Nitrogen cycling in Louisiana Gulf Coast brackish marshes

Nitrogen fixation and nitrogen accumulation were measured in a Louisiana Spartina patens brackish marsh. Using the acetylene reduction technique calibrated with direct ¹⁵N₂ assimilation, an equivalent of 90.0 µ g N g^–1 yr^–1 was fixed. Fixation was greater in the summer months and in the upper portion of the soil profile. Extractable ammonium increased with depth and was negatively correlated with ethylene production. Average ammonium concentration in the sediment was 39 µg NH₄ ⁺-N g^–1 sediment. Cesium-137 dating of the soil profile showed the marsh was vertically accreting at a rate of 0.60 cm yr^–1. Calculations using vertical accretion rate, bulk density, and total nitrogen content of sediment indicate that the marshes are accumumating 7.2 g Nm^–2 yr^–1 thus serving as a major nitrogen sink. Measured nitrogen fluxes were incorporated with existing flux measurement in developing a nitrogen budget for the marsh.     

Seasonal flooding,nitrogen mineralization and nitrogen utilization in a prairie marsh

Flooding can be an important control of nitrogen (N) biogeochemistry in wetland ecosystems. In North American prairie marshes, spring flooding is a dominant feature of the physical environment that increases emergent plant production and could influence N cycling. I investigated how spring flooding affects N availability and plant N utilization in whitetop (Scolochloa festucacea) marshes in Manitoba, Canada by comparing experimentally spring-flooded marsh inside an impoundment with adjacent nonflooded marsh. The spring-flooded marsh had net N mineralization rates up to 4 times greater than nonflooded marsh. Total growing season net N mineralization was 124 kg N ha^–1 in the spring-flooded marsh compared with 62 kg N ha^–1 in the nonflooded marsh. Summer water level drawdown in the spring-flooded marsh decreased net N mineralization rates. Net nitrification rates increased in the nonflooded marsh following a lowering of the water table during mid summer. Growing season net nitrification was 33 kg N ha^–1 in the nonflooded marsh but < 1 kg N ha^–1 in the spring-flooded marsh. Added NO₃ ^–1 induced nitrate reductase (NRA) activity in whitetop grown in pot culture. Field-collected plants showed higher NRA in the nonflooded marsh. Nitrate comprised 40% of total plant N uptake in the nonflooded marsh but <1% of total N uptake in the spring-flooded marsh. Higher plant N demand caused by higher whitetop production in the spring-flooded marsh approximately balanced greater net N mineralization. A close association between the presence of spring flooding and net N mineralization and net nitrification rates indicated that modifications to prairie marshes that change the pattern of spring inundation will lead to rapid and significant changes in marsh N cycling patterns.     

Co-development of wetland soils and benthic invertebrate communities following salt marsh creation

The development of wetland soil characteristics andbenthic invertebrate communities were evaluated increated Spartina alterniflorasalt marshes inNorth Carolina ranging in age from 1 to 25 years-old.A combination of measurements from different-agecreated marshes as well as periodic measurements overtime on two marshes were used to (1) document rates ofwetland pedogenesis, especially soil organic matter,and, (2) explore relationships between soil andbenthic invertebrate community development. Soilmacro-organic matter (MOM, the living and dead rootand rhizome mat), organic C and N increased and bulkdensity decreased during the 25 years following marshestablishment. The most dramatic changes in bulkdensity, MOM, C and N occurred within the upper 10 cmof the soil with lesser changes below this depth.Created marshes were sinks for organic C (90–140g·m^-2·yr^-1) and N (7–11g·m^-2·yr^-1) but not for P (0–1g·m^-2·yr^-1). The density of benthicinvertebrates (>250 m) and subsurface-depositfeeding oligochaetes also increased over time oncreated salt marshes. Invertebrate and oligochaetedensity were strongly related to MOM content(r²= 0.83–0.87) and soil organic C(r²= 0.52–0.82) and N (r²= 0.62–0.84). Thesefindings suggest that, in created salt marshes,development of the benthic invertebrate community istied to marsh soil formation, especially accumulationof organic matter as MOM and soil. Field studies thatmanipulate the quantity and quality of soil organicmatter are needed to elucidate the relationshipbetween salt marsh pedogenesis and benthicinvertebrate community development.     

Retention of nutrients in river basins

In Denmark, as in many other European countries, the diffuse losses of nitrogen (N) and phosphorus (P) from the rural landscape are the major causes of surface water eutrophication and groundwater pollution. The export of total N and total P from the Gjern river basin amounted to 18.2 kg ha^–1 and 0.63 kg P ha^–1 during June 1994 to May 1995. Diffuse losses of N and P from agricultural areas were the main nutrient source in the river basin contributing 76% and 51%, respectively, of the total export.Investigations of nutrient cycling in the Gjern river basin have revealed the importance of permanent nutrient sinks (denitrification and overbank sedimentation) and temporary nutrient storage in watercourses. Temporary retention of N and P in the watercourses thus amounted to 7.2–16.1 g N m^–2 yr^–1 and 3.7–8.3 g P m^–2 yr–1 during low-flow periods. Deposition of P on temporarily flooded riparian areas amounted from 0.16 to 6.50 g P m^–2 during single irrigation and overbank flood events, whereas denitrification of nitrate amounted on average to 7.96 kg N yr^–1 per running metre watercourse in a minerotrophic fen and 1.53 kg N yr^–1 per linear metre watercourse in a wet meadow. On average, annual retention of N and P in 18 Danish shallow lakes amounted to 32.5 g N m^–2 yr^–1 and 0.30 g P m^–2 yr^–1, respectively, during the period 1989–1995.The results indicate that permanent nutrient sinks and temporary nutrient storage in river systems represent an important component of river basin nutrient budgets. Model estimates of the natural retention potential of the Gjern river basin revealed an increase from 38.8 to 81.4 tonnes yr^–1 and that P-retention increased from –0.80 to 0.90 tonnes yr^–1 following restoration of the water courses, riparian areas and a shallow lake. Catchment management measures such as nature restoration at the river basin scale can thus help to combat diffuse nutrient pollution.     

Hydrological and nutrient budgets of freshwater and estuarine wetlands of Taylor Slough in Southern Everglades, Florida (U.S.A.)

Hydrological restoration of the Southern Everglades will result in increased freshwater flow to the freshwater and estuarine wetlands bordering Florida Bay. We evaluated the contribution of surface freshwater runoff versus atmospheric deposition and ground water on the water and nutrient budgets of these wetlands. These estimates were used to assess the importance of hydrologic inputs and losses relative to sediment burial, denitrification, and nitrogen fixation. We calculated seasonal inputs and outputs of water, total phosphorus (TP) and total nitrogen (TN) from surface water, precipitation, and evapotranspiration in the Taylor Slough/C-111 basin wetlands for 1.5 years. Atmospheric deposition was the dominant source of water and TP for these oligotrophic, phosphorus-limited wetlands. Surface water was the major TN source of during the wet season, but on an annual basis was equal to the atmospheric TN deposition. We calculated a net annual import of 31.4 mg m^–2 yr^–1 P and 694 mg m^–2 yr^–1N into the wetland from hydrologic sources. Hydrologic import of P was within range of estimates of sediment P burial (33–70 mg m^–2 yr^–1 P), while sediment burial of N (1890–4027 mg m^–2 yr^–1 N) greatly exceeded estimated hydrologic N import. High nitrogen fixation rates or an underestimation of groundwater N flux may explain the discrepancy between estimates of hydrologic N import and sediment N burial rates.     

Climatic variability,hydrologic anomaly,and methane emission can turn productive freshwater marshes into net carbon sources

Freshwater marshes are well‐known for their ecological functions in carbon sequestration, but complete carbon budgets that include both methane (CH₄) and lateral carbon fluxes for these ecosystems are rarely available. To the best of our knowledge, this is the first full carbon balance for a freshwater marsh where vertical gaseous [carbon dioxide (CO₂) and CH₄] and lateral hydrologic fluxes (dissolved and particulate organic carbon) have been simultaneously measured for multiple years (2011–2013). Carbon accumulation in the sediments suggested that the marsh was a long‐term carbon sink and accumulated ~96.9 ± 10.3 (±95% CI) g C m^?2 yr^?1 during the last ~50 years. However, abnormal climate conditions in the last 3 years turned the marsh to a source of carbon (42.7 ± 23.4 g C m^?2 yr^?1). Gross ecosystem production and ecosystem respiration were the two largest fluxes in the annual carbon budget. Yet, these two fluxes compensated each other to a large extent and led to the marsh being a CO₂ sink in 2011 (?78.8 ± 33.6 g C m^?2 yr^?1), near CO₂‐neutral in 2012 (29.7 ± 37.2 g C m^?2 yr^?1), and a CO₂ source in 2013 (92.9 ± 28.0 g C m^?2 yr^?1). The CH₄ emission was consistently high with a three‐year average of 50.8 ± 1.0 g C m^?2 yr^?1. Considerable hydrologic carbon flowed laterally both into and out of the marsh (108.3 ± 5.4 and 86.2 ± 10.5 g C m^?2 yr^?1, respectively). In total, hydrologic carbon fluxes contributed ~23 ± 13 g C m^?2 yr^?1 to the three‐year carbon budget. Our findings highlight the importance of lateral hydrologic inflows/outflows in wetland carbon budgets, especially in those characterized by a flow‐through hydrologic regime. In addition, different carbon fluxes responded unequally to climate variability/anomalies and, thus, the total carbon budgets may vary drastically among years.     

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.     

Nitrogen mineralization in heathland ecosystems dominated by different plant species

Net N mineralization rates were measured in heathlands still dominated by ericaceous dwarf shrubs (Calluna vulgaris or Erica tetralix) and in heathlands that have become dominated by grasses (Molinia caerulea or Deschampsia flexuosa). Net N mineralization was measuredin situ by sequential soil incubations during the year. In the wet area (gravimetric soil moisture content 74–130%), the net N mineralization rates were 4.4 g N m^–2 yr^–1 in the Erica soil and 7.8 g N m^–2 yr^–1 in the Molinia soil. The net nitrification rate was negligibly slow in either soil. In the dry area (gravimetric soil moisture content 7–38%), net N mineralization rates were 6.2 g N M-2 yr^–1 in the Calluna soil, 10.9 g N m^–2 yr^–1 in the Molinia soil and 12.6 g N m^–2 yr^–1 in the Deschampsia soil. The Calluna soil was consistently drier throughout the year, which may partly explain its slower mineralization rate. Net nitrification was 0.3 g N m^–2 yr^–1 in the Calluna soil, 3.6 g N m^–2 yr^–1 in the Molinia soil and 5.4 g N m^–2 yr^–1 in the Deschampsia soil. The net nitrification rate increased proportionally with the net N mineralization rate suggesting ammonium availability may control nitrification rates in these soils. In the dry area, the faster net N mineralization rates in sites dominated by grasses than in the site dominated by Calluna may be explained by the greater amounts of organic N in the soil of sites dominated by grasses. In both areas, however, the net amount of N mineralized per gram total soil N was greater in sites dominated by Molinia or Deschampsia than in sites dominated by Calluna or Erica. This suggests that in heathlands invaded by grasses the quality of the soil organic matter may be increased resulting in more rapid rates of soil N cycling.     

Nitrogen sequestration capacity of two salt marshes from the Tagus estuary

The role of salt marshes as nitrogen sink is examined taking into consideration the seasonal variation of above and belowground biomass of Spartina martima and Halimione portulacoides in two marshes from Tagus estuary, Pancas and Corroios, and the degradation rates of belowground litter. Total nitrogen was determined in plant components, decomposing litter and sediment. Biomass was higher in Corroios, the saltier marsh, with 7190 g m⁻² y⁻¹ dw of S. maritima and 6593 g m⁻² y⁻¹ dw of H. portulacoides and the belowground component contributed to 96% and 90% of total biomass, respectively. In the other marsh, Pancas, belowground biomass contributed to 56% and 76% of total biomass for S. maritima and H. portulacoides, respectively. Litterbag experiment showed that between 25% and 50% of nitrogen is lost within the first month and remained relatively constant in the next four months. Slower decomposition is observed in sediments with higher nitrogen concentration (max. 0.7% N in the saltier marsh). Higher concentrations of N were found in the sediment upper layers. Considering the sediment-root system, most of the nitrogen is stored in the sediment compartment and only about 1–4% of the total N was found in the roots. Considering these results, Tagus salt marshes act as a sink for nitrogen.     

Sediment nutrient accumulation and nutrient availability in two tidal freshwater marshes along the Mattaponi River, Virginia, USA

Sediment deposition is the main mechanism of nutrient delivery to tidal freshwater marshes (TFMs). We quantified sediment nutrient accumulation in TFMs upstream and downstream of a proposed water withdrawal project on the Mattaponi River, Virginia. Our goal was to assess nutrient availability by comparing relative rates of carbon (C), nitrogen (N), and phosphorus (P) accumulated in sediments with the C, N, and P stoichiometries of surface soils and above ground plant tissues. Surface soil nutrient contents (0.60–0.92% N and 0.09–0.13% P) were low but within reported ranges for TFMs in the eastern US. In both marshes, soil nutrient pools and C, N, and P stoichiometries were closely associated with sedimentation patterns. Differences between marshes were more striking than spatial variations within marshes: both C, N, and P accumulation during summer, and annual P accumulation rates (0.16 and 0.04 g P m^–2 year^–1, respectively) in sediments were significantly higher at the downstream than at the upstream marsh. Nitrogen:P ratios <14 in above ground biomass, surface soils, and sediments suggest that N limits primary production in these marshes, but experimental additions of N and/or P did not significantly increase above ground productivity in either marsh. Lower soil N:P ratios are consistent with higher rates of sediment P accumulation at the downstream site, perhaps due to its greater proximity to the estuarine turbidity maximum.     

Benithic-pelagic coupling of nutrient metabolism along an estuarine eutrophication gradient

The shallow, brackish (11–18% salinity) Roskilde Fjord represents a eutrophication gradient with annual averages of chlorophyll, ranging from 3 to 25 mg chl a m^–3. Nutrient loadings in 1985 were 11.3–62.4 g N m^–2 yr^–1 and 0.4–7.3 g P m^–2 yr^–1. A simple one-layer advection-diffusion model was used to calculate mass balances for 7 boxes in the fjord. Net loss rates varied from –32.2 to 17.9 g P m^–2 yr^–1 and from –3.3 to 66.8 g N m^–2, corresponding to 74% of the external P-loading and 88% of the external N-loading to the entire estuary.Gross sedimentation rates measured by sediment traps were between 7 and 52 g p m^–2 yr^–1 and 50 and 426 g N M^–2 yr^–1, respectively. Exchangeable sediment phosphorus varied in annual average between 2.0 and 4.8 g P m^–2 and exchangeable sediment nitrogen varied from 1.9 to 33.1 g N m–1. Amplitudes in the exchangeable pools followed sedimentation peaks with delays corresponding to settling rates of 0.3 m d^–1. Short term nutrient exchange experiments performed in the laboratory with simultaneous measurements of sediment oxygen uptake showed a release pattern following the oxygen uptake, the changes in the exchangeable pools and the sedimentation peaks.The close benthic-pelagic coupling also exists for the denitrification with maxima during spring of 5 to 20 mmol N m^–2 d–1. Denitrification during the nitrogen-limited summer period suggests dependence on nitrification. Comparisons with denitrification from other shallow estuaries indicate a maximum for denitrification in estuaries of about 250 µmol N m^–2 h^–2 achieved at loading rates of about 25–125 g N m^–2 yr^–1.     

Forest nitrogen sinks in large eastern U.S. watersheds: estimates from forest inventory and an ecosystem model

The eastern U.S. receives elevated rates of Ndeposition compared to preindustrial times, yetrelatively little of this N is exported indrainage waters. Net uptake of N into forestbiomass and soils could account for asubstantial portion of the difference between Ndeposition and solution exports. We quantifiedforest N sinks in biomass accumulation andharvest export for 16 large river basins in theeastern U.S. with two separate approaches: (1)using growth data from the USDA ForestService's Forest Inventory and Analysis (FIA)program, and (2) using a model of forestnitrogen cycling (PnET-CN) linked to FIAinformation on forest age-class structure. Themodel was also used to quantify N sinks in soiland dead wood, and nitrate losses below therooting zone. Both methods agreed that netgrowth rates were highest in the relativelyyoung forests on the Schuylkill watershed, andlowest in the cool forests of northern Maine. Across the 16 watersheds, wood export removedan average of 2.7 kg N ha^–1 yr^–1(range: 1–5 kg N ha^–1 yr^–1), andstanding stocks increased by 4.0 kg N ha^–1yr^–1 (–3 to 8 kg N ha^–1 yr^–1). Together, these sinks for N in woody biomassamounted to a mean of 6.7 kg N ha^–1yr^–1 (2–9 kg N ha^–1 yr^–1), or73% (15–115%) of atmospheric N deposition. Modeled rates of net N sinks in dead wood andsoil were small; soils were only a significantnet sink for N during simulations ofreforestation of degraded agricultural sites. Predicted losses of nitrate depended on thecombined effects of N deposition, and bothshort- and long-term effects of disturbance. Linking the model with forest inventoryinformation on age-class structure provided auseful step toward incorporating realisticpatterns of forest disturbance status acrossthe landscape.     

The biogeochemistry of nitrogen in freshwater wetlands

The biogeochemistry of N in freshwater wetlands is complicated by vegetation characteristics that range from annual herbs to perennial woodlands; by hydrologic characteristics that range from closed, precipitation-driven to tidal, riverine wetlands; and by the diversity of the nitrogen cycle itself. It is clear that sediments are the single largest pool of nitrogen in wetland ecosystems (100's to 1000's g N m^-2) followed in rough order-of-magnitude decreases by plants and available inorganic nitrogen. Precipitation inputs (< 1–2 g N m^-2 yr^-1) are well known but other atmospheric inputs, e.g. dry deposition, are essentially unknown and could be as large or larger than wet deposition. Nitrogen fixation (acetylene reduction) is an important supplementary input in some wetlands (< < 1–3 g N m^-2 yr^-1) but is probably limited by the excess of fixed nitrogen usually present in wetland sediments.Plant uptake normally ranges from a few g N m^-2 yr^-1 to 35 g N m^-2 yr^-1 with extreme values of up to 100g N m^-2 yr^-1 Results of translocation experiments done to date may be misleading and may call for a reassessment of the magnitude of both plant uptake and leaching rates. Interactions between plant litter and decomposer microorganisms tend, over the short-term, to conserve nitrogen within the system in immobile forms. Later, decomposers release this nitrogen in forms and at rates that plants can efficiently reassimilate.The NO₃ formed by nitrification (< 0.1 to 10 g N m^-2 yr^-1 has several fates which may tend to either conserve nitrogen (uptake and dissimilatory reduction to ammonium) or lead to its loss (denitrification). Both nitrification and denitrification operate at rates far below their potential and under proper conditions (e.g. draining or fluctuating water levels) may accelerate. However, virtually all estimates of denitrification rates in freshwater wetlands are based on measurements of potential denitrification, not actual denitrification and, as a consequence, the importance of denitrification in these ecosystems may have been greatly over estimated.In general, larger amounts of nitrogen cycle within freshwater wetlands than flow in or out. Except for closed, ombrotrophic systems this might seem an unusual characteristic for ecosystems that are dominated by the flux of water, however, two factors limit the opportunity for N loss. At any given time the fraction of nitrogen in wetlands that could be lost by hydrologic export is probably a small fraction of the potentially mineralizable nitrogen and is certainly a negligible fraction of the total nitrogen in the system. Second, in some cases freshwater wetlands may be hydrologically isolated so that the bulk of upland water flow may pass under (in the case of floating mats) or by (in the case of riparian systems) the biotically active components of the wetland. This may explain the rather limited range of N loading rates real wetlands can accept in comparison to, for example, percolation columns or engineered marshes.     

The nitrogen cycle in lodgepole pine forests,southeastern Wyoming

Storage and flux of nitrogen were studied in several contrasting lodgepole pine (Pinus contorta spp.latifolia) forests in southeastern Wyoming. The mineral soil contained most of the N in these ecosystems (range of 315–860 g · m^–2), with aboveground detritus (37.5–48.8g · m^–2) and living biomass (19.5–24.0 g · m^–2) storing much smaller amounts. About 60–70% of the total N in vegetation was aboveground, and N concentrations in plant tissues were unusually low (foliage = 0.7% N), as were N input via wet precipitation (0.25 g · m^–2 · yr^–1), and biological fixation of atmospheric N (<0.03 g · m^–2 · yr^–1, except locally in some stands at low elevations where symbiotic fixation by the leguminous herbLupinus argenteus probably exceeded 0.1 g · m^–2 · yr^–1).Because of low concentrations in litterfall and limited opportunity for leaching, N accumulated in decaying leaves for 6–7 yr following leaf fall. This process represented an annual flux of about 0.5g · m^–2 to the 01 horizon. Only 20% of this flux was provided by throughfall, with the remaining 0.4g · m^–2 · yr^–1 apparently added from layers below. Low mineralization and small amounts of N uptake from the 02 are likely because of minimal rooting in the forest floor (as defined herein) and negligible mineral N (< 0.05 mg · L^–1) in 02 leachate. A critical transport process was solubilization of organic N, mostly fulvic acids. Most of the organic N from the forest floor was retained within the major tree rooting zone (0–40 cm), and mineralization of soil organic N provided NH₄ for tree uptake. Nitrate was at trace levels in soil solutions, and a long lag in nitrification was always observed under disturbed conditions. Total root nitrogen uptake was calculated to be 1.25 gN · m^–2 · yr^–1 with estimated root turnover of 0.37-gN · m^–2 · yr^–1, and the soil horizons appeared to be nearly in balance with respect to N. The high demand for mineralized N and the precipitation of fulvic acid in the mineral soil resulted in minimal deep leaching in most stands (< 0.02 g · m^–2 · yr^–1). These forests provide an extreme example of nitrogen behavior in dry, infertile forests.     

Nitrogen transformations in two nitrogen saturated forest ecosystems subjected to an experimental decrease in nitrogen deposition

Nitrogen transformations were studied in the forest floor and mineral soil (0–5 cm) of a Douglas fir forest (Pseudotsuga menziesii (Mirb.) Franco.) and a Scots pine forest (Pinus sylvestris L.) in the Netherlands. Curren nitrogen depositions (40 and 56 kg N ha^-1 yr^-1, respectively) were reduced to natural background levels (1–2 kg N ha^-1 yr^-1) by a roof construction. The study concentrated on rates and dynamic properties of nitrogen transformations and their link with the leaching pattern and nitrogen uptake of the vegetation under high and reduced nitrogen deposition levels. Results of an in situ field incubation experiment and laboratory incubations were compared. No effect of the reduced N deposition on nitrogen transformations was found in the Douglas fir forest. In the Scots pine forest, however, during some periods of the year nitrogen transformations were significantly decreased under the low nitrogen deposition level. At low nitrogen inputs a net immobilization occurred during most of the year leading to a very small net mineralization for the whole year. In laboratory and in individual field plots nitrogen transformations were negatively correlated with initial inorganic nitrogen concentrations. Nitrogen budget estimates showed that nitrogen transformations were probably underestimated by the in situ incubation technique. Nevertheless less nitrogen was available for plant uptake and leaching at the low deposition plots.     

Vernal nitrogen and phosphorus retention by forest understory vegetation and soil microbes

Northern hardwood forests experience annual maximal loss of nutrients during spring. The vernal dam hypothesis predicts that spring ephemeral herbs in northern hardwood forests serve as sinks for nutrients during this season and reduce the loss of nutrients from the terrestrial ecosystem. Soil microbes of northern hardwood forests also sequester nutrients during spring. We compared the vernal nutrient acquisition ability of a soil microbial community and an understory plant community with species of mixed leaf phenology. We monitored nitrogen and phosphorus pool sizes in understory vegetation and soil microbes during spring from 1999 through 2001 in a northern hardwood forest in the Catskill Mountains, New York. Vegetation nutrient content increased during two spring seasons by an average of 3.07 g N m^–2 and 0.19 g P m^–2 and decreased during one spring by 0.81 g N m^–2 and 0.10 g P m^–2. Evergreen, wintergreen, and deciduous plant species were able to sequester nutrients during spring. Soil microbial nutrient content decreased during one spring by 1.29 g N m^–2 and remained constant during the other two springs. Streamwater nitrogen losses were not correlated with biotic nutrient uptake suggesting a temporal disconnect between the two processes. We conclude that understory vegetation is a larger potential sink for vernal nutrients than are soil microbes in this northern hardwood forest and understory and species representing multiple phenologies are capable of vernal nutrient uptake.     

Nitrogen fluxes and budget seasonality in the Ria Vigo [2pt] (NW Iberian Peninsula)

Inorganic and organic nitrogen fluxes in the Ria Vigo have been quantified in order to recognise the contrasting nitrogen budget scenarios and understand the biogeochemical response to eutrophication events. According to the nitrogen biogeochemical pathways of the ria reservoir (photosynthesis, remineralization, denitrification, PON rain rate and sedimentation), three main seasonal behavioural trends are emphasised: (1) low inorganic nitrogen inputs and low organic nitrogen fluxes, (2) high inorganic nitrogen input and output, (3) high inorganic nitrogen input and high organic nitrogen output. The first scenario occurs in late spring and in summer during non-upwelling situations. The consumption of inorganic nitrogen by net photosynthesis is approximately 2 mol N s^–1 and the ria is oligotrophic (12 mgC m^–2 h^–1). The outgoing estuarine residual current transports phytoplanktonic material towards the mouth of the ria whereupon it sediments and is remineralized as it falls to the lower water layers and the incoming residual current. The regenerated nitrogen is reintroduced to the photic ria layer which leads to the greatest reduction in dissolved oxygen concentration (50% of saturation). Recycled nutrients play an important role in primary production during this oligotrophic state of the ria. Thus, approximately half of the inorganic nitrogen utilised by photosynthesis is ammonium. The majority of PON is deposited inside the ria (0.8 mmol N m^–2 d^–1) and the denitrification rate is 0.3 mmol N₂ m^–2 d^–1. The other two cases occur in winter and spring–summer with upwelling. In winter, estuarine circulation and freshwater contributions control the nitrogen cycle. The ria mainly exports nitrate (up to 14 mol N s^–1) and so there is fertilisation but no eutrophication. In spring and summer, the nitrogen cycle is controlled by upwelling circulation. The inorganic nitrogen consumption by net photosynthesis is high, 7–14 mmol N m^–2 d^–1, and the ria is a natural eutrophic system (70 mgC m^–2 h^–1). Accordingly, 90% of organic nitrogen is synthesised from nitrate and the upwelling-increased circulation exports 6.5 mol N s^–1 of organic nitrogen.     

Dynamics of nitrogen and phosphorus retention during wetland ecosystem succession

We compared the mechanisms of nitrogen (N) and phosphorus (P) removal in four young (<15 years old) constructed estuarine marshes with paired mature natural marshes to determine how nutrient retention changes during wetland ecosystem succession. In constructed wetlands, N retention begins as soon as emergent vegetation becomes established and soil organic matter starts to accumulate, which is usually within the first 1–3 years. Accumulation of organic carbon in the soil sets the stage for denitrification which, after 5–10 years, removes approximately the same amount of N as accumulating organic matter, 5–10 g/m²/yr each, under conditions of low N loadings. Under high N loadings, the amount of N stored in accumulating organic matter doubles while N removal from denitrification may increase by an order of magnitude or more. Both organic N accumulation and denitrification provide for long-term reliable N removal regardless of N loading rates. Phosphorus removal, on the other hand, is greatest during the first 1–3 years of succession when sediment deposition and sorption/precipitation of P are greatest. During this time, constructed marshes may retain from 3 g P/m²/yr under low P loadings to as much as 30 g P/m²/yr under high loadings. However, as sedimentation decreases and sorption sites become saturated, P retention decreases to levels supported by organic P accumulation (1–2 g P/m²/yr) and sorption/precipitation with incoming aqueous and particulate Fe, Al and Ca. Phosphorus cycling in wetlands differs from forest and other terrestrial ecosystems in that conservation of P is greatest during the early years of succession, not during the middle or late stages. Conservation of P by wetlands is largely regulated by geochemical processes (sorption, precipitation) which operate independently of succession. In contrast, the conservation of N is controlled by biological processes (organic matter accumulation, denitrification) that change as succession proceeds.     

A function of cyanobacterial mats in phosphorus-limited tropical wetlands

Cyanobacterial mats are important components of oligotrophic wetland ecosystems in the limestone-based regions of the Caribbean. Our goals were to: (1) Estimate the biomass and primary production of cyanobacterial mats, quantify the extent of nitrogen fixation and measure the activity of alkaline phosphatase (APA) in representative marshes of northern Belize; (2) Record changes in these variables following nutrient additions. The mat biomass ranged from 200 to 700 g m^–2 AFDM, with the epipelon contributing up to 87% of the total. Tissue nitrogen was similar in all marshes (1.1–1.5%), while tissue phosphorus was extremely low (0.0055–0.0129%) and well correlated with the N:P ratio in water. Nitrogen fixation expressed as nitrogenase activity was high in some marshes (17.5 nmol C₂H₄ cm^–2 h^–1) and low (< 5 nmol C₂H₄ cm^–2 h^–1) in others depending mainly on the proportion of heterocyst-forming cyanobacteria (Nostocales, Stigonematales) in the mat. Alkaline phosphatase activity was positively correlated with the N:P ratio of the mat. Experimental addition of phosphorus resulted in significant increase in primary production and nitrogen fixation while it suppressed the APA activity. The presented data clearly showed that oligotrophic marshes of northern Belize are strongly P limited. Increased input of phosphorus would profoundly change their structure and functions.     

Effects of a catch crop on leaching of nitrogen from a sandy soil: Simulations and measurements

The effects of an undersown catch crop on the dynamics and leaching of nitrogen in cropping systems with spring cereals were investigated in southern Sweden. Field measurements of soil mineral nitrogen and nitrogen concentrations in drainage water were made for 4 years in a sandy soil. The experiment was performed on four tile-drained field plots sown with spring cereals. On two of the plots, Italian rye grass was undersown and ploughed down the following spring during three of the years. The other two plots were treated in a conventional way and served as controls. Soil nitrate levels were substantially reduced in the catch-crop treatment, but increased during the fourth year when no catch crop was grown. The differences between the treatments in soil nitrate were reflected in the nitrate concentrations measured in the drainage water. A mathematical model was used to simulate nitrogen dynamics in corresponding treatments. There was good agreement between measurements and simulations with regard to patterns of change in soil mineral nitrogen and nitrate concentrations in drainage water for each treatment. Simulated leaching of nitrate in the conventional treatment was 1.9–3.9 g N m^–2 y^–1 during the first three years while calculated leaching based on the measurements was 2.7–4.4 g N m^–2 y^–1. In the catch-crop treatment leaching of nitrate was reduced by 1.4–2.6 g m^–2 y^–1 according to the simulations and by 2.2–4.1 g m^–2 y^–1 according to calculations based on the measurements. Measurements showed that leaching of nitrogen compounds other than nitrate was hardly affected by the catch crop. In the simulations the ploughed-down catch crop resulted in temporary increases of the litter pool, a net increase of the humus pool and a reduced C-N ratio of the litter pool. Simulated net mineralization from the litter pool was substantially higher in the catch-crop treatment compared with the conventional treatment. In the fourth year, the yield of the main crop was 20–25% higher in the catch-crop treatment, and leaching was higher than in the conventional treatment.