Large Canopy Exchange Fluxes of Inorganic and Organic Nitrogen and Preferential Retention of Nitrogen by Epiphytes in a Tropical Lowland Rainforest

Little is known about how tropical forest canopies interact with atmospheric nitrogen deposition and how this affects the internal nutrient dynamics and the processing of external nutrient inputs. The objectives of this study therefore were (1) to investigate gross and net canopy nitrogen (N) fluxes (retention and leaching) and (2) the effect of canopy components on net canopy N retention. Tracers were applied on detached branches in a tropical wet lowland rainforest, Costa Rica. A novel ¹⁵N pool dilution method showed that gross canopy fluxes (retention and leaching) of NO₃ ^?, NH₄ ⁺, and dissolved organic nitrogen (DON) were remarkably higher than net throughfall fluxes. Gross fluxes of NH₄ ⁺ and NO₃ ^? resulted in a negligible net flux whereas DON showed net uptake by the canopy. The highest quantity of ¹⁵N was recovered in epiphytic bryophytes (16.4%) although the largest biomass fraction was made up of leaves. The study demonstrates that tracer applications allow investigation of the dynamic and complex canopy exchange processes and that epiphytic communities play a major role in solute fluxes in tree canopies and therefore in the nutrient dynamics of tropical rain forests.     

Distribution of nitrogen-15 tracers applied to the canopy of a mature spruce-hemlock stand,Howland, Maine,USA

In N-limited ecosystems, fertilization by N deposition may enhance plant growth and thus impact C sequestration. In many N deposition–C sequestration experiments, N is added directly to the soil, bypassing canopy processes and potentially favoring N immobilization by the soil. To understand the impact of enhanced N deposition on a low fertility unmanaged forest and better emulate natural N deposition processes, we added 18 kg N ha⁻¹ year⁻¹ as dissolved NH₄NO₃ directly to the canopy of 21 ha of spruce-hemlock forest. In two 0.3-ha subplots, the added N was isotopically labeled as ¹⁵NH₄ ⁺ or ¹⁵NO₃ ⁻ (1% final enrichment). Among ecosystem pools, we recovered 38 and 67% of the ¹⁵N added as ¹⁵NH₄ ⁺ and ¹⁵NO₃ ⁻, respectively. Of ¹⁵N recoverable in plant biomass, only 3–6% was recovered in live foliage and bole wood. Tree twigs, branches, and bark constituted the most important plant sinks for both NO₃ ⁻ and NH₄ ⁺, together accounting for 25–50% of ¹⁵N recovery for these ions, respectively. Forest floor and soil ¹⁵N retention was small compared to previous studies; the litter layer and well-humified O horizon were important sinks for NH₄ ⁺ (9%) and NO₃ ⁻ (7%). Retention by canopy elements (surfaces of branches and boles) provided a substantial sink for N that may have been through physico-chemical processes rather than by N assimilation as indicated by poor recoveries in wood tissues. Canopy retention of precipitation-borne N added in this particular manner may thus not become plant-available N for several years. Despite a large canopy N retention potential in this forest, C sequestration into new wood growth as a result of the N addition was only ~16 g C m⁻² year⁻¹ or about 10% above the current net annual C sequestration for this site.     

Functional role of DNRA and nitrite reduction in a pristine south Chilean <Emphasis Type="Italic">Nothofagus</Emphasis> forest

Nitrite (NO₂ ⁻) is an intermediate in a variety of soil N cycling processes. However, NO₂ ⁻ dynamics are often not included in studies that explore the N cycle in soil. Within the presented study, nitrite dynamics were investigated in a Nothofagus betuloides forest on an Andisol in southern Chile. We carried out a ¹⁵N tracing study with six ¹⁵N labeling treatments, including combinations of NO₃ ⁻, NH₄ ⁺ and NO₂ ⁻. Gross N transformation rates were quantified with a ¹⁵N tracing model in combination with a Markov chain Monte Carlo optimization routine. Our results indicate the occurrence of functional links between (1) NH₄ ⁺ oxidation, the main process for NO₂ ⁻ production (nitritation), and NO₂ ⁻ reduction, and (2) oxidation of soil organic N, the dominant NO₃ ⁻ production process in this soil, and dissimilatory NO₃ ⁻ reduction to NH₄ ⁺ (DNRA). The production of NH₄ ⁺ via DNRA was approximately ten times higher than direct mineralization from recalcitrant soil organic matter. Moreover, the rate of DNRA was several magnitudes higher than the rate of other NO₃ ⁻ reducing processes, indicating that DNRA is able to outcompete denitrification, which is most likely not an important process in this ecosystem. These functional links are most likely adaptations of the microbial community to the prevailing pedo-climatic conditions of this Nothofagus ecosystem.     

Abiotic immobilization of nitrate in two soils of relic <Emphasis Type="Italic">Abies pinsapo</Emphasis>-fir forests under Mediterranean climate

Evidence for abiotic immobilization of nitrogen (N) in soil is accumulating, but remains controversial. Identifying the fate of N from atmospheric deposition is important for understanding the N cycle of forest ecosystems. We studied soils of two Abies pinsapo fir forests under Mediterranean climate seasonality in southern Spain—one with low N availability and the other with symptoms of N saturation. We hypothesized that biotic and abiotic immobilization of nitrate (NO₃ ⁻) would be lower in soils under these forests compared to more mesic temperate forests, and that the N saturated stand would have the lowest rates of NO₃ ⁻ immobilization. Live and autoclaved soils were incubated with added ¹⁵NO₃ ⁻ (10 μg N g⁻¹ dry soil; 99% enriched) for 24 h, and the label was recovered as total dissolved-N, NO₃ ⁻, ammonium (NH₄ ⁺), or dissolved organic-N (DON). To evaluate concerns about possible iron interference in analysis of NO₃ ⁻ concentrations, both flow injection analysis (FIA) and ion chromatography (IC) were applied to water extracts, soluble iron was measured in both water and salt extracts, and standard additions of NO₃ ⁻ to salt extracts were analyzed. Good agreement between FIA and IC analysis, low concentrations of soluble Fe, and 100% (±3%) recovery of NO₃ ⁻ standard additions all pointed to absence of an interference problem for NO₃ ⁻ quantification. On average, 85% of the added ¹⁵NO₃ ⁻ label was recovered as ¹⁵NO₃ ⁻, which supports our hypothesis that rates of immobilization were generally low in these soils. A small amount (mean = 0.06 μg N g⁻¹ dry soil) was recovered as ¹⁵NH₄ ⁺ in live soils and none in sterilized soils. Mean recovery as DO¹⁵N ranged from 0.6 to 1.5 μg N g⁻¹ dry soil, with no statistically significant effect of sterilization or soil type, indicating that this was an abiotic process that occurred at similar rates in both soils. These results demonstrate a detectable, but modest rate of abiotic immobilization of NO₃ ⁻ to DON, supporting our first hypothesis. These mineral soils may not have adequate carbon availability to support the regeneration of reducing microsites needed for high rates of NO₃ ⁻ reduction. Our second hypothesis regarding lower expected abiotic immobilization in soils from the N-saturated site was not supported. The rates of N deposition in this region may not be high enough to have swamped the capacity for soil NO₃ ⁻ immobilization, even in the stand showing some symptoms of N saturation. A growing body of evidence suggests that soil abiotic NO₃ ⁻ immobilization is common, but that rates are influenced by a combination of factors, including the presence of plentiful available carbon, reduced minerals in anaerobic microsites and adequate NO₃ ⁻ supply.     

Flow of Deposited Inorganic N in Two Gleysol-dominated Mountain Catchments Traced with <Superscript>15</Superscript>NO<Subscript>3</Subscript><Superscript>−</Superscript> and <Superscript>15</Superscript>NH<Subscript>4</Subscript><Superscript>+</Superscript>

In two mountain ecosystems at the Alptal research site in central Switzerland, pulses of ¹⁵NO₃ and ¹⁵NH₄ were separately applied to trace deposited inorganic N. One forested and one litter meadow catchment, each approximately 1600 m², were delimited by trenches in the Gleysols. K¹⁵NO₃ was applied weekly or fortnightly over one year with a backpack sprayer, thus labelling the atmospheric nitrate deposition. After the sampling and a one-year break, ¹⁵NH₄Cl was applied as a second one-year pulse, followed by a second sampling campaign. Trees (needles, branches and bole wood), ground vegetation, litter layer and soil (LF, A and B horizon) were sampled at the end of each labelling period. Extractable inorganic N, microbial N, and immobilised soil N were analysed in the LF and A horizons. During the whole labelling period, the runoff water was sampled as well. Most of the added tracer remained in both ecosystems. More NO₃⁻ than NH₄⁺ tracer was retained, especially in the forest. The highest recovery was in the soil, mainly in the organic horizon, and in the ground vegetation, especially in the mosses. Event-based runoff analyses showed an immediate response of ¹⁵NO₃⁻ in runoff, with sharp ¹⁵N peaks corresponding to discharge peaks. NO₃⁻ leaching showed a clear seasonal pattern, being highest in spring during snowmelt. The high capacity of N retention in these ecosystems leads to the assumption that deposited N accumulates in the soil organic matter, causing a progressive decline of its C:N ratio.     

Natural 15N abundance of soil N pools and N2O reflect the nitrogen dynamics of forest soils

Natural ¹⁵N abundance measurements of ecosystem nitrogen (N) pools and ¹⁵N pool dilution assays of gross N transformation rates were applied to investigate the potential of δ¹⁵N signatures of soil N pools to reflect the dynamics in the forest soil N cycle. Intact soil cores were collected from pure spruce (Picea abies (L.) Karst.) and mixed spruce-beech (Fagus sylvatica L.) stands on stagnic gleysol in Austria. Soil δ¹⁵N values of both forest sites increased with depth to 50 cm, but then decreased below this zone. δ¹⁵N values of microbial biomass (mixed stand: 4.7 ± 0.8‰, spruce stand: 5.9 ± 0.9‰) and of dissolved organic N (DON; mixed stand: 5.3 ± 1.7‰, spruce stand: 2.6 ± 3.3‰) were not significantly different; these pools were most enriched in ¹⁵N of all soil N pools. Denitrification represented the main N₂O-producing process in the mixed forest stand as we detected a significant ¹⁵N enrichment of its substrate NO₃⁻ (3.6 ± 4.5‰) compared to NH₄⁺ (−4.6 ± 2.6‰) and its product N₂O (−11.8 ± 3.2‰). In a ¹⁵N-labelling experiment in the spruce stand, nitrification contributed more to N₂O production than denitrification. Moreover, in natural abundance measurements the NH₄⁺ pool was slightly ¹⁵N-enriched (−0.4 ± 2.0 ‰) compared to NO₃⁻ (−3.0 ± 0.6 ‰) and N₂O (−2.1 ± 1.1 ‰) in the spruce stand, indicating nitrification and denitrification operated in parallel to produce N₂O. The more positive δ¹⁵N values of N₂O in the spruce stand than in the mixed stand point to extensive microbial N₂O reduction in the spruce stand. Combining natural ¹⁵N abundance and ¹⁵N tracer experiments provided a more complete picture of soil N dynamics than possible with either measurement done separately.     

Microbial Cycling of C and N in Northern Hardwood Forests Receiving Chronic Atmospheric NO 3 − Deposition

Sugar maple (Acer saccharum Marsh.)-dominated northern hardwood forests in the upper Lakes States region appear to be particularly sensitive to chronic atmospheric NO₃⁻ deposition. Experimental NO₃⁻ deposition (3 g NO₃⁻ N m⁻² y⁻¹) has significantly reduced soil respiration and increased the export of DOC/DON and NO₃⁻ across the region. Here, we evaluate the possibility that diminished microbial activity in mineral soil was responsible for these ecosystem-level responses to NO₃⁻ deposition. To test this alternative, we measured microbial biomass, respiration, and N transformations in the mineral soil of four northern hardwood stands that have received 9 years of experimental NO₃⁻ deposition. Microbial biomass, microbial respiration, and daily rates of gross and net N transformations were not changed by NO₃⁻ deposition. We also observed no effect of NO₃⁻ deposition on annual rates of net N mineralization. However, NO₃⁻ deposition significantly increased (27%) annual net nitrification, a response that resulted from rapid microbial NO₃⁻ assimilation, the subsequent turnover of NH₄⁺, and increased substrate availability for this process. Nonetheless, greater rates of net nitrification were insufficient to produce the 10-fold observed increase in NO₃⁻ export, suggesting that much of the exported NO₃⁻ resulted directly from the NO₃⁻ deposition treatment. Results suggest that declines in soil respiration and increases in DOC/DON export cannot be attributed to NO₃⁻-induced physiological changes in mineral soil microbial activity. Given the lack of response we have observed in mineral soil, our results point to the potential importance of microbial communities in forest floor, including both saprotrophs and mycorrhizae, in mediating ecosystem-level responses to chronic NO₃⁻ deposition in Lake States northern hardwood forests.     

Chronic Atmospheric NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack> Deposition Does Not Induce NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack> Use by <Emphasis Type="Italic">Acer saccharum</Emphasis> Marsh.

The ability of an ecosystem to retain anthropogenic nitrogen (N) deposition is dependent upon plant and soil sinks for N, the strengths of which may be altered by chronic atmospheric N deposition. Sugar maple (Acer saccharum Marsh.), the dominant overstory tree in northern hardwood forests of the Lake States region, has a limited capacity to take up and assimilate NO₃⁻. However, it is uncertain whether long-term exposure to NO₃⁻ deposition might induce NO₃⁻ uptake by this ecologically important overstory tree. Here, we investigate whether 10 years of experimental NO₃⁻ deposition (30 kg N ha⁻¹ y⁻¹) could induce NO₃⁻ uptake and assimilation in overstory sugar maple (approximately 90 years old), which would enable this species to function as a direct sink for atmospheric NO₃⁻ deposition. Kinetic parameters for NH₄⁺ and NO₃⁻ uptake in fine roots, as well as leaf and root NO₃⁻ reductase activity, were measured under conditions of ambient and experimental NO₃⁻ deposition in four sugar maple-dominated stands spanning the geographic distribution of northern hardwood forests in the Upper Lake States. Chronic NO₃⁻ deposition did not alter the V _max or K _m for NO₃⁻ and NH₄⁺ uptake nor did it influence NO₃⁻ reductase activity in leaves and fine roots. Moreover, the mean V _max for NH₄⁺ uptake (5.15 μmol ¹⁵N g⁻¹ h⁻¹) was eight times greater than the V _max for NO₃⁻ uptake (0.63 μmol ¹⁵N g⁻¹ h⁻¹), indicating a much greater physiological capacity for NH₄⁺ uptake in this species. Additionally, NO₃⁻ reductase activity was lower than most values for woody plants previously reported in the literature, further indicating a low physiological potential for NO₃⁻ assimilation in sugar maple. Our results demonstrate that chronic NO₃⁻ deposition has not induced the physiological capacity for NO₃⁻ uptake and assimilation by sugar maple, making this dominant species an unlikely direct sink for anthropogenic NO₃⁻ deposition.     

Vertical transport of dissolved organic C and N under long-term N amendments in pine and hardwood forests

At the Harvard Forest, Massachusetts, a long-term effort is under way to study responses in ecosystem biogeochemistry to chronic inputs of N in atmospheric deposition in the region. Since 1988, experimental additions of NH₄NO₃ (0, 5 and 15 g N m^–2 yr^–1) have been made in two forest stands:Pinus resinosa (red pine) and mixed hardwood. In the seventh year of the study, we measured solute concentrations and estimated solute fluxes in throughfall and at two soil depths, beneath the forest floors (Oa) and beneath the B horizons.Beneath the Oa, concentrations and fluxes of dissolved organic C and N (DOC and DON) were higher in the coniferous stand than in the hardwood stand. The mineral soil exerted a strong homogenizing effect on concentrations beneath the B horizons. In reference plots (no N additions), DON composed 56% (pine) and 67% (hardwood) of the total dissolved nitrogen (TDN) transported downward from the forest floor to the mineral soil, and 98% of the TDN exported from the solums. Under N amendments, fluxes of DON from the forest floor correlated positively with rates of N addition, but fluxes of inorganic N from the Oa exceeded those of DON. Export of DON from the solums appeared unaffected by 7 years of N amendments, but as in the Oa, DON composed smaller fractions of TDN exports under N amendments. DOC fluxes were not strongly related to N amendment rates, but ratios of DOC:DON often decreased.The hardwood forest floor exhibited a much stronger sink for inorganic N than did the pine forest floor, making the inputs of dissolved N to mineral soil much greater in the pine stand. Under the high-N treatment, exports of inorganic N from the solum of the pine stand were increased >500-fold over reference (5.2 vs. 0.01 g N m^–2 yr^–1), consistent with other manifestations of nitrogen saturation. Exports of N from the solum in the pine forest decreased in the order NO₃-N> NH₄-N> DON, with exports of inorganic N 14-fold higher than exports of DON. In the hardwood forest, in contrast, increased sinks for inorganic N under N amendments resulted in exports of inorganic N that remained lower than DON exports in N-amended plots as well as the reference plot.     

The kinetics of NH4 + and NO3 − uptake by Douglas fir from single N-solutions and from solutions containing both NH4 + and NO3 −

The kinetics of NH₄ ⁺ and NO₃ ⁻ uptake in young Douglas fir trees (Pseudotsuga menziesii [Mirb.] Franco) were studied in solutions, containing either one or both N species. Using solutions containing a single N species, the V_max of NH₄ ⁺ uptake was higher than that of NO₃ ⁻ uptake. The K_m of NH₄ ⁺ uptake and K_m of NO₃ ⁻ uptake differed not significantly. When both NH₄ ⁺ and NO₃ ⁻ were present, the V_max for NH₄ ⁺ uptake became slightly higher, and the K_m for NH₄ ⁺ uptake remained in the same order. Under these conditions the NO₃ ⁻ uptake was almost totally inhibited over the whole range of concentrations used (10–1000 μM total N). This inhibition by NH₄ ⁺ occurred during the first two hours after addition. ei]{gnA C}{fnBorstlap}     

Site-dependent N uptake from N-form mixtures by arctic plants,soil microbes and ectomycorrhizal fungi

Soil microbes constitute an important control on nitrogen (N) turnover and retention in arctic ecosystems where N availability is the main constraint on primary production. Ectomycorrhizal (ECM) symbioses may facilitate plant competition for the specific N pools available in various arctic ecosystems. We report here our study on the N uptake patterns of coexisting plants and microbes at two tundra sites with contrasting dominance of the circumpolar ECM shrub Betula nana. We added equimolar mixtures of glycine-N, NH₄⁺–N and NO₃⁻–N, with one N form labelled with ¹⁵N at a time, and in the case of glycine, also labelled with ¹³C, either directly to the soil or to ECM fungal ingrowth bags. After 2 days, the vegetation contained 5.6, 7.7 and 9.1% (heath tundra) and 7.1, 14.3 and 12.5% (shrub tundra) of the glycine-, NH₄⁺- and NO₃⁻–¹⁵N, respectively, recovered in the plant–soil system, and the major part of ¹⁵N in the soil was immobilized by microbes (chloroform fumigation-extraction). In the subsequent 24 days, microbial N turnover transferred about half of the immobilized ¹⁵N to the non-extractable soil organic N pool, demonstrating that soil microbes played a major role in N turnover and retention in both tundra types. The ECM mycelial communities at the two tundras differed in N-form preferences, with a higher contribution of glycine to total N uptake at the heath tundra; however, the ECM mycelial communities at both sites strongly discriminated against NO₃⁻. Betula nana did not directly reflect ECM mycelial N uptake, and we conclude that N uptake by ECM plants is modulated by the N uptake patterns of both fungal and plant components of the symbiosis and by competitive interactions in the soil. Our field study furthermore showed that intact free amino acids are potentially important N sources for arctic ECM fungi and plants as well as for soil microorganisms. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Microbial transformations of C and N in a boreal forest floor as affected by temperature

The effects of temperature on N mineralization were studied in two organic surface horizons (LF and H) of soil from a boreal forest. The soil was incubated at 5 °C and 15 °C after adding ¹⁵ N and gross N fluxes were calculated using a numerical simulation model. The model was calibrated on microbial C and N, basal respiration, and KCl-extractable NH₄ ⁺, NO₃ ⁻, ¹⁵NH₄ ⁺ and ¹⁵ NO₃ ⁻. In the LF layer, increased temperature resulted in a faster turnover of all N pools. In both layers net N mineralization did not increase at elevated temperature because both gross NH₄ ⁺ mineralization and NH₄ ⁺ immobilization increased. In the H layer, however, both gross NH₄ ⁺ mineralization and NH₄ ⁺ immobilization were lower at 15 °C than at 5 °C and the model predicted a decrease in microbial turnover rate at higher temperature although measured microbial activity was higher. The decrease in gross N fluxes in spite of increased microbial activity in the H layer at elevated temperature may have been caused by uptake of organic N. The model predicted a decrease in pool size of labile organic matter and microbial biomass at elevated temperature whereas the amount of refractory organic matter increased. Temperature averaged microbial C/N ratio was 14.7 in the LF layer suggesting a fungi-dominated decomposer community whereas it was 7.3 in the H layer, probably due to predominance of bacteria. Respiration and microbial C were difficult to fit using the model if the microbial C/N ratio was kept constant with time. A separate ¹⁵N-enrichment study with the addition of glucose showed that glucose was metabolized faster in the LF than in the H layer. In both layers, decomposition of organic matter appeared to be limited by C availability. This revised version was published online in June 2006 with corrections to the Cover Date.     

Organic matter quality and management effects on enrichment of soil organic matter fractions in contrasting soils in Zimbabwe

Spatial variability of soil total nitrogen (N), available N (KCl extractable NH₄⁺ and NO₃⁻), and spatial patterns of N mineralization and nitrification at a stand scale were characterized with geostatistical and univariate analysis. Two extensive soil spatial samplings were conducted in an evergreen broadleaf forest in Sichuan province, southwestern China in June and August 2000. In a study area of 90 × 105 m², three soil samples were collected from each 5 × 5 m² plot (n = 378) in June and August, and were analyzed for total N and available N contents. Net N mineralization and nitrification were measured by in situ core incubation and the rates were estimated based on the difference of NH₄⁺ and NO₃⁻ contents between the two sampling dates. Total N, NH₄⁺, and NO₃⁻ were all spatially structured with different semivariogram ranges (from high to low: NH₄⁺, NO₃⁻, and total N). The semivariograms of mineralization and nitrification were not as spatially structured as available N. NH₄⁺ was the dominant soil inorganic N form in the system. Both NH₄⁺ and NO₃⁻ affected spatial patterns of soil available N, but their relative importance switched in August, probably due to high nitrification as indicated by greatly increased soil NO₃⁻ content. High spatial auto-correlations (>0.7) were found between available N and NH4⁺, available N and NO₃⁻ on both sampling dates, as well as total N measurements between both sampling dates. Although significant, the spatial auto-correlation between NH₄⁺ and NO₃⁻ were generally low. Topography had significant but low correlations with mineralization (r = −0.16) and nitrification (r = −0.14), while soil moisture did not. The large nugget values of the calculated semivariograms and high-semivariance values, particularly for mineralization and nitrification, indicate that some fine scale (<5 m) variability may lie below the threshold for detection in this study.     

Effects of Succession on Nitrogen Export in the West-Central Cascades, Oregon

This study examined impacts of succession on N export from 20 headwater stream systems in the west central Cascades of Oregon, a region of low anthropogenic N inputs. The seasonal and successional patterns of nitrate (NO₃−N) concentrations drove differences in total dissolved N concentrations because ammonium (NH₄−N) concentrations were very low (usually < 0.005 mg L⁻¹) and mean dissolved organic nitrogen (DON) concentrations were less variable than nitrate concentrations. In contrast to studies suggesting that DON levels strongly dominate in pristine watersheds, DON accounted for 24, 52, and 51% of the overall mean TDN concentration of our young (defined as predominantly in stand initiation and stem exclusion phases), middle-aged (defined as mixes of mostly understory reinitiation and older phases) and old-growth watersheds, respectively. Although other studies of cutting in unpolluted forests have suggested a harvest effect lasting 5 years or less, our young successional watersheds that were all older than 10 years still lost significantly more N, primarily as NO₃−N, than did watersheds containing more mature forests, even though all forest floor and mineral soil C:N ratios were well above levels reported in the literature for leaching of dissolved inorganic nitrogen. The influence of alder may contribute to these patterns, although hardwood cover was quite low in all watersheds; it is possible that in forested ecosystems with very low anthropogenic N inputs, even very low alder cover in riparian zones can cause elevated N exports. Only the youngest watersheds, with the highest nitrate losses, exhibited seasonal patterns of increased summer uptake by vegetation as well as flushing at the onset of fall freshets. Older watersheds with lower N losses did not exhibit seasonal patterns for any N species. The results, taken together, suggest a role for both vegetation and hydrology in N retention and loss, and add to our understanding of N cycling by successional forest ecosystems influenced by disturbance at various spatial and temporal scales in a region of relatively low anthropogenic N input.     

Nitrogen cycling in canopy soils of tropical montane forests responds rapidly to indirect N and P fertilization

Although the canopy can play an important role in forest nutrient cycles, canopy‐based processes are often overlooked in studies on nutrient deposition. In areas of nitrogen (N) and phosphorus (P) deposition, canopy soils may retain a significant proportion of atmospheric inputs, and also receive indirect enrichment through root uptake followed by throughfall or recycling of plant litter in the canopy. We measured net and gross rates of N cycling in canopy soils of tropical montane forests along an elevation gradient and assessed indirect effects of elevated nutrient inputs to the forest floor. Net N cycling rates were measured using the buried bag method. Gross N cycling rates were measured using ¹⁵N pool dilution techniques. Measurements took place in the field, in the wet and dry season, using intact cores of canopy soil from three elevations (1000, 2000 and 3000 m). The forest floor had been fertilized biannually with moderate amounts of N and P for 4 years; treatments included control, N, P, and N + P. In control plots, gross rates of NH₄⁺ transformations decreased with increasing elevation; gross rates of NO₃^? transformations did not exhibit a clear elevation trend, but were significantly affected by season. Nutrient‐addition effects were different at each elevation, but combined N + P generally increased N cycling rates at all elevations. Results showed that canopy soils could be a significant N source for epiphytes as well as contributing up to 23% of total (canopy + forest floor) mineral N production in our forests. In contrast to theories that canopy soils are decoupled from nutrient cycling in forest floor soil, N cycling in our canopy soils was sensitive to slight changes in forest floor nutrient availability. Long‐term atmospheric N and P deposition may lead to increased N cycling, but also increased mineral N losses from the canopy soil system.     

NO<Subscript>3</Subscript><Superscript>−</Superscript>-Driven SO<Subscript>4</Subscript><Superscript>2−</Superscript> Production in Freshwater Ecosystems: Implications for N and S Cycling

Massive anthropogenic acceleration of the global nitrogen (N) cycle has stimulated interest in understanding the fate of excess N loading to aquatic ecosystems. Nitrate (NO₃ ⁻) is traditionally thought to be removed mainly by microbial respiratory denitrification coupled to carbon (C) oxidation, or through biomass assimilation. Alternatively, chemolithoautotrophic bacterial metabolism may remove NO₃ ⁻ by coupling its reduction with the oxidation of sulfide to sulfate (SO₄ ²⁻). The NO₃ ⁻ may be reduced to N₂ or to NH₄ ⁺, a form of dissimilatory nitrate reduction to ammonium (DNRA). The objectives of this study were to investigate the importance of S oxidation as a NO₃ ⁻ removal process across diverse freshwater streams, lakes, and wetlands in southwestern Michigan (USA). Simultaneous NO₃ ⁻ removal and SO₄ ²⁻ production were observed in situ using modified “push-pull” methods in nine streams, nine wetlands, and three lakes. The measured SO₄ ²⁻ production can account for a significant fraction (25–40%) of the overall NO₃ ⁻ removal. Addition of ¹⁵NO₃ ⁻ and measurement of ¹⁵NH₄ ⁺ production using the push–pull method revealed that DNRA was a potentially important process of NO₃ ⁻ removal, particularly in wetland sediments. Enrichment cultures suggest that Thiomicrospira denitrificans may be one of the organisms responsible for this metabolism. These results indicate that NO₃ ⁻-driven SO₄ ²⁻ production could be widespread and biogeochemically important in freshwater sediments. Removal of NO₃ ⁻ by DNRA may not ameliorate problems such as eutrophication because the N remains bio-available. Additionally, if sulfur (S) pollution enhances NO₃ ⁻ removal in freshwaters, then controls on N processing in landscapes subject to S and N pollution are more complex than previously appreciated. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Ammonium Transformation in a Nitrogen-Rich Tidal Freshwater Marsh

The fate and transport of watershed-derived ammonium in a tidal freshwater marsh fringing the nutrient rich Scheldt River, Belgium, was quantified in a whole ecosystem ¹⁵N labeling experiment. In late summer (September) we added ¹⁵N-NH₄⁺ to the flood water entering a 3477 m² tidal freshwater marsh area, and traced the ammonium processing and retention in four subsequent tide cycles. In this paper we present the results for the water-phase components of the marsh system and compare them to a similar experiment conducted in spring/early summer (May). Changes in concentration and isotopic enrichment of NO₃⁻ + NO₂⁻, N₂O, N₂, NH₄⁺ and suspended particulate nitrogen (SPN) were measured in concert with a mass balance study. All analyzed N-pools were labeled, and 49% of the added ¹⁵NH₄⁺ was retained or transformed. The most important pool for ¹⁵N was nitrate, accounting for 17% of ¹⁵N-transformation. N₂, N₂O and SPN accounted for 2.4, 0.02 and 1.4%, respectively. The temporal and spatial patterns of ¹⁵N transformation in the water phase component of the system were remarkably similar to those observed in May, indicating good reproducibility of the whole ecosystem labeling approach, but the absolute ammonium transformation rate was 3 times higher in May. While the marsh surface area was crucial for nitrification in May this was less pronounced in September. Denitrification, on the other hand, appeared more important in September compared to May.     

Effects of forest management on soil N cycling in beech forests stocking on calcareous soils

The effects of forest management (thinning) on gross and net N conversion, the balance of inorganic N production and consumption, inorganic N concentrations and on soil microbial biomass in the Ah layer were studied in situ during eight intensive field measuring campaigns in the years 2002–2004 at three beech (Fagus sylvatica L.) forest sites. At all sites adjacent thinning plots (“T”) and untreated control plots (“C”) were established. Since the sites are characterized either by cool-moist microclimate (NE site and NW site) or by warm-dry microclimate (SW site) and thinning took place in the year 1999 at the NE and SW sites and in the year 2003 at the NW site the experimental design allowed to evaluate (1) short-term effects (years 1–2) of thinning at the NW site and (2) medium-term effects (years 4–6) of thinning under different microclimate at the SW and NE site. Microbial biomass N was consistently higher at the thinning plots of all sites during most of the field campaigns and was overall significantly higher at the SWT and NWT plots as compared to the corresponding untreated control plots. The size of the microbial biomass N pool was found to correlate positively with both gross ammonification and gross nitrification as well as with extractable soil NO₃⁻ concentrations. At the SW site neither gross ammonification, gross nitrification, gross ammonium (NH₄⁺) immobilization and gross nitrate (NO₃⁻) immobilization nor net ammonification, net nitrification and extractable NH₄⁺ and NO₃⁻ contents were significantly different between control and thinning plot. At the NET plot lower gross ammonification and gross NH₄⁺ immobilization in conjunction with constant nitrification rates coincided with higher net nitrification and significantly higher extractable NO₃⁻ concentrations. Thus, the medium-term effects of thinning varied with different microclimate. The most striking thinning effects were found at the newly thinned NW site, where gross ammonification and gross NH₄⁺ immobilization were dramatically higher immediately after thinning. However, they subsequently tended to decrease in favor of gross nitrification, which was significantly higher at the NWT plot as compared to␣the␣NWC plot during all field campaigns after␣thinning except for April 2004. This increase␣in␣gross nitrification at the NWT plot (1.73 mg N kg⁻¹ sdw day⁻¹ versus 0.48 mg N kg⁻¹ sdw day⁻¹ at the NWC plot) coincided with significantly higher extractable NO₃⁻ concentrations (4.59 mg N kg⁻¹ sdw at the NWT plot versus 0.96 mg N kg⁻¹ sdw at the NWC plot). Pronounced differences in relative N retention (the ratio of gross NH₄⁺ immobilization + gross NO₃⁻ immobilization to gross ammonification + gross nitrification) were found across the six research plots investigated and could be positively correlated to the soil C/N ratio (R = 0.94; p = 0.005). In sum, the results obtained in this study show that (1) thinning can lead to a shift in the balance of microbial inorganic N production and consumption causing a clear decrease in the N retention capacity in the monitored forest soils especially in the first two years after thinning, (2)␣the resistance of the investigated forest ecosystems to disturbances of N cycling by thinning may vary with different soil C contents and C/N ratios, e. g. caused by differences in microclimate, (3) thinning effects tend to decline with the growth of understorey vegetation in the years 4–6 after thinning.     

Natural 15N abundance in two nitrogen saturated forest ecosystems

Natural ¹⁵N abundance values were measured in needles, twigs, wood, soil, bulk precipitation, throughfall and soil water in a Douglas fir (Pseudotsuga menziesii (Mirb.) and a Scots pine (Pinus sylvestris L.) stand receiving high loads of nitrogen in throughfall (>50 kg N ha⁻¹ year⁻¹). In the Douglas fir stand δ¹⁵N values of the vegetation ranged between −5.7 and −4.2‰ with little variation between different compartments. The vegetation of the Scots pine stand was less depleted in ¹⁵N and varied from −3.3 to −1.2‰δ¹⁵N. At both sites δ¹⁵N values increased with soil depth, from −5.7‰ and −1.2‰ in the organic layer to +4.1‰ and +4.7‰ at 70 cm soil depth in the Douglas fir and Scots pine stand, respectively. The δ¹⁵N values of inorganic nitrogen in bulk precipitation showed a seasonal variation with a mean in NH₄ ⁺-N of −0.6‰ at the Douglas fir stand and +10.8‰ at the Scots pine stand. In soil water below the organic layer NH₄ ⁺-N was enriched and NO₃ ⁻-N depleted in ¹⁵N, which was interpreted as being caused by isotope fractionation accompanying high nitrification rates in the organic layers. Mean δ¹⁵N values of NH₄ ⁺ and NO₃ ⁻ were very similar in the drainage water at 90 cm soil depth at both sites (−7.1 to −3.8‰). A dynamic N cycling model was used to test the sensitivity of the natural abundance values for the amount of N deposition, the ¹⁵N ratio of atmospheric N deposited and for the intrinsic isotope discrimination factors associated with N transformation processes. Simulated δ¹⁵N values for the N saturated ecosystems appeared particularly sensitive to the ¹⁵N ratio of atmospheric N inputs and discrimination factors during nitrification and mineralization. The N-saturated coniferous forest ecosystems studied were not characterized by elevated natural ¹⁵N abundance values. The results indicated that the natural ¹⁵N abundance values can only be used as indicators for the stage of nitrogen saturation of an ecosystem if the δ¹⁵N values of the deposited N and isotope fractionation factors are taken into consideration. Combining dynamic isotope models and natural ¹⁵N abundance values seems a promising technique for interpreting natural ¹⁵N abundance values found in these forest ecosystems. Received: 5 May 1996 / Accepted: 10 April 1997     

Dissolved organic nitrogen budgets for upland, forested ecosystems in New England

Relatively high deposition ofnitrogen (N) in the northeastern United States hascaused concern because sites could become N saturated.In the past, mass-balance studies have been used tomonitor the N status of sites and to investigate theimpact of increased N deposition. Typically, theseefforts have focused on dissolved inorganic forms ofN (DIN = NH₄-N + NO₃-N) and have largelyignored dissolved organic nitrogen (DON) due todifficulties in its analysis. Recent advances in themeasurement of total dissolved nitrogen (TDN) havefacilitated measurement of DON as the residual of TDN– DIN. We calculated DON and DIN budgets using data onprecipitation and streamwater chemistry collected from9 forested watersheds at 4 sites in New England. TDNin precipitation was composed primarily of DIN. Netretention of TDN ranged from 62 to 89% (4.7 to 10 kgha^{minus 1} yr^{minus 1}) of annual inputs. DON made up themajority of TDN in stream exports, suggesting thatinclusion of DON is critical to assessing N dynamicseven in areas with large anthropogenic inputs of DIN.Despite the dominance of DON in streamwater,precipitation inputs of DON were approximately equalto outputs. DON concentrations in streamwater did notappear significantly influenced by seasonal biologicalcontrols, but did increase with discharge on somewatersheds. Streamwater NO₃-N was the onlyfraction of N that exhibited a seasonal pattern, withconcentrations increasing during the winter months andpeaking during snowmelt runoff. Concentrations ofNO₃-N varied considerably among watersheds andare related to DOC:DON ratios in streamwater. AnnualDIN exports were negatively correlated withstreamwater DOC:DON ratios, indicating that theseratios might be a useful index of N status of uplandforests.