Growth responses of the perennial legume Sesbania sesban to NH4 and NO3 nutrition and effects on root nodulation

Preference for NH₄⁺ or NO₃⁻ nutrition by the perennial legume Sesbania sesban (L.) Merr. was assessed by supplying plants with NH₄⁺ and NO₃⁻ alone or mixed at equal concentrations (0.5 mM) in hydroponic culture. In addition, growth responses of S. sesban to NH₄⁺ and NO₃⁻ nutrition and the effects on root nodulation and nutrient and mineral composition of the plant tissues were evaluated in a hydroponic setup at a range of external concentration of NH₄⁺ and NO₃⁻ (0, 0.1, 0.2, 0.5, 2 and 5 mM). Seedlings of S. sesban grew equally well when supplied with either NH₄⁺ or NO₃⁻ alone or mixed and had high relative growth rates (RGRs) ranging between 0.19 and 0.21 d⁻¹. When larger plants of S. sesban were supplied with NH₄⁺ or NO₃⁻ alone, the RGRs and shoot elongation rates were not affected by the external concentration of inorganic N. At external N concentrations up to 0.5 mM nodulation occurred and contributed to the N nutrition through fixation of gaseous N₂ from the atmosphere. For both NH₄⁺ and NO₃⁻-fed plants the N concentration in the plant tissues, particularly water-extractable NO₃⁻, increased at high supply concentrations, and concentrations of mineral cations generally decreased. It is concluded that S. sesban can grow without an external inorganic N supply by fixing atmospheric N₂ gas via root nodules. Also, S. sesban grows well on both NH₄⁺ and NO₃⁻ as the external N source and the plant can tolerate relatively high concentrations of NH₄⁺. This wide ecological amplitude concerning N nutrition makes S. sesban very useful as a N₂-fixing fallow crop in N deficient areas and also a candidate species for use in constructed wetland systems for the treatment of NH₄⁺ rich waters.     

Dissolved oxygen and nutrient budgets in a phytotreatment pond colonised by <Emphasis Type="Italic">Ulva</Emphasis> spp.

Net daily budgets of dissolved oxygen (O₂), dissolved inorganic carbon (DIC), dissolved inorganic nitrogen (DIN = NH₄⁺+NO₂⁻+NO₃⁻) and soluble reactive phosphorus (SRP) were determined in a pond colonised by Ulva spp. This pond received wastewater from a land-based fish farm and was used as a phytotreatment plant. Three consecutive 24-h cycles of measurements were performed with 8–14 samplings per day. Water samples were collected at the inlet and outlet of the pond and budgets were estimated from differences between inlet and outlet loadings. The first cycle was started when Ulva biomass was 8 kg m⁻², as wet weight. The second cycle was performed after the harvest of ~20% of the macroalgal biomass and the third after the harvest of another ~20% of the remaining biomass. Ulva removal was very fast (<1 h) and samplings for cycles 2 and 3 were started two hours after harvesting, so that the whole experiment lasted ~80 h. When Ulva biomass was at its maximum, the aquatic system was heterotrophic with an O₂ demand of 519 mol d⁻¹ and a net regeneration of DIC (2686 mol d⁻¹), NH₄⁺ (49 mol d⁻¹) and SRP (2.5 mol d⁻¹). The DIC to O₂ ratio was an indicator of persistent anaerobic metabolism. Following the first harvest intervention, this system displayed a prompt response and shifted toward a lower O₂ demand (from −519 to −13 mol d⁻¹), with a lesser regeneration degree of NH₄⁺ (11.4 mol d⁻¹) and DIC (1066 mol d⁻¹). After the second Ulva removal the net budget of SRP became negative (−1.0 mol d⁻¹). By integrating these results over the three days cycle we estimated that in order to operate an efficient nutrient control and maintain macroalgal mats in a healthy status the optimal Ulva biomass should be well below ~4 kg m⁻² as wet weight. Above this threshold, self-limitation would render most of the algal mat unable to exploit light and nutrients. An efficient removal of nitrogen and phosphorus could be attained through the management of macroalgal biomass only with an optimisation of recipient surface to nutrient loading ratio.     

Influence of bed media characteristics on ammonia and nitrate removal in shallow horizontal subsurface flow constructed wetlands

Two bed media were tested (gravel and Filtralite) in shallow horizontal subsurface flow (HSSF) constructed wetlands in order to evaluate the removal of ammonia and nitrate for different types of wastewater (acetate-based and domestic wastewater) and different COD/N ratios. The use of Filtralite allowed both higher mass removal rates (1.1 g NH₄–N m⁻² d⁻¹ and 3 g NO₃–N m⁻² d⁻¹) and removal efficiencies (>62% for ammonia, 90–100% for nitrate), in less than 2 weeks, when compared to the ones observed with gravel. The COD/N ratio seems to have no significant influence on nitrate removal and the removal of both ammonia and nitrate seems to have involved not only the conventional pathways of nitrification–denitrification. The nitrogen loading rate of both ammonia (0.8–2.4 g NH₄–N m⁻² d⁻¹) and nitrate (0.6–3.2 g NO₃–N m⁻² d⁻¹) seem to have influenced the respective removal rates.     

Nitrogen removal from secondary effluent by using integrated constructed wetland system

The treatment capacity of an integrated constructed wetland system (CWS) that was designed to reduce nitrogen (N) from secondary effluent was explored. The integrated CWS consisted of vertical-flow constructed wetland, floating bed and sand filter. The vertical-flow wetland was filled with gravel, steel slag and peat from the bottom to the top. Vetiver zizanioides was selected to grow in the vertical-flow constructed wetland and Coix lacrymajobi L. was grown in the floating bed. The results showed that the integrated CWS displayed superior removal efficiency for nitrate nitrogen (NO₃⁻-N), ammonia nitrogen (NH₄⁺-N), nitrite nitrogen (NO₂⁻-N), and total nitrogen (TN). The average NO₃⁻-N, NO₂⁻-N, NH₄⁺-N and TN removal efficiencies of the integrated CWS were 98.83%, 95.60%, 98.05% and 92.41%, respectively, during the whole experimental operation. The integrated CWS may have a good potential for removing N from secondary effluent.     

Nitrate removal and DO levels in batch wetland mesocosms: Cattail (Typha spp.) versus bulrush (Scirpus spp.)

Nitrate removal rates and dissolved oxygen (DO) levels were evaluated in small batch-mode wetland mesocosms with two different plant species, cattail (Typha spp.) and bulrush (Scirpus spp.), and associated mineral-dominated sediment collected from a mature treatment wetland. Nitrate loss in both cattail and bulrush mesocosms was first-order in nature. First-order volumetric rate constants (k_V) were 0.30 d⁻¹ for cattail and 0.21 d⁻¹ for bulrush and rates of nitrate loss were significantly different between plant treatments (p < 0.005). On an areal basis, maximum rates of nitrate removal were around 500 mg N/(m² d) early in the experiment when nitrate levels were high (> 15 mg N/L). Areal removal rates were on average 25% higher in cattail versus bulrush mesocosms. DO in mesocosm water was significantly higher in bulrush versus cattail (p < 0.001). DO in bulrush generally ranged between 0.5 and 2 mg/L, while DO in cattail mesocosms was consistently below 0.3 mg/L. Based on cumulative frequency analysis, DO exceeded 1 mg/L around 50% of the time in bulrush, but only 2% of the time in cattail. DO in bulrush exhibited a statistically significant diel cycle with DO peaks in the late afternoon and DO minimums in the early morning hours. Difference in nitrate removal rates between wetland plant treatments may have been due to differing plant carbon quality. Cattail litter, which has been shown in other studies to exhibit superior biodegradability, may have enhanced biological denitrification by fueling heterotrophic microbial activity, which in turn may have depressed DO levels, a prerequisite for denitrification. Our results show that the cattail is more effective than bulrush for treating nitrate-dominant wastewaters.     

Removal of municipal solid waste COD and NH4–N by phyto-reduction: A laboratory–scale comparison of terrestrial and aquatic species at different organic loads

Laboratory scale tests on phytodepuration of raw and pre-treated leachate from municipal sanitary waste were carried out with four vegetable aquatic and terrestrial species at different organic loads. We used the terrestrial species Stenotaphrum secundatum and the free-floating aquatic species Lemna minor, Eichhornia crassipes and Myriophyllum verticellatum to purify leachate from municipal solid waste. The organic load characterized by COD varied from 2–30 g m⁻² day⁻¹. Blanks using tap water served as controls. Duration of the experiments varied from 9–90 days. Maximum concentrations in the experiments were 1600 mg l⁻¹ COD and 300 mg l⁻¹ NH₄–N for S. secundatum. Best results in terms of COD, BOD, and ammonia removal were obtained for raw leachate with COD=2 g m⁻² day⁻¹ in free water surface (FWS) wetlands, and with 2 and 5 g m⁻² day⁻¹ in subsurface flow (SSF) wetlands. Results show that for pretreated leachate (labeled c) low in BOD and NH₄–N, the aquatic species showed low removal and stress even at the lowest load of COD=2 g m⁻² day⁻¹. We cannot say if this is due to the pretreatment itself or the chemical or microbial composition of this leachate. The Stenotaphrum system operated well with this load of leachate c. For untreated leachate (type a and b) the removal and plant growing conditions seemed good at COD=2 g m⁻² day⁻¹. For S. secundatum a load of COD=5 g m⁻² day⁻¹ operated well. All loads above COD=5 g m⁻² day⁻¹ caused low removal and stress, and the green parts of the plants disappeared. Oxygen was, however, consumed throughout the experimental period. For pretreated leachate (type c), the removal of COD were low (−24 to 17%) but good for NH₄–N (52–91%). This leachate also experienced high ammonia removal from the beginning of the experiments, probably due to existing consortia of nitrifying bacteria in it. Statistical analysis shows that the S. secundatum and L. minor systems maintained higher oxygen levels than the M. verticellatum and E. crassipes systems, when operated with tap water. For Lemna minor, this may be due to a better capacity for transporting oxygen into the water. With leachate all S. secundatum systems have higher oxygen levels than the aquatic systems, basically because the water content of the soil has been kept well below saturation. S. secundatum shows a significantly lower removal of COD than did the aquatic systems at a loading of COD=2 g m⁻² day⁻¹ of raw leachate. There is no significant difference between the systems in the removal of NH₄–N at a loading of COD=2 g m⁻² day⁻¹ of both types of leachate. E. crassipes has a lower removal of NH₄–N than M. verticellatum and S. secundatum at a loading of 5 g m⁻² day⁻¹ of COD of both types of leachate. In our experiments, it appears that the amount of free ammonia explains the toxicity of the leachate to the plants. This, however, does not exclude other possible toxic factors.     

Effect of water level fluctuation on nitrogen removal from constructed wetland mesocosms

Nitrogen removal processes were investigated at three frequencies of water level fluctuation, static, low and high (0, 2 and 6 d⁻¹), in duplicate gravel-bed constructed wetland mesocosms (0.145 m³) with and without plants (Schoenoplectus tabernaemontani). Fluctuation was achieved by temporarily pumping wastewater into a separate tank (total drain time ∼35 min). Intensive sampling of the mesocosms, batch-fed weekly with ammonium-rich (∼100 g m⁻³ NH₄-N) farm dairy wastewaters, showed rates of chemical oxygen demand (COD) and total Kjeldahl nitrogen (TKN) removal increased markedly with fluctuation frequency and in the presence of plants. Nearly complete removal of NH₄-N was recorded over the 7 day batch period at the highest level of fluctuation, with minimal enhancement by plants. Redox potentials (Eh) at 100 mm depth rose from initial levels of around −100 to >350 mV and oxidised forms of N (NO₂ and NO₃) increased to ∼40 g m⁻³, suggesting conditions were conducive to microbial nitrification at this level of fluctuation. In the unplanted mesocosms with low or zero fluctuation, mean NH₄-N removals were only 28 and 10%, respectively, and redox potentials in the media remained low for a substantial part of the batch periods (mid-batch Eh ∼+100 and −100 mV, respectively). In the presence of wetland plants, mean NH₄-N removal in the mesocosms with low or zero fluctuation rose to 71 and 54%, respectively, and COD removal (>70%) and redox potential (mid-batch Eh>200 mV) were markedly higher than in the unplanted mesocosms. Negligible increases in oxidised N were recorded at these fluctuation frequencies, but total nitrogen levels declined at mean rates of 2.4 and 1.8 g m⁻² d⁻¹, respectively. NH₄-N removal from the bulk water in the mesocosms was well described (R²=0.97–0.99) by a sorption-plant uptake-microbial model. First-order volumetric removal rate constants (k_v) rose with increasing fluctuation frequency from 0.026 to 0.46 d⁻¹ without plants and from 0.042 to 0.62 d⁻¹ with plants. As fluctuation frequency increased, reversible sorption of NH₄-N to the media, and associated biofilms and organic matter, became an increasingly important moderator of bulk water concentrations during the batch periods. TN mass balances for the full batch periods suggested that measured plant uptake estimates of between 0.52 and 1.07 g N m⁻² d⁻¹ (inversely related to fluctuation frequency) could fully account for the increased overall removal of TN recorded in the planted systems. By difference, microbial nitrification-denitrification losses were therefore estimated to be approximately doubled by low-level fluctuation from 0.7 to 1.4 g N m⁻² d⁻¹ (both with and without plants), rising to a maximum rate of 2.1 g N m⁻² d⁻¹ at high fluctuation, in the absence of competitive uptake by plants.     

The role of plants in the removal of nutrients at a constructed wetland treating agricultural (dairy) wastewater,Ontario, Canada

The South Nation River Watershed, in eastern Ontario, Canada, is an agricultural watershed impacted by excess nutrient loading primarily from agricultural activities. A constructed wetland for the treatment of agricultural wastewater from a 150-cow dairy operation in this watershed was monitored in its eighth operating season to evaluate the proportion of total nitrogen (TN) (approximated by total Kjeldahl nitrogen (TKN) due to low NO₃⁻) and total phosphorus (TP) removal that could be attributed to storage in Typha latifolia L. and Typha angustifolia L., which dominate this system. Nutrient loading rates were high, with 16.2 kg ha⁻¹ d⁻¹ N and 3.4 kg ha⁻¹ d⁻¹ P entering the wetland and loading the first wetland cell. Plant uptake accounted for 0.7% of TKN removal when the vegetated free water surface cells were considered together. However, separately, in the second wetland cell with lower N and P loading rates, plants accounted for 9% of TKN, 21% of NH₄⁺ and 5% of TP removal. Plant uptake was significant to overall removal given wetland age and nutrient loading. Nutrient storage during the growing season at this constructed wetland helped reduce the nutrient load entering the watershed, already stressed by intensive local agriculture.     

Drought‐induced saltwater incursion leads to increased wetland nitrogen export

Coastal wetlands have the capacity to retain and denitrify large quantities of reactive nitrogen (N), making them important in attenuating increased anthropogenic N flux to coastal ecosystems. The ability of coastal wetlands to retain and transform N is being reduced by wetland losses resulting from land development. Nitrogen retention in coastal wetlands is further threatened by the increasing frequency and spatial extent of saltwater inundation in historically freshwater ecosystems, due to the combined effects of dredging, declining river discharge to coastal areas due to human water use, increased drought frequency, and accelerating sea‐level rise. Because saltwater incursion may affect N cycling through multiple mechanisms, the impacts of salinization on coastal freshwater wetland N retention and transformation are not well understood. Here, we show that repeated annual saltwater incursion during late summer droughts in the coastal plain of North Carolina changed N export from organic to inorganic forms and led to a doubling of annual NH₄⁺ export from a 440 hectare former agricultural field undergoing wetland restoration. Soil solution NH₄⁺ concentrations in two mature wetlands also increased with salinization, but the magnitude of increase was smaller than that in the former agricultural field. Long‐term saltwater exposure experiments with intact soil columns demonstrated that much of the increase in reactive N released could be explained by exchange of salt cations with sediment NH₄⁺. Using these findings together with the predicted flooding of 1661 km² of wetlands along the NC coast by 2100, we estimate that saltwater incursion into these coastal areas could release up to 18 077 Mg N, or approximately half the annual NH₄⁺ flux of the Mississippi River. Our results suggest that saltwater incursion into coastal freshwater wetlands globally could lead to increased N loading to sensitive coastal waters.     

Impact of substrates acclimation strategy on simultaneous biodegradation of hydrogen sulfide and ammonia

Three columns were differentiated with feeding mixture of H₂S and NH₃ (MFC), feeding NH₃ followed by H₂S (NFC), and feeding H₂S followed by NH₃ (SFC). Removal performance, biodegradation capacity and microbial community structures in the three columns were compared. The results show that NFC has a shorter acclimation period for the removal of NH₃ gas and nitrification than MFC. Under the high loading of H₂S and NH₃ at 164 and 82 g m⁻³ h⁻¹, respectively, NFC exhibited high removal efficiency of NH₃ (>95%) while the removal efficiencies were obtained at 63 and 75% in MFC and SFC, respectively. The removal of NH₃ gas in NFC was significantly attributed to nitrification (over 50%), while adsorption and chemical reaction contributed to the removal of NH₃ in MFC and SFC. The different biodegradation capacities of NH₃ could be due to the dissimilarity in the microbial population presented in each column.     

Biodegradation of 1,2-dichloroethane (1,2-DCA) by cometabolism in a nitrifying biofilm reactor

Biodegradation of 1,2-dichloroethane (1,2-DCA) by cometabolism was investigated in a continuous-flow nitrifying biofilm reactor over a time period of 218 days. The removal efficiency of 1,2-DCA ranged between 70 and 90%. Using the generation of chloride (Cl⁻) as an indicator of 1,2-DCA mineralization, it was shown that the cometabolic degradation of 1,2-DCA was initiated through oxidative dechlorination. However, Cl⁻ production rates were observed to be lower than the stoichiometric ones which indicated the partial mineralization of 1,2-DCA and the possibility of by-product formation due to incomplete dechlorination. At high 1,2-DCA removal rates, Cl⁻ release seemed to reach a saturation due to 1,2-DCA-dependent inactivation of NH₄–N oxidation. The cometabolic 1,2-DCA degradation capacity of nitrifiers was quite comparable to metabolic 1,2-DCA degradation capacities of pure cultures. A strong linear relationship was found between 1,2-DCA transformation yields and NH₄–N and 1,2-DCA loadings. The effect of 1,2-DCA loading on nitrifier population was monitored using molecular microbiological tools. Long-term input of 1,2-DCA to the biofilm reactor resulted in no significant changes in the quantities of Nitrosomonas, Nitrobacter and Nitrospira species and no shift in the diversities of ammonia oxidizing species. Those findings provide an insight into both the operation and the community structure in natural and managed nitrifying biofilm systems where cometabolic 1,2-DCA takes place.     

Les Stephanodiscus (Diatomées) du Lac Léman: Description d'une nouvelle espèce,Stephanodiscus irregularis; mise au point taxonomique et distributions saisonnières

Intact sediment-water columns from a flowing cypress swamp receiving primary wastewater effluent were used to evaluate inorganic N removal and to determine the fate of ¹⁵NH _inf4 ^sup+ -N added to the floodwater. Treatments represented wetland sites which had received 0 (initial application), 2, and 50 years of primary wastewater application. The rate of inorganic-N decrease in the floodwater was greatest for the initial application columns, primarily due to sediment adsorption of NH _inf4 ^sup+ , followed by 2-year and 50-yr-columns. Maximum removal rates were 318, 296, and 148 mg N m^–2 day^–1, respectively. At the end of the 21-day study period, only 0.5 to 2.3% of applied ¹⁵N was recovered in the floodwater and 11.4 to 17.3% was recovered in the sediment, with the remaining 82.2 to 86.3% being lost from the sediment-water system. Results of the study indicated that N removal efficiency did not decrease with prolonged wastewater application, despite reduced sediment adsorption capacity, because of the significance of gaseous N losses (nitrification-denitrification, NH₃ volatilization) as an N sink in the sediment-water system.Florida Agricultural Experiment Stations Journal Series No. 7712     

Nitrogen utilization by the raphidophyte Heterosigma akashiwo: Growth and uptake kinetics in laboratory cultures

The nitrogen uptake and growth capabilities of the potentially harmful, raphidophycean flagellate Heterosigma akashiwo (Hada) Sournia were examined in unialgal batch cultures (strain CCMP 1912). Growth rates as a function of three nitrogen substrates (ammonium, nitrate and urea) were determined at saturating and sub-saturating photosynthetic photon flux densities (PPFDs). At saturating PPFD (110 μE m⁻² s⁻¹), the growth rate of H. akashiwo was slightly greater for cells grown on NH₄⁺ (0.89 d⁻¹) compared to cells grown on NO₃⁻ or urea, which had identical growth rates (0.82 d⁻¹). At sub-saturating PPFD (40 μE m⁻² s⁻¹), both urea- and NH₄⁺-grown cells grew faster than NO₃⁻-grown cells (0.61, 0.57 and 0.46 d⁻¹, respectively). The N uptake kinetic parameters were investigated using exponentially growing batch cultures of H. akashiwo and the ¹⁵N-tracer technique. Maximum specific uptake rates (V_max) for unialgal cultures grown at 15 °C and saturating PPFD (110 μE m⁻² s⁻¹) were 28.0, 18.0 and 2.89 × 10⁻³ h⁻¹ for NH₄⁺, NO₃⁻ and urea, respectively. The traditional measure of nutrient affinity—the half saturation constants (K_s) were similar for NH₄⁺ and NO₃⁻ (1.44 and 1.47 μg-at N L⁻¹), but substantially lower for urea (0.42 μg-at N L⁻¹). Whereas the α parameter (α = V_max/K_s), which is considered a more robust indicator for substrate affinity when substrate concentrations are low (<K_s), were 19.4, 12.2 and 6.88 × 10⁻³ h⁻¹/(μg-at N L⁻¹) for NH₄⁺, NO₃⁻ and urea, respectively. These laboratory results demonstrate that at both saturating and sub-saturating N concentrations, N uptake preference follows the order: NH₄⁺ > NO₃⁻ > urea, and suggests that natural blooms of H. akashiwo may be initiated or maintained by any of the three nitrogen substrates examined.     

Nutrient and faecal contamination and retention in wetland enclosures (gei wais) in the Mai Po Marshes,Hong Kong

Nutrient and faecal contamination is an increasing problem to the shrimp productivity and wildlife conservation at the internationally important wetland ecosystems of the Mai Po Marshes (Hong Kong, P.R. China). The present study examined the nutrient status and faecal bacteria loading and potential retention capacity of contaminants of two wetland enclosures. Water in the wetland enclosures was eutrophicated with high concentrations of dissolved inorganic nitrogen (inorg–N_diss= 15.0 mg l^–1) and orthophosphate phosphorus (o-P = 1.89 mg l^–1) and was loaded with high levels of faecal coliforms (172 ×10³ cfu in 100 ml) and faecal streptococci (1.94 ×10³ cfu in 100 ml). The pattern of nutrient enrichment of two wetland enclosures is related to a north-to-south pollution gradient from the Shenzhen River to the wetlands. By retaining tidal water in the wetland for an 8-day period, water quality was greatly improved; NH₄–N was removed by 83%, o-P by 45% and faecal bacteria by 100%. This implies a self-purification capability of the wetland enclosures and a potentiality of using them as an alternative sewage treatment.     

Removal of nutrients from wastewater with Canna indica L. under different vertical-flow constructed wetland conditions

Constructed wetlands are becoming increasingly popular worldwide for removing contaminants from domestic wastewater. This study investigated the removal efficiency of nitrogen (N) and phosphorus (P) from wastewater with the simulated vertical-flow constructed wetlands (VFCWs) under three different substrates (i.e., BFAS or blast furnace artificial slag, CBAS or coal burn artificial slag, and MSAS or midsized sand artificial slag), hydraulic loading rates (i.e., 7, 14, and 21 cm d^?1), and wetland operational periods (0.5, 1, and 2 years) as well as with and without planting Canna indica L. The wastewater was collected from the campus of South China Agricultural University, Guangzhou, China. Results show that the percent removal of total P (TP) and ammonium N (NH₄⁺-N) by the substrates was BFAS > CBAS > MSAS due to the high contents of Ca and Al in substrate BFAS. In contrast, the percent removal of total N (TN) by the substrates was CBAS > MSAS > BFAS due to the complicated nitrification/denitrification processes. The percent removal of nutrients by all of the substrates was TP > NH₄⁺-N > TN. About 10% more TN was removed from the wastewater after planting Canna indica L. A lower hydraulic loading rate or longer hydraulic retention time (HRT) resulted in a higher removal of TP, NH₄⁺-N, and TN because of more contacts and interactions among nutrients, substrates, and roots under the longer HRT. Removal of NO₃^?N from the simulated VFCWs is a complex process. A high concentration of NO₃^?N in the effluent was observed under the high hydraulic loading rate because more NH₄⁺-N and oxygen were available for nitrification and a shorter HRT was unfavorable for denitrification. In general, a longer operational period had a highest removal rate for nutrients in the VFCWs.     

Influence of nitrogen species (NH4 + and NO3 −) on the dynamics of P in water–sediment–Salvinia herzogii systems

Water – Salvinia herzogii – sediment systems were exposed to different phosphorus and nitrogen combinations in outdoor experiments. The aim was to estimate the amounts of P immobilized in macrophytes and sediments, as well as to elucidate whether or not the presence of N affects the retention of P. The following components were added: o-P, o-P + NH₄ ⁺, o-P + NO₃ ^– + NH₄ ⁺, o-P + NO₃ ^–. The concentration of nutrients was periodically determined throughout the experiment (28 days). The concentrations of P and N in plant tissues and sediments were determined at the beginning and the end of the experiment. Sequential extractions of P-fractions in sediment were performed using the EDTA method (Golterman, 1996). The removal efficiency of P in water was 95–99%. The removal of NH₄ ⁺ (97–98%) was more effective than that of NO₃ ^– (44–86%). The presence of nitrogen species increased the removal velocity of o-P from water, NH₄ ⁺ was the most effective species. Sediments not only had higher P removal rates than macrophytes but, in the control treatment without macrophytes, they reached the values obtained by macrophytes plus sediments in the other treatments. The adsorption of P takes place at the surface layer of the sediment (1 cm). Most of the P incorporated into the sediment during the experiment was sorbed by the fraction Fe(OOH)P. The addition of nutrients to water modified the leaves/lacinias weight ratio.     

Rapid immobilization of applied nitrogen in saline–alkaline soils

The dynamics of inorganic N are important in soil, and this applies particularly to the saline–alkaline soils of the former lake Texcoco in Mexico with high pH and salinity where a forestation program was started in the 1970s. In soils of lake Texcoco, in Mexico, more than 50% of applied N could not be accounted for one day after application of 200 mg kg^–1 soil along with glucose amendment. It was not clear whether this was due to abiotic or biotic processes, the form of inorganic N applied or the result of applying an easily decomposable substrate. We investigated this by adding glucose and 200 mg kg^–1 soil as (NH₄)₂SO₄-N or KNO₃-N to sterilized and unsterilized soil. The changes in inorganic and ninhydrin N, microbial biomass C and production of CO₂ were then monitored. Between the time of applying N and extraction with 0.5 M K₂SO₄, i.e., after ca 2 h, approximately 110 mg NH₄ ⁺-N kg^–1 dry soil could not be accounted for in the unsterilized and sterilized soil and that remained so for the entire incubation in the sterilized soil. After 1 day this increased to 140 mg NH₄ ⁺-N kg^–1 dry soil in the unsterilized control and 170 mg NH₄ ⁺-N kg^–1 dry soil in C amended soil. Volatilization of NH₃ accounted for 56 mg NH₄ ⁺-N kg^–1 so the rest appeared to be adsorbed on the soil matrix. The NH₃ volatilization and NH₄ ⁺ fixed in the soil matrix remained constant over time and no oxidation to NO₂ ^– or NO₃ ^– had occurred, so unaccounted N in unsterilized soil was probably incorporated into the microbial biomass in excess of what was required for metabolic activity. The unaccounted N was ca 70 mg NO₃ ^––N in nitrate amended soil after 3 days and 138 NO₃ ^––N when glucose was additionally added. Losses through abiotic processes were absent as inferred from changes in sterilized soil and the aerobic incubation inhibited possible losses through denitrification. It was inferred that NO₃ ^– that could not be accounted for was taken up by micro-organisms in excess of what was required for metabolic activity.     

Simulated Nitrogen Deposition Reduces CH4 Uptake and Increases N2O Emission from a Subtropical Plantation Forest Soil in Southern China

To date, few studies are conducted to quantify the effects of reduced ammonium (NH₄ ⁺) and oxidized nitrate (NO₃ ⁻) on soil CH₄ uptake and N₂O emission in the subtropical forests. In this study, NH₄Cl and NaNO₃ fertilizers were applied at three rates: 0, 40 and 120 kg N ha⁻¹ yr⁻¹. Soil CH₄ and N₂O fluxes were determined twice a week using the static chamber technique and gas chromatography. Soil temperature and moisture were simultaneously measured. Soil dissolved N concentration in 0–20 cm depth was measured weekly to examine the regulation to soil CH₄ and N₂O fluxes. Our results showed that one year of N addition did not affect soil temperature, soil moisture, soil total dissolved N (TDN) and NH₄ ⁺-N concentrations, but high levels of applied NH₄Cl and NaNO₃ fertilizers significantly increased soil NO₃ ⁻-N concentration by 124% and 157%, respectively. Nitrogen addition tended to inhibit soil CH₄ uptake, but significantly promoted soil N₂O emission by 403% to 762%. Furthermore, NH₄ ⁺-N fertilizer application had a stronger inhibition to soil CH₄ uptake and a stronger promotion to soil N₂O emission than NO₃ ⁻-N application. Also, both soil CH₄ and N₂O fluxes were driven by soil temperature and moisture, but soil inorganic N availability was a key integrator of soil CH₄ uptake and N₂O emission. These results suggest that the subtropical plantation soil sensitively responses to atmospheric N deposition, and inorganic N rather than organic N is the regulator to soil CH₄ uptake and N₂O emission.     

Macroalgal blooms on southeast Florida coral reefs: I. Nutrient stoichiometry of the invasive green alga Codium isthmocladum in the wider Caribbean indicates nutrient enrichment

Invasive blooms of the siphonaceous green algae Codium spp. have been considered a symptom of coastal eutrophication but, to date, only limited biochemical evidence supports a linkage to land-based nutrient pollution. Beginning in the summer of 1990, spectacular blooms of unattached Codium isthmocladum developed on deep coral reef habitats in southern Palm Beach County and northern Broward County, and in subsequent years, attached populations formed on reefs in northern Palm Beach County.To better understand the nutrition of these HABs, we collected C. isthmocladum and other reef macroalgae from various locations in southeast Florida as well as the wider Caribbean region for tissue C:N:P analysis in order to gauge variability in the type and degree of N- and/or P-limited growth. Widespread nutrient enrichment in floridian C. isthmocladum populations was evidenced by significantly higher tissue P (0.06% versus 0.04% of dry weight) and lower C:N (12 versus 19), C:P (425 versus 980), and N:P (35 versus 50) ratios compared to more nutrient-depleted Caribbean populations. To determine nutrient availability on southeast Florida's reefs, we sampled near-bottom waters at a variety of locations for DIN (NH₄⁺ + NO₃⁻ + NO₂⁻) and SRP analysis. In general, concentrations of NH₄⁺, NO₃⁻and SRP were all high on southeast Florida's reefs compared to values reported for Caribbean coral reefs. Although summertime upwelling provides episodic NO₃⁻ and SRP enrichment to reefs in southeast Florida, these transient nutrient pulses have not historically supported C. isthmocladum blooms.We suggest that the widespread P enrichment of C. isthmocladum tissue and water column DIN:SRP ratios <16:1 in southeast Florida drive this system toward N limitation where low level NH₄⁺ enrichment becomes of paramount importance. Hence, the recent C. isthmocladum blooms appear to be supported by increasing land-based nutrient pollution, particularly, sewage that is enriched in NH₄⁺ and SRP at a low N:P ratio (<10:1) critical to sustaining balanced growth during bloom formation.     

Heterotrophic nitrogen removal by <Emphasis Type="Italic">Providencia rettgeri</Emphasis> strain YL

Providencia rettgeri strain YL was found to be efficient in heterotrophic nitrogen removal under aerobic conditions. Maximum removal of NH₄ ⁺–N occurred under the conditions of pH 7 and supplemented with glucose as the carbon source. Inorganic ions such as Mg²⁺, Mn²⁺, and Zn²⁺ largely influenced the growth and nitrogen removal efficiency. A quantitative detection of nitrogen gas by gas chromatography was conducted to evaluate the nitrogen removal by strain YL. From the nitrogen balance during heterotrophic growth with 180 mg/l of NH₄ ⁺–N, 44.5% of NH₄ ⁺–N was in the form of N₂ and 49.7% was found in biomass, with only a trace amount of either nitrite or nitrate. The utilization of nitrite and nitrate during the ammonium removal process demonstrated that the nitrogen removal pathway by strain YL was heterotrophic nitrification-aerobic denitrification. A further enzyme assay of nitrate reductase and nitrite reductase activity under the aerobic condition confirmed this nitrogen removal pathway.