Nitrogen is an essential nutrient in the environment that exists in multiple oxidation states in nature. Numerous microbial processes are involved in its transformation. Knowledge about very complex N cycling has been growing rapidly in recent years, with new information about associated isotope effects and about the microbes involved in particular processes. Furthermore, molecular methods that are able to detect and quantify particular processes are being developed, applied and combined with other analytical approaches, which opens up new opportunities to enhance understanding of nitrogen transformation pathways. This review presents a summary of the microbial nitrogen transformation, including the respective isotope effects of nitrogen and oxygen on different nitrogen-bearing compounds (including nitrates, nitrites, ammonia and nitrous oxide), and the microbiological characteristics of these processes. It is supplemented by an overview of molecular methods applied for detecting and quantifying the activity of particular enzymes involved in N transformation pathways. This summary should help in the planning and interpretation of complex research studies applying isotope analyses of different N compounds and combining microbiological and isotopic methods in tracking complex N cycling, and in the integration of these results in modelling approaches.

What can the stable isotope values of human and animal faeces tell us? This often under-appreciated waste product is gaining recognition across a variety of disciplines. Faecal isotopes provide a means of monitoring diet, resource partitioning, landscape use, tracking nutrient inputs and cycling, and reconstructing past climate and environment. Here, we review what faeces are composed of, their temporal resolution, and how these factors may be impacted by digestive physiology and efficiency. As faeces are often used to explore diet, we clarify how isotopic offsets between diet and faeces can be calculated, as well as some differences among commonly used calculations that can lead to confusion. Generally, faecal carbon isotope (δ13 C) values are lower than those of the diet, while faecal nitrogen isotope values (δ15 N) values are higher than in the diet. However, there is considerable variability both within and among species. We explore the role of study design and how limitations stemming from a variety of factors can affect both the reliability and interpretability of faecal isotope data sets. Finally, we summarise the various ways in which faecal isotopes have been applied to date and provide some suggestions for future research. Despite remaining challenges, faecal isotope data are poised to continue to contribute meaningfully to a variety of fields.

Accurately quantifying trophic positions (TP) to describe food web structure is an important element in studying pollutant bioaccumulation. In recent years, compound-specific nitrogen isotopic analysis of amino acids (AAs-N-CSIA) has been progressively applied as a potentially reliable tool for quantifying TP, facilitating a better understanding of pollutant food web transfer. Therefore, this review provides an overview of the analytical procedures, applications, and limitations of AAs-N-CSIA in pollutant (halogenated organic pollutants (HOPs) and heavy metals) bioaccumulation studies. We first summarize studies on the analytical techniques of AAs-N-CSIA, including derivatization, instrumental analysis, and data processing methods. The N-pivaloyl-i-propyl-amino acid ester method is a more suitable AAs derivatization method for quantifying TP. The AAs-N-CSIA application in pollutant bioaccumulation studies (e.g., Hg, MeHg, and HOPs) is discussed, and its application in conjunction with various techniques (e.g., spatial analysis, food source analysis, and compound tracking techniques, etc.) to research the influence of pollutant levels on organisms is summarized. Finally, the limitations of AAs-N-CSIA in pollutant bioaccumulation studies are discussed, including the use of single empirical values of βglu/phe and TDFglu/phe that result in large errors in TP quantification. The weighted βglu/phe and the multi-TDFglu/phe models are still challenging to solve for accurate TP quantification of omnivores; however, factors affecting the variation of βglu/phe and TDFglu/phe are unclear, especially the effect of pollutant bioaccumulation in organisms on internal AA metabolic processes.

Researchers have long been committed to identify nitrate sources in groundwater and to develop an advanced technique for its remediation because better apply remediation solution and management of water quality is highly dependent on the identification of the NO3- sources contamination in water. In this review, we systematically introduce nitrate source tracking tools used over the past ten years including dual isotope and multi isotope techniques, water chemistry profile, Bayesian mixing model, microbial tracers and land use/cover data. These techniques can be combined and exploited to track the source of NO3- as mineral or organic fertilizer, sewage, or atmospheric deposition. These available data have significant implications for making an appropriate measures and decisions by water managers. A continuous remediation strategy of groundwater was among the main management strategies that need to be applied in the contaminated area. Nitrate removal from groundwater can be accomplished using either separation or reduction based process. The application of these processes to nitrate removal is discussed in this review and some novel methods were presented for the first time. Moreover, the advantages and limitations of each approach are critically summarized and based on our own understanding of the subject some solutions to overcomes their drawbacks are recommended. Advanced techniques are capable to attain significantly higher nitrate and other co-contaminants removal from groundwater. However, the challenges of by-products generation and high energy consumption need to be addressed in implementing these technologies for groundwater remediation for potable use.

Clean water and sanitation for the world population is one of the most important challenges established by the Sustainable Development Goals of the United Nations since worldwide, one in three people do not have access to safe drinking water. Groundwater, one of the main sources of fresh water, has been considerably damaged by human activities. Nevertheless, while numerous plants are globally aimed at removing pollutants from surface waters, a much scarcer number of facilities have focused on groundwater remediation. Nowadays, there is increasing concern about the presence of nitrates (NO3-) in groundwaters as a consequence of the intensive use of fertilizers and other anthropogenic sources, such as sewage or industrial wastewater discharge. In this context, the selection and development of highly effective and low-cost solutions for the sustainable management of groundwater resources need to be addressed. Thus, this work collects data from the literature regarding the presence of nitrates in groundwater, and, simultaneously, it reviews the main alternatives available to remove NO3- from groundwater sources. A total of 292 sites have been analyzed categorized by continents, carefully discussing the possible origins of nitrate pollution. In addition, a discussion is carried out of the different technologies currently employed to treat groundwater, highlighting the progress made and the main challenges to be overcome. Finally, the review gathers the data available in the literature for nitrate treatment plants at full-scale.

Developing the rumen\'s capacity to utilise recalcitrant and low-value feed resources is important for ruminant production systems. Early-life nutrition and management practices have been shown to influence development of the rumen in young animals with long-term consequences on their performance. Therefore, there has been increasing interest to understand ruminal development and function in young ruminants to improve feed efficiency, health, welfare, and performance of both young and adult ruminants. However, due to the small size, rapid morphological changes and low initial microbial populations of the rumen, it is difficult to study ruminal function in young ruminants without major invasive approaches or slaughter studies. In this review, we discuss the usefulness of a range of proxies and markers to monitor ruminal function and nitrogen use efficiency (a major part of feed efficiency) in young ruminants. Breath sulphide and methane emissions showed the greatest potential as simple markers of a developing microbiota in young ruminants. However, there is only limited evidence for robust indicators of feed efficiency at this stage. The use of nitrogen isotopic discrimination based on plasma samples appeared to be the most promising proxy for feed efficiency in young ruminants. More research is needed to explore and refine potential proxies and markers to indicate ruminal function and feed efficiency in young ruminants, particularly for neonatal ruminants.

Roots of the higher plants can assimilate inorganic nitrogen by an enzymatic reduction of the most oxidized form (+6) nitrate to the reduced form (-2) glutamate. For such reactions, the substrates (originated from photosynthates) must be imported to supply energy through the reductant-generating systems within the root cells. Intensive studies over last 70 years (reviewed here) revealed the precise mechanisms of nitrate-to-glutamate transformation in roots with elaborate searches of 15N-tracing, enzymes involved, the reductant-supplying system, and nitrate signaling. In the 1970s, the tracing of 15N-labeled nitrate and ammonia in the roots demonstrated the sequential reduction and assimilation of nitrate to nitrite, ammonia, glutamine amide, and then glutamate. These reactions involve nitrate reductase (NADH-NR, EC 1.7.1.1) in the cytosol, nitrite reductase (ferredoxin [Fd]-NiR, EC 1.7.7.1), glutamine synthetase (GS2, EC 6.3.1.2), and glutamate synthase (Fd-GOGAT, EC 1.4.7.1) in the plastids. NADH for NR is generated by glycolysis in the cytosol, and NADPH for Fd-NIR and Fd-GOGAT are produced by the oxidative pentose phosphate pathway (OPPP). Electrons from NADPH are conveyed to reduce NIR and Fd-GOGAT through Fd-NADP+ reductase (FNR, EC 1.6.7.1) specifically in the roots. Physiological and molecular analyses showed the parallel inductions of NR, NIR, GS2, Fd-GOGAT, OPPP enzymes, FNR, and Fd in response to a short-term nitrate supply. Recent studies proposed a molecular mechanism of nitrate-induction of these genes and proteins. Roots can also assimilate the reduced form of inorganic ammonia by the combination of cytosolic GS1 and plastidic NADH-GOGAT.

In the past several decades, a variety of methods have been developed for measuring the isotopic composition of ammonium (δ15N) and nitrate (δ15N and δ18O). This review summarized the advantages and disadvantages of these methods. Nowadays, the most popular method for measu-ring δ15N of ammonium is the combined hypobromite (BrO-) and hydroxylamine (NH2OH) me-thod, while for δ15N and δ18O of nitrate is the denitrifier method and the sodium azide (NaN3) me-thod. These methods convert NH4+ or NO3- into nitrous oxide (N2O) and measure its isotopic compositions, with higher analytical precision because of the lower background concentration of atmospheric N2O. Accordingly, these methods are suitable for the samples with lower N concentration, and normally require 10-60 nmol N. The development of new methods for measuring N isotopic composition has greatly stimulated the studies in nitrogen cycling worldwide.
综述了过去几十年来铵盐和硝酸盐稳定同位素丰度测定方法的历史发展变化,分析了各种方法的优缺点,并对新方法作了介绍和推荐.目前铵盐稳定同位素丰度的最新测定方法为次溴酸盐氧化结合羟胺还原法,硝酸盐氮氧同位素丰度主流的测定方法为反硝化细菌法和镉粉叠氮酸还原化学法.这些方法的主要共同特点是以N2O为分析物,分析精度高,对样品的含氮量需求小,一般只需要10～60 nmol N,适用于低浓度样品.新方法的建立对于国内外开展氮素循环研究将起到极大的推动作用.

During the last several decades, persistent organic pollutants and metals cause great concern for their toxicity in organisms as well as for their bioaccumulation and/or trophic transfer through the food chains in ecosystems. A large number of studies therefore have focused on the trophic levels of organisms to illustrate food web structure, as a critical component in the study of pollutant dynamics and biomagnification. The trends in biomagnification of pollutants in food webs indeed provide fundamental information about the properties and fates of pollutants in ecosystems. The trophic magnification supports the establishment of a reliable trophic structure, which can further aid the understanding of the transport and exposure routes of contaminants in accumulation and risk assessments. Recently, efforts to interpret the food web structure using carbon and nitrogen stable isotope ratios have contributed to better understanding of the fate of pollutants in the ecosystem. However, it is known that this isotope analysis of bulk ones has many weaknesses, particularly for uncertainties on the estimate of trophic levels and therefore of magnification factors for studied organisms, enough to support a regulatory interpretation. In this review, we collate studies that investigated biomagnification characteristics of pollutants in aquatic ecosystems, along with calculated trophic magnification factors. Moreover, we introduce a novel approach, compound-specific stable isotope analysis of nitrogen in amino acids, to establish reliable food web structures and accurate trophic levels for biomagnification studies. This method promises to provide sound results for interpreting the influence of the pollutant in organisms, along with their bioaccumulation and magnification characteristics, as well as that in ecosystem.

Methodologies based on (15)N enrichment (E) and (15)N natural abundance (NA) have been used to obtain quantitative estimates of the response of biological N2 fixation (BNF) of legumes (woody, grain and forage) and actinorhizal plants grown in artificial media or in soil exposed to elevated atmospheric concentrations of carbon dioxide e[CO2] for extended periods of time, in growth rooms, greenhouses, open top chambers or free-air CO2 enrichment (FACE) facilities. (15)N2 has also been used to quantify the response of endophytic and free-living diazotrophs to e[CO2]. The primary criterion of response was the proportional dependence of the N2-fixing system on the atmosphere as a source of N. i.e. the symbiotic dependence (Patm). The unique feature of (15)N-based methods is their ability to provide time-integrated and yield-independent estimates of Patm. In studies conducted in artificial media or in soil using the E methodology there was either no response or a positive response of Patm to e[CO2]. The interpretation of results obtained in artificial media or with (15)N2 is straight forward, not being subject to the assumptions on which the E and NA soil-cultured methods are based. A variety of methods have been used to estimate isotopic fractionation attendant on the NA technique, the so-called \'B value\', which attaches a degree of uncertainty to the results obtained. Using the NA technique, a suite of responses of Patm to e[CO2] has been published, from positive to neutral to sometimes negative effects. Several factors which interact with the response of N2-fixing species to e[CO2] were identified.