Precursors of glutamic acid nitrogen in primary neuronal cultures: Studies with15N

We utilized gas chromatography-mass spectrometry to study the transfer of¹⁵N from [2-¹⁵N]glutamine, [¹⁵N]leucine, [¹⁵N]alanine, or¹⁵NH₄Cl to [¹⁵N]glutamate and [¹⁵N]aspartate in cultured cerebrocortical GABA-ergic neurons from the mouse. Initial rates of¹⁵N appearance (atom % excess) were somewhat higher with 2mM [2-¹⁵N]glutamine as a precursor than with 1mM [¹⁵N]leucine or 1mM [¹⁵N]alanine, but initial net formation (nmol [¹⁵N]glutamate/mg protein.min^–1) was roughly comparable with all precursors. At steady-state¹⁵N labeling was about two times greater with 2mM [2-¹⁵N]glutamine as precursor. The subsequent transfer of¹⁵N from glutamate to aspartate was extremely rapid, the labelling pattern of these two amino acid pools being virtually indistinguishable. We observed little reductive amination of 2-oxo-glutarate to yield [¹⁵N]glutamate in the presence of 0.3mM¹⁵NH₄Cl. Reductive amination through glutamate dehydrogenase was much more prominent at a concentration of 3.0mM¹⁵NH₄Cl. Glutamate formation via reductive amination was unaffected by inclusion of 1 mM 2-oxo-glutarate in the incubation medium. These results indicate that glutamate synthesis in cultured GABA-ergic neurons is derived not only from the glutaminase reaction, but also from transamination reactions in which both leucine and alamine are efficient N donors. Reductive amination of 2-oxo-glutarate in the glutamate dehydrogenase pathway plays a relatively minor role at lower concentrations of extracellular ammonia but becomes quite active at 3mM ammonia.     

15N自然丰度法在陆地生态系统氮循环研究中的应用

随着氮沉降的不断增加以及人们对全球变化问题的日益关注, 稳定同位素技术在全球变化研究中得到广泛的应用。因为植物和土壤的氮同位素组成记录了氮循环影响因子的综合作用, 并且具有测量简单以及不受取样时间和空间限制的优点, 所以氮同位素自然丰度法被用于氮循环的研究中。该文从氮循环过程中植物和土壤的氮分馏入手, 总结国内外相关文献, 阐述了植物和土壤氮自然丰度在预测生态系统氮饱和和氮循环长期变化趋势中的应用; 总结了利用树轮δ ¹⁵N法研究氮循环过程中应该注意的事项以及目前尚未解决的问题。     

Techniques of preparing plant material for chromatographic separation and analysis

This paper discusses preparation techniques of samples of plant material for chromatographic analysis. Individual steps of the procedures used in sample preparation, including sample collection from the environment or from tissue cultures, drying, comminution, homogenization, leaching, extraction, distillation and condensation, analyte enrichment, and obtaining the final extracts for chromatographic analysis are discussed. The techniques most often used for isolation of analytes from homogenized plant material, i.e., Soxhlet extraction, ultrasonic solvent extraction (sonication), accelerated solvent extraction, microwave-assisted extraction, supercritical-fluid extraction, steam distillation, as well as membrane processes are emphasized. Sorptive methods of sample enrichment and removal of interferences, i.e., solid-phase extraction, and solid-phase micro-extraction are also discussed.     

Effects of storage conditions on total carbon and nitrogen contents of soil and plant samples

Aims The storage of soil and plant samples has important significance for ecological studies, but has not been widely used. This study aims to compare total carbon/nitrogen content of soil and plant samples before and after long term storage, and further to investigate the feasibility of archiving samples for time series ecological studies at large temporal scales.Methods Soil and plant samples were collected in the growing season in 2011. Carbon/nitrogen mass fraction were analyzed after four years of storage, and were compared with the data obtained before storage using pairwise t-test and linear regression.Important findings Nitrogen mass fractions of stored samples were linearly correlated to the data before storage along the 1:1 line under different storage conditions, and the correlation coefficient r was greater than 0.98 (except for soil samples stored at temperature lower than 20 °C and with particle size <2 mm, r = 0.91). The carbon mass fraction after storage was changed by the storage conditions. Carbon mass fractions of stored samples with particle size <0.15 mm were linearly correlated to the data before storage along the 1:1 line (r > 0.98). Carbon mass fractions of samples with particle size <2 mm increased after storage, and the slope of the linear relationship was 1.26 and 1.04 for soil and plant samples respectively. These results indicated that, nitrogen content of stored samples was stable under different storage conditions, while the stability of carbon content was affected by sample particle size but by storage temperature. Archived samples used for carbon/nitrogen analysis were suggested to be ground to particle size <0.15 mm under fully dry and completely sealed conditions.     

Solid-state 15N NMR analysis of highly 15N-enriched plant materials

Solid-state ¹⁵N NMR spectroscopy was used to determine the chemical nature of nitrogen in ¹⁵N-enriched material from the roots and stems of wheat (Triticum aesitivum), field pea (Pisum sativum) and kikuyu grass (Pennisetum clandestinum) and from the roots, stems and leaves of a eucalyptus species (Eucalyptus globulus). Nitrogen-15 cross polarization (CP) spectra of the materials were all very similar, with 64–75% of total signal assigned to amide N. Spin counting analysis indicated that 37–80% of potential signal was accounted for in the CP spectra, and that NMR observability using the CP technique (N _obs -CP) was higher for stems and leaves than for roots, and higher for wheat and eucalyptus than for peas and kikuyu. The ¹⁵N direct polarization (DP) spectra contained higher proportions of signal assigned to amine (up to 22%) and nitrate (up to 17%), and less assigned to amide N (50–72%) than the corresponding CP spectra. Spin counting analysis indicated that 68–93% of potential signal was accounted for in the DP spectra, confirming the DP technique to be more quantitatively reliable than CP.     

The influence of biological soil crusts on 15N translocation in soil and vascular plant in a temperate desert of northwestern China

Aims Desert ecosystems are often characterized by patchy distribution of vascular plants, with biological soil crusts (BSC) covering interplant spaces. However, few studies have comprehensively examined the linkage between BSC and vascular plants through nitrogen (N) or element translocation. The objective of this study was to evaluate the ecological roles of BSC on N translocation from soil to the dominant herb Erodium oxyrrhynchum Bieb. (Geraniaceae) in a temperate desert in China.Methods Isotopes (including 15 N-Glu, 15 N-NH 4 Cl and 15 N-NaNO 3) were used as a tracer to detect translocation of N in two types of desert soil (BSC covered; bare) to the dominant herb E. oxyrrhynchum. Three different forms of 15 N-enriched N compounds were applied as a point source to small patches of BSC and to bare soil. And we measured isotopes (14 N and 15 N) and obtained the concentration of labeled- 15 N in both vascular plants and soils at different distances from substrate applicationImportant findings Plants of E. oxyrrhynchum growing in BSC-covered plots accumulated more δ 15 N than those growing in the bare soil. Similarly, soil from BSC-covered plots showed a higher concentration of labeled-N irrespective of form of isotope, than did the bare soil. The concentration of dissolved organic N (15 N-Glu) in E. oxyrrhynchum was higher than that of dissolved inorganic N (15 N-NH 4 Cl and 15 N-NaNO 3). Soil covered by BSC also accumulated considerably more dissolved organic N than bare soil, whereas the dominant form of 15 N concentrated in bare soil was dissolved inorganic N. Correlation analysis showed that the concentration of labeled-N in plants was positively related to the concentration of labeled-N in soils and the N% recorded in E. oxyrrhynchum. Our study supports the hypothesis that BSC facilitates 15 N translocation in soils and vascular plants in a temperate desert of northwestern China.     

Influence of biochemical quality on C and N mineralisation from a broad variety of plant materials in soil

We studied C and N mineralisation patterns from a large number of plant materials (76 samples, covering 37 species and several plant parts), and quantified how these patterns related to biological origin and selected indicators of chemical composition. We determined C and N contents of whole plant material, in water soluble material and in fractions (neutral detergent soluble material, cellulose, hemicellulose and lignin) obtained by stepwise chemical digestion (modified van Soest method). Plant materials were incubated in a sandy soil under standardised conditions (15 °C, optimal water content, no N limitation) for 217days, and CO₂ evolution and soil mineral N contents were monitored regularly. The chemical composition of the plant materials was very diverse, as indicated by total N ranging from 2 to 59 mg N g^–1, (i.e. C/N-ratios between 7 and 227). Few materials were lignified (median lignin=4% of total C). A large proportion of plant N was found in the neutral detergent soluble (NDS) fraction (average 84%) but less of the plant C (average 46%). Over the entire incubation period, holocellulose C content was the single factor that best explained the variability of C mineralisation (r=–0.73 to –0.82). Overall, lignin C explained only a small proportion of the variability in C mineralisation (r=–0.44 to –0.51), but the higher the lignin content, the narrower the range of cumulative C mineralisation. Initial net N mineralisation rate was most closely correlated (r=0.76) to water soluble N content of the plant materials, but from Day 22, net N mineralisation was most closely correlated to total plant N and NDS-N contents (r varied between 0.90 and 0.94). The NDS-N content could thus be used to roughly categorise the net N mineralisation patterns into (i) sustained net N immobilisation for several months; (ii) initial net N immobilisation, followed by some re-mineralisation; and (iii) initially rapid and substantial net N mineralisation. Contrary to other studies, we did not find plant residue C/N or lignin/N-ratio to be closely correlated to decomposition and N mineralisation.     

Assessing crop N status of fertigated vegetable crops using plant and soil monitoring techniques

Evaluation of crop N status will assist optimal N management of intensive vegetable production. Simple procedures for monitoring crop N status such as petiole sap [NO₃^?–N], leaf N content and soil solution [NO₃^?] were evaluated with indeterminate tomato and muskmelon. Their sensitivity to assess crop N status throughout each crop was evaluated using linear regression analysis against nitrogen nutrition index (NNI) and crop N content. NNI is the ratio between the actual and the critical crop N contents (critical N content is the minimum N content necessary to achieve maximum growth), and is an established indicator of crop N status. Nutrient solutions with four different N concentrations (treatments N1–N4) were applied throughout each crop. Average applied N concentrations were 1, 5, 13 and 22 mmol L^?1 in tomato, and 2, 7, 13 and 21 mmol L^?1 in muskmelon. Respective rates of N were 23, 147, 421 and 672 kg N ha^?1 in tomato, and 28, 124, 245 and 380 kg N ha^?1 in muskmelon. For each N treatment in each crop, petiole sap [NO₃^?–N] was relatively constant throughout the crop. During both crops, there were very significant (P < 0.001) linear relationships between both petiole sap [NO₃^?–N] and leaf N content with NNI and with crop N content. In indeterminate tomato, petiole sap [NO₃^?–N] was very strongly linearly related to NNI (R² = 0.88–0.95, P < 0.001) with very similar slope and intercept values on all dates. Very similar relationships were obtained from published data of processing tomato. A single linear regression (R² = 0.77, P < 0.001) described the relationship between sap [NO₃^?–N] and NNI for both indeterminate and processing tomato, each grown under very different conditions. A single sap [NO₃^?–N] sufficiency value of 1050 mg N L^?1 was subsequently derived for optimal crop N nutrition (at NNI = 1) of tomato grown under different conditions. In muskmelon, petiole sap [NO₃^?–N] was strongly linearly related to NNI (R² = 0.75 – 0.88, P < 0.001) with very similar slope and intercept values for much of the crop (44–72 DAT, days after transplanting). A single linear relationship between sap [NO₃^?–N] and NNI (R² = 0.77, P < 0.001) was derived for this period, but sap sufficiency values could not be derived for muskmelon as NNI values were >1. Relationships between petiole sap [NO₃^?–N] with crop N content, and leaf N content with both NNI and crop N content had variable slopes and intercept values during the indeterminate tomato and the muskmelon crops. Soil solution [NO₃^?] in the root zone was not a sensitive indicator of crop N status. Of the three systems examined for monitoring crop/soil N status, petiole sap [NO₃^?–N] is suggested to be the most useful because of its sensitivity to crop N status and because it can be rapidly analysed on the farm.     

Measurement of fluorine pollutant in plant leaves and soil using nuclear reaction analysis

In this article, the soil and the leaves of plants, parasol, cotton, and glossy privet around a fluorine-polluted area were taken as the samples, and fluorine concentration of the samples were studied using the nuclear reaction¹⁹F(P,α)¹⁶O, and some results were given.     

Increased soil moisture content increases plant N uptake and the abundance of 15N in plant biomass

The natural abundance of ¹⁵N in plant biomass has been used to infer how N dynamics change with elevated atmospheric CO₂ and changing water availability. However, it remains unclear if atmospheric CO₂ effects on plant biomass ¹⁵N are driven by CO₂-induced changes in soil moisture. We tested whether ¹⁵N abundance (expressed as δ¹⁵N) in plant biomass would increase with increasing soil moisture content at two atmospheric CO₂ levels. In a greenhouse experiment we grew sunflower (Helianthus annuus) at ambient and elevated CO₂ (760 ppm) with three soil moisture levels maintained at 45, 65, and 85% of field capacity, thereby eliminating potential CO₂-induced soil moisture effects. The δ¹⁵N value of total plant biomass increased significantly with increased soil moisture content at both CO₂ levels, possibly due to increased uptake of ¹⁵N-rich organic N. Although not adequately replicated, plant biomass δ¹⁵N was lower under elevated than under ambient CO₂ after adjusting for plant N uptake effects. Thus, increases in soil moisture can increase plant biomass δ¹⁵N, while elevated CO₂ can decrease plant biomass δ¹⁵N other than by modifying soil moisture.     

A method to optimize N-application in relation to soil supply of N,and yield of potato

Mathematical models of crop growth can provide estimates of the potential yield of potato, and also the minimum, critical N-concentration required, [N_c], to attain that yield. Efficient use of nitrogen requires that the crop incorporates sufficient nitrogen to attain its potential yield and that excess uptake is avoided. Predictions of the rate of supply of nitrogen from the soil are imprecise and so it has been difficult to estimate accurately the required application of fertilizer-N. Our work has shown the feasibility of using the growing crop as a monitor of the rate of supply of N from the soil. Using a low initial application rate of N at planting and monitoring uptake rate, we can estimate the contribution from the soil, and couple that information with estimates of yield and the related [N_c] to give an estimate of the requirement for supplementary applied-N. The method can be seen, therefore, as a means to determine the size of a second or subsequent part of a split application of fertilizer. This approach avoids much of the uncertainty over the fate of applied nitrogen and should offer growers the double benefits of economic use of fertilizer and of minimizing leaching losses. Further, by tailoring applications of N-fertilizer to the crop's requirements the grower will be better able to ensure the quality considerations in his crop.     

集约化生产对农田土壤碳氮含量及δ13C和δ15N同位素丰度的影响

集约化生产下农田土壤碳、氮含量变化是衡量土壤肥力持久性的重要指标.对常规水稻-蚕豆轮作地、露地蔬菜地、3年塑料大棚地和10年以上塑料大棚地的土壤pH、电导率(EC)、土壤有机碳(SOC)和总氮(TN)含量及δ13C和δ15N同位素丰度进行测定,研究了集约化生产程度对土壤特性的影响.结果表明:与水稻-蚕豆轮作地相比,露地蔬菜地、3年塑料大棚地和10年以上塑料大棚地0 ～20 cm耕层土壤pH分别降低1.1、0.8和0.7,而土壤EC分别是水稻-蚕豆轮作地的4.2、4.9和5.2倍;土壤碳、氮含量随塑料大棚地生产年限的增加总体上呈先增大后减小的趋势.与水稻-蚕豆轮作地相比,10年以上塑料大棚地0～20、20～40、40 ～60、60 ～ 80、80 ～ 100 cm土层的土壤SOC含量分别下降了54％、46％、60％、63％和59％,土壤TN含量分别下降了53％、53％、71％、82％和85％.农田集约化生产程度显著影响土壤SOC、TN含量和δ13C、δ15N丰度,土壤δ13C丰度与SOC含量呈显著负相关.土壤δ13C丰度可作为评价农田土壤碳循环受人为干扰强度的指标.     

地震滑坡迹地的植物群落与土壤恢复效果评价

地震滑坡是大陆山区常见的次生地质灾害,其会带来植被损毁、土壤破坏、生物栖息地破碎化等一系列生态问题。已有不少学者对地震滑坡迹地的植物群落特征和土壤性质进行调查研究,但对其恢复效果的综合评价鲜有报道。以龙门山东坡的地震滑坡迹地为研究对象,通过调查分析不同恢复方式下的植物群落特征和土壤性质,并基于模糊数学模型,结合主成分分析,定量地综合评价地震滑坡迹地恢复十年的植物群落与土壤恢复效果。结果表明：（1）地震滑坡迹地的乔木层平均高度和胸径显著小于未受损区（P<0.05）,灌木层平均基径和草本层α多样性与未受损区无显著差异（P>0.05）,人工恢复与自然恢复的灌草层α多样性均无显著性差异（P>0.05）;（2）地震滑坡迹地的土壤容重显著大于未受损区（P<0.05）,孔隙度和碱解氮含量显著小于未受损区（P<0.05）,土壤微生物生物量碳氮以及脱氢酶、蛋白酶、脲酶、磷酸酶和蔗糖酶活性小于未受损区;（3）地震滑坡迹地的植物群落特征与土壤性质综合得分小于邻近未受损区,人工恢复滑坡迹地的综合得分小于自然恢复;（4）与自然恢复相比,水平阶整地滑坡迹地的综合恢复率小于0,栽植不同树种对其恢复率无显著影响（P>0.05）;穴状整地后,栽植刺槐(Robinia pseudoacacia)滑坡迹地的综合恢复率大于0,且显著高于栽植柳杉(Cryptomeria fortunei)的滑坡迹地（P<0.05）;进行穴状整地的综合恢复率显著高于水平阶整地的滑坡迹地（P<0.05）。龙门山东坡地震滑坡迹地恢复十年,其植物群落与土壤还未恢复到未受损区水平,通过穴状整地并栽植适宜树种可在一定程度上提高其恢复率。     

A simple urea leaf-feeding method for the production of 13C and 15N labelled plant material

The use of ¹³C isotope tracer techniques in terrestrial ecology has been restricted by the technical requirements and high costs associated with the production of ¹³C enriched plant material by ¹³CO₂ release in labelling chambers. We describe a novel, simple and relatively inexpensive method for the small-scale production of ¹³C and ¹⁵N labelled plant material. The method is based on foliar feeding of plants with a urea solution (97 atom% ¹³C, 2 atom% ¹⁵N) by daily misting. Maize was grown in a greenhouse in a compost–soil mixture and enclosed in clear polythene bags between urea applications. Final enrichment in 27 d old maize shoots was 211 ¹³C (1.34 atom% ¹³C) and 434 ¹⁵N (0.52 atom% ¹⁵N). Enrichments of hot-water extractable fractions (289 ¹³C, 469 ¹⁵N) were only slightly higher than those observed in plant bulk material, which suggests that daily urea applications ensured fairly uniform labelling of different biochemical fractions and plant tissues. Recovery of applied excess ¹³C and ¹⁵N in plant shoots was 22% and 42%, respectively. Roots were less enriched (21 ¹³C and 277 ¹⁵N), but no attempts were made to recover roots quantitatively.     

增温、增雨对荒漠草原植物和土壤总氮及稳定氮同位素的影响

剖析气候变化下植物和土壤氮含量关系,有助于了解植物—土壤氮循环,揭示植物群落和土壤演替趋势。收集了增温(环境温度、增温2℃和4℃)增雨(自然降水量、降水量增加25%和50%)6年析因实验处理后荒漠草原优势物种和土壤样品,通过对叶片和土壤的总氮(Total nitrogen, TN)、有效氮(Available nitrogen, AN)以及稳定氮同位素(Stable nitrogen isotopes, δ¹⁵N)测定分析,探究了增温增雨后荒漠草原植物和土壤氮含量变化趋势以及控制植物—土壤氮循环的主要因子。结果表明：(1)单独增温和同时增温增雨使土壤TN、AN和δ¹⁵N值升高但不显著(P>0.05);单独增雨显著降低了土壤AN(P<0.05)。(2)单独增温使短花针茅(Stipa breviflora)和栉叶蒿(Neopallasia pectinata)叶片TN升高,而冷蒿(Artemisia frigida)和蒙古葱(Allium mongolicum)叶片TN降低;单独增雨和同时增温增雨显著降低了4个物种叶片TN(P<...     

A simplified procedure for GC/C/IRMS analysis of underivatized 19-norandrosterone in urine following HPLC purification

Nandrolone and/or its precursors are included in the World Anti-doping Agency (WADA) list of forbidden substances and methods and as such their use is banned in sport. 19-Norandrosterone (19-NA) the main metabolite of these compounds can also be produced endogenously. The need to establish the origin of 19-NA in human urine samples obliges the antidoping laboratories to use isotope ratio mass spectrometry (IRMS) coupled to gas chromatography (GC/C/IRMS). In this work a simple liquid chromatographic method without any additional derivatization step is proposed, allowing to drastically simplify the urine pretreatment procedure, leading to extracts free of interferences permitting precise and accurate IRMS analysis. The purity of the extracts was verified by parallel analysis by gas chromatography coupled to mass spectrometry with GC conditions identical to those of the GC/C/IRMS assay. The method has been validated according to ISO17025 requirements (within assay precision of ±0.3‰ and between assay precision of ±0.4‰). The method has been tested with samples obtained after the administration of synthetic 19-norandrostenediol and samples collected during pregnancy where 19-NA is known to be produced endogenously. Twelve drugs and synthetic standards able to produce through metabolism 19-NA have shown to present δ¹³C values around −29‰ being quite homogeneous (−28.8 ± 1.5; mean ± standard deviation) while endogenously produced 19-NA has shown values comparable to other endogenous produced steroids in the range −21 to −24‰ as already reported. The efficacy of the method was tested on real samples from routine antidoping analyses.     

植物代谢组学中几种重要的次生代谢物液质分析技术研究进展

代谢组学（metabolomics）主要是研究生物体、组织、细胞的代谢物组分及检测其动态变化过程,是继基因组和蛋白组学后新兴的一门组学技术。代谢物是细胞调节过程中的最终产物,其水平被视为生物系统对遗传或环境变化的最终反映。通过合适的分析平台,准确定性、定量在复杂的生物中具有化学多样性的次生代谢物是代谢组学的一项重要工作。液相色谱-串联质谱技术（liquid chromatography-tandem mass spectrometry,LC-MS/MS）是代谢物质检测平台最常用的方法,也为植物次生代谢物的广泛应用研究提供了基础。本文主要从植物激素类、叶酸类、黄酮类等次生代谢物方面进行阐述,结合液质联用技术,简要论述不同次生代谢物检测技术的研究进展。     

Nitrogen mineralization and crop uptake of N from decomposing 15N labelled red clover and yellow sweetclover plant fractions of different age

Nitrogen mineralization from 15N labelled red clover (Trifolium pratense L.) and yellow sweetclover (Melilotus officinalis Lam.) plant fractions of three different ages (8-, 14- and 20-week old) was studied in an out-door pot experiment during 8.5 months. Individual plant fractions (leaves/stems/roots/flowers), 23 g dry matter pot-1 (corresponding to 7300 kg ha-1), were incorporated into a sandy soil. The net mineralization of N was measured as 15N recovery in spring wheat (Triticum æstivum L.), perennial ryegrass (Lolium perenne L.) following the wheat and in the soil mineral N pool. Dry matter and N yields of the wheat crop were largest in pots receiving the legume leaf materials and the oldest root fractions. The largest amount of net N mineralized was obtained after application of sweetclover leaves, 381 mg N pot-1 (38% of added N), and a smaller amount was measured from red clover leaves, 215 mg N pot-1 (26% of added N). The N release was much smaller from the stems, being on average 63 mg N pot-1 (15% of added N), with intermediate values obtained from roots, 152 mg N pot-1 (26% of added N). The effects of age of the legume fractions on net N mineralization were more pronounced for sweetclover than for red clover materials. Greater net N release was obtained from sweetclover leaves and roots with inceasing age, but the opposite was valid for stems. At final harvest of the ryegrass, an additional 2.8% of added legume N was mineralized, compared with at wheat harvest. The net N mineralized proportion of added N was significantly related to concentrations of N and cell wall constituents in the plant material. Differences in net N mineralization estimates were generally larger between plant fractions than between materials of different age, implying that leaf proportion of the above-ground biomass is of great importance when predicting net N mineralization from green-manure plant materials. In addition, the contribution from roots to net mineralized legume N could be substantial.     

15N2 incorporation by rhizosphere soil influence of rice variety,organic matter and combined nitrogen

Summary Heterotrophic nitrogen fixation by rhizosphere soil samples from 20 rice cultivars grown under uniform field conditions was estimated employing¹⁵N-tracer technique. Rhizosphere soil samples from different rice cultivars showed striking differences with regard to their ability to incorporate¹⁵N₂. Rhizosphere samples from rice straw-amended (3 and 6 tons/ha) soil exhibited more pronounced nitrogen-fixing activity than the samples from unamended soil; while the activity of the rhizosphere samples from soils receiving combined nitrogen (40 and 80 kg N/ha) was relatively low. However, the inhibitory effect of combined nitrogen was not expressed in the presence of rice straw at 6 tons/ha. Results suggest that plant variety, application of combined nitrogen and organic matter influence the rhizosphere nitrogen fixation.     

In situ studies of soil mineral N fluxes: Some comments on the applicability of the sequential soil coring method in arable soils

Is the sequential in situ incubation of undisturbed soil cores, developed for forest stands applicable to arable soils? The incubation of covered and uncovered soil cores allows the estimation of net nitrogen mineralization (NNM), plant nitrogen uptake (N_uptake) and potential leaching losses (N_trans). The amounts and temporal dynamics of these N fluxes were determined at four arable soils in a two-year study. Results suggest that: (i) the method can not be recommended for the estimation of N uptake and leaching losses, but (ii) it is suitable for the estimation of NNM; (iii) incubations should preferably be started when soil is moist; (iv) the length of incubation periods should be reduced (<4 weeks); (v) dynamics of NNM is mainly determined by temperature and moisture conditions if there is no interference by agricultural management. Inputs of straw, manure, slurry or green manure strongly influence the amount and the dynamics of NNM.