The Role of Dissolved Organic Carbon, Dissolved Organic Nitrogen, and Dissolved Inorganic Nitrogen in a Tropical Wet Forest Ecosystem

Although tropical wet forests play an important role in the global carbon (C) and nitrogen (N) cycles, little is known about the origin, composition, and fate of dissolved organic C (DOC) and N (DON) in these ecosystems. We quantified and characterized fluxes of DOC, DON, and dissolved inorganic N (DIN) in throughfall, litter leachate, and soil solution of an old-growth tropical wet forest to assess their contribution to C stabilization (DOC) and to N export (DON and DIN) from this ecosystem. We found that the forest canopy was a major source of DOC (232 kg C ha^–1 y^–1). Dissolved organic C fluxes decreased with soil depth from 277 kg C ha^–1 y^–1 below the litter layer to around 50 kg C kg C ha^–1 y^–1 between 0.75 and 3.5m depth. Laboratory experiments to quantify biodegradable DOC and DON and to estimate the DOC sorption capacity of the soil, combined with chemical analyses of DOC, revealed that sorption was the dominant process controlling the observed DOC profiles in the soil. This sorption of DOC by the soil matrix has probably led to large soil organic C stores, especially below the rooting zone. Dissolved N fluxes in all strata were dominated by mineral N (mainly NO₃⁻). The dominance of NO₃^– relative to the total amount nitrate of N leaching from the soil shows that NO₃^– is dominant not only in forest ecosystems receiving large anthropogenic nitrogen inputs but also in this old-growth forest ecosystem, which is not N-limited.     

Nitrogen biogeochemistry of three hardwood ecosystems in the Adirondack Region of New York

The biogeochemistry of nitrogen (N)was evaluated for three forest ecosystems[Woods Lake (WL), Pancake-Hall Creek (PHC) andHuntington Forest (HF)] in the Adirondackregion of New York, U.S.A. to evaluate theresponse of a range of N atmospheric inputsand experimental N additions. Bulk Ndeposition was higher at sites in the westthan those in the central and easternAdirondacks. These higher atmospheric N inputswere reflected in higher bulk throughfallfluxes of N (WL and PHC, 10.1 and 12.0 kg Nha^–1 yr^–1, respectively) in thewestern Adirondacks than at HF (4.6 kg Nha^–1 yr^–1) in the centralAdirondacks. Nitrogen was added to plots as(NH₄)₂SO₄ at 14 and 28 kg Nha^–1 yr^–1 or as HNO₃ at 14 kg Nha^–1 yr^–1. Litter decompositionrates of Fagus grandifolia and Acerrubrum were substantially higher at WL andPHC compared to HF but were not affected byexperimental N additions. Results usingmineral soil bags showed no effects of Naddition on N and C concentrations in soilorganic matter, but C and N concentrationincreases were less at WL and PHC compared toHF. Soil solution nitrate (NO₃ ^–)concentrations at 15-cm depth in the referenceplots were higher at PHC than at WL and HFwhile at 50-cm concentrations were higher atPHC and WL than at HF. The reference plots atthe two sites (WL and PHC) with the highestatmospheric inputs of N exhibited lower Nretention (53 and 33%, respectively) than HF(68%) in reference plots. The greatestincrease in NO₃ ^– loss in response tothe experimental treatments occurred at HFwhere the HNO₃ additions resulted in thehighest NO₃ ^– concentrations andlowest N retentions. In contrast, at WL andPHC increases in soil water NO₃ ^–were not evident in response to experimental Nadditions. The results suggest that the twosites (WL and PHC) in the western Adirondacksdid not respond to additional N inputsalthough they have experienced elevatedatmospheric N inputs and higher N drainagelosses in reference plots than the HF site inthe central Adirondacks. Some of thesedifferences in site response may have alsobeen a function of stand age of WL and PHCthat were younger (24 and 33 years,respectively) than the HF (age 70).Highest NO₃ ^– fluxes in thereference plots across the sites correspondedto higher ¹⁵N values in soil andplants. An experimental addition experimentat PHC found that the forest floor and themineral soil were the largest sinks forexperimentally added N.     

Nitrogen input–output budgets for lake-containing watersheds in the Adirondack region of New York

The Adirondack region of New York is characterized by soils and surface waters that are sensitive to inputs of strong acids, receiving among the highest rates of atmospheric nitrogen (N) deposition in the United States. Atmospheric N deposition to Adirondack ecosystems may contribute to the acidification of soils through losses of exchangeable basic cations and the acidification of surface waters in part due to increased mobility of nitrate (NO₃^–). This response is particularly evident in watersheds that exhibit nitrogen saturation. To evaluate the contribution of atmospheric N deposition to the N export and the capacity of lake-containing watersheds to remove, store, or release N, annual N input–output budgets were estimated for 52 lake-containing watersheds in the Adirondack region from 1998 to 2000. Wet N deposition was used as the N input and the lake N discharge loss was used as the N output based on modeled hydrology and measured monthly solute concentrations. Annual outputs were also estimated for dissolved organic carbon (DOC). Wet N deposition increased from the northeast to the southwest across the region. Lake N drainage losses, which exhibited a wider range of values than wet N deposition, did not show any distinctive spatial pattern, although there was some evidence of a relationship between wet N deposition and the lake N drainage loss. Wet N deposition was also related to the fraction of N removed or retained within the watersheds (i.e., the fraction of net N hydrologic flux relative to wet N deposition, calculated as [(wet N deposition minus lake N drainage loss)/wet N deposition]). In addition to wet N deposition, watershed attributes also had effects on the exports of NO₃^–, ammonium (NH₄⁺), dissolved organic nitrogen (DON), and DOC, the DOC/DON export ratio, and the N flux removed or retained within the watersheds (i.e., net N hydrologic flux, calculated as [wet N deposition less lake N drainage loss]). Elevation was strongly related with the lake drainage losses of NO₃^–, NH₄⁺, and DON, net NO₃^– hydrologic flux (i.e., NO₃^– deposition less NO₃^– drainage loss), and the fraction of net NO₃^– hydrologic flux, but not with the DOC drainage loss. Both DON and DOC drainage losses from the lakes increased with the proportion of watershed area occupied by wetlands, with a stronger relationship for DOC. The effects of wetlands and forest type on NO₃^– flux were evident for the estimated NO₃^– fluxes flowing from the watershed drainage area into the lakes, but were masked in the drainage losses flowing out of the lakes. The DOC/DON export ratios from the lake-containing watersheds were in general lower than those from forest floor leachates or streams in New England and were intermediate between the values of autochthonous and allochthonous dissolved organic matter (DOM) reported for various lakes. The DOC/DON ratios for seepage lakes were lower than those for drainage lakes. In-lake processes regulating N exports may include denitrification, planktonic depletion, degradation of DOM, and the contribution of autochthonous DOM and the influences of in-lake processes were also reflected in the relationships with hydraulic retention time. The N fluxes removed or stored within the lakes substantially varied among the lakes. Our analysis demonstrates that for these northern temperate lake-containing watershed ecosystems, many factors, including atmospheric N deposition, landscape features, hydrologic flowpaths, and retention in ponded waters, regulated the spatial patterns of net N hydrologic flux within the lake-containing watersheds and the loss of N solutes through drainage waters.     

Minimal response in watershed nitrate export to severe soil frost raises questions about nutrient dynamics in the Hubbard Brook experimental forest

Experimental and theoretical work emphasize the role of plant nutrient uptake in regulating ecosystem nutrient losses and predict that forest succession, ecosystem disturbance, and continued inputs of atmospheric nitrogen (N) will increase watershed N export. In ecosystems where snowpack insulates soils, soil-frost disturbances resulting from low or absent snowpack are thought to increase watershed N export and may become more common under climate-change scenarios. This study monitored watershed N export from the Hubbard Brook Experimental Forest (HBEF) in response to a widespread, severe soil-frost event in the winter of 2006. We predicted that nitrate (NO₃ ⁻) export following the disturbance would be high compared to low background streamwater NO₃ ⁻ export in recent years. However, post-disturbance annual NO₃ ⁻ export was the lowest on record from both reference (undisturbed) and treated experimental harvest or CaSiO₃ addition watersheds. These results are consistent with other studies finding greater than expected forest NO₃ ⁻ retention throughout the northeastern US and suggest that changes over the last five decades have reduced impacts of frost events on watershed NO₃ ⁻ export. While it is difficult to parse out causes from a complicated array of potential factors, based on long-term records and watershed-scale experiments conducted at the HBEF, we propose that reduced N losses in response to frost are due to a combination of factors including the long-term legacies of land use, process-level alterations in N pathways, climate-driven hydrologic changes, and depletion of base cations and/or reduced soil pH due to cumulative effects of acid deposition.     

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.     

Leaching of nitrate and ammonium in heathland and forest ecosystems in Northwest Germany under the influence of enhanced nitrogen deposition

To study the impact of high atmospheric nitrogen deposition on the leaching of NO₃^– and NH₄⁺ beneath forest and heathland vegetation, investigations were carried out in adjacent forest and heathland ecosystems in Northwest Germany. The study area is subjected to high deposition of nitrogen ranging from 15.9 kg ha^–1 yr^–1 in bulk precipitation to 65.3 kg ha^–1 yr^–1 beneath a stand of Pinus sylvestris L. with NH₄–N accounting for 70–80% of the nitrogen deposited. Considerable leaching of nitrogen compounds from the upper horizons of the soil, mostly as nitrate, occurred at most of the forest sites and below a mixed stand of Calluna vulgaris (L.) Hull. and Erica tetralix, but was low in a Betula pubescens Ehrh. swamp forest as well as beneath Erica tetralix L. wet heath and heath dominated by Molinia caerulea(L.) Moench. Ground water concentrations of both NO₃–N and NH₄–N did not exceed 1 mg L^–1 at most of the sites investigated.     

Inorganic Nitrogen Losses from a Forested Ecosystem in Responseto Physical,Chemical, Biotic,and Climatic Perturbations

Nitrate leaching to streams is a sensitive indicator of the biogeochemical status of forest ecosystems. Two primary theories predicting long-term (decadal) changes in nitrate loss rates (N saturation theory and the nutrient retention hypothesis) both predict increasing dissolved inorganic nitrogen (DIN) losses for watershed 6 (W6), the biogeochemical reference watershed at the Hubbard Brook Experimental Forest (HBEF). Measured values, however, have declined substantially since measurements began in the mid-1960s. Are these theories wrong, or are there other important controls on DIN losses at the annual to decadal time scale that have obscured the tendency toward higher losses over time? We tested the individual and combined effects of several forms of disturbance on DIN loss rates from northern hardwood forests by comparing predictions from a relatively simple model of forest carbon, nitrogen, and water dynamics (PnET-CN) with the long-term record of annual DIN loss from W6 at HBEF. Perturbations tested include interannual climate variation, changes in atmospheric chemistry (CO₂, O₃, N deposition), and physical and biotic disturbances (two harvests, a hurricane salvage, and a defoliation event). No single disturbance caused changes in DIN losses to mimic measured values. Only when run with all of the disturbances combined did the model-predicted pattern of interannual change in DIN loss approach the measured record. Single-disturbance simulations allow an estimation of the role of each in the total pattern of DIN loss. We conclude that DIN losses from W6 were elevated in the 1960s by a combination of recovery from extreme drought and a significant defoliation event. N deposition alone, in the absence of other disturbances, would have increased DIN losses by 0.35 g N m^?2y^?1. These findings indicate that predictions of DIN losses must take into account the full spectrum of disturbance events and changes in environmental conditions impacting the systems examined.     

Nitrogen Fluxes and Retention in Urban Watershed Ecosystems

Although the watershed approach has long been used to study whole-ecosystem function, it has seldom been applied to study human-dominated systems, especially those dominated by urban and suburban land uses. Here we present 3 years of data on nitrogen (N) losses from one completely forested, one agricultural, and six urban/suburban watersheds, and input–output N budgets for suburban, forested, and agricultural watersheds. The work is a product of the Baltimore Ecosystem Study, a long-term study of urban and suburban ecosystems, and a component of the US National Science Foundations long-term ecological research (LTER) network. As expected, urban and suburban watersheds had much higher N losses than did the completely forested watershed, with N yields ranging from 2.9 to 7.9 kg N ha^–1 y^–1 in the urban and suburban watersheds compared with less than 1 kg N ha^–1 y^–1 in the completely forested watershed. Yields from urban and suburban watersheds were lower than those from an agricultural watershed (13–19.8 kg N ha^–1 y^–1). Retention of N in the suburban watershed was surprisingly high, 75% of inputs, which were dominated by home lawn fertilizer (14.4 kg N ha^–1 y^–1) and atmospheric deposition (11.2 kg N ha^–1 y^–1). Detailed analysis of mechanisms of N retention, which must occur in the significant amounts of pervious surface present in urban and suburban watersheds, and which include storage in soils and vegetation and gaseous loss, is clearly warranted.     

Effects of soil freezing disturbance on soil solution nitrogen, phosphorus, and carbon chemistry in a northern hardwood ecosystem

Reductions in snow cover undera warmer climate may cause soil freezing eventsto become more common in northern temperateecosystems. In this experiment, snow cover wasmanipulated to simulate the late development ofsnowpack and to induce soil freezing. Thismanipulation was used to examine the effects ofsoil freezing disturbance on soil solutionnitrogen (N), phosphorus (P), and carbon (C)chemistry in four experimental stands (twosugar maple and two yellow birch) at theHubbard Brook Experimental Forest (HBEF) in theWhite Mountains of New Hampshire. Soilfreezing enhanced soil solution Nconcentrations and transport from the forestfloor. Nitrate (NO₃ ^–) was thedominant N species mobilized in the forestfloor of sugar maple stands after soilfreezing, while ammonium (NH₄ ⁺) anddissolved organic nitrogen (DON) were thedominant forms of N leaching from the forestfloor of treated yellow birch stands. Rates ofN leaching at stands subjected to soil freezingranged from 490 to 4,600 mol ha^–1yr^–1, significant in comparison to wet Ndeposition (530 mol ha^–1 yr^–1) andstream NO₃ ^– export (25 mol ha^–1yr^–1) in this northern forest ecosystem. Soil solution fluxes of P_i from the forestfloor of sugar maple stands after soil freezingranged from 15 to 32 mol ha^–1 yr^–1;this elevated mobilization of P_i coincidedwith heightened NO₃ ^– leaching. Elevated leaching of P_i from the forestfloor was coupled with enhanced retention ofP_i in the mineral soil Bs horizon. Thequantities of P_i mobilized from the forestfloor were significant relative to theavailable P pool (22 mol ha^–1) as well asnet P mineralization rates in the forest floor(180 mol ha^–1 yr^–1). Increased fineroot mortality was likely an important sourceof mobile N and P_i from the forest floor,but other factors (decreased N and P uptake byroots and increased physical disruption of soilaggregates) may also have contributed to theenhanced leaching of nutrients. Microbialmortality did not contribute to the acceleratedN and P leaching after soil freezing. Resultssuggest that soil freezing events may increaserates of N and P loss, with potential effectson soil N and P availability, ecosystemproductivity, as well as surface wateracidification and eutrophication.     

Nitrogen relations of a low nitrate uptake inbred line of white clover (Trifolium repens L.)

The nitrogen relations of an inbred line of white clover (Trifolium repens L.) thought to exhibit an abnormally low capacity for NO₃ ^– uptake (line LNU) were compared with a line regarded as normal with respect to NO₃ ^– uptake (line NNU). Growth, nodulation, N₂ fixation and NO₃ ^– uptake were measured over 7 weeks in flowing solution culture (Experiment 1) by plants dependent for N acquisition on either (i) NO₃ ^– uptake, (ii) NO₃ ^– uptake +N₂ fixation, or (iii) N₂ fixation only. Effects of plant N status on the short-term uptake and translocation of ¹⁵NH₄ ⁺ and ¹⁵NO₃ ^– were also investigated (Experiment 2). Nitrate uptake per plant by –fix/+NO₃ ^– line LNU was 50% of uptake by line NNU over 35 days, and there were significant differences in specific uptake rates of NO₃ ^– between the lines over the first 24 days. The `low NO<sub>3</sub> <sup>–</sup> uptake' phenotype was indistinct under +fix/+NO<sub>3</sub> <sup>–</sup> treatment. Nitrate lowered specific rates of nitrogen fixation by line NNU but had no effect on line LNU. Only low N status line LNU plants had lower short-term rates of NH<sub>4</sub> <sup>+</sup> and NO<sub>3</sub> <sup>–</sup> uptake than line NNU. It is concluded that the `low NO₃ ^– uptake' phenotype of line LNU is inconsistently expressed. Circumstantial evidence points to increased NO₃ ^– efflux and decreased xylem translocation of NO₃ ^– as possible explanations for the lower NO₃ ^– uptake by line LNU.     

Influence of Earthworm Invasion on Redistribution and Retention of Soil Carbon and Nitrogen in Northern Temperate Forests

We analyzed soil organic matter distribution and soil solution chemistry in plots with and without earthworms at two sugar maple (Acer saccharum)–dominated forests in New York State, USA, with differing land-use histories to assess the influence of earthworm invasion on the retention or loss of soil carbon (C) and nitrogen (N) in northern temperate forests. Our objectives were to assess the influence of exotic earthworm invasion on (a) the amount and depth distribution of soil C and N, (b) soil ¹³C and ¹⁵N, and (c) soil solution chemistry and leaching of C and N in forests with different land-use histories. At a relatively undisturbed forest site (Arnot Forest), earthworms eliminated the thick forest floor, decreased soil C storage in the upper 12 cm by 28%, and reduced soil C:N ratios from 19.2 to 15.3. At a previously cultivated forest site with little forest floor (Tompkins Farm), earthworms did not influence the storage of soil C or N or soil C:N ratios. Earthworms altered the stable isotopic signature of soil at Arnot Forest but not at Tompkins Farm; the alteration of stable isotopes indicated that earthworms significantly increased the loss of forest floor C but not N from the soil profile at Arnot Forest. Nitrate (NO₃^–) concentrations in tension and zero-tension lysimeters were much greater at Tompkins Farm than Arnot Forest, and earthworms increased NO₃^– leaching at Tompkins Farm. The results suggest that the effect of earthworm invasion on the distribution, retention, and solution chemistry of soil C and N in northern temperate forests may depend on the initial quantity and quality of soil organic matter at invaded sites.     

Nitrogen deposition and forest nitrogen cycling along an urban–rural transect in southern China

There is increasing concern over the impact of atmospheric nitrogen (N) deposition on forest ecosystems in the tropical and subtropical areas. In this study, we quantified atmospheric N deposition and revealed current plant and soil N status in 14 forests along a 150 km urban to rural transect in southern China, with an emphasis on examining whether foliar δ¹⁵N can be used as an indicator of N saturation. Bulk deposition ranged from 16.2 to 38.2 kg N ha^?1 yr^?1, while the throughfall covered a larger range of 11.7–65.1 kg N ha^?1 yr^?1. Foliar N concentration, NO₃^? leaching to stream, and soil NO₃^? concentration were low and NO₃^? production was negligible in some rural forests, indicating that primary production in these forests may be limited by N supply. But all these N variables were enhanced in suburban and urban forests. Across the study transect, throughfall N input was correlated positively with soil nitrification and NO₃^? leaching to stream, and negatively with pH values in soil and stream water. Foliar δ¹⁵N was between ?6.6‰ and 0.7‰, and was negatively correlated with soil NO₃^? concentration and NO₃^? leaching to stream across the entire transect, demonstrating that an increased N supply does not necessarily increase forest δ¹⁵N values. We proposed several potential mechanism that could contribute to the δ¹⁵N pattern, including (1) increased plant uptake of ¹⁵N‐depleted soil NO₃^?, (2) foliage uptake of ¹⁵N‐depleted NH₄⁺, (3) increased utilization of soil inorganic N relative to dissolved organic N, and (4) increased fractionation during plant N uptake under higher soil N availability.     

Nitrate reduction in sediments of lowland tropical streams draining swamp forest in Costa Rica: An ecosystem perspective

Nitrate reduction and denitrification were measured in swamp forest streams draining lowland rain forest on Costa Rica's Atlantic slope foothills using the C₂H₂-block assay and sediment-water nutrient fluxes. Denitrification assays using the C₂H₂-block technique indicated that the full suite of denitrifying enzymes were present in the sediment but that only a small fraction of the functional activity could be expressed without adding NO₃ ^–. Under optimal conditions, denitrification enzyme activity averaged 15 nmoles cm^–3 sediment h^–1. Areal NO₃ ^– reduction rates measured from NO₃ ^– loss in the overlying water of sediment-water flux chambers ranged from 65 to 470 umoles m^–2 h^–1. Oxygen loss rates accompanying NO₃ ^– depletion averaged 750 umoles m^–2 h^–1. Corrected for denitrification of NO₃ ^– oxidized from NH₄ ⁺ in the sediment, gross NO₃ ^– reduction rates increase by 130 umoles m^–2 h^–1, indicating nitrification may be the predominant source of NO₃ ^– for NO₃ ^– reduction in swamp forest stream sediments. Under field conditions approximately 80% of the increase in inorganic N mass along a 1250-m reach of the Salto River was in the form of NO₃ ^– with the balance NH₄ ⁺ . Scrutiny of potential inorganic N sources suggested that mineralized N released from the streambed was a major source of the inorganic N increase. Despite significant NO₃ ^– reduction potential, swamp forest stream sediments appear to be a source of inorganic N to downstream communities.     

Large Loss of Dissolved Organic Nitrogen from Nitrogen-Saturated Forests in Subtropical China

Dissolved organic nitrogen (DON) has recently been recognized as an important component of terrestrial N cycling, especially under N-limited conditions; however, the effect of increased atmospheric N deposition on DON production and loss from forest soils remains controversial. Here we report DON and dissolved organic carbon (DOC) losses from forest soils receiving very high long-term ambient atmospheric N deposition with or without additional experimental N inputs, to investigate DON biogeochemistry under N-saturated conditions. We studied an old-growth forest, a young pine forest, and a young mixed pine/broadleaf forest in subtropical southern China. All three forests have previously been shown to have high nitrate (NO₃⁻) leaching losses, with the highest loss found in the old-growth forest. We hypothesized that DON leaching loss would be forest specific and that the strongest response to experimental N input would be in the N-saturated old-growth forest. Our results showed that under ambient deposition (35–50 kg N ha⁻¹ y⁻¹ as throughfall input), DON leaching below the major rooting zone in all three forests was high (6.5–16.9 kg N ha⁻¹ y⁻¹). DON leaching increased 35–162% following 2.5 years of experimental input of 50–150 kg N ha⁻¹ y⁻¹. The fertilizer-driven increase of DON leaching comprised 4–17% of the added N. A concurrent increase in DOC loss was observed only in the pine forest, even though DOC:DON ratios declined in all three forests. Our data showed that DON accounted for 23–38% of total dissolved N in leaching, highlighting that DON could be a significant pathway of N loss from forests moving toward N saturation. The most pronounced N treatment effect on DON fluxes was not found in the old-growth forest that had the highest DON loss under ambient conditions. DON leaching was highly correlated with NO₃⁻ leaching in all three forests. We hypothesize that abiotic incorporation of excess NO₃⁻ (through chemically reactive NO₂⁻) into soil organic matter and the consequent production of N-enriched dissolved organic matter is a major mechanism for the consistent and large DON loss in the N-saturated subtropical forests of southern China. Dr. YT Fang performed research, analyzed data, and wrote the paper; Prof. WX Zhu participated in the initial experimental design, analyzed data, and took part in writing the paper; Prof. P Gundersen conceived the study and took part in writing; Prof. JM Mo and Prof. GY Zhou conceived study; Prof. M Yoh analyzed part of the data and contributed to the development of DON model.     

Nitrogen nutrition of rice plants under salinity

Two rice (Oryza sativa L.) cultivars, Koshihikari and Pokkali, were grown in solution culture at three concentrations of NaCl or Na₂SO₄ [0 (S0), 50 (S1), and 100 (S2) mmol dm^–3] and three N contents [0.7 (N1), 7 (N2) and 14 (N3) mmol dm^–3]. Salinity significantly decreased dry matter of both cultivars. Pokkali had better growth than Koshihikari under both saline and non-saline conditions. Applications of N enhanced development of shoot dry mass under S0 and S1 treatments up to N2. Under S2, N application had no effect on shoot dry mass of both cultivars. Root dry mass of both cultivars decreased with increasing N application at S1 and S2. Shoot and root NO₃-N content in both rice cultivars increased with increasing N concentration in the nutrient solutions. The absorption of NO₃-N was less in Koshihikari than Pokkali plants, and also was much less in Cl^– than SO₄ ^2– salinity suggesting the antagonism between Cl^– and NO₃ ^–. In addition a significant negative correlation between concentrations of NO₃-N and Cl^– in the shoots or roots was observed in both cultivars     

Dissolved organic carbon and nitrate concentrations in streams: a useful index indicating carbon and nitrogen availability in catchments

Dissolved organic carbon (DOC) and NO₃^– are important forms of C and N in stream water. Hypotheses concerning relationships between DOC and NO₃^– concentrations have been proposed, but there are no reports demonstrating a relationship between them in stream water. We observed 35 natural streams in the Lake Biwa watershed, central Japan, and found an inverse relationship between DOC and NO₃^– concentrations. This relationship was also found in observations of their seasonal variations in the Lake Biwa watershed. Moreover, this relationship was also found to apply to watersheds in other regions in Japan. These results suggest that forest biogeochemical processes which control DOC and NO₃^– concentrations in Japanese streams are closely related. Excess N availability together with a C (energy) deficit in a soil environment may explain this relationship. DOC and NO₃^– concentrations in streams will thus be a useful index indicating C and N availability in catchments.     

Nitrogen cycling in an acid forest ecosystem in the Netherlands under increased atmospheric nitrogen input

Within a long-term research project studying the biogeochemical budget of an oak-beech forest ecosystem in the eastern part of the Netherlands, the nitrogen transformations and solute fluxes were determined in order to trace the fate of atmospherically deposited NH₄ ⁺ and to determine the contribution of nitrogen transformations to soil acidification.The oak-beech forest studied received an annual input of nitrogen via throughfall and stemflow of 45 kg N ha^–1 yr^–1, mainly as NH₄ ⁺, whereas 8 kg N ha^–1 yr^–1 was taken up by the canopy. Due to the specific hydrological regime resulting in periodically occurring high groundwater levels, denitrification was found to be the dominant output flux (35 kg N ha^–1 yr^–1). N₂0 emmission rate measurements indicated that 57% of this gaseous nitrogen loss (20 kg N ha^–1 yr^–1) was as N₂O. The forest lost an annual amount of 11 kg N ha^–1 yr^–1 via streamwater output, mainly as N0₃ ^–.Despite the acid conditions, high nitrification rates were measured. Nitrification occurred mainly in the litter layer and in the organic rich part of the mineral soil and was found to be closely correlated with soil temperature. The large amount of NH₄ ⁺ deposited on the forest floor via atmospheric deposition and produced by mineralization was to a large extent nitrified in the litter layer. Almost no NH₄ ⁺ reached the subsurface soil horizons. The N0₃ ^– was retained, taken up or transformed mainly in the mineral soil. A small amount of N0₃ ^– (9 kg N ha^–1 yr^–1) was removed from the system in streamwater output. A relatively small amount of nitrogen was measured in the soil water as Dissolved Organic Nitrogen.On the basis of these data the proton budget of the system was calculated using two different approaches. In both cases net proton production rates were high in the vegetation and in the litter layer of the forest ecosystem. Nitrogen transformations induced a net proton production rate of 2.4 kmol ha^–1 yr^–1 in the soil compartment.     

Suppression of nitrate formation within an exotic conifer plantation

Summary Nitrate-N losses to stream waters and soil inorganic N pools, nitrifying potentials and NO₃-N production rates were measured in 2 adjacent watersheds, one used as pasture and the other planted in exotic conifer forest (Pinus radiata D. Don). Estimated NO₃-N loss to stream waters draining the pine and pasture watersheds were 0.6kg ha⁻¹ y⁻¹ and 7.6 kg ha⁻¹ y⁻¹ respectively. Ammonium-N pool sizes were not significantly different between soils in the two watersheds but NO₃−N pools and nitrifying potentials were always lower in the pine watershed soil samples. Laboratory incubation experiments indicated that suppression of NO₃−N formation in pine watershed soils required the presence of live tree roots and was not due to the direct action of allelopathic chemicals on nitrifiers.     

Nitrogen mineralization,nitrification and denitrification in upland and wetland ecosystems

Summary Nitrogen mineralization, nitrification, denitrification, and microbial biomass were evaluated in four representative ecosystems in east-central Minnesota. The study ecosystems included: old field, swamp forest, savanna, and upland pin oak forest. Due to a high regional water table and permeable soils, the upland and wetland ecosystems were separated by relatively short distances (2 to 5 m). Two randomly selected sites within each ecosystem were sampled for an entire growing season. Soil samples were collected at 5-week intervals to determine rates of N cycling processes and changes in microbial biomass. Mean daily N mineralization rates during five-week in situ soil incubations were significantly different among sampling dates and ecosystems. The highest annual rates were measured in the upland pin oak ecosystem (8.6 g N m^–2 yr^–1), and the lowest rates in the swamp forest (1.5 g N m^–2 yr^–1); nitrification followed an identical pattern. Denitrification was relatively high in the swamp forest during early spring (8040 g N₂O–N m^–2 d^–1) and late autumn (2525 g N₂O–N m^–2 d^–1); nitrification occurred at rates sufficient to sustain these losses. In the well-drained uplands, rates of denitrification were generally lower and equivalent to rates of atmospheric N inputs. Microbial C and N were consistently higher in the swamp forest than in the other ecosystems; both were positively correlated with average daily rates of N mineralization. In the subtle landscape of east-central Minnesota, rates of N cycling can differ by an order of magnitude across relatively short distances.     

Nitrogen Export from Forested Watersheds in the Oregon Coast Range: The Role of N<Subscript>2</Subscript>-fixing Red Alder

Variations in plant community composition across the landscape can influence nutrient retention and loss at the watershed scale. A striking example of plant species importance is the influence of N₂-fixing red alder (Alnus rubra) on nutrient cycling in the forests of the Pacific Northwest. To understand the influence of red alder on watershed nutrient export, we studied the chemistry of 26 small watershed streams within the Salmon River basin of the Oregon Coast Range. Nitrate and dissolved organic nitrogen (DON) concentrations were positively related to broadleaf cover (dominated by red alder: 94% of basal area), particularly when near-coastal sites were excluded (r ² = 0.65 and 0.68 for nitrate-N and DON, respectively). Nitrate and DON concentrations were more strongly related to broadleaf cover within entire watersheds than broadleaf cover within the riparian area alone, which indicates that leaching from upland alder stands plays an important role in watershed nitrogen (N) export. Nitrate dominated over DON in hydrologic export (92% of total dissolved N), and nitrate and DON concentrations were strongly correlated. Annual N export was highly variable among watersheds (2.4–30.8 kg N ha^–1 y^–1), described by a multiple linear regression combining broadleaf and mixed broadleaf–conifer cover (r² = 0.74). Base cation concentrations were positively related to nitrate concentrations, which suggests that nitrate leaching increases cation losses. Our findings provide evidence for strong control of ecosystem function by a single plant species, where leaching from N saturated red alder stands is a major control on N export from these coastal watersheds.