¹⁵N enrichment factor (ε) of NO₃^? of about ?5‰ in the GDZ is similar to values previously reported for efficient denitrification in riparian and groundwater systems. Episode studies in the summers of 2009, 2010 and 2013 revealed that the spatial pattern of δ¹⁵N and δ¹⁸O‐NO₃^? in soil water was remarkably similar from year to year. The importance of denitrification as a major N sink was also seen at the catchment scale, as largest δ¹⁵N‐NO₃^? values in stream water were observed at lowest discharge, confirming the importance of the relatively small GDZ for N removal under base flow conditions. This study, explicitly recognizing hydrologically connected landscape elements, reveals an overlooked but robust N sink in N‐saturated, subtropical forests with important implications for regional N budgets.     

High retention of 15N‐labeled nitrogen deposition in a nitrogen saturated old‐growth tropical forest

The effects of increased reactive nitrogen (N) deposition in forests depend largely on its fate in the ecosystems. However, our knowledge on the fates of deposited N in tropical forest ecosystems and its retention mechanisms is limited. Here, we report the results from the first whole ecosystem ¹⁵N labeling experiment performed in a N‐rich old‐growth tropical forest in southern China. We added ¹⁵N tracer monthly as ¹⁵NH₄¹⁵NO₃ for 1 year to control plots and to N‐fertilized plots (N‐plots, receiving additions of 50 kg N ha^?1 yr^?1 for 10 years). Tracer recoveries in major ecosystem compartments were quantified 4 months after the last addition. Tracer recoveries in soil solution were monitored monthly to quantify leaching losses. Total tracer recovery in plant and soil (N retention) in the control plots was 72% and similar to those observed in temperate forests. The retention decreased to 52% in the N‐plots. Soil was the dominant sink, retaining 37% and 28% of the labeled N input in the control and N‐plots, respectively. Leaching below 20 cm was 50 kg N ha^?1 yr^?1 in the control plots and was close to the N input (51 kg N ha^?1 yr^?1), indicating N saturation of the top soil. Nitrogen addition increased N leaching to 73 kg N ha^?1 yr^?1. However, of these only 7 and 23 kg N ha^?1 yr^?1 in the control and N‐plots, respectively, originated from the labeled N input. Our findings indicate that deposited N, like in temperate forests, is largely incorporated into plant and soil pools in the short term, although the forest is N‐saturated, but high cycling rates may later release the N for leaching and/or gaseous loss. Thus, N cycling rates rather than short‐term N retention represent the main difference between temperate forests and the studied tropical forest.     

Stable N isotope composition of nitrate reflects N transformations during the passage of water through a montane rain forest in Ecuador

Knowledge of the fate of deposited N in the possibly N-limited, highly biodiverse north Andean forests is important because of the possible effects of N inputs on plant performance and species composition. We analyzed concentrations and fluxes of NO₃ ^??CN, NH₄ ⁺?CN and dissolved organic N (DON) in rainfall, throughfall, litter leachate, mineral soil solutions (0.15?C0.30 m depths) and stream water in a montane forest in Ecuador during four consecutive quarters and used the natural ¹⁵N abundance in NO₃ ^? during the passage of rain water through the ecosystem and bulk ??¹⁵N values in soil to detect N transformations. Depletion of ¹⁵N in NO₃ ^? and increased NO₃ ^??CN fluxes during the passage through the canopy and the organic layer indicated nitrification in these compartments. During leaching from the organic layer to mineral soil and stream, NO₃ ^? concentrations progressively decreased and were enriched in ¹⁵N but did not reach the ??¹⁵N values of solid phase organic matter (??¹⁵N = 5.6?C6.7??). This suggested a combination of nitrification and denitrification in mineral soil. In the wettest quarter, the ??¹⁵N value of NO₃ ^? in litter leachate was smaller (??¹⁵N = ?1.58??) than in the other quarters (??¹⁵N = ?9.38 ± SE 0.46??) probably because of reduced mineralization and associated fractionation against ¹⁵N. Nitrogen isotope fractionation of NO₃ ^? between litter leachate and stream water was smaller in the wettest period than in the other periods probably because of a higher rate of denitrification and continuous dilution by isotopically lighter NO₃ ^??CN from throughfall and nitrification in the organic layer during the wettest period. The stable N isotope composition of NO₃ ^? gave valuable indications of N transformations during the passage of water through the forest ecosystem from rainfall to the stream.     

Mycorrhizal associations of dominant trees influence nitrate leaching responses to N deposition

Temperate forests receive some of the highest rates of nitrogen (N) deposition in the world. While numerous studies have investigated the effects of N enrichment on forests, there is little consensus on why some forests become N saturated while others do not. To investigate this, we used a multi-factor meta-analysis to simultaneously estimate the relative importance of several environmental, experimental, and anthropogenic variables on nitrate (NO₃ ^?) leaching in response to experimental N addition. Given that overstory tree species composition and soil C:N ratio influence forest responses to N, we hypothesized that forests dominated by arbuscular mycorrhizal (AM) trees would respond differently than forests dominated by ectomycorrhizal (ECM) trees in the context of forest susceptibility to NO₃ ^? leaching. We found that mycorrhizal association is an important predictor of NO₃ ^? leaching, and AM-dominated forests leach more NO₃ ^? in response to N deposition than ECM forests. Additionally, we found that the amount of total N added, ambient N deposition rates, and the form of N added influenced the magnitude of the NO₃ ^? leaching response. Given that the mycorrhizal associations of most temperate trees are known, our results suggest that this functional grouping may be useful in identifying forests that are most susceptible to NO₃ ^? leaching.     

Nitrogen addition alters mineralization dynamics of 13C‐depleted leaf and twig litter and reduces leaching of older DOC from mineral soil

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.     

Nitrogen Cycling and Mass Balance for a Forested Catchment in the Italian Alps. Assessment of Nitrogen Status

During 1999–2001 the chemical composition and fluxes were measured in rainfall, throughfall, soil solution and stream water in a remote forested site in the Italian Alps. The analysis of temporal patterns revealed the differential behaviour of nitrogen and sulphur and suggested that different mechanisms controlled their flux. No important changes in sulphate concentration and fluxes emerged as the solution passed through the various components of the forest ecosystem, and temporal variations of SO₄ in the soil solution and stream were likely driven by the physical process of dilution. The availability of nitrate and ammonia, by contrast, was drastically reduced as throughfall water entered the soil and passed through the mineral layers, irrespective of season. The calculated hydrochemical budget based on throughfall and soil solution N fluxes revealed that ~80% N retention in the forest soil, corresponding to 12 kg ha⁻¹ yr⁻¹, despite a relatively high N deposition loading (15 kg ha⁻¹ yr⁻¹). Most of the leached nitrogen (90%) was in the organic form. Indicators of the N status of this ecosystem, such as C/N ratio in solid and solution phase of the soil and N foliage content as well as land use history were examined. Despite the strong N retention in the forested part of the catchment, the stream water N–NO₃ levels were consistently above 10 μg l⁻¹ suggesting that the Val Masino catchment as a whole was less efficient in processing atmospheric N inputs. This contrasting N behaviour illustrates the role of landscape features, such as the soil cover and vegetation type, that is characteristic of an alpine catchment.     

Natural 15N abundance of epiphytes depends on the position within the forest canopy: source signals and isotope fractionation

The natural ¹⁵N abundance (δ¹⁵N) of epiphytes and its N sources were studied in the canopy of a lowland rainforest in Costa Rica. Vascular and non‐vascular epiphytes and canopy soils were collected from four canopy zones and analysed for N contents and δ¹⁵N signals. In addition, the N concentrations and δ¹⁵N signatures of bulk precipitation, throughfall and stemflow were measured during the wet and the dry season. The δ¹⁵N values of epiphyte leaves decreased significantly from the lower zones (means of ?3·9 and ?4·3‰) to the upper zones (means of ?5·4 and ?6·1‰) of the canopy. In contrast, δ¹⁵N signatures of canopy soils (average ?0·3‰) differed little between the zones. Bulk deposition was enriched in ¹⁵N (+4·3‰) compared to all other potential N sources and was higher than throughfall and stemflow (+0·5 to ?1·3‰). δ¹⁵N values of atmospheric deposition were inversely related to those of the epiphyte leaves, whereas N isotopic composition of canopy soils did not vary significantly. Consequently, it is concluded that the variations in foliar N isotope composition of epiphytes were not simply caused by utilization of isotopically different N sources, but by different ¹⁵N discrimination during N acquisition.     

Nitrogen deposition contributes to soil acidification in tropical ecosystems

Elevated anthropogenic nitrogen (N) deposition has greatly altered terrestrial ecosystem functioning, threatening ecosystem health via acidification and eutrophication in temperate and boreal forests across the northern hemisphere. However, response of forest soil acidification to N deposition has been less studied in humid tropics compared to other forest types. This study was designed to explore impacts of long‐term N deposition on soil acidification processes in tropical forests. We have established a long‐term N‐deposition experiment in an N‐rich lowland tropical forest of Southern China since 2002 with N addition as NH₄NO₃ of 0, 50, 100 and 150 kg N ha^?1 yr^?1. We measured soil acidification status and element leaching in soil drainage solution after 6‐year N addition. Results showed that our study site has been experiencing serious soil acidification and was quite acid‐sensitive showing high acidification (pH_(H2O)<4.0), negative water‐extracted acid neutralizing capacity (ANC) and low base saturation (BS,< 8%) throughout soil profiles. Long‐term N addition significantly accelerated soil acidification, leading to depleted base cations and decreased BS, and further lowered ANC. However, N addition did not alter exchangeable Al³⁺, but increased cation exchange capacity (CEC). Nitrogen addition‐induced increase in SOC is suggested to contribute to both higher CEC and lower pH. We further found that increased N addition greatly decreased soil solution pH at 20 cm depth, but not at 40 cm. Furthermore, there was no evidence that Al³⁺ was leaching out from the deeper soils. These unique responses in tropical climate likely resulted from: exchangeable H⁺ dominating changes of soil cation pool, an exhausted base cation pool, N‐addition stimulating SOC production, and N saturation. Our results suggest that long‐term N addition can contribute measurably to soil acidification, and that shortage of Ca and Mg should receive more attention than soil exchangeable Al in tropical forests with elevated N deposition in the future.     

The impact of nitrogen deposition on carbon sequestration in European forests and forest soils

An estimate of net carbon (C) pool changes and long‐term C sequestration in trees and soils was made at more than 100 intensively monitored forest plots (level II plots) and scaled up to Europe based on data for more than 6000 forested plots in a systematic 16 km × 16 km grid (level I plots). C pool changes in trees at the level II plots were based on repeated forest growth surveys At the level I plots, an estimate of the mean annual C pool changes was derived from stand age and available site quality characteristics. C sequestration, being equal to the long‐term C pool changes accounting for CO₂ emissions because of harvest and forest fires, was assumed 33% of the overall C pool changes by growth. C sequestration in the soil were based on calculated nitrogen (N) retention (N deposition minus net N uptake minus N leaching) rates in soils, multiplied by the C/N ratio of the forest soils, using measured data only (level II plots) or a combination of measurements and model calculations (level I plots). Net C sequestration by forests in Europe (both trees and soil) was estimated at 0.117 Gton yr^?1, with the C sequestration in stem wood being approximately four times as high (0.094 Gton yr^?1) as the C sequestration in the soil (0.023 Gton yr^?1). The European average impact of an additional N input on the net C sequestration was estimated at approximately 25 kg C kg^?1 N for both tree wood and soil. The contribution of an average additional N deposition on European forests of 2.8 kg ha^?1 yr^?1 in the period 1960–2000 was estimated at 0.0118 Gton yr^?1, being equal to 10% of the net C sequestration in both trees and soil in that period (0.117 Gton yr^?1). The C sequestration in trees increased from Northern to Central Europe, whereas the C sequestration in soil was high in Central Europe and low in Northern and Southern Europe. The result of this study implies that the impact of forest management on tree growth is most important in explaining the C pool changes in European forests.     

The effects of gap disturbance on nitrogen cycling and retention in late-successional northern hardwood–hemlock forests

Late-successional forests in the upper Great Lakes region are susceptible to nitrogen (N) saturation and subsequent nitrate (NO₃⁻) leaching loss. Endemic wind disturbances (i.e., treefall gaps) alter tree uptake and soil N dynamics; and, gaps are particular susceptible to NO₃⁻ leaching loss. Inorganic N was measured throughout two snow-free periods in throughfall, forest floor leachates, and mineral soil leachates in gaps (300–2,000 m², 6–9 years old), gap-edges, and closed forest plots in late-successional northern hardwood, hemlock, and northern hardwood–hemlock stands. Differences in forest water inorganic N among gaps, edges, and closed forest plots were consistent across these cover types: NO₃⁻ inputs in throughfall were significantly greater in undisturbed forest plots compared with gaps and edges; forest floor leachate NO₃⁻ was significantly greater in gaps compared to edges and closed forest plots; and soil leachate NO₃⁻ was significantly greater in gaps compared to the closed forest. Significant differences in forest water ammonium and pH were not detected. Compared to suspected N-saturated forests with high soil NO₃⁻ leaching, undisturbed forest plots in these late-successional forests are not losing NO₃⁻ (net annual gain of 2.8 kg ha⁻¹) and are likely not N-saturated. Net annual NO₃⁻ losses were observed in gaps (1.3 kg ha⁻¹) and gap-edges (0.2 kg ha⁻¹), but we suspect these N leaching losses are a result of decreased plant uptake and increased soil N mineralization associated with disturbance, and not N-saturation.     

Role of six European tree species and land‐use legacy for nitrogen and water budgets in forests

Water and nutrient fluxes for single stands of different tree species have been reported in numerous studies, but comparative studies of nutrient and hydrological budgets of common European deciduous tree species are rare. Annual fluxes of water and inorganic nitrogen (N) were established in a 30‐year‐old common garden design with stands of common ash (Fraxinus excelsior), European beech (Fagus sylvatica L.), pedunculate oak (Quercus robur), small‐leaved lime (Tilia cordata Mill.), sycamore maple (Acer pseudoplatanus) and Norway spruce (Picea abies [L.] Karst.) replicated at two sites in Denmark, Mattrup and Vallø during 2 years. Mean annual percolation below the root zone (mm yr^?1±SE, n=4) ranked in the following order: maple (351±38)>lime (284±32), oak (271±25), beech (257±30), ash (307±69)? spruce (75±24). There were few significant tree species effects on N fluxes. However, the annual mean N throughfall flux (kg N ha^?1 yr^?1±SE, n=4) for spruce (28±2) was significantly larger than for maple (12±1), beech (11±1) and oak (9±1) stands but not different from that of lime (15±3). Ash had a low mean annual inorganic N throughfall deposition of 9.1 kg ha^?1, but was only present at Mattrup. Annual mean of inorganic N leaching (kg ha^?1 yr^?1±SE, n=4) did not differ significantly between species despite of contrasting tree species mean values; beech (25±9)>oak (16±10), spruce (15±8), lime (14±8)? maple (1.9±1), ash (2.0±1). The two sites had similar throughfall N fluxes, whereas the annual leaching of N was significantly higher at Mattrup than at Vallø. Accordingly, the sites differed in soil properties in relation to rates and dynamics of N cycling. We conclude that tree species affect the N cycle differently but the legacy of land use exerted a dominant control on the N cycle within the short‐term perspective (30 years) of these stands.     

Ammonium,nitrate and dissolved organic nitrogen in seepage water as affected by compost amendment to European beech,Norway spruce,and Scots pine forests

Fertilization of nutrient-depleted and degraded forest soils may be required to sustain utilization of forests. In some European countries, the application of composts may now be an alternative to the application of inorganic fertilizers because commercial compost production has increased and compost quality has been improved. There is, however, concern that compost amendments may cause increased leaching of nitrogen, trace metals and toxic organic compounds to groundwater. The objective of this study was to assess the risk of ammonium (NH₄ ⁺), nitrate (NO₃ ^–) and dissolved organic nitrogen (DON) leaching following a single compost application to silty and sandy soils in mature beech (Fagus sylvatica L.), pine (Pinus silvestris L.) and spruce (Picea abies Karst.) forests at Solling and Unterlüß in Lower Saxony, Germany. Mature compost from separately collected organic household waste was applied to the soil surface at a rate of 6.3 kg m^–2 in the summer of 1997 and changes in NH₄ ⁺, NO₃ ^– and DON concentrations in throughfall and soil water at 10 and 100 cm soil depths were determined for 32 months. The spruce forests had the highest N inputs by throughfall water and the highest N outputs in both the control and compost plots compared with the pine and beech forests. Overall, the differences in total N outputs at 100 cm soil depth between the control and compost plots ranged between 0.3 and 11.2 g N m^–2 for the entire 32-month period. The major leaching of these amounts occurred during the first 17 months after compost amendments, but there was no significant difference in total N outputs (–0.2 to 1.8 g N m^–2) between the control and compost plots during the remaining 15 months. Most of the mineral soils acted as a significant sink for NO₃ ^– and DON as shown by a reduction of their outputs from 10 to 100 cm depth. Based on these results, we conclude that application of mature compost with high inorganic N contents could diminish the groundwater quality in the first months after the amendments. A partial, moderate application of mature compost with low inorganic N content to nutrient depleted forest soils can minimize the risk of NO₃ ^– leaching.     

Simulated chronic NO3− deposition reduces soil respiration in northern hardwood forests

Chronic N additions to forest ecosystems can enhance soil N availability, potentially leading to reduced C allocation to root systems. This in turn could decrease soil CO₂ efflux. We measured soil respiration during the first, fifth, sixth and eighth years of simulated atmospheric NO₃^? deposition (3 g N m^?2 yr^?1) to four sugar maple‐dominated northern hardwood forests in Michigan to assess these possibilities. During the first year, soil respiration rates were slightly, but not significantly, higher in the NO₃^?‐amended plots. In all subsequent measurement years, soil respiration rates from NO₃^?‐amended soils were significantly depressed. Soil temperature and soil matric potential were measured concurrently with soil respiration and used to develop regression relationships for predicting soil respiration rates. Estimates of growing season and annual soil CO₂ efflux made using these relationships indicate that these C fluxes were depressed by 15% in the eighth year of chronic NO₃^? additions. The decrease in soil respiration was not due to reduced C allocation to roots, as root respiration rates, root biomass, and root turnover were not significantly affected by N additions. Aboveground litter also was unchanged by the 8 years of treatment. Of the remaining potential causes for the decline in soil CO₂ efflux, reduced microbial respiration appears to be the most likely possibility. Documented reductions in microbial biomass and the activities of extracellular enzymes used for litter degradation on the NO₃^?‐amended plots are consistent with this explanation.     

Long-term changes of the δ15N natural abundance of plants and soil in a temperate grassland

Tracing back the N use efficiency of long-term fertilizer trials is important for future management recommendations. Here we tested the changes in natural N-isotope composition as an indicator for N- management within a long-term fertilization lysimeter experiment in a low mountain range pasture ecosystem at Rengen (Eifel Mountains), Germany. Cattle slurry (δ¹⁵N?=?8.9?±?0.5‰) and mineral fertilizers (calcium ammonium nitrate; δ¹⁵N?=??1.0?±?0.2‰) were applied at a rate between 0 and 480 kg N ha^?1?yr^?1 throughout 20 years from 1985 onwards. In 2006, samples were taken from different grass species, coarse and fine particulate soil organic matter, bulk soil and leachates. Total soil N content hardly changed during fertilization experiment. As also N leaching has been small within the stagnant water regime, most N was lost through the gaseous phase beside plant uptake and cutting. Unlike N uptake by plants, the process of N volatilization resulted in strong discrimination against the ¹⁵N isotope. As a consequence, the δ¹⁵N values of top soil samples increased from 1.8?±?0.4‰ to 6.0?±?0.4‰ and that of the plants from ?1.2?±?1.3‰ to 4.8?±?1.2‰ with increasing N fertilizer rate. Samples receiving organic fertilizer were most enriched in δ¹⁵N. The results suggest that parts of the fertilizer N signal was preserved in soils and even discovered in soil organic matter pools with slow N turnover. However, a ¹⁵N/¹⁴N isotope fractionation of up to 1.5‰ added to the δ¹⁵N values recovered in soils and plants, rendering the increase in δ¹⁵N value a powerful indicator to long-term inefficient N usage and past N management in the terrestrial environment.     

Exotic grasses and nitrate enrichment alter soil carbon cycling along an urban–rural tropical forest gradient

Urban areas are expanding rapidly in tropical regions, with potential to alter ecosystem dynamics. In particular, exotic grasses and atmospheric nitrogen (N) deposition simultaneously affect tropical urbanized landscapes, with unknown effects on properties like soil carbon (C) storage. We hypothesized that (H1) soil nitrate (NO₃^?) is elevated nearer to the urban core, reflecting N deposition gradients. (H2) Exotic grasslands have elevated soil NO₃^? and decreased soil C relative to secondary forests, with higher N promoting decomposer activity. (H3) Exotic grasslands have greater seasonality in soil NO₃^? vs. secondary forests, due to higher sensitivity of grassland soil moisture to rainfall. We predicted that NO₃^? would be positively related to dissolved organic C (DOC) production via changes in decomposer activity. We measured six paired grassland/secondary forest sites along a tropical urban‐to‐rural gradient during the three dominant seasons (hurricane, dry, and early wet). We found that (1) soil NO₃^? was generally elevated nearer to the urban core, with particularly clear spatial trends for grasslands. (2) Exotic grasslands had lower soil C than secondary forests, which was related to elevated decomposer enzyme activities and soil respiration. Unexpectedly, soil NO₃^? was negatively related to enzyme activities, and was lower in grasslands than forests. (3) Grasslands had greater soil NO₃^? seasonality vs. forests, but this was not strongly linked to shifts in soil moisture or DOC. Our results suggest that exotic grasses in tropical regions are likely to drastically reduce soil C storage, but that N deposition may have an opposite effect via suppression of enzyme activities. However, soil NO₃^? accumulation here was higher in urban forests than grasslands, potentially related to of aboveground N interception. Net urban effects on C storage across tropical landscapes will likely vary depending on the mosaic of grass cover, rates of N deposition, and responses by local decomposer communities.     

Dissolved carbon leaching from soil is a crucial component of the net ecosystem carbon balance

Estimates of carbon leaching losses from different land use systems are few and their contribution to the net ecosystem carbon balance is uncertain. We investigated leaching of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and dissolved methane (CH₄), at forests, grasslands, and croplands across Europe. Biogenic contributions to DIC were estimated by means of its δ¹³C signature. Leaching of biogenic DIC was 8.3±4.9 g m^?2 yr^?1 for forests, 24.1±7.2 g m^?2 yr^?1 for grasslands, and 14.6±4.8 g m^?2 yr^?1 for croplands. DOC leaching equalled 3.5±1.3 g m^?2 yr^?1 for forests, 5.3±2.0 g m^?2 yr^?1 for grasslands, and 4.1±1.3 g m^?2 yr^?1 for croplands. The average flux of total biogenic carbon across land use systems was 19.4±4.0 g C m^?2 yr^?1. Production of DOC in topsoils was positively related to their C/N ratio and DOC retention in subsoils was inversely related to the ratio of organic carbon to iron plus aluminium (hydr)oxides. Partial pressures of CO₂ in soil air and soil pH determined DIC concentrations and fluxes, but soil solutions were often supersaturated with DIC relative to soil air CO₂. Leaching losses of biogenic carbon (DOC plus biogenic DIC) from grasslands equalled 5–98% (median: 22%) of net ecosystem exchange (NEE) plus carbon inputs with fertilization minus carbon removal with harvest. Carbon leaching increased the net losses from cropland soils by 24–105% (median: 25%). For the majority of forest sites, leaching hardly affected actual net ecosystem carbon balances because of the small solubility of CO₂ in acidic forest soil solutions and large NEE. Leaching of CH₄ proved to be insignificant compared with other fluxes of carbon. Overall, our results show that leaching losses are particularly important for the carbon balance of agricultural systems.     

Soil microbial community indices as predictors of soil solution chemistry and N leaching in Picea abies (L.) Karst. forests in S. Sweden

High rates of inorganic nitrogen (N) deposition or internal N turnover increases the risks of N loss from forests with negative effects on stream water quality. We hypothesized that soil fungi may be more important N sinks than bacteria, and thus examined the impact of soil microbial community composition on N leaching from forests. We studied 19 spruce stands to examine relationships between microbial community composition, stem growth, soil-, and lysimeter-collected soil solution characteristics, and N leaching. We used nitrate concentration in the soil solution below the rooting zone as an N leaching index and phospholipid fatty acid (PLFA) analysis for characterisation of microbial communities. Microbial community composition in the organic horizon and soil solution chemistry below the rooting zone was highly correlated. Stands with low concentrations of nitrate (NO₃ ^?) and aluminium (Al) had higher fungi: bacteria ratio compared with stands with higher concentrations of NO₃ ^? and Al. Stem growth and fungi: bacteria ratio explained 70 % of the variation in N and Al leaching. We identified three microbial predictors of variation in soil solution chemistry, of which the fungi: bacteria was the strongest. The other two were putative indicators of microbial C limitation, a condition known to stimulate N mineralisation and nitrification.