Soil nitrate accumulation explains the nonlinear responses of soil CO2 and CH4 fluxes to nitrogen addition in a temperate needle-broadleaved mixed forest

The responses of soil-atmosphere carbon (C) exchange fluxes to growing atmospheric nitrogen (N) deposition are controversial, leading to large uncertainty in the estimated C sink of global forest ecosystems experiencing substantial N inputs. However, it is challenging to quantify critical load of N input for the alteration of the soil C fluxes, and what factors controlled the changes in soil CO₂ and CH₄ fluxes under N enrichment. Nine levels of urea addition experiment (0, 10, 20, 40, 60, 80, 100, 120, 140 kg N ha⁻¹ yr⁻¹) were conducted in the needle-broadleaved mixed forest in Changbai Mountain, Northeast China. Soil CO₂ and CH₄ fluxes were monitored weekly using the static chamber and gas chromatograph technique. Environmental variables (soil temperature and moisture in the 0–10 cm depth) and dissolved N (NH₄⁺-N, NO₃⁻-N, total dissolved N (TDN), and dissolved organic N (DON)) in the organic layer and the 0–10 cm mineral soil layer were simultaneously measured. High rates of N addition (≥60 kg N ha⁻¹ yr⁻¹) significantly increased soil NO₃⁻-N contents in the organic layer and the mineral layer by 120%-180% and 56.4%-84.6%, respectively. However, N application did not lead to a significant accumulation of soil NH₄⁺-N contents in the two soil layers except for a few treatments. N addition at a low rate of 10 kg N ha⁻¹ yr⁻¹ significantly stimulated, whereas high rate of N addition (140 kg N ha⁻¹ yr⁻¹) significantly inhibited soil CO₂ emission and CH₄ uptake. Significant negative relationships were observed between changes in soil CO₂ emission and CH₄ uptake and changes in soil NO₃⁻-N and moisture contents under N enrichment. These results suggest that soil nitrification and NO₃⁻-N accumulation could be important regulators of soil CO₂ emission and CH₄ uptake in the temperate needle-broadleaved mixed forest. The nonlinear responses to exogenous N inputs and the critical level of N in terms of soil C fluxes should be considered in the ecological process models and ecosystem management.     

Effect of different macrophytes in abating nitrogen from a synthetic wastewater

An experiment conducted in an unheated glasshouse from October 2006 to March 2008 studied the efficiency of different macrophytes in reducing NO₃-N and NH₄-N concentrations and loads in synthetic wastewaters. The experimental setup consisted of plastic tanks, filled with gravel and vegetated with Carex elata All., Juncus effusus L., Phragmites australis (Cav.) Trin., Typhoides arundinacea L. Moench (syn Phalaris arundinacea L.) var. picta and Typha latifolia L. There was also a control without vegetation. From January to July, a solution of 50–60 ppm of NH₄-N and NO₃-N was applied monthly; then the input concentration was doubled. The total load at the end of the experimental period was 70.4 g/m² of NO₃-N and 67.3 of NH₄-N. At the end of each month, water was discharged from the tanks and analysed to determine the two nitrogen forms. At the end of the experiment, 33 g/m² of total N (almost 24% of applied N) had disappeared in the control. Among species, the highest abatement was detected in T. latifolia (72 g/m², almost 52% of applied N) and the lowest in J. effusus (35%).A weekly chemical analysis in July showed that a large amount of NH₄-N quickly disappeared in all treatments, while NO₃-N only decreased in the vegetated tanks. In December, NH₄-N had similar dynamics, while NO₃-N increased.All water volumes entering and exiting the tanks were measured in order to evaluate evapotranspiration. T. latifolia showed the highest water consumption, reaching a cumulative value of above 1000 mm.At the end of the experiment, J. effusus presented the highest amount of nitrogen stored in the aerial parts (5.63 g/m²) and T. latifolia the lowest (1.92 g/m²).     

The effects of land use change and irrigation water resource on nitrate contamination in shallow groundwater at county scale

The objective of this study was to assess the impacts of land use changes and irrigation water resource on the nitrate contamination in shallow groundwater. 394 water samples were sampled from the same irrigation wells during a period of five years (from 2002 to 2007) in Huantai County in the North China Plain. NO₃-N concentration in irrigation wells was measured. Geostatistical method combined with GIS technique was used to analyze the spatio-temporal distribution of groundwater NO₃-N concentrations in Huantai County. Land use type and irrigation water resource were combined with the variation of NO₃-N concentrations by statistical approach to investigate the relationship between them. The distribution map showed that the percentages of area increased by 13.06%, 14.37%, 12.23% and 3.85% for that had nitrate concentrations of 10–15, 15–20, 20–30 mg L^?1 and greater than 30 mg L^?1 for shallow groundwater, respectively, while decreased by 28.87% and 14.63% for 0–5 and 5–10 mg L^?1. In the well-irrigated field, the NO₃-N concentrations in shallow groundwater had increased for vegetables, wheat–vegetables and wheat–maize rotations. In contrast, fast-growing tree system could act as a buffer to retain shallow groundwater nitrate content which resulted in reduced NO₃-N concentrations. Under the same land use condition, irrigation with sewage, or well and sewage by turns would both enormously add nitrate to groundwater.     

The sensitivity of water chemistry to climate in a forested,nitrogen-saturated catchment recovering from acidification

Fluxes of major ions and nutrients were measured in the N-saturated mountain forest catchment-lake system of Čertovo Lake (Czech Republic) from 1998 to 2014. The lake has been rapidly recovering from atmospheric acidification due to a 90% decrease in sulphate (SO₄²⁻) deposition since the late 1980s and nitrate (NO₃⁻) contribution to the pool of strong acid anion and leaching of dissolved organic carbon (DOC) have increased. Present concentrations of base cations, phosphorus (P), total organic N (TON), and ionic (Al_i) and organically bound (Al_o) aluminium in tributaries are thus predominantly governed by NO₃⁻ and DOC leaching. Despite a continuing recovery lasting 25 years, the Čertovo catchment is still a net source of protons (H⁺), producing 44 mmol m⁻² yr⁻¹ H⁺ on a catchment-area basis (corresponding to 35 μmol L⁻¹ on a concentration basis). Retention of the deposited inorganic N in the catchment averages 20%, and ammonium consumption (51 mmol m⁻² yr⁻¹) and net NO₃⁻ production (28 mol m⁻² yr⁻¹) are together the dominant terrestrial H⁺ generating processes. In contrast, the importance of SO₄²⁻ release from the soils on terrestrial H⁺ production is continuously decreasing, with an average of 47 mmol m⁻² yr⁻¹ during the study. The in-lake biogeochemical processes reduce the incoming acidity by ∼40%, neutralizing 23 μmol L⁻¹ H⁺ (i.e., 225 mmol m⁻² yr⁻¹ on a lake-area basis). Denitrification and photochemical and microbial decomposition of DOC are the most important in-lake H⁺ consuming processes (50 and 39%, respectively), while hydrolysis of Al_i (from tributaries and photochemically liberated from Al_o) is the dominant in-lake H⁺ generating process. Because the trends in water chemistry and H⁺ balance in the catchment-lake system are increasingly related to variability in NO₃⁻ and DOC leaching, they have become sensitive to climate-related factors (drought, elevated runoff) and forest damage that significantly modify the leaching of these anions. During the study period, increased exports of NO₃⁻ (accompanied by Al_i and base cations) from the Čertovo catchment occurred after a dry and hot summer, after forest damage, and during elevated winter runoff. Increasing DOC export due to decreasing acid deposition was further elevated during years with higher runoff (and especially during events with lateral flow), and was accompanied by P, TON, and Al_o leaching. The climate-related processes, which originally “only” confounded chemical trends in waters recovering from acidification, may soon become the dominant variables controlling water composition in N-saturated catchments.     

Effects of wetland construction on water quality in a semi-arid catchment degraded by intensive agricultural use

Many factors can influence the improvement of water quality in surface-flow constructed wetlands (SFW). To test if water quality was improved, especially in nutrient and salt content, after passage through SFW, 11 wetland plots of various sizes (50, 200, 800 and 5000 m²) were established within constructed wetlands on agricultural soils in the Ebro River basin (NE Spain) that had been affected by salinization. A set of 15 water quality parameters (e.g., nutrients, salts, sediments, and alkalinity) was obtained from samples collected at the inflow and outflow of the wetlands during the first 4 years after the wetlands were constructed. NO₃-N retention rates were as high as 99% in the largest (5000 m²) wetlands. After 4 years, total phosphorus was still being released from the wetlands but not salts. Over the same period, in small wetlands (50, 200, and 800 m²), retention rate relative to the input of NO₃-N increased from 40% to almost 60%. Retention of NO₃-N amounted to up to 500 g N m^?2 per year, for an average load concentration at inflow of ～20 mg l^?1. Release of Na⁺ declined from 16% to 0–2% by volume, for an average load concentration at inflow of ～70 mg l^?1. At the current retention rate of NO₃-N (76–227 g m^?2 per year), 1.5–4% of the catchment should be converted into wetlands to optimize the elimination of NO₃-N.     

Nitrogen retention efficiency and nitrogen losses of a managed and phytodiverse temperate grassland

Maintaining nitrogen retention efficiency (NRE) is crucial in minimizing N losses when intensifying management of temperate grasslands. Our aim was to evaluate how grassland management practices and sward compositions affect NRE (1 − N losses/soil available N), defined as the efficiency with which soil available N is retained in an ecosystem. A three-factorial grassland management experiment was established with two fertilization treatments (without and combined N, phosphorus and potassium fertilization), two mowing frequencies (cut once and thrice per year) and three sward compositions (control, monocot- and dicot-enhanced swards). We measured N losses as leaching and nitrous oxide emissions, and soil available N as gross N mineralization rates. Fertilization increased N losses due to increased nitrification and decreased microbial N immobilization, and consequently decreased NRE. Intensive mowing partly dampened high N losses following fertilization. Sward compositions influenced NRE but not N losses: control swards that developed for decades under extensive management had the highest NRE, whereas monocot-enhanced sward had the lowest NRE. NRE was highly correlated with microbial NH₄⁺ immobilization and microbial biomass and only marginally correlated with plant N uptake, underlining the importance of microbial N retention in the soil-plant system. Microbial N retention is reflected in NRE but not in indices commonly used to reflect plant response. NRE was able to capture the effects of sward composition and fertilization whereas N losses were only sensitive to fertilization; thus, NRE is a better index when evaluating environmental sustainability of sward compositions and management practices of grasslands.     

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.     

Acute and chronic nitrite toxicity in juvenile pike-perch (Sander lucioperca) and its compensation by chloride

Pike-perch Sander lucioperca is currently considered as one of the most promising candidates for production in freshwater recirculation aquaculture systems (RAS). Here, due to the lack of studies on nitrite (NO₂^?) toxicity in pike-perch, a flow-through exposure at 0, 0.44, 0.88, 1.75, 3.5, 7, 14 and 28 mg/L NO₂^?–N was carried out to determine the acute and chronic toxicity over a period of 32 days. In juvenile pike-perch, 120 h LC₅₀ was 6.1 mg/L NO₂^?–N and at ≥ 14 mg/L NO₂^?–N all fish had died within 24 h. Chronic exposure revealed a significant build up of NO₂^? in the plasma as well as in the muscles at ≥ 0.44 mg/L NO₂^?–N peaking in fish exposed to the highest concentration of 3.5 mg/L NO₂^?–N after 32 days. Still, due to high individual variation methemoglobin (MetHb) was only significantly increased (p < 0.01) at 3.5 mg/L NO₂^?–N. No adverse effects on red blood cells (RBC) and hematocrit were observed in any of the treatments. In a second experiment, compensation of NO₂^? toxicity at increasing chloride concentrations (40 (freshwater), 65, 90, 140, 240, 440 mg/L Cl^?) was observed at a constant exposure of 10 mg/L NO₂^?–N for 42 days. At ≥ 240 mg/L Cl^?, NO₂^? build-up in blood plasma and muscle was completely inhibited. At lower Cl^? concentrations (≤ 140 mg/L), NO₂^? was significantly increased in plasma, but only insignificantly elevated in muscle due to high individual variation. MetHb was increased significantly difference only at 40 mg/L Cl^? (freshwater control) compared to the control. Again, high individual variations were observed. As a conclusion, S. lucioperca is moderately sensitive towards NO₂^? and acceptable levels in RAS should hence not exceed 1.75 mg/L NO₂^?–N to avoid MetHb formation. However, based on the 120 h LC₅₀ and a factor of 0.01 according to Sprague (1971), a NO₂^? concentration of ≤ 0.061 mg/L NO₂^?–N is considered as “safe.” Thereby, no NO₂^? should accumulate in the plasma or muscle tissue during chronic exposure. For 10 mg/L NO₂^?–N, ≥ 240 mg/L chloride compensates for NO₂^? uptake in plasma and muscle.     

The effects of NH4+ and NO3− on growth,resource allocation and nitrogen uptake kinetics of Phragmites australis and Glyceria maxima

The effects of NH₄⁺ or NO₃⁻ on growth, resource allocation and nitrogen (N) uptake kinetics of two common helophytes Phragmites australis (Cav.) Trin. ex Steudel and Glyceria maxima (Hartm.) Holmb. were studied in semi steady-state hydroponic cultures. At a steady-state nitrogen availability of 34 μM the growth rate of Phragmites was not affected by the N form (mean RGR = 35.4 mg g⁻¹ d⁻¹), whereas the growth rate of Glyceria was 16% higher in NH₄⁺-N cultures than in NO₃⁻-N cultures (mean = 66.7 and 57.4 mg g⁻¹ d⁻¹ of NH₄⁺ and NO₃⁻ treated plants, respectively). Phragmites and Glyceria had higher S/R ratio in NH₄⁺ cultures than in NO₃⁻ cultures, 123.5 and 129.7%, respectively.Species differed in the nitrogen utilisation. In Glyceria, the relative tissue N content was higher than in Phragmites and was increased in NH₄⁺ treated plants by 16%. The tissue NH₄⁺ concentration (mean = 1.6 μmol g fresh wt⁻¹) was not affected by N treatment, whereas NO₃⁻ contents were higher in NO₃⁻ (mean = 1.5 μmol g fresh wt⁻¹) than in NH₄⁺ (mean = 0.4 μmol g fresh wt⁻¹) treated plants. In Phragmites, NH₄⁺ (mean = 1.6 μmol g fresh wt⁻¹) and NO₃⁻ (mean = 0.2 μmol g fresh wt⁻¹) contents were not affected by the N regime. Species did not differ in NH₄⁺ (mean = 56.5 μmol g⁻¹ root dry wt h⁻¹) and NO₃⁻ (mean = 34.5 μmol g⁻¹ root dry wt h⁻¹) maximum uptake rates (V_max), and V_max for NH₄⁺ uptake was not affected by N treatment. The uptake rate of NO₃⁻ was low in NH₄⁺ treated plants, and an induction phase for NO₃⁻ was observed in NH₄⁺ treated Phragmites but not in Glyceria. Phragmites had low K_m (mean = 4.5 μM) and high affinity (10.3 l g⁻¹ root dry wt h⁻¹) for both ions compared to Glyceria (K_m = 6.3 μM, affinity = 8.0 l g⁻¹ root dry wt h⁻¹). The results showed different plasticity of Phragmites and Glyceria toward N source. The positive response to NH₄⁺-N source may participates in the observed success of Glyceria at NH₄⁺ rich sites, although other factors have to be considered. Higher plasticity of Phragmites toward low nutrient availability may favour this species at oligotrophic sites.     

Control and optimization of nitrifying communities for nitritation from domestic wastewater at room temperatures

To achieve nitritation from complete-nitrification seed sludge at room temperature of 19 ± 1 °C, a lab-scale sequencing batch reactor (SBR) treating domestic wastewater with low C/N ratios was operated to investigate the control and optimization of nitrifying communities. Ammonia oxidizing bacteria (AOB) dominance was enhanced through the combination of low DO concentrations (<1.0 mg/L) and preset short-cycle control of aeration time. Nitritation was successfully established with NO₂^?-N/NO_x^?-N over 95%. To avoid the adverse impact of low DO concentrations on AOB activities, DO concentrations were increased to 1–2 mg/L. At the normal DO levels and temperatures, on-line control strategy of aerobic durations maintained the stability of nitritation with nitrite accumulation rate over 95% and ammonia removal above 97%. Fluorescence in-situ hybridization (FISH) analysis presented that the maximal percentage of AOB in biomass reached 10.9% and nitrite oxidizing bacteria (NOB) were washed out.     

Proton production from nitrogen transformation drives stream export of base cations in acid-sensitive forested watersheds

The biogeochemical cycles of nitrogen (N) and base cations (BCs), (i.e., K⁺, Na⁺, Ca²⁺, and Mg²⁺), play critical roles in plant nutrition and ecosystem function. Empirical correlations between large experimental N fertilizer additions to forest ecosystems and increased BCs loss in stream water are well demonstrated, but the mechanisms driving this coupling remain poorly understood. We hypothesized that protons generated through N transformation (PPR_N)—quantified as the balance of NH₄⁺ (H⁺ source) and NO₃⁻ (H⁺ sink) in precipitation versus the stream output will impact BCs loss in acid-sensitive ecosystems. To test this hypothesis, we monitored precipitation input and stream export of inorganic N and BCs for three years in an acid-sensitive forested watershed in a granite area of subtropical China. We found the precipitation input of inorganic N (17.71 kg N ha⁻¹ year⁻¹ with 54% as NH₄⁺–N) was considerably higher than stream exported inorganic N (5.99 kg N ha⁻¹ year⁻¹ with 83% as NO₃⁻–N), making the watershed a net N sink. The stream export of BCs (151, 1518, 851, and 252 mol ha⁻¹ year⁻¹ for K⁺, Na⁺, Ca²⁺, and Mg²⁺, respectively) was positively correlated (r = 0.80, 0.90, 0.84, and 0.84 for K⁺, Na⁺, Ca²⁺, and Mg²⁺ on a monthly scale, respectively, P < 0.001, n = 36) with PPR_N (389 mol ha⁻¹ year⁻¹) over the three years, suggesting that PPR_N drives loss of BCs in the acid-sensitive ecosystem. A global meta-analysis of 15 watershed studies from non-calcareous ecosystems further supports this hypothesis by showing a similarly strong correlation between ∑BCs output and PPR_N (r = 0.89, P < 0.001, n = 15), in spite of the pronounced differences in environmental settings. Collectively, our results suggest that N transformations rather than anions (NO₃⁻ and/or SO₄²⁻) leaching specifically, are an important mediator of BCs loss in acid-senstive ecosystems. Our study provides the first definitive evidence that the chronic N deposition and subsequent transformation within the watershed drive stream export of BCs through proton production in acid-sensitive ecosystems, irrespective of their current relatively high N retention. Our findings suggest the N-transformation-based proton production can be used as an indicator of watershed outflow quality in the acid-sensitive ecosystems.     

Two ways to achieve an anammox influent from real reject water treatment at lab-scale: Partial SBR nitrification and SHARON process

A comparative study to produce the correct influent for Anammox process from anaerobic sludge reject water (700–800 mg NH₄⁺-N L⁻¹) was considered here. The influent for the Anammox process must be composed of NH₄⁺-N and NO₂⁻-N in a ratio 1:1 and therefore only a partial nitrification of ammonium to nitrite is required. The modifications of parameters (temperature, ammonium concentration, pH and solid retention time) allows to achieve this partial nitrification with a final effluent only composed by NH₄⁺-N and NO₂⁻-N at the right stoichiometric ratio. The equal ratio of HCO₃⁻/NH₄⁺ in reject water results in a natural pH decrease when approximately 50% of NH₄⁺ is oxidised. A Sequencing batch reactor (SBR) and a chemostat type of reactor (single-reactor high activity ammonia removal over nitrite (SHARON) process) were studied to obtain the required Anammox influent. At steady state conditions, both systems had a specific conversion rate around 40 mg NH₄⁺-N g⁻¹ volatile suspended solids (VSS) h⁻¹, but in terms of absolute nitrogen removal the SBR conversion was 1.1 kg N day⁻¹ m⁻³, whereas in the SHARON chemostat was 0.35 kg N day⁻¹ m⁻³ due to the different hydraulic retention time (HRT) used. Both systems are compared from operational (including starvation experiments) and kinetic point of view and their advantages/disadvantages are discussed.     

Spatiotemporal modeling of watershed nutrient transport dynamics: Implications for eutrophication abatement

The main objective of this study was to quantify nutrient transport dynamics of a previously ungauged, temperate watershed (145 km²) surrounding a shallow eutrophic lake and discern lake response to external nutrient loading, based on soil water assessment tool (SWAT) and the Organization of Economic Cooperation and Development (OECD) empirical lake models, respectively. A SWAT model was used to simulate baseline nutrient dynamics after its calibration and validation against daily tributary flow, total dissolved phosphorus (TDP), total phosphorus (TP), and nitrate (NO₃) loads. On the watershed scale, median annual TDP, TP, and NO₃ losses were 0.4, 1.1, and 2.0 kg ha^− 1, respectively. The highest median annual TP and NO₃ losses were estimated at 3.7 and 7.7 kg ha^− 1 for pastureland and 1.7 and 3.8 kg ha^− 1 for cropland and mixed forests, respectively. Baseflow was the major nutrient transport pathway over a wide range of precipitation events (450 to 900 mm yr^− 1). Erosion was the predominant surface process exporting P across the watershed. Critical source areas (CSAs) of TP and NO₃ comprised 17% and 4% of the watershed, respectively. Annual mean TP, and mean and maximum chlorophyll content indicated a hyper-eutrophication risk for the lake. An external P load reduction by excess of 80% could be necessary to restore mesotrophy in the lake. Our results suggested that subsurface P transport should not be overlooked a priori when groundwater-dependent and extensively farmed watersheds are managed for eutrophication abatement.     

Nitrogen transformations in vertical flow systems with and without rice (Oryza sativa) studied with a high-resolution soil–water profiler

Nitrogen transformations were studied in flooded and non-flooded vertical flow columns with and without a rice plant. Influent (average concentration: NH₄⁺-N: 40 mg L^?1; NO₃^?-N: 0.15 mg L^?1; and NO₂^?-N: 4.0 mg L^?1) was supplied at 1.25 cm d^?1 during stage 1 (20 May–5 August) and at 2.50 cm d^?1 at stage 2 (6 August–26 October), which resulted in an average nitrogen loading of 156 g m^?2 during the entire experimental period. Total nitrogen (T-N) removal efficiencies exceeded 90% in vertical flow systems with rice plants. Nitrogen assimilated by the rice plants in the flooded column accounted for 60% of the total input nitrogen, while that in the non-flooded column accounted for 36% of the total input. The remaining nitrogen appeared to be removed through biogeochemical pathways. Although some nitrogen flowed out, most input nitrogen was also removed even in the flooded and non-flooded unplanted columns.A high-resolution vertical distribution investigation showed the changes of nitrogen forms in soil water. In the flooded condition, there were high ammonium and high nitrite concentrations in the upper layers. The concentrations of ammonium and nitrite simultaneously decreased with depth increasing, suggesting that anaerobic ammonia oxidation (anammox) may occur in these anaerobic conditions. In contrast, the distributions of nitrogen in the non-flooded columns with elevated water level suggested that nitrification–denitrification route was the major removal mechanism, whether or not rice plants were present.     

Effect of hydraulic retention time on the treatment of secondary effluent in a subsurface flow constructed wetland

This study evaluates the potential of subsurface flow (SSF) constructed wetlands (CWs) for tertiary treatment of wastewater at four shorter HRTs (1–4 days). The CWs were planted with Typha angustata, which was observed in our earlier study to be more efficient than Phragmites karka and Scirpus littoralis. The CWs comprised four rectangular treatment cells (2.14 m × 0.76 m × 0.61 m) filled with layers of gravel of two different sizes (approximately 2.5 cm and 1.5 cm diameter) to a depth of 0.61 m. The inflow rates of the secondary effluent in the four cells were accordingly fixed at 300 L d^?1, 150 L d^?1, 100 L d^?1 and 75 L d^?1, respectively, for 1, 2, 3 and 4 days HRT. The hydraulic loads ranged between 59.05 mm d^?1 and 236.22 mm d^?1.The wastewater inflow into the CW system as well as the treated effluent were analyzed, using standard methods, at regular intervals for various forms of nitrogen (NH₄-N, NO₃-N and TKN), orthophosphate-P and organic matter (BOD and COD) concentrations over a period of five weeks after the development of a dense stand.The higher HRT of 4 days not only helped maximum removal of all the pollutants but also maintained the stability of the treatment efficiency throughout the monitoring period. For the nutrients (NH₄-N, NO₃-N and TKN), HRT played a more significant role in their removal than in case of organic matter (BOD₃ and COD). More than 90% of NO₃-N and TKN and 100% of NH₄-N were removed from the wastewater at 4 days HRT.At lower HRTs, the mass loading rate was higher with greater fluctuation. However mass reduction efficiency of the T. angustata CW for all forms of nitrogen was >80% with the HRTs of 2, 3 and 4 days.     

Two-decade wetland cultivation and its effects on soil properties in salt marshes in the Yellow River Delta,China

Wetland cultivation and its effects on soil properties in salt marshes in the Yellow River Delta, China were examined by using a combination of the satellite imageries and field experiments. Results showed that the conversions mainly occurred between dry lands and Phragmites australis–Suaeda salsa–Tamarix chinensis marshes (PSTMs). The total area of marsh wetland was reduced by 65.09 km² during the period from 1986 to 2005, and these conversions might be attributed to a combination of farming, oil exploration and water extraction, as well as soil salinization. Significant differences were observed in bulk density, pH, salinity and NO₃⁻-N between different land-use types (P < 0.05). After the conversions from marsh wetlands to dry lands, bulk density, pH, salinity and NH₄⁺-N decreased slightly, while a significant increase in NO₃⁻-N, TN (total nitrogen), and AP (available phosphorus) (P < 0.05) was observed. The more loss of soil nutrient storage also occurred after the maximal area conversion from PSTMs to dry lands compared to other conversions during the study period. The storages of soil organic matter, NH₄⁺-N and total phosphorus decreased greatly under the conversion from three types of marshes to dry lands, while those of NO₃⁻-N, AP and TN showed an obvious increase during the whole study period.     

Denitrification activity,wood loss,and N2O emissions over 9 years from a wood chip bioreactor

Loss of nitrate in subsurface drainage water from agricultural fields is an important problem in the Midwestern United States and elsewhere. One possible strategy for reducing nitrate export is the use of denitrification bioreactors. A variety of experimental bioreactor designs have been shown to reduce nitrate losses in drainage water for periods up to several years. This research reports on the denitrification activity of a wood chip-based bioreactor operating in the field for over 9 years. Potential denitrification activity was sustained over the 9-year period, which was consistent with nitrate removal from drainage water in the field. Denitrification potentials ranged from 8.2 to 34 mg N kg^?1 wood during the last 5 years of bioreactor operation. Populations of denitrifying bacteria were greater in the wood chips than in adjacent subsoil. Loss of wood through decomposition reached 75% at the 90–100 cm depth with a wood half-life of 4.6 years. However, wood loss was less than 20% at 155–170 cm depth and the half-life of this wood was 36.6 years. The differential wood loss at these two depths appears to result from sustained anaerobic conditions below the tile drainage line at 120 cm depth. Pore space concentrations of oxygen and methane support this conjecture. Nitrous oxide exported in tile water from the wood chip bioreactor plots was not significantly higher than N₂O exports in tile water from the untreated control plots, and loss of N₂O from tile water exiting the bioreactor accounted for 0.0062 kg N₂O-N kg^?1 NO₃-N.     

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.     

Denitrifying bioreactors—An approach for reducing nitrate loads to receiving waters

Low-cost and simple technologies are needed to reduce watershed export of excess nitrogen to sensitive aquatic ecosystems. Denitrifying bioreactors are an approach where solid carbon substrates are added into the flow path of contaminated water. These carbon (C) substrates (often fragmented wood-products) act as a C and energy source to support denitrification; the conversion of nitrate (NO₃^?) to nitrogen gases. Here, we summarize the different designs of denitrifying bioreactors that use a solid C substrate, their hydrological connections, effectiveness, and factors that limit their performance. The main denitrifying bioreactors are: denitrification walls (intercepting shallow groundwater), denitrifying beds (intercepting concentrated discharges) and denitrifying layers (intercepting soil leachate). Both denitrifcation walls and beds have proven successful in appropriate field settings with NO₃^? removal rates generally ranging from 0.01 to 3.6 g N m^?3 day^?1 for walls and 2–22 g N m^?3 day^?1 for beds, with the lower rates often associated with nitrate-limitations. Nitrate removal is also limited by the rate of C supply from degrading substrate and removal is operationally zero-order with respect to NO₃^? concentration primarily because the inputs of NO₃^? into studied bioreactors have been generally high. In bioreactors where NO₃^? is not fully depleted, removal rates generally increase with increasing temperature. Nitrate removal has been supported for up to 15 years without further maintenance or C supplementation because wood chips degrade sufficiently slowly under anoxic conditions. There have been few field-based comparisons of alternative C substrates to increase NO₃^? removal rates but laboratory trials suggest that some alternatives could support greater rates of NO₃^? removal (e.g., corn cobs and wheat straw). Denitrifying bioreactors may have a number of adverse effects, such as production of nitrous oxide and leaching of dissolved organic matter (usually only for the first few months after construction and start-up). The relatively small amount of field data suggests that these problems can be adequately managed or minimized. An initial cost/benefit analysis demonstrates that denitrifying bioreactors are cost effective and complementary to other agricultural management practices aimed at decreasing nitrogen loads to surface waters. We conclude with recommendations for further research to enhance performance of denitrifying bioreactors.     

The consideration of time step in calculating grey water footprints of agricultural cropping systems

A water footprint considers both the water volumes involved in production processes and the resulting waste water generated. The grey water (GW) footprint represents the volume of fresh water required to assimilate pollutants to acceptable concentrations—a concept proposed by the water footprint network—but it faces several difficulties when applied to agricultural production systems. Crop production cannot be fully controlled and it is weather-dependent, which greatly affects the year-to-year GW calculations.In this study, we examined the effect of time step on the calculation of annual GW footprints by utilizing 30 years of daily average nitrate-nitrogen (NO₃-N) concentrations in drainage water (both leachate and runoff water derived from a process-based model) from corn and soybean production systems. For each crop year, the volume of water required to assimilate NO₃-N to an acceptable threshold concentration (i.e. <10 mg L⁻¹) was calculated over different time steps (daily, weekly, monthly, seasonally and yearly), and each case was summed to an annual GW value. Daily average NO₃-N concentrations in the effluent water were generally below the acceptable threshold concentrations, with intermittent exceedances. Thus, the fields often provided their own ‘dilution’ water, and annual average concentrations were only 2.0 mg L⁻¹ and 0.4 mg L⁻¹ for corn and soybean, respectively.The GW footprint varied significantly when calculated for different time steps. The greatest annual footprint occurred when calculated daily (shortest time step). The GW footprint for corn ranged from 2.7 × 10³ m³ ha⁻¹, or 2700 mm of water, when estimated daily to zero for the yearly time step. For soybean it ranged from 0.5 × 10³ m³ ha⁻¹, or 500 mm of water, to zero. The GW footprint results are therefore highly dependent on the time step of calculation. The effect of this issue extends beyond crop production as it is exported and amplified through feed rations to affect the GW footprint from animal production. To be able to reconcile these problems, the GW calculation pathways should be reconsidered and standardized.