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.     

Response of Douglas fir seedlings to nitrate and ammonium nitrogen sources at different levels of pH and iron supply

Douglas fir seedlings were grown for two to three months in sand and soil cultures in a greenhouse to examine their growth response to nitrogen (N) source at different levels of pH and iron (Fe) supply. In the first two experiments nutrient solutions of known pH were automatically applied to the top of the sand cultures and allowed to run to waste from the bottom. Under these conditions seedlings made most growth on nitrate (NO₃–N) under acid (pH4) conditions, but most growth on ammonium (NH₄–N) under neutral (pH7) conditions. Calcium carbonate (CaCO₃) was used to create a range of pH conditions (from 4.0 to 7.2) in a peat and sand artificial soil. Over the pH range 4 to 6 NH₄–N or NO₃+NH₄–N produced larger seedlings than NO₃–N alone, but above pH6 growth on all N sources was depressed. Chemical analysis showed that seedling Ca concentration had increased and Fe concentration had decreased with increase in CaCO₃ application. Both Ca and Fe concentrations were higher in seedlings receiving NO₃–N than in those receiving NH₄ or NO₃+NH₄.In sub-irrigated sand cultures, Doughlas fir seedlings receiving NO₃–N were shown to respond to additions of Fe chelate, but seedlings receiving NH₄–N responded little to Fe chelate. At pH5 seedlings receiving NO₃–N did not grow as big as seedlings receiving NH₄–N in the absence of Fe chelate, but addition of Fe chelate resulted in NO₃-fed seedlings growing larger than NH₄-fed seedlings. The relationship between seedling Fe concentration and N nutrition is discussed.The relatively larger root dry weight and surface area of seedlings grown on NO₃–N, as compared to NH₄–N, in sand culture, was noted.     

Changing nitrogen source on the yield and nitrogen content of soybeans

Summary Soybean plants were grown in nutrient culture solutions containing 150 ppm of N either as an equal concentration of NH₄ ⁺ or NO₃ ^–, or all NO₃ ^–. At the R2 stage of growth for some plants, the N form was changed to either all NO₃ ^– or all NH₄ ⁺, but at the same total N concentration as before. Highest seed yield was obtained with all NO₃ ^– over the entire growth period, the poorest when the N form was switched from an equal ratio of NH₄ ⁺ and NO₃ ^– to all NH₄ ⁺ at the R2 stage. Kjeldahl N concentrations in the plant leaves and seed were highest when NH₄ ⁺ was part or all of the N source in the nutrient solution. These results may partially explain why the literature is inconsistent on the effect of added fertilizer N on soybean seed yield, and may pose a problem in using leaf Kjeldahl N concentration to determine plant N sufficiency.     

Plant-mediated processess to acquire nutrients: nitrogen uptake by rice plants

The ways in which root–soil interactions can control nutrient acquisition by plants is illustrated by reference to the N nutrition of rice. Model calculations and experiments are used to assess how uptake is affected by root properties and N transport through the soil. Measurements of the kinetics of N absorption and assimilation and their regulation, and of interactions between NH₄ ⁺ and NO₃ ^– nutrition, are described. It is shown that uptake of N from the soil–-as opposed to N in ricefield floodwater which can be absorbed very rapidly but is otherwise lost by gaseous emission–-will often be limited by root uptake properties. Rice roots are particularly efficient in absorbing and assimilating NO₃ ^–, and NH₄ ⁺ absorption and assimilation are stimulated by NO₃ ^–. The uptake of NO₃ ^– formed in the rice rhizosphere by root-released O₂ may be more important than previously thought, with beneficial consequences for rice growth. Other root-induced changes in the rice rhizosphere and their consequences are discussed.     

A Non-Native Riparian Tree (<Emphasis Type="Italic">Elaeagnus angustifolia</Emphasis>) Changes Nutrient Dynamics in Streams

Russian olive (Elaeagnus angustifolia) is a non-native riparian tree that has become common and continues to rapidly spread throughout the western United States. Due to its dinitrogen (N₂)-fixing ability and proximity to streams, Russian olive has the potential to subsidize stream ecosystems with nitrogen (N), which may in turn alter nutrient processing in these systems. We tested these potential effects by comparing background N concentrations; nutrient limitation of biofilms; and uptake of ammonium (NH₄-N), nitrate (NO₃-N), and phosphate (PO₄-P) in paired upstream-reference and downstream-invaded reaches in streams in southeastern Idaho and central Wyoming. We found that stream reaches invaded by Russian olive had higher organic N concentrations and exhibited reduced N limitation of biofilms compared to reference reaches. However, at low inorganic N background concentrations, reaches invaded by Russian olive exhibited higher demand for both NH₄-N and NO₃-N compared to their paired reference reaches, suggesting these streams have the potential to retain the N subsidy from Russian olive N₂ fixation and diminish its downstream export and effects. Our findings demonstrate the potential for a non-native riparian plant to significantly alter biogeochemical cycling in streams. Finally, we used our results to develop a conceptual model that describes predicted effects of Russian olive and other non-native riparian N₂ fixers on in-stream N dynamics.     

The influence of nitrogen concentration and ammonium/nitrate ratio on N-uptake,mineral composition and yield of citrus

In short-term water culture experiments with different ¹⁵N labeled ammonium or nitrate concentrations, citrus seedlings absorbed NH₄ ⁺ at a higher rate than NO₃ ^–. Maximum NO₃ ^– uptake by the whole plant occurred at 120 mg L^–1 NO₃ ^–-N, whereas NH₄ ⁺ absorption was saturated at 240 mg L^–1 NH₄ ⁺-N. ¹⁵NH₄ ⁺ accumulated in roots and to a lesser degree in both leaves and stems. However, ¹⁵NO₃ ^– was mostly partitioned between leaves and roots.Adding increasing amounts of unlabeled NH₄ ⁺ (15–60 mg L^–1 N) to nutrient solutions containing 120 mg L^–1 N as ¹⁵N labeled nitrate reduced ¹⁵NO₃ ^– uptake. Maximum inhibition of ¹⁵NO₃ ^– uptake was about 55% at 2.14 mM NH₄ ⁺ (30 mg L^–1 NH₄ ⁺-N) and it did not increase any further at higher NH₄ ⁺ proportions.In a long-term experiment, the effects of concentration and source of added N (NO₃ ^– or NH₄ ⁺) on nutrient concentrations in leaves from plants grown in sand were evaluated. Leaf concentration of N, P, Mg, Fe and Cu were increased by NH₄ ⁺ versus NO₃ ^– nutrition, whereas the reverse was true for Ca, K, Zn and Mn.The effects of different NO₃ ^–-N:NH₄ ⁺-N ratios (100:0, 75:25, 50:50, 25:75 and 0:100) at 120 mg L^–1 total N on leaf nutrient concentrations, fruit yield and fruit characteristics were investigated in another long-term experiment with plants grown in sand cultures. Nitrogen concentrations in leaves were highest when plants were provided with either NO₃ ^– or NH₄ ⁺ as a sole source of N. Lowest N concentration in leaves was found with a 75:25 NO₃ ^–-N/NH₄ ⁺-N ratio. With increasing proportions of NH₄ ⁺ in the N supply, leaf nutrients such as P, Mg, Fe and Cu increased, whereas Ca, K, Mn and Zn decreased. Yield in number of fruits per tree was increased significantly by supplying all N as NH₄ ⁺, although fruit weight was reduced. The number of fruits per tree was lowest with the 75:25 NO₃ ^–-N:NH₄ ⁺-N ratio, but in this treatment fruits reached their highest weight. Rind thickness, juice acidity, and colour index of fruits decreased with increasing NH₄ ⁺ in the N supply, whereas the % pulp and maturity index increased. Percent of juice in fruits and total soluble solids were only slightly affected by NO₃ ^–:NH₄ ⁺ ratio.     

An artificial neural network approach for studying phytoplankton succession

Artificial neural networks are used to model phytoplankton succession and gain insight into the relative strengths of bottom-up and top-down forces shaping seasonal patterns in phytoplankton biomass and community composition. Model comparisons indicate that patterns in chlorophyll aconcentrations response instantaneously to patterns in nutrient concentrations (phosphorous (P), nitrite and nitrate (NO₂/NO₃–N) and ammonium (NH₄–H) concentrations) and zooplankton biomass (daphnid cladocera and copepoda biomass); whereas lagged responses in an index of algal community composition are evident. A randomization approach to neural networks is employed to reveal individual and interacting contributions of nutrient concentrations and zooplankton biomass to predictions of phytoplankton biomass and community composition. The results show that patterns in chlorophyll aconcentrations are directly associated with P, NO₂/NO₃–N and daphnid cladocera biomass, as well as related to interactions between daphnid cladocera biomass, and NO₂/NO₃–N and P. Similarly, patterns in phytoplankton community composition are associated with NO₂/NO₃–N and daphnid cladocera biomass; however show contrasting patterns in nutrient– zooplankton and zooplankton–zooplankton interactions. Together, the results provide correlative evidence for the importance of nutrient limitation, zooplankton grazing and nutrient regeneration in shaping phytoplankton community dynamics. This study shows that artificial neural networks can provide a powerful tool for studying phytoplankton succession by aiding in the quantification and interpretation of the individual and interacting contributions of nutrient limitation and zooplankton herbivory on phytoplankton biomass and community composition under natural conditions.     

Biogeochemistry of unpolluted forested watersheds in the Oregon Cascades: temporal patterns of precipitation and stream nitrogen fluxes

We analyzed long-term organic and inorganic nitrogen inputs and outputs in precipitation and streamwater in six watersheds at the H.J. Andrews Experimental Forest in the central Cascade Mountains of Oregon. Total bulk N deposition, averaging 1.6 to 2.0 kg N ha^–1 yr^–1, is low compared to other sites in the United States and little influenced by anthropogenic N sources. Streamwater N export is also low, averaging <1 kg ha^–1 yr^–1. DON is the predominant form of N exported from all watersheds, followed by PON, NH₄-N, and NO₃-N. Total annual stream discharge was a positive predictor of annual DON output in all six watersheds, suggesting that DON export is related to regional precipitation. In contrast, annual discharge was a positive predictor of annual NO₃-N output in one watershed, annual NH₄-N output in three watersheds, and annual PON output in three watersheds. Of the four forms of N, only DON had consistent seasonal concentration patterns in all watersheds. Peak streamwater DON concentrations occurred in November-December after the onset of fall rains but before the peak in the hydrograph, probably due to flushing of products of decomposition that had built up during the dry summer. Multiple biotic controls on the more labile nitrate and ammonium concentrations in streams may obscure temporal DIN flux patterns from the terrestrial environment. Results from this study underscore the value of using several watersheds from a single climatic zone to make inferences about controls on stream N chemistry; analysis of a single watershed may preclude identification of geographically extensive mechanisms controlling N dynamics.     

Amazon deforestation alters small stream structure, nitrogen biogeochemistry and connectivity to larger rivers

Human activities that modify land cover can alter the structure and biogeochemistry of small streams but these effects are poorly known over large regions of the humid tropics where rates of forest clearing are high. We examined how conversion of Amazon lowland tropical forest to cattle pasture influenced the physical and chemical structure, organic matter stocks and N cycling of small streams. We combined a regional ground survey of small streams with an intensive study of nutrient cycling using ¹⁵N additions in three representative streams: a second-order forest stream, a second-order pasture stream and a third-order pasture stream. These three streams were within several km of each other and on similar soils. Replacement of forest with pasture decreased stream habitat complexity by changing streams from run and pool channels with forest leaf detritus (50% cover) to grass-filled (63% cover) channel with runs of slow-moving water. In the survey, pasture streams consistently had lower concentrations of dissolved oxygen and nitrate (NO₃ ^?) compared with similar-sized forest streams. Stable isotope additions revealed that second-order pasture stream had a shorter NH₄ ⁺ uptake length, higher uptake rates into organic matter components and a shorter ¹⁵NH₄ ⁺ residence time than the second-order forest stream or the third-order pasture stream. Nitrification was significant in the forest stream (19% of the added ¹⁵NH₄ ⁺) but not in the second-order pasture (0%) or third-order (6%) pasture stream. The forest stream retained 7% of added ¹⁵N in organic matter compartments and exported 53% (¹⁵NH₄ ⁺?=?34%; ¹⁵NO₃ ^??=?19%). In contrast, the second-order pasture stream retained 75% of added ¹⁵N, predominantly in grasses (69%) and exported only 4% as ¹⁵NH₄ ⁺. The fate of tracer ¹⁵N in the third-order pasture stream more closely resembled that in the forest stream, with 5% of added N retained and 26% exported (¹⁵NH₄ ⁺?=?9%; ¹⁵NO₃ ^??=?6%). These findings indicate that the widespread infilling by grass in small streams in areas deforested for pasture greatly increases the retention of inorganic N in the first- and second-order streams, which make up roughly three-fourths of total stream channel length in Amazon basin watersheds. The importance of this phenomenon and its effect on N transport to larger rivers across the larger areas of the Amazon Basin will depend on better evaluation of both the extent and the scale at which stream infilling by grass occurs, but our analysis suggests the phenomenon is widespread.     

Effects of light quality,CO2 tensions and NO 3 +− concentrations on the inorganic nitrogen metabolism of Chlamydomonas reinhardii

The blue light dependent utilization of nitrate by green algae under common air and high irradiances, besides its assimilatory nature, is associated with the release of NO₂ ^– and NH₄ ⁺ to the culture medium. If the CO₂ content of the sparging air was increased up to 2%, previously excreted NO₂ ^– and NH₄ ⁺ were rapidly assimilated. When under air and high irradiances the cell density in the culture reached values corresponding to 25 g Ch 1.ml^-1, no further growth was observed and the highest values of NO₃ ^– consumption and NO₂ ^– and NH₄ ⁺ release were attained. Besides low CO₂ tensions, increasing NO₃ ^– concentrations in the medium stimulated the release of NO₃ ^– and NH₄ ⁺. Under CO₂-free air the consumption of NO₃ ^– and the release of NO₂ ^– and NH₄ ⁺ on a total N bases were almost stoichiometric and their rates saturated at much lower irradiances than under air. Under CO₂-free air high rates of NO₂ ^– release were only observed under the blue radiations that were effectively absorbed by photosynthetically active pigments, i.e. 460 nm, but not under 404 and 630 nm radiations. However, the simultaneous illumination of the cells with 404 and 630 nm monochromatic light showed a remarkable synergistic effect on NO₂ ^– release.The results are discussed in terms of the close relationship between C and N metabolism, the photosynthetic reducing power required to convert NO _inf3 ^sup± -N into R – NH₂-N and the blue light activation of nitrate reductase.     

Soil N mineralization and nitrification in relation to nitrogen solution chemistry in a small forested watershed

Spatial variations in soil processes regulating mineral N losses to streams were studied in a small watershed near Toronto, Ontario. Annual net N mineralization in the 0–8 cm soil was measured in adjacent upland and riparian forest stands using in situ soil incubations from April 1985 to 1987. Mean annual rates of soil N mineralization and nitrification were higher in a maple soil (93.8 and 87.0 kg.ha^–1) than in a pine soil (23.3 and 8.2 kg.ha^–1 ). Very low mean rates of mineralization (3.3 kg.ha^–1) and nitrification (3.4 kg.ha^–1) were found in a riparian hemlock stand. Average NO₃-N concentrations in soil solutions were 0.3–1.0 mg.L^–1 in the maple stand and >0.06mg.L^–1 in the pine stand. Concentrations of NO₃–N in shallow ground water and stream water were 3–4× greater in a maple subwatershed than in a pine subwatershed. Rapid N uptake by vegetation was an important mechanism reducing solution losses of NO₃–N in the maple stand. Low rates of nitrification were mainly responsible for negligible NO₃–N solution losses in the pine stand.     

Using porewater profiles to assess nutrient availability in seagrass-vegetated carbonate sediments

We measured porewater profiles of inorganic (NH₄ ⁺, NO₃ ^–(+NO₂ ^–), PO₄ ^3– (hereafter referred to as DIP)) and organic (DON, DOP) nutrients in seagrass-vegetated sediments at two sites in a shallow bay in Bermuda within close proximity (200 m) but subject to different nutrient loading. At both sites, total dissolved and inorganic nutrient concentrations were usually 1–2 orders of magnitude higher in the sediments than in the water column, with the exception of NO₃ ^–. Organic N and P were significant components of the total dissolved nutrient pools both in the sediment porewater and in the overlying water column (up to 75% for DON and 40% for DOP), and may be important in meeting plant nutrient demands. We used two approaches to examine how well porewater nutrient concentrations reflected the relative availabilities of N and P for seagrasses: (1) a simple stoichiometric nutrient regeneration model based on the N:P ratio of decomposing organic matter and porewater NH₄ ⁺ concentrations to predict porewater DIP, and (2) fitting of the porewater profiles to estimate rates of net nutrient production (or consumption), which reflects the balance between nutrient sources and sinks in the rhizosphere. The stoichiometric model indicated that sediment porewaters were depleted in P relative to N in the low-nutrient outer bay site, and enriched in P relative to N in the higher-nutrient inner bay site. These results are consistent with the mechanism of carbonate sediments in oligotrophic tropical environments being a strong sink for dissolved inorganic P and our previous work suggesting that nutrient enrichment causes P to become disproportionately more available than N. Net nutrient production rates of porewater P at both sites and N at the inner bay site were low (typically < 2%) relative to the nutrient demands of the seagrasses. The implications of the profile interpretation are two-fold: (1) the low rates of net nutrient production indicate diffusive losses from the root zone were insignificant and that nutrient turnover rates were high, except in the P-limited outer bay where N accumulated in sediment porewaters; and (2) because standing stock nutrient concentrations often represent a small fraction of the total nutrients cycled in the sediments, they are in many cases a poor indicator of nutrient availability. Based on our estimates of losses from the root zone, decomposition, and plant uptake we have constructed a rough budget for the cycling of P and N at our two sites.     

Nitrogen dynamics in two antarctic streams

The many glacier meltwater streams of southern Victoria Land flow through catchments where life forms are almost entirely microbial. Allochthonous inputs of nitrogen from two study streams near McMurdo Sound were derived mostly from the melting glaciers (ca. 100–200 mg N m^–3) with some originating from N₂-fixation by heterocystous cyanobacteria (max. 939 mg N m^–2 year^–1). Thirty to fifty per cent of the glacier derived N was dissolved organic N and a major proportion of this was identified as urea N which was utilised by the rich algal and cyanobacterial mats in the streams. A nutrient budget for Fryxell Stream was estimated, quantifying uptake of urea-N and dissolved inorganic N and the release of dissolved organic (non urea) and particulate N by the stream communities. An index of in-stream nitrogen processing, the Net Uptake Length Constant in these streams was compared with that from temperate climates and was found to be similar. Despite the influence of low temperatures on microbial activity (mean daily water temperature = 5 °C) nutrient removal rates from these antarctic streams are high because of the large standing stock of microbial biomass there.     

Nutrient retention function of a stream wetland complex—A high-frequency monitoring approach

The ability of riverine ecosystems to retain nutrients depends on different hydrological, chemical and biological conditions including exchange processes between streams and wetlands. We investigated nutrient retention in a stream wetland complex on the time scale of daily hydrological exchange between both systems. Daily mass balances of NO₃-N, NH₄-N, TP and SRP were calculated with data obtained by two automated measurement stations in a stream reach upstream and downstream of a wetland. The pattern of hydrological exchange between stream and wetland was used to classify characteristic hydrological periods like floods, base and low flows. The nutrient retention function of the stream wetland complex varied considerably during phases of similar hydrologic conditions. Despite re-wetting measures in the wetland, an overall net export of all nutrients except for NH₄-N characterised the whole growing season. Nitrate retention occurred during summer flood (retention in the wetland, 23 kg NO₃-N d^?1, 17% of the input load) and low flow (retention in the stream, 1 kg NO₃-N d^?1, 2% of the input load). TP retention during summer could be assigned to sedimentation (0.7 kg TP d^?1, 7% during flooding in the wetland, 0.2 kg TP d^?1, 4% during low flow in the stream). SRP retention was only intermittent. We concluded that the nutrient retention of streams and wetlands can only be optimised by restoration measures that regard both systems as one functional unit in terms of nutrient retention.     

Leaf litter identity alters the timing of lotic nutrient dynamics

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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.     

Assessment of the effectiveness of best management practices for streams draining agricultural landscapes using diatoms and macroinvertebrates

In this study, a bioassessment was conducted to determine the effectiveness of best management practices (BMPs) implemented in farms in the Upper Delaware River watershed, NY (USA). Diatom and macroinvertebrate communities were analyzed across 17 low-order streams, designated as reference, BMP, or non-BMP. Streams lacking improvements (non-BMP) had significantly greater specific conductance, pH, TDP, NH₄ ⁺-N, and NO₃ ⁻-N than did reference streams. Diatom model affinity (DMA) values were significantly greater in reference and BMP streams than in non-BMP streams; non-BMP streams bordered on a “severely impacted” rating. The Trophic Diatom Index (TDI) varied two-fold among stream classes, with non-BMP > BMP > reference. TDI and DMA values were highly correlated, and both varied significantly with conductance, TDP, NH₄ ⁺-N, and NO₃ ⁻-N. Macroinvertebrate taxa, EPT richness, and Simpson’s diversity did not differ significantly among stream classes. Macroinvertebrate metrics (HBI, Bioassessment Profile, Percent Model Affinity) varied by stream class, but none indicated greater water quality in BMP sites. Nonetheless, each correlated significantly with conductance and TDP in the directions predicted by each model. Our data suggest that diatoms are more sensitive to moderate increases in nutrients, conductivity, and pH in high-gradient agricultural streams, and may be more useful in assessing stream management practices.     

Seasonal Photochemical Transformations of Nitrogen Species in a Forest Stream and Lake

The photochemical release of inorganic nitrogen from dissolved organic matter is an important source of bio-available nitrogen (N) in N-limited aquatic ecosystems. We conducted photochemical experiments and used mathematical models based on pseudo-first-order reaction kinetics to quantify the photochemical transformations of individual N species and their seasonal effects on N cycling in a mountain forest stream and lake (Plešné Lake, Czech Republic). Results from laboratory experiments on photochemical changes in N speciation were compared to measured lake N budgets. Concentrations of organic nitrogen (N_org; 40–58 µmol L⁻¹) decreased from 3 to 26% during 48-hour laboratory irradiation (an equivalent of 4–5 days of natural solar insolation) due to photochemical mineralization to ammonium (NH₄ ⁺) and other N forms (N_x; possibly N oxides and N₂). In addition to N_org mineralization, N_x also originated from photochemical nitrate (NO₃ ⁻) reduction. Laboratory exposure of a first-order forest stream water samples showed a high amount of seasonality, with the maximum rates of N_org mineralization and NH₄ ⁺ production in winter and spring, and the maximum NO₃ ⁻ reduction occurring in summer. These photochemical changes could have an ecologically significant effect on NH₄ ⁺ concentrations in streams (doubling their terrestrial fluxes from soils) and on concentrations of dissolved N_org in the lake. In contrast, photochemical reactions reduced NO₃ ⁻ fluxes by a negligible (<1%) amount and had a negligible effect on the aquatic cycle of this N form.     

Combination Effect of NaCl Salinity and Nitrogen Form on Mineral Composition of Sunflower Plants

The effect of two N-forms (NH₄ ⁺ and NO₃ ^–) and NaCl on pattern of accumulation of some essential inorganic nutrients was examined in sunflower (Helianthus annuus L.) cv. Hisun 33. Eight-day-old plants of were subjected for 21 d to Hoagland's nutrient solution containing 8 mM N as NH₄ ⁺ or NO₃ ^·, and salinized with and addition of NaCl to the growth medium had no significant effect on total leaf N. However, root N of NH₄-supplied plants decreased significantly with increase in NaCl concentration, whereas that of NO₃-supplied plants remained unaffected. There was no significant effect of NaCl on leaf or root P, but the NO₃-supplied plants had significa concentration of leaf P than that of NH₄-supplied plants at varying salt treatments. Salinity of the rooting med did not show any significant effect on Na⁺ concentrations of leaves or roots of plants subjected to two differen N. NH₄-treated plants generally had greater concentrations of Cl^– in leaves and roots and lower K⁺ content in leaves than NO₃-supplied plants. Ca²⁺ concentrations of leaves and roots and Mg²⁺ concentrations of leaves decreased in NH₄-supplied plants due to NaCl, but they remained unaffected in NO₃-treated plants.     

Clinoptilolite zeolite and cellulose amendments to reduce ammonia volatilization in a calcareous sandy soil

Leaching of nitrate (NO₃ ^–) below the root zone and gaseous losses of nitrogen (N) such as ammonia (NH₃) volatilization, are major mechanisms of N loss from agricultural soils. New techniques to minimize such losses are needed to maximize N uptake efficiency and minimize production costs and the risk of potential N contamination of ground and surface waters. The effects of cellulose (C), clinoptilolite zeolite (CZ), or a combination of both (C+CZ) on NH₃ volatilization and N transformation in a calcareous Riviera fine sand (loamy, siliceous, hyperthermic, Arenic Glossaqualf) from a citrus grove were investigated in a laboratory incubation study. Ammonia volatilization from NH₄NO₃ (AN), (NH₄)₂SO₄(AS), and urea (U) applied at 200 mg N kg^–1 soil decreased by 2.5-, 2.1- and 0.9-fold, respectively, with cellulose application at 15 g kg^–1 and by 4.4-, 2.9- and 3.0-fold, respectively, with CZ application at 15 g kg^–1 as compared with that from the respective sources without the amendments. Application of cellulose plus CZ (each at 15 g kg^–1) was the most effective in decreasing NH₃ volatilization. Application of cellulose increased the microbial biomass, which was responsible for immobilization of N, and thus decreased volatilization loss of NH₃–N. The effect of CZ, on the other hand, may be due to increased retention of NH₄ in the ion-exchange sites. The positive effect of interaction between cellulose and CZ amendment on microbial biomass was probably due to improved nutrient retention and availability to microorganisms in the soil. Thus, the amendments provide favorable conditions for microbial growth. These results indicate that soil amendment of CZ or CZ plus organic materials such as cellulose has great potential in reducing fertilizer N loss in sandy soils.