Search

Reactive nitrogen (Nr) plays a key role in the atmospheric environment and its deposition has induced large negative impacts on ecosystem health and services. Five-year continuous in-situ monitoring of N deposition, including wet (total nitrogen ( W TN), total dissolved nitrogen ( W TDN), dissolved organic nitrogen ( W DON), ammonium nitrogen ( W AN) and nitrate nitrogen ( W NN)) and dry ( D NH 3 , D HNO 3 , D pNH 4 + , D pNO 3 − and D NO 2 ) deposition, had been conducted since August 2008 to December 2013 (wet) and May 2011 to December 2013 (dry) in Yan-ting, China, a typical agricultural area in the central Sichuan Basin. Mean annual total N deposition from 2011 to 2013 was 30.8 kg N ha −1 yr −1 , and speculated that of 2009 and 2010 was averaged 28.2 kg N ha −1 yr −1 , respectively. Wet and dry N deposition accounted for 76.3% and 23.7% of annual N deposition, respectively. Reduced N ( W AN, D NH 3 and D pNH 4 + ) was 1.7 times of oxidized N ( W NN, D HNO 3 , D NO 2 and D pNO 3 − ) which accounted for 50.9% and 30.3% of TN, respectively. Maximum loadings of all N forms of wet deposition, gaseous NH 3 , HNO 3 and particulate NH 4 + in dry deposition occurred in summer and minimum loadings in winter. Whether monthly, seasonal or annual averaged, dissolved N accounted for more than 70% of the total. N deposition in the central Sichuan Basin increased during the sampling period, especially that of ammonium compounds, and has become a serious threat to local aquatic ecosystems, the surrounding forest and other natural or semi-natural ecosystems in the upper reaches of the Yangtze River. [ABSTRACT FROM AUTHOR]

Nitrogen (N) input from atmospheric deposition is a concern in the North China Plain (NCP). To quantify the magnitude of the problem, the total airborne N deposition to agroecosystems in the NCP was quantified using a manual ITNI (integrated total nitrogen input) system and a long-term experiment (LTE). Total estimated N deposition averaged 83. 3kgNha−1 across the sites Dongbeiwang (DBW) of Beijing, Wuqiao (WQ) and Quzhou (QZ) of Hebei, with atmospheric N input 14kgNha−1 in the period from flowering to maturity, when maize (Zea mays) was the monitoring crop. The 15N-supply level in the ITNI system seemed not to affect the estimation of N deposition significantly. However, the N deposition value was only 48. 6kgNha−1 on average when ryegrass (Lolium multiflorum) was cultivated during the same period of maize (May–September), suggesting that total airborne N input varies with the monitoring crop, largely because of the different surface areas. The total annual N deposition obtained from zero-N plots of LTE at Changping (82. 8kgNha−1) supported the N deposition results from the maize-based ITNI systems. [Copyright &y& Elsevier]

Abstract: There is increasing interest in nitrogen (N) deposition because of its importance as a nutrient resource and a component of acid deposition within the overall global N cycle. Precipitation samples were collected for periods varying from 6 months to 6 years (1998–2004) from seven sites in the Beijing area to determine the amount and seasonal distribution of N (bulk/wet) deposition. Bulk deposition of N ranged from 26.6 to 38.5kgNha−1 year−1 and averaged 30.6kgNha−1 year−1. Bulk deposition of NH4–N was, on average, 2.1 times the NO3–N deposition, suggesting that reduced N is the major form of N deposition in the study area. Concentrations of NH4–N and NO3–N in rainfall averaged 4.8 and 2.2mgNL−1 and showed great temporal variation from month to month. A negative relationship between rainfall and NH4–N or NO3–N concentration in rainwater was observed by an exponential equation, indicating dilution of NH4–N and NO3–N with increasing precipitation. Bulk deposition of inorganic N occurred mainly from April to September (>80% of total bulk deposition), which was consistent with both the monthly distribution of precipitation and the times of fertilizer applications in local agricultural land. Wet-only deposition of inorganic N, however, was 8.3–8.4kgNha−1 lower than that of bulk deposition during similar periods in 2003 (June–November) and 2004 (April–November), suggesting the potential contribution of dry deposition to total N deposition in the Beijing area. [Copyright &y& Elsevier]

Nitrogen dynamics and budgets in a clay loam soil (Meadow Aqualf) in the North China Plain were investigated in a winter wheat (Triticum aestivum L.) and maize (Zea mays L.) cropping system comparing the effects of four N rates (0, 120, 240 and 360 kg N ha−1 as urea) applied twice to each crop over 2 years. Ammonium nitrogen (NH4-N) in the soil profile remained at a low and constant level (except in the surface 20 cm layer) following application of fertilizer N. In contrast, nitrate nitrogen (NO3-N) levels were significantly altered by the rate of applied N. A strong tendency of NO3-N to move from the surface layer to the lower layers (20–100 cm) was observed during the wheat and maize growth seasons in treatments of 240 and 360 kg N ha−1 per crop (N240 and N360). The amounts of NO3-N accumulated in the soil profile were significantly higher in N240 and N360 than those in N0 and N120 (treatments receiving 0 and 120 kg N ha−1 per crop). After 2 years, soil NO3-N levels at 0–300 cm depth in N120, N240 and N360 amounted to 336, 815 and 1141 kg ha−1, respectively, with more than half of these amounts distributed in the 100–300 cm layer. The calculated total N balance indicates that most fertilizer N was available as NO3-N in the top 300 cm of the soil profile using traditional fertilization and irrigation practices. Over the subsequent 2 years, N losses were calculated to be relatively low in N120 but significantly higher in N240 and N360. Measured gaseous N losses showed that NH3 volatilization and denitrification comprised only a small fraction of total N losses during the 2-year rotation, while NO3-N leaching from the top 100 cm of the soil profile accounted for most N losses across all N rates and experimental years. The N budget showed that accumulation and/or leaching of NO3-N below 100 cm depth (beyond the reach of most roots) was the main pathway for N losses in the winter wheat–maize cropping system. The recommended N application rate of 120 kg N ha−1 minimized soil NO3-N accumulation and leaching losses while maintaining high yields and N utilization by winter wheat and maize. [Copyright &y& Elsevier]

In a recent publication in the journal Science of the Total Environment, Zhu et al. (2015) reported the composition, spatial patterns, and factors influencing atmospheric wet nitrogen (N) deposition based on one year's data from 41-monitoring sites in China. We suspect their results may largely underestimate dissolved organic N (DON) but overestimate total particulate N (TPN) in wet deposition due to the uncertainty resulting from the sampling, storage and analysis methods in their study. Our suspicions are based mainly on our experience from earlier measurements and the literature. We therefore suggest that enhanced data quality control on atmospheric N deposition measurements should be taken into account in future studies. [ABSTRACT FROM AUTHOR]

Abstract While the ongoing increases in the deposition of atmospheric nitrogen (N) in China have attracted ever more attention, to date there has been little research in N deposition in inland aquatic ecosystems. In this study, we quantified the bulk and dry atmospheric N deposition around the Dongting Lake, which is connected to the Yangtze River and is the second largest freshwater lake in China. Bulk N deposition averaged 48.0 kg N ha−1 yr−1, of which ammonium (NH 4 N), nitrate (NO 3 N), and dissolved organic nitrogen (DON) accounted for 41.6%, 35.1%, and 23.3%, respectively. Dry N deposition averaged 27.0 kg N ha−1 yr−1, and was dominated by NH 3. Bulk N deposition was higher in spring and summer than in autumn and winter at the sites around the lake because of the high precipitation. NH 3 deposition was the highest in spring and summer due to fertilization, and NO x deposition was the highest in winter because of the combustion of coal for domestic heating. The concentrations of NH 3 significantly decreased with the concentration of NH 4 N, indicating below-cloud scavenging and local sources. The concentrations of NO x and NO 3 N were not correlated, indicating long-distance transfer of NO x. Bulk N deposition in our study was generally twice that reported in other studies, even when only inorganic N species in bulk deposition were considered. Total N deposition averaged 75.0 kg N ha−1 yr−1, which was much higher than that reported in other studies. Both N deposited directly into the lake and N deposited across the lake basin that was subsequently transported to the lake in runoff could enrich the N concentration of the lake water and increase the gross primary productivity (GPP). Our results highlighted the importance of integrated management that includes both aquatic and atmospheric systems at either the lake basin scale or regional scale, rather than at the individual lake-scale. Graphical abstract Image 1 Highlights • Total N deposition was much higher than that reported in other studies around the Dongting Lake. • DON deposition, which usually be ignored, is a significant contributor. • Potential eutrophication effects of N loading to the Dongting Lake from atmospheric N deposition should be re-evaluated. [ABSTRACT FROM AUTHOR]

Quantifying the similarities and differences in atmospheric nitrogen (N) deposition between different ecosystems is important to develop effective measures to reduce air pollution and maintain biodiversity. Here we show that the constitution of N deposition differed significantly between a grassland and a desert ecosystem in Northwestern China. Flux of bulk (wet plus part of dry deposition) and dry (gaseous NH 3 and NO 2) deposition were continuously monitored from 2018 to 2020. The grassland and desert sites had similar amount of total N deposition, being 7.29 and 6.33 kg N ha−1 yr−1, respectively. However, N deposition at the grassland was dominated by the bulk deposition (4.44 kg N ha−1 yr−1, 61% of the total N deposition), whereas that at the desert was dominated by dry deposition (4.20 kg N ha−1 yr−1, 66% of total deposition). The desert had greater ambient concentrations of NH 3 (3.66 μg N m−3) and NO 2 (1.52 μg N m−3) than the grassland (2.73 μg NH 3 –N m−3 and 0.72 μg NO 2 –N m−3). The amount of reduced N deposition (NH 4 + and NH 3) was around 3 times of that of oxidized N deposition (NO 3 − and NO 2) in both ecosystems. The N deposition rates in both ecosystems have exceeded the critical load for the fragile ecosystems (5–10 kg N ha−1 yr−1), highlighting the importance of reducing N emission sources that are related with anthropogenic disturbance. [Display omitted] • The boundary of Tarim Basin and Tibet Plateau has relatively low N deposition. • Bulk and dry deposition dominated in grassland and desert, respectively. • The reduced/oxidized N ratio in N deposition was close to 3 in both ecosystems. • N deposition exceeded the minimum critical load value for sensitive ecosystems. [ABSTRACT FROM AUTHOR]

Organic nitrogen (N) is an important component of the atmospheric deposition of reactive N, but its sources are essentially unknown. Assessing whether this dissolved organic N (DON) is of natural, anthropogenic or mixed origin is critically important in attempting to determine the scale of human perturbation of the global N cycle. Here we report evidence for atmospheric organic N deposition and its anthropogenic sources in China. Precipitation samples were collected and analyzed from 15 rural, suburban and urban sites during 2005 and 2006. The average deposition of DON was 8.6kgha−1 yr−1 with a volume-weighted concentration of 111μmolL−1, which was much higher than in other regions of the world. The contribution of DON to total dissolved N (TDN) was approximately 30% on average, agreeing well with other reported data in the literature. Parallel collections of wet-only and bulk deposition showed wet deposition to be 68% on average, indicating a significant dry deposition component. Combining data from the Chinese sites with those from elsewhere in the world, significant (p<0.0001) correlations between DON and NH4–N, NO3–N and TDN suggest that atmospheric organic N originates from similar sources to dissolved inorganic N (DIN) (NH4–N and NO3–N), which are largely attributed to anthropogenic emissions from both agricultural and industrial sources. [Copyright &y& Elsevier]

Abstract Very few comparative studies on atmospheric dry and wet nitrogen (N) deposition have been conducted in agro-pastoral catchment areas. In an effort to fill this knowledge gap, three interconnected land use types (cropland, mountain pasture, and plain pasture) with six sampling sites (Chinese cropland (CC), Chinese mountain pasture (CM), Chinese plain pasture (CP) Mongolia cropland (MC), Mongolia mountain pasture (MM) and Mongolia plain pasture (MP)) were selected in the transboundary regions of northwest China and western Mongolia. During 16 months from May 2014 to September 2015 atmospheric and precipitation samples were collected to assess levels of nitrogen deposition. Cropland had the highest NO 3 −N (1.0 mg N L−1 in China and 1.2 mg N L−1 in Mongolia) and NH 4 +-N concentrations (1.6 mg N L−1 in China and 2.0 mg N L−1 in Mongolia) in our six sampling sites. The CC experienced the highest wet deposition (5.2 kg N ha−1 yr−1), followed by MM pasture (3.2 kg N ha−1 yr−1) while wet deposition in other sites ranged from 2.1 to 2.8 kg N ha−1 yr−1. The ambient concentrations of NH 3 , NO 2 and dry N deposition were significant higher in cropland than being other land use with 3.0, 3.8 and 1.2 μg N m−3 respectively in China and 2.2, 2.8 and 0.6 μg N m−3 respectively in Mongolia. The highest total N deposition was at CC (14.5 kg N ha−1 yr−1) and MC (9.4 kg N ha−1 yr−1), followed by the mountain pasture (5.8–6.8 kg N ha−1 yr−1), and the plains pasture (5.6–5.7 kg N ha−1 yr−1). Overall, agro-pastoral catchments areas had relatively low total N deposition compared to other studies, and cropland had the highest wet and dry N deposition than mountain and plain pastures, and dry N deposition absolutely dominated the percentage (64–70% in cropland and 53–64% in pasture areas) of total atmospheric N deposition. Comparing the total N deposition in China and Mongolia, we found the CC had 54% higher value than in MC while the reverse was true for mountain pastures Mongolia received 17% more N. Graphical abstract Image 1 Highlights • Passive samplers, PM 10 samplers and rain gauges equipments were used to quantify the nitrogen deposition in six typical land use types. • The cropland sites had significant higher NH 3 and NO 2 concentration and total N deposition than pasture areas in our research area. • Dry deposition absolutely dominated the percentage of total atmospheric N deposition in agro-pastoral catchment areas of China and Mongolia. • Nitrogen deposition in CC had 54% higher value than in MC while the reverse was true for mountain pastures Mongolia received 17% more N. [ABSTRACT FROM AUTHOR]

Remote sensing technology has great potential to expand the observation of ground-level nitrogen deposition from local monitoring sites to a regional scale, with high spatial and temporal resolutions. A new methodology is developed to estimate the spatial distribution of monthly bulk nitrogen deposition on a regional scale, based on precipitation amounts and HNO 3 and aerosol nitrate (NO 3 − ) columns, derived from OMI NO 2 columns and the relationship of NO 2 , HNO 3 and NO 3 − from MOZART. The accuracy assessment shows that the proposed model has achieved a reasonably high predictive power for monthly NO 3 − -N deposition (slope = 0.96, intercept = 0.35, R = 0.83, RMSE = 0.72) across China. The spatial NO 3 − -N deposition shows a significant gradient from industrial areas to undeveloped regions, ranging from 0.01 to 26.76 kg N ha − 1 y − 1 with an average of 5.77 kg N ha − 1 y − 1 over China during 2010–2012. The bulk NO 3 − -N deposition shows a clear seasonal variation, with high depositions occurring in the warm season (March–November) and peaking in July and August, and low bulk NO 3 − -N deposition appearing in winter (December–February). This study proves for the first time that the atmospheric boundary layer (ABL) HNO 3 and NO 3 − columns and precipitation are powerful to predict the bulk nitrate deposition. [ABSTRACT FROM AUTHOR]

Field experiments were conducted at three typical wheat–maize double-cropping sites in north and northwest China to investigate ammonia (NH 3 ) volatilization from urea and from urea amended with 0.12% (w/w) Limus ® (a new urease inhibitor consisting of 75% N-(n-butyl) thiophosphoric triamide (NBPT) and 25% N-(n-propyl) thiophosphoric triamide (NPPT)) in winter wheat. Grain yields and nitrogen (N) budgets of all N treatments were also evaluated to investigate the effects of urea-N application rates and Limus during one wheat season. Cumulative NH 3 losses after two weeks for conventional urea were 11–25% of applied N, while those for urea amended with Limus were only 0–6% of applied N. The urease inhibitor increased fertilizer N retention more strongly when soil and environmental conditions promoted extensive NH 3 losses. However, grain yields were not significantly ( P > 0.05) enhanced by Limus compared to conventional urea at all three sites. A clear increase in apparent N recovery efficiency (RE N ) with Limus (ranging from 10 to 16%) resulted at the Quzhou (QZ) site compared to equal amounts of optimized urea-N, with an increase in RE N of up to 65% under a further 20% N-reduced urea amended with Limus treatment. The study also demonstrated the urgency of appropriate fertilizer N management in China. [ABSTRACT FROM AUTHOR]

Functional genes involved in nitrogen (N) cycling regulate soil nitrification, denitrification and N 2 O emissions. However, the general patterns and variability of N functional genes in response to N addition, and their association with N 2 O emission have not been synthesized for terrestrial ecosystems. We synthesized 2068 observations from 144 papers to explore the impact of N addition on the abundance and diversity of N functional genes, and their relationship to N 2 O emissions in croplands, grasslands and forests on a global scale. In croplands, N addition increased N 2 O emissions (109%), the abundance of ammonia-oxidizing archaea (AOA) (19%) , ammonia-oxidizing bacteria (AOB) (95%) , nirK (52%), nirS (40%) and nosZ (41%), and the diversity of AOB (15%), nirS (12%) and nosZ (11%). In grassland, N addition increased AOB abundance (130%) and decreased the abundance of nirS (−99%) and nosZ (−58%) genes, but in forests, significant effects were only found for the abundance of AOA (35%) and AOB (121%). N 2 O emission was negatively correlated with the abundance of nosZ , but positively correlated with the abundance of AOA and AOB. Apart from the abundance of functional AOA, AOB and nosZ genes, climate variables (precipitation and temperature), and available N concentrations were the main factors explaining the variation in N 2 O emission with N addition, as shown by random forest analysis. These findings indicate that impacts on N functional genes that encode enzymes involved in nitrification (AOA, AOB) and in the transformation of N 2 O to N 2 (nos Z) are the main mechanisms behind the effect on N fertilizer-induced N 2 O emissions. • Nitrogen addition increased the abundance of AOA, AOB, nirK , nirS and nosZ. • N 2 O emission was negatively correlated with the abundance of nosZ gene. • N 2 O emission was positively correlated with the AOA and AOB abundance. • AOA, AOB and nosZ abundance were main factors affecting N 2 O emission. • Climate variables and available N concentrations were additional main factors. [ABSTRACT FROM AUTHOR]

• N enrichment enhanced foliar N, P concentration and N:P, but decreased NRE and PRE. • N enrichment notably changed plant N and P coupling in mature foliar. • NRE could regulate foliar N and P coupling under N enrichment. • Ambient N deposition notably affected foliar nutrient status and resorption. A meta-analysis was preformed to reveal how environmental and experimental factors alter the effects of N enrichment on the concentrations and stoichiometry of N and P in both mature and senesced foliage and nutrient resorption, with published studies from Chinese grasslands. N enrichment, when averaged across the studies, significantly increased the mature leaf N, P and N:P by 27.89%, 2.55% and 20.85%, respectively. The N, P and N:P ratio for senesced foliage under N enrichment increased by 41.35%, 3.28% and 20.11%, respectively, but N enrichment significantly decreased the leaf N resorption efficiency (NRE) and P resorption efficiency (PRE) by 6.85% and 3.41%, respectively. They were significantly controlled by the rates of addition of N, humidity index and the ratio of N addition rates to ambient N deposition, which suggested that ambient N deposition can indirectly influence plant internal nutrient cycling in this system. The effects of N enrichment on the N, P, stoichiometry and nutrient resorption in plants varied greatly among the types of grassland. In addition, the rates of N addition, forms of N application, experimental duration and application frequency of N had different effects on the leaf nutrient status. Specifically, the response of a critical threshold value of leaf P to N addition is 50–100 kg N ha−1 year−1, at which range with a shift from negative to positive impacts. Moreover, the addition of N significantly changed N and P coupling in mature leaves but not of senesced leaves, which were primarily regulated by N enrichment impacts on nutrient resorption. Our findings suggest that N enrichment significantly affected the status of leaf nutrients and then altered plant internal nutrient cycling across the grasslands of northern China. [ABSTRACT FROM AUTHOR]

China has the largest reactive nitrogen deposition worldwide. Understanding the change of nitrogen deposition and its socioeconomic drivers in China is crucial for the function and sustainability of regional and global ecosystems. Here we presented satellite-derived nitrogen deposition constrained with the national ground-based measurements over China. We found that oxidized nitrogen deposition peaked at 7.0 kg N ha−1 in 2012, then decreased to 6.2 kg N ha−1 in 2017. In contrast, reduced nitrogen deposition increased continuously, and reached 12.6 kg N ha−1 in 2017. The Chinese Clean Air Act is responsible for the reduction of oxidized nitrogen emissions, while there is no current effective control policy for ammonia emissions in place resulting in reduced nitrogen deposition reaching levels of approximately twice that of oxidized nitrogen in 2017. The ratio of reduced to oxidized nitrogen deposition decreased from 1.30 in 2008 to 1.08 in 2011, while this ratio began to increase from 1.08 in 2011 to 1.56 in 2017. This suggests reduced nitrogen deposition would become more important in the air quality and their ecological consequences in the future. The total nitrogen deposition in China was about 3–4 times that in the US and Europe. The policy of controlling nitrogen pollution is urgent, especially for reducing ammonia emissions, while ensuring the sustainable production of agriculture and maintaining food security. Image 1 • Methods were developed to obtain N deposition using satellite observations. • The ratio of reduced to oxidized N deposition in China increased since 2011. • N deposition in China was about 3–4 times that in the US and Europe. [ABSTRACT FROM AUTHOR]

• Atmospheric N deposition in China was evaluated by two time-scale deposition databases. • Bulk and dry N deposition reached a peak between 2000 and 2010 and then began a steady decline. • Significant decrease of oxidized N deposition proved the effectiveness of NO x emissions control. • Increase of NH 3 dry deposition highlighted the importance of regional NH 3 mitigation. China has experienced a dramatic change in atmospheric reactive nitrogen (Nr) emissions over the past four decades. However, it remains unclear how nitrogen (N) deposition has responded to increases and/or decreases in Nr emissions. This study quantitatively assesses temporal and spatial variations in measurements of bulk and calculated dry N deposition in China from 1980 to 2018. A long-term database (1980–2018) shows that bulk N deposition peaked in around 2000, and had declined by 45% by 2016–2018. Recent bulk and dry N deposition (based on monitoring from 2011 to 2018) decreased from 2011 to 2018, with current average values of 19.4 ± 0.8 and 20.6 ± 0.4 kg N ha−1 yr−1, respectively. Oxidized N deposition, especially dry deposition, decreased after 2010 due to NO x emission controls. In contrast, reduced N deposition was approximately constant, with reductions in bulk NH 4 +-N deposition offset by a continuous increase in dry NH 3 deposition. Elevated NH 3 concentrations were found at nationwide monitoring sites even at urban sites, suggesting a strong influence of both agricultural and non-agricultural sources. Current emission controls are reducing Nr emissions and deposition but further mitigation measures are needed, especially of NH 3 , built on broader regional emission control strategies. [ABSTRACT FROM AUTHOR]

• Up to 93.45% of total nitrogen transformed into ammonium by hydrothermal process. • Activated hydrochar was synthesized to capture ammonium nitrogen. • 10.14% of total nitrogen was recovered after solubilization, capture, and available. • Nitrogen sorption by activated hydrochar was assessed by FTIR, SEM and BET. Nitrogen recovery from bio-based digestate has attracted considerable interests recently, but has been limited by the indirect capture of organic or insoluble nitrogen and the lower sorption ability of hydrochar. To mitigate these challenges, we used a hydrothermal pretreatment to promote the conversion of insoluble or organic nitrogen. Activated hydrochar was synthesized by using potassium hydroxide to enhance its sorption capacity, and the hydrochar was characterized by Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller surface-area analysis, and scanning electron microscopy. An optimal efficiency was achieved at 210 °C, with 93.45% total nitrogen solubilization into ammonium. The sorption data of ammonium on the activated hydrochar fitted the pseudo second order kinetics and Langmuir model well. This reaction was dominated by chemisorption, which referring to the formation of bonds between (oxygen-containing) functional groups and ammonium. The maximum ammonium adsorption on the activated hydrochar at 260 °C (260AH) was 108.57 mg g−1. The ammonium sorption was endothermic, spontaneous and associated with a disorder increase on the solid-liquid interface. Once saturated, the ammonium was released continually from activated hydrochar and is anticipated to show an optimal performance in an alkaline environment. The amount of ammonium that was released from the saturated activated hydrochar at 260 °C after five extractions was 13.09 mg g−1. The nitrogen mass-balance calculations show that 10.14% total nitrogen was recovered after nitrogen solubilization, capture, and release. Therefore, potassium-hydroxide-activated hydrochar performed as a promising composite sorbent for ammonium. [ABSTRACT FROM AUTHOR]