Organic matter flow in the food web at a temperate heath under multifactorial climate change

The rising atmospheric CO(2) concentration, increasing temperature and changed patterns of precipitation currently expose terrestrial ecosystems to altered environmental conditions. This may affect belowground nutrient cycling through its intimate relationship with the belowground decomposers. Three climate change factors (elevated CO(2), increased temperature and drought) were investigated in a full factorial field experiment at a temperate heathland location. The combined effect of biotic and abiotic factors on nitrogen and carbon flows was traced in plant root → litter → microbe → detritivore/omnivore → predator food-web for one year after amendment with (15)N(13)C(2)-glycine. Isotope ratio mass spectrometry (IRMS) measurement of (15)N/(14)N and (13)C/(12)C in soil extracts and functional ecosystem compartments revealed that the recovery of (15)N sometimes decreased through the chain of consumption, with the largest amount of bioactive (15)N label pool accumulated in the microbial biomass. The elevated CO(2) concentration at the site for 2 years increased the biomass, the (15)N enrichment and the (15)N recovery in detritivores. This suggests that detritivore consumption was controlled by both the availability of the microbial biomass, a likely major food source, and the climatic factors. Furthermore, the natural abundance δ(13)C of enchytraeids was significantly altered in CO(2)-fumigated plots, showing that even small changes in δ(13)C-CO(2) can be used to detect transfer of carbon from primary producers to detritivores. We conclude that, in the short term, the climate change treatments affected soil organism activity, possibly with labile carbohydrate production controlling the microbial and detritivore biomass, with potential consequences for the decomposition of detritus and nutrient cycling. Hence, there appears to be a strong coupling of responses in carbon and nitrogen cycling at this temperate heath.     

Das geheime Leben der Pflanzenwurzeln: Liebigs Erben

The mineral theory consitutes the basis of modern plant nutrition. Plant nutrients are chemical elements, which are essential for growth and normal development of plants. Part of them is taken up from the atmosphere. In the case of molecular atmospheric nitrogen plants have to establish a symbiosis with specific bacteria to utilize this form of nitrogen by N2 fixation. For this purpose, special organs, the nodules, are developed that fulfill the structural requirements for the activity of the key enzyme of N2 fixation, namely the nitrogenase. The prevailing part of nutrients is acquired by plants via the root system. Since the uptake depends on solubilized nutrients in the soil solution, the mobilization of nutrients is of particular importance. The most important strategies of soil phosphate mobilization are descibed for white lupin, which serves as a model plant in modern research.     

光催化固氮:人工光合成的新途径（英文）

氨不仅是一种广泛使用的化工原料,还可用作重要的能源载体.哈伯法合成氨被认为是20世纪最伟大的发明之一,为人类社会的发展做出了巨大贡献.同时,氨合成过程每年需要消耗世界总能源的1%–2%.因此,开发绿色清洁的氨合成方法一直是世界范围内工业界和学术界关注的热点.随着人工光合成太阳燃料研究的蓬勃发展,利用太阳能光催化的方式实现在温和条件下合成氨吸引了越来越多研究者的兴趣,因为这是一条最为理想的能源利用途径,即直接利用太阳能将氮气和水转化为氨.近期,该研究领域涌现了一系列有代表性的研究工作,报道了利用半导体光催化剂实现太阳能到氨的转化,虽然整体效率仍很低,但是已经证明了利用太阳能直接将氮气转化为氨的可能性.光催化合成氨过程中,最具挑战的是氮气分子在半导体光催化剂表面的吸附和活化.研究表明,通过在半导体光催化剂表面引入空位或者缺陷可有效地增加氮气的吸附,且很可能成为氮气分子活化并参与反应的活性中心.此外,借鉴自然界豆类植物固氮酶的独特结构,利用其对于氮气分子高效活化的独特优势,构建自然-人工杂化体系也是提升氮气吸附与活化的有效策略之一.本综述将从合成氨过程中氮气的吸附与活化问题入手,分别从缺陷与空位调控和固氮酶两个方面的策略考虑,结合几个典型的光催化剂体系(如卤氧化铋,二氧化钛及水滑石等)作为示例,介绍空位调控与模拟固氮酶策略对太阳能光催化固氮的影响并分析其可能的机理.虽然人工光合成固氮研究取得了一些进展,但是目前效率太低,亟需从基础科学问题的认识和理解上有新的突破,如氮气分子的吸附与活化微观过程、空位可控调变策略、新型光催化剂的开发与表界面修饰、氨氧化逆反应的抑制策略及精确的理论模拟指导人工光合成固氮体系的构建等.最后,对人工光合成固氮研究方向面临的挑战和未来的发展方向进行了总结与展望.     

An improved analytical method for the determination of urea nitrogen isotopomers in biological samples utilizing continuous flow isotope ratio mass spectrometry

Over the past few years numerous dual inlet isotope ratio mass spectrometry (IRMS) applications have been adapted to continuous flow systems which allow the automation of sample admission and a higher throughput. The isotopomer analysis of urea nitrogen by IRMS requires the offline conversion of urea into nitrogen gas before analysis. The oxidation of urea with LiOBr results in the monomolecular degradation of urea, which preserves the identity of the parent urea molecule, and has to be conducted under vacuum to prevent contamination with atmospheric nitrogen. We have developed an offline system of urea degradation utilizing disposable Exetainers, in which atmospheric nitrogen is displaced by helium. Recovery of urea nitrogen was linear within the range of the standards tested (0 to 420 microg nitrogen) and standard curves for 15N15N-urea standards showed high coefficients of determination (R2 > 0.9998). A small portion of urea degrades in a non-monomolecular fashion and has been shown to depend on the concentration of urea in the sample. Long-term storage of prepared samples showed a decline in 15N15N enrichment, suggesting air contamination. However, samples were stable for 24 h, which allows for the analysis of large sample batches. Interest in urea metabolism, particularly in ruminant species, has increased recently due to the environmental implications of urea and nitrogen excretion by farm animals. This novel analytical method will allow for accurate measurements and the rapid throughput needed in order to support these field studies.     

Activation and functionalization of molecular nitrogen by metal complexes

Molecular nitrogen is intrinsically unreactive, so much so that it has confounded chemists for decades in attempts to functionalize this abundant diatomic molecule. While biological systems and industrial processes can fix nitrogen to form ammonia, the challenge is to discover a process that involves a homogeneous catalyst that can utilize N(2) as a feedstock to generate higher value organonitrogen materials. In this review, the activation of molecular nitrogen by transition metal complexes is reviewed with the view to present new kinds of transformations for coordinated dinitrogen. Moreover, some reaction types that are as yet unknown are outlined to try and stimulate further research in this area.     

Natural bond orbital analysis of some para-substituted N-nitrosoacetanilide biological molecules

Theoretical study of several para-substituted N-nitrosoacetanilide biological molecules has been performed using density functional B3LYP method with 6-31G(d,p) basis set. Geometries obtained from DFT calculation were used to perform natural bond orbital analysis. The p characters of two nitrogen natural hybrid orbital (NHO) σ _N3–N2 bond orbitals increase with increasing σ _p values of the para substituent group on the benzene, which results in a lengthening of the N₃–N₂ bond. The p characters of oxygen NHO σ _O1–N2 and nitrogen NHO σ _O1–N2 bond orbitals decrease with increasing σ _p values of the para substituent group on the benzene, which results in a shortening of the N₂=O₁ bond. It is also noted that decreased occupancy of the localized σ _N3–N2 orbital in the idealized Lewis structure, or increased occupancy of s_N3-N2^* \sigma_{\rm N3-N2}^{\ast} of the non-Lewis orbital, and their subsequent impact on molecular stability and geometry (bond lengths) are also related with the resulting p character of the corresponding nitrogen NHO of σ _N3–N2 bond orbital.     

Stable isotope natural abundance of nitrous oxide emitted from Antarctic tundra soils: effects of sea animal excrement depositions

Nitrous oxide (N2O), a greenhouse gas, is mainly emitted from soils during the nitrification and denitrification processes. N2O stable isotope investigations can help to characterize the N2O sources and N2O production mechanisms. N2O isotope measurements have been conducted for different types of global terrestrial ecosystems. However, no isotopic data of N2O emitted from Antarctic tundra ecosystems have been reported although the coastal ice-free tundra around Antarctic continent is the largest sea animal colony on the global scale. Here, we report for the first time stable isotope composition of N2O emitted from Antarctic sea animal colonies (including penguin, seal and skua colonies) and normal tundra soils using in situ field observations and laboratory incubations, and we have analyzed the effects of sea animal excrement depositions on stable isotope natural abundance of N2O. For all the field sites, the soil-emitted N2O was 15N- and 18O-depleted compared with N2O in local ambient air. The mean delta values of the soil-emitted N2O were delta15N = -13.5 +/- 3.2 per thousand and delta18O = 26.2 +/- 1.4 per thousand for the penguin colony, delta15N = -11.5 +/- 5.1 per thousand and delta18O = 26.4 +/- 3.5 per thousand for the skua colony and delta15N = -18.9 +/- 0.7 per thousand and delta18O = 28.8 +/- 1.3 per thousand for the seal colony. In the soil incubations, the isotopic composition of N2O was measured under N2 and under ambient air conditions. The soils incubated under the ambient air emitted very little N2O (2.93 microg N2O--N kg(-1)). Under N2 conditions, much more N2O was formed (9.74 microg N2O--N kg(-1)), and the mean delta15N and delta18O values of N2O were -19.1 +/- 8.0 per thousand and 21.3 +/- 4.3 per thousand, respectively, from penguin colony soils, and -17.0 +/- 4.2 per thousand and 20.6 +/- 3.5 per thousand, respectively, from seal colony soils. The data from in situ field observations and laboratory experiments point to denitrification as the predominant N2O source from Antarctic sea animal colonies.     

用激光诱导击穿光谱在空气中定量测量土壤中氮元素方法的研究-基于CN分子谱线

土壤中氮元素的快速检测在现代农业中有重要意义,传统方法前处理复杂不适合现场快速检测。激光诱导击穿光谱法已被证明可以用来对土壤中的元素进行检测,但对于土壤中的氮元素检测,如何规避大气环境中氮气的影响是不可回避的问题。提出了一种基于激光诱导击穿光谱法的空气中土壤全氮检测方法,通过对不同土壤标准样品的CN(388.3nm)分子特征谱线对比分析并建立了分析模型,结果显示,可以很好地规避大气中氮元素的影响,其检出限小于0.14%。对比于N(746.8nm)处的原子特征峰在空气中和氩气中的分析结果,其优势非常明显。     

Retention of pesticides and nutrients in a vegetated buffer root zone compared to soil with low biological activity

A laboratory study has been conducted to examine the retention processes of nitrogen and pesticides through vegetated buffer zones compared to bare soil. Soil columns with low biological activity and vegetation columns with normal biological activity were tested. Pesticides frequently used in vegetable production (namely aclonifen, azinphos-methyl, chlorpropham, diazinon, dimethoate, fluazinam, iprodione, linuron, metalaxyl, metamitron, metribuzin and propachlor) equal to 1/50 to 1/5 part of recommended doses, and nutrients equal to 1, 5 and 20 mg N/L and 0.2 mg P/L, were added. The pesticide retention was more than 60% for all pesticides, except dimethoate, with a retention of about 30% in columns with low microbial activity. Biological transformation and plant uptake were important for removal of nitrogen and organic matter. Nitrogen retention was high (over 90%) in vegetation columns. Plant uptake and phosphorus content in soil were important for phosphorus retention.     

Determination of the natural abundance δ15N of taurine by gas chromatography-isotope ratio measurement mass spectrometry

The measurement of the nitrogen isotope ratio of taurine (2-aminoethanesulphonic acid) in biological samples has a large number of potential applications. Taurine is a small water-soluble molecule which is notoriously difficult to analyze due to its polarity and functionality. A method is described which allows the determination of the natural abundance δ(15)N values of taurine and structural analogues, such as 3-amino-1-propanesulphonic acid (APSA), by isotope ratio mass spectrometry interfaced to gas chromatography (GC-irm-MS). The one-step protocol exploits the simultaneous derivatization of both functionalities of these aminosulphonic acids by reaction with triethylorthoacetate (TEOA). Conditions have been established which ensure quantitative reaction thus avoiding any nitrogen isotope fractionation during derivatization and workup. The differences in the δ(15)N values of derivatized and non-derivatized taurine and APSA all fall within the working range of 0.4‰ (-0.02 to 0.39‰). When applied to four sources of taurine with various δ(15)N values, the method achieved excellent reproducibility and accuracy. The optimized method enables the determination of the natural abundance δ(15)N values of taurine over the concentration range 1.5-7.84 μmol.mL(-1) in samples of biological origin.     

乙腈溶液中锇氮化合物电化学诱导桥氮偶联过程的原位FTIR反射光谱研究(英)

采用原位 红外反射 光谱（ｉｎ＿ｓｉｔｕ Ｆ Ｔ Ｉ Ｒ Ｓ） 结合紫 外可见 光谱（ Ｕ Ｖ／ Ｖｉｓ） 和 电化学循 环伏安技术（ Ｃ Ｖ） ,研究了［ Ｏｓ Ｖ Ｉ（ Ｎ）（ Ｎ Ｈ３） ４］（ Ｃ Ｆ３ Ｓ Ｏ３） ３ 的 电化 学诱 导桥 氮偶 联过 程． 首次 在 Ｐｔ 电 极上检测到桥 氮 混 合 价 锇 物 种［ Ｏｓ＿ Ｎ ≡ Ｎ＿ Ｏｓ］ 及 其 随 电 位 的 变 化 过 程 ． 在 约 ２ ｍ ｍ ｏｌ／ Ｌ 〔 Ｏｓ Ｖ Ｉ（ Ｎ）（ Ｎ Ｈ３） ４〕（ Ｃ Ｆ３ Ｓ Ｏ３） ３ ＋ ０ ．１ ｍ ｏｌ／ Ｌ Ｔ Ｂ Ａ Ｈ 的乙腈溶 液中,选 取０ ．４ ～－ １ ．０ Ｖ 电位 区间 １００ ｍ Ｖ／ｓ 扫描速度 ,对 Ｐｔ 或 Ｇ Ｃ 电 极进行电 化学循环 伏安处 理,处 理 后的 电极 表 面均 可积 累 一层 深绿 色 的沉积物,表 明电化 学诱导 Ｎ＿ Ｎ 偶联效 应已在 电极上 发生, 并形 成了 混合 价 桥氮 络合 物． 同时 ,在 上述过程中 所生成 的混合价 锇氮物种 ,有可能 较强地 吸附在电 极表 面,且 形成 一定 厚度 的表 面层, 从而减缓了 体系中 Ｏｓ Ｖ Ｉ≡ Ｎ 物种在电 极上的 继续还 原．同 时可 以看 到,随 着 Ｃ Ｖ 的 不 断进 行,溶 液 颜色将逐渐 地由黄变 绿．经过 长时间（ 约     

哒嗪酮的乙酰胺类化合物的合成及生物活性

2-叔丁基-4-氯-5-羟基-哒嗪-3(2H)-酮(3)与相应的N-烷基或N,N-二烷基氯乙酰胺(4a-4l)反应,合成了2-叔丁基-5-[(N-烷基或N,N-二烷基乙酰胺)氧基]-4-氯-哒嗪-3(2H)-酮(1a-1l),生物活性测定结果表明部分化合物(1a-1l)具有杀虫活性。     

Methods to adjust for the interference of N2O on delta13C and delta18O measurements of CO2 from soil mineralization

In this paper we present an overview of the present knowledge relating to methods that avoid interference of N2O on delta13C and delta18O measurements of CO2. The main focus of research to date has been on atmospheric samples. However, N2O is predominantly generated by soil processes. Isotope analyses related to soil trace gas emissions are often performed with continuous flow isotope ratio mass spectrometers, which do not necessarily have the high precision needed for atmospheric research. However, it was shown by using laboratory and field samples that a correction to obtain reliable delta13C and delta18O values is also required for a commercial continuous flow isotope ratio mass spectrometer. The capillary gas chromatography column of the original equipment was changed to a packed Porapak Q column. This adaptation resulted in an improved accuracy and precision of delta13C (standard deviation(Ghent): from 0.2 to 0.08 per thousand; standard deviation(Lincoln): from 0.2 to 0.13 per thousand) of CO2 for N2O/CO2 ratios up to 0.1. For delta18O there was an improvement for the standard deviation measured at Ghent University (0.13 to 0.08 per thousand) but not for the measurements at Lincoln University (0.08 to 0.23 per thousand). The benefits of using the packed Porapak Q column compared with the theoretical correction method meant that samples were not limited to small N(2)O concentrations, they did not require an extra N2O concentration measurement, and measurements were independent of the variable isotopic composition of N2O from soil.     

Chemical synthesis and biological properties of pyridine epothilones

BACKGROUND: Numerous analogs of the antitumor agents epothilones A and B have been synthesized in search of better pharmacological profiles. Insights into the structure-activity relationships within the epothilone family are still needed and more potent and selective analogs of these compounds are in demand, both as biological tools and as chemotherapeutic agents, especially against drug-resistant tumors. RESULTS: A series of pyridine epothilone B analogs were designed, synthesized and screened. The synthesized compounds exhibited varying degrees of tubulin polymerization and cytotoxicity properties against a number of human cancer cell lines depending on the location of the nitrogen atom and the methyl substituent within the pyridine nucleus. CONCLUSIONS: The biological screening results in this study established the importance of the nitrogen atom at the ortho position as well as the beneficial effect of a methyl substituent at the 4- or 5-position of the pyridine ring. Two pyridine epothilone B analogs (i.e. compounds 3 and 4) possessing higher potencies against drug-resistant tumor cells than epothilone B, the most powerful of the naturally occurring epothilones, were identified.     

Gross and net rates of nitrogen mineralisation in soil amended with composted olive mill pomace

Olive mill pomace is the major waste product in the olive oil industry and composting these by-products for the purpose of recycling nutrients and organic matter is a sound environmental strategy. Yet little is known about the quantity and timing of nitrogen (N) release from composted olive mill pomace. This paper assesses both gross (using the (15)N dilution technique) and net (aerobic incubation) nitrogen (N) mineralisation and N(2)O emissions of soil amended with seven commercially available composts of olive mill pomace (COMP). All are currently produced in Andalusia and differ in the proportions of raw materials co-composted with the pomace. The absence of significant differences in net N or gross mineralisation and nitrification in COMP-amended soil compared with a control, except for COMP combined with poultry manure, highlighted the recalcitrant nature of the COMP-N. Applications of COMP are hence unlikely to supply available N in available forms, at least in the short-term. Furthermore, N(2)O emissions from COMP-amended soil were negligible and, therefore, applications in the field should not result in increased N loss through denitrification.     

Assessment of effects of the rising atmospheric nitrogen deposition on nitrogen uptake and long-term water-use efficiency of plants using nitrogen and carbon stable isotopes

This study assesses the effects of the atmospheric nitrogen (N) deposition on the N uptake and the long-term water-use efficiency of two C(3) plants (Agropyron cristatum and Leymus chinensis) and two C(4) plants (Amaranthus retroflexus and Setaria viridis) using N and C stable isotopes. In addition, this study explores the potential correlation between leaf N isotope (δ(15)N) values and leaf C isotope (δ(13)C) values. This experiment shows that the atmospheric N deposition has significant effects on the N uptake, δ(15)N and leaf N content (N(m)) of C(3) plants. As the atmospheric N deposition rises, the proportion and the amount of N absorbed from the simulated atmospheric deposition become higher, and the δ(15)N and N(m) of the two C(3) plants both also increase, suggesting that the rising atmospheric N deposition is beneficial for C(3) plants. However, C(4) plants display different patterns in their N uptake and in their variations of δ(15)N and N(m) from those of C(3) plants. C(4) plants absorb less N from the atmospheric deposition, and the leaf N(m) does not change with the elevated atmospheric N deposition. Photosynthetic pathways may account for the differences between C(3) and C(4) plants. This study also shows that atmospheric N deposition does not play a role in determining the δ(13)C and in the long-term water-use efficiency of C(3) and C(4) plants, suggesting that the long-term water-use pattern of the plants does not change with the atmospheric N input. In addition, this study does not observe any relationship between leaf δ(15)N and leaf δ(13)C in both C(3) and C(4) plants.     

Use of a novel nitrification inhibitor to reduce nitrous oxide emission from (15)N-labelled dairy slurry injected into soil

Recent recommendations for environmentally sound use of liquid animal manure often include injection of slurry into soil. Two of the most important undesired side effects, ammonia (NH(3)) volatilisation and odour emissions, are usually significantly reduced by slurry injection. On the other hand, because of the higher amount of nitrogen (N) remaining in soil, the risk of nitrate (NO(3)(-)) leaching and nitrous oxide (N(2)O) emissions is increased. Thus, the reduction of local effects caused by NH(3) deposition, e.g. N enrichment and soil acidification, may be at the cost of large-scale effects such as ozone depletion and global warming as a result of emitted N(2)O. In this context, nitrification inhibitors can contribute significantly to a reduction in NO(3)(-) leaching and N(2)O production. A field experiment was carried out at IGER, North Wyke, which aimed to evaluate the effect of the new nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP/ENTEC). For this experiment, (15)N enriched dairy slurry was used and the isotopic label in soil N as well as in N(2)O were studied. After slurry injection into the grassland soil in August 2000, the major emissions of N(2)O occurred during the first ten days. As expected, high N(2)O emission rates and (15)N content of the emissions were concentrated on the slurry injection slots, showing a steep decrease towards the untreated centre-point between slurry injection slots. The nitrification inhibitor DMPP proved to be very efficient in reducing N(2)O emissions. At a rate of 2 kg DMPP ha(-1), the total amount of N(2)O emitted was reduced by 32%, when compared with slurry injection without DMPP. The isotopic label of the emitted N(2)O showed that during the 22-day experimental period, emissions from the slurry N pool were strongly reduced by DMPP from 0.93 kg N(2)O-N ha(-1) (-DMPP) to 0.50 kg N(2)O-N ha(-1) (+DMPP), while only a minor effect on emissions from the soil N pool was observed (0.69 to 0.60 kg N(2)O-N ha(-1); -DMPP, +DMPP, respectively).     

Vanadium nitrogenase: a two-hit wonder?

Nitrogenase catalyzes the biological conversion of atmospheric dinitrogen to bioavailable ammonia. The molybdenum (Mo)- and vanadium (V)-dependent nitrogenases are two homologous members of this metalloenzyme family. However, despite their similarities in structure and function, the characterization of V-nitrogenase has taken a much longer and more winding path than that of its Mo-counterpart. From the initial discovery of this nitrogen-fixing system, to the recent finding of its CO-reducing capacity, V-nitrogenase has proven to be a two-hit wonder in the over-a-century-long research of nitrogen fixation. This perspective provides a brief account of the catalytic function and structural basis of V-nitrogenase, as well as a short discussion of the theoretical and practical potentials of this unique metalloenzyme.     

Coordination chemistry of the antitumor metallocene molybdocene dichloride with biological ligands

The relative affinity of molybdocene dichloride (Cp(2)MoCl(2)) for the thiol, amino, carboxylate, phosphate(O) and heterocyclic(N) donor ligands present in amino acids and nucleotides, has been studied in aqueous solutions at pH 2-7, using (1)H, (13)C and (31)P NMR spectroscopy. Molybdocene dichloride forms the highly water soluble, air-stable complexes Cp(2)Mo(Cys)(2) and Cp(2)Mo(GS)(2) with cysteine and glutathione respectively, via coordination of the deprotonated thiol groups. While coordination to the imidazole nitrogen in histidine was observed, no evidence for coordination of the amino or carboxylate groups in the amino acids cysteine, histidine, alanine or lysine to Cp(2)MoCl(2) was detected. Competition experiments with dAMP, ribose monophosphate and histidine showed preferential coordination to the cysteine thiol over the phosphate(O) and heterocyclic(N) groups. Cp(2)Mo(Cys)(2) is stable in the presence of excess dAMP or ribose monophosphate and Cys displaces coordinated histidine, dAMP or ribose monophosphate to give Cp(2)Mo(Cys)(2). These results provide further evidence against interaction with DNA as the key interaction that is related to the antitumor activity of molybdocene dichloride. The implications of these results for the biological activity of the antitumor metallocene and the likely species formed in vivo are discussed.     

Belowground fate of 15N injected into sweetgum trees (Liquidambar styraciflua) at the ORNL FACE Experiment

Nitrogen (N) cycling can be an important constraint on forest ecosystem response to elevated atmospheric CO₂. Our objective was to trace the movement of ¹⁵N, injected into tree sap, to labile and stable forms of soil organic matter derived partly from the turnover of tree roots under elevated (545 ppm) and ambient (394 ppm) atmospheric CO₂ concentrations at the Oak Ridge National Laboratory (ORNL) FACE (Free‐Air Carbon Dioxide Enrichment) Experiment. Twenty‐four sweetgum trees, divided equally between CO₂ treatments, were injected with 3.2 g ¹⁵N‐ammonium sulfate (99 atom %), and soil samples were collected beneath the trees over a period of 89 weeks. For 16 cm deep soil samples collected beneath the study trees, there was 28% more fine root (less than or equal to 2 mm diameter) biomass under elevated CO₂ (P = 0.001), but no significant treatment effect on the amounts of necromass, coarse root biomass, or on the N concentrations in tree roots and necromass. Nitrogen‐15 moved quickly into roots from the stem injection site and the ¹⁵N content of roots, necromass, and labile organic matter (i.e. particulate organic matter, POM) increased over time. At 89 weeks post‐injection, approximately 76% of the necromass ¹⁵N originated from fine root turnover. Nitrogen‐15 in POM had a relatively long turnover time (47 weeks) compared with ¹⁵N in roots (16 to 22 weeks). Over the 1.7 year period of the study, ¹⁵N moved from roots into slower cycling POM and the disparity in turnover times between root N and N in POM could impose progressive limitations on soil N availability with stand maturation irrespective of atmospheric CO₂, especially if the release of N through the decomposition of POM is essential to sustain forest net primary production. Published in 2009 by John Wiley & Sons, Ltd.