Comparative assessment of nitrogen fixation rate by ^(15)N_(2) tracer assays in the South China Sea

Nitrogen fixation is one of the most important sources of new nitrogen in the ocean and thus profoundly affects the nitrogen and carbon biogeochemical processes.The distribution,controlling factors,and flux of N2 fixation in the global ocean remain uncertain,partly because of the lack of methodological uniformity.The^(15)N_(2)tracer assay(the original bubble method→the^(15)N_(2)-enriched seawater method→the modified bubble method)is the mainstream method for field measurements of N2 fixation rates(NFRs),among which the original bubble method is the most frequently used.However,accumulating evidence has suggested an underestimation of NFRs when using this method.To improve the availability of previous data,we compared NFRs measured by three^(15)N_(2)tracer assays in the South China Sea.Our results indicate that the relationship between NFRs measured by the original bubble method and the^(15)N_(2)-enriched seawater method varies obviously with area and season,which may be influenced by incubation time,diazotrophic composition,and environmental factors.In comparison,the relationship between NFRs measured by the original bubble method and the modified bubble method is more stable,indicating that the N2 fixation rates based on the original bubble methods may be underestimated by approximately 50%.Based on this result,we revised the flux of N2 fixation in the South China Sea to 40 mmol/(m2·a).Our results improve the availability and comparability of literature NFR data in the South China Sea.The comparison of the^(15)N_(2)tracer assay for NFRs measurements on a larger scale is urgently necessary over the global ocean for a more robust understanding of the role of N2 fixation in the marine nitrogen cycle.

基于^(15)N示踪的炭氮耦合对番茄氮素吸收利用效益的影响

为探讨分根交替灌溉下,不同生物炭添加量和施氮量对番茄氮素吸收利用规律的影响,应用^(15)N示踪技术,以番茄为研究对象开展盆栽试验,试验设置3种生物炭添加水平和3种施氮水平.结果表明,株高和茎粗在生物炭添加量为2%、施氮量为450 kg/hm^(2)时最大,干物质质量在生物炭添加量为2%、施氮量为300 kg/hm^(2)时最大;添加生物炭和增加施氮量均能促进番茄各器官对^(15)N肥料和全氮的积累,增加^(15)N残留量和^(15)N吸收量;添加生物炭会降低肥料贡献率,但能够显著增加番茄吸收土壤氮的比例、^(15)N回收率和^(15)N利用率.添加生物炭可有效提高番茄产量、水分利用效率和品质,其中处理B2N2的产量最高,为2.61 kg/株,品质也较其他处理有显著提高.基于组合赋权的TOPSIS法评价番茄综合效益结果表明,分根交替灌溉下,生物炭添加量为2%、施氮量为300 kg/hm^(2)(处理B2N2)为试验条件下的最佳种植模式.

^(15)N示踪技术在植物N素营养研究中的应用及进展

近年来,由于氮肥利用效率的不断下降,植物氮素的营养学研究更是日益受到重视。氮肥利用率受施氮水平、施用方法、土壤性状、气象条件、品种类型等多种因素的影响,平均利用率只有30%-35%。减少氨的挥发和反硝化作用,选育氮高效、耐低氮品种是提高氮素利用率、减少环境污染与资源浪费和实现农业可持续发展的主要途径。本文简要概述15N标记技术尤其是示踪技术在植物氮素的吸收、利用及分配等方面的应用及近年来的研究进展。

用^(15)N示踪法研究不同土壤水分条件下小麦对氮的吸收利用

本研究结果表明，同一施氮量处理小麦地上生物量、茎叶产量和籽粒产量随土壤水分含量的增加而提高，在同一土壤含水量下，这３ 种产量则随氮素肥料施用量的增加而增加。小麦成熟后地上部吸收总氮量的方差分析显示出施氮处理和水分处理对小麦吸氮量的影响达到显著水平，同一施氮处理小麦地上部吸氮总量随土壤含水量的增加而增加，在３ 种土壤水分含量条件下，小麦的吸氮量均随施尿素氮肥量的增加而提高。施尿素处理的土壤Ａ 值随土壤含水量的增加而降低；尿素与猪粪配施处理的土壤Ａ 值则随土壤含水量的增加而增加。在相同施肥量下，尿素氮的利用率随土壤含水量的增加而提高，当田间持水量分别为５０ ％ 、７０ ％ 和９０ ％ 时，小麦对土壤中氮素利用率分别为１３３０ ％ 、２７９７ ％ 和３２２６ ％ ，尿素与猪粪配施处理在３ 种土壤水分条件下的氮肥利用率分别为１９９６ ％ 、２９９０ ％ 和３４３９ ％ 。说明在水分缺乏的情况下，尿素氮的利用率受土壤水分条件的制约，土壤水分充足及无机氮肥与有机肥配施可以提高氮肥的利用率。

等氮条件下磷钾互作对氮肥利用率及烤烟产量和质量的影响——^(15)N同位素示踪法

肥料合理施用在全球控制肥料施用量、降低环境污染背景下对于农业的绿色发展具有更为重要的意义,适宜的肥料配比是合理利用肥料的关键措施之一。为进一步明确不同氮磷钾比例条件下的烟叶质量特征,摸清不同肥料配比下肥料的利用率,以云烟87为材料,采用^(15)N同位素示踪法,研究了等氮条件下不同氮磷钾配比,即不施处理(T1),常规施肥处理(1∶0.5∶2,CK),增钾处理(1∶0.5∶3,T2),增磷处理(1∶1∶2,T3)和提磷增钾处理(1∶1∶3,T4)下烤烟产质量及氮肥利用率特征。结果表明:(1)等氮条件下提磷增钾的措施显著提高烟叶的产量和产值。产量表现为T4>T3>T2>CK>T1,产值表现为T4>T2>T3>CK>T1。与CK相比,T2、T3和T4处理的产量分别增加了11.81%、3.48%和20.72%,产值分别增加了9.66%、1.17%和19.81%。(2)^(15)N同位素示踪标记结果表明,等氮条件下提磷增钾措施能促进烟株对硝态氮和铵态氮的吸收,提高氮肥利用率。上、中、下部叶的铵态氮和硝态氮含量(氮分布)分别为:T4>T2>T3>CK。与CK相比,T2、T3和T4处理的氮肥利用率分别增加了16.87%、15.88%和31.00%。(3)等氮条件下增钾处理的氮肥利用率高于增磷处理,产量和产值较高。结果显示,等氮条件下提磷增钾的施肥措施促进烟株对肥料的吸收,有效提高烟株的氮肥利用率,提高烤烟产量和增加农民经济收入,为优质烟叶的生产提供一定的理论依据。

The fate of ^(15)N-labeled nitrogen inputs to pot cultured beech seedlings

The partitioning of nitrogen deposition among forest soil （including forest floor）, leachate and above- and belowground biomass of pot cultured beech seedlings in comparison to non-cultured treatments were investigated by adding 1.92 g.m^-2 ^15N tracer in throughfall for two successive growing seasons at a greenhouse experiment. Ammonium and nitrate depositions were simulated on four treatments （cultured and non-cultured） and each treatment was labeled with either ^15N-NH4^＋ or ^15N-NO3^-. Total recovery rates of the applied ^15N in the whole system accounted for 74.9% to 67.3% after ^15N-NH4^＋ and 85.3% to 88.1% after ^15N-NO3^-in cultured and non-cultured treatments, respectively. The main sink for both ^15N tracers was the forest soil （including forest floor）, where 34.6% to 33.7% of ^15N-NH4^＋ and 13.1% to 9.0% of ^15N-NO3^-were found in cultured and non-cultured treatments, respectively, suggesting strong immobilization of both N forms by hetero- trophic microorganisms. Nitrogen immobilization by microorganisms in the forest soil （including forest floor） was three times higher when ^15N-NH4^＋ was applied compared to ^15N-NO3^-. The preferential heterotrophic use of ammonium resulted in a two times higher retention of deposited ^15N-NH4^＋ in the forest soil as compared to plants. In contrast, nitrate immobilization in the forest soil was lower compared to plants, although statistically it was not significantly different. In total the immobilization of ammonium in the plant-soil system was about 60% higher than nitrate, indicating the importance of the N-forms deposition for retention in forest ecosystems.

改性尿素N在土壤-烟株系统中的分布规律研究

在盆栽烟草条件下采用15N示踪方法研究尿素改性前后N在土壤烟株中的分布规律 ,并比较尿素改性前后N的利用率大小。结果表明 ,尿素改性后比改性前烟叶增产 2 5 1 6% ,达 1 %显著水准。烟株全N量 ,改性后是改性前处理的 1 4 5 7%。从肥料中吸收的N量 ,改性后是改性前处理的 1 2 7 9%。从土壤中吸收的N量 ,改性后是改性前处理的 1 58 3 %。N肥利用率 ,改性后处理为 45 8% ,比改性前处理提高1 0 1个百分点。N的回收率 ,改性后处理达 98 3 % ,比改性前处理提高 1 2 8个百分点。改性尿素省肥增产的主要原因是刺激作物从土壤N库中吸收了较多N素 。

Symbiotic nitrogen fixation in black locust (Robinia pseudoacacia L.) seedlings from four seed sources

We conducted a greenhouse experiment to investigate the role of seed source in growth and symbiotic nitrogen fixation of black locust （Robinia pseudoacacia L）. Seeds from different sources were planted in the same environmental conditions and inoculated with a suspension of mixed Rhizobium. We used the modified ^15N isotope dilution method to estimate biological nitrogen fixation of Robinia trees. Different Robinia seed sources differed significantly in terms of tissue dry weight （50.6-80.1 g）, total N （1.31-2.16 g） and proportion of nitrogen derived from the atmosphere （ 0-51%）. A higher nitrogen fixation rate of Robinia trees was associated with higher dry weight. Moreover, the leaves of Robinia proved to adequately represent the nitrogen fixation capacity of entire plants. Our results confirmed that assessment of seed sources is a useful way to improve the nitrogen fixation capacity and therefore the growth rate of Robinia.

稻草和稻根还田对作物的尿素肥效的作用

用^(15)N 示踪法进行盆栽试验,研究稻草和稻根对于尿素肥效的作用.结果表明:在水稻和小麦上,稻草处理都有增加尿素^(15)N 在土壤中的生物固持作用和减少其损失的效果.分析植株上谷/草的 dff 比值,表明稻草处理提高尿素^(15)N 运转到稻谷和麦粒的相对比率.当水稻迟栽情况下,植株对肥料的依赖性增强,因而稻草和稻根处理更明显地促进植株对尿素^(15)N 的吸收量和转化率.试验得出的“激发效应”,在水稻上为正的而在小麦上为负的.

旱地不同覆盖方式对冬小麦氮肥利用的影响

为了解不同覆盖方式下西北旱地冬小麦产量和氮肥利用的差异,在长期定位试验基础上,设置^(15)N微区试验,分析常规播种、垄覆沟播、全膜穴播和秸秆覆盖4种栽培条件下冬小麦的产量、氮素累积及氮肥利用率。结果表明,与常规播种相比,垄覆沟播、全膜穴播和秸秆覆盖下小麦产量分别提高4.9%、15.4%和12.4%,其中全膜穴播和秸秆覆盖的增产效果达到显著水平;地上部总生物量分别提高4.6%、23.9%和16.8%;籽粒氮含量分别降低19.6%、8.9%和7.2%。籽粒氮素累积量和地上部氮素总累积量在不同覆盖方式下变化趋势一致,全膜穴播和秸秆覆盖的籽粒氮素累积量较常规播种分别提高6.0%和5.6%,地上部氮素总累积量分别提高8.0%和5.5%,而垄覆沟播的籽粒氮素累积量和地上部氮素总累积量较常规播种分别降低13.9%和10.8%。垄覆沟播、全膜穴播和秸秆覆盖的籽粒氮素中肥料氮的比例较常规播种分别提高22.5%、29.8%和16.3%,籽粒中肥料氮累积量分别提高4.2%、39.3%和20.5%;地上部肥料氮累积量分别提高7.1%、38.8%和19.3%;氮肥利用率分别提高7.1%、38.8%和19.3%。因此,在本试验条件下,全膜穴播和秸秆覆盖可以明显促进小麦增产、氮素累积和氮肥高效利用,其中从环境因素考虑,秸秆覆盖的效果更好。

外源氮素形态对植烟土壤酸解有机态氮组分的影响

【目的】为明确外源氮形态对植烟土壤酸解有机态氮(AON)组分的影响,为植烟土壤保育与定向培肥提供理论依据。【方法】利用^(15)N示踪技术,以湖北恩施植烟土壤为研究对象,采取土壤培养试验,分析外源添加氨态氮、硝态氮和氨基酸态氮对植烟土壤AON及其组分酸解铵态氮(AN)、氨基酸态氮(ANN)、氨基糖态氮(ASN)和酸解未知态氮(UAN)中的氮素转化的影响。【结果】(1)在0~10 d,氨态氮^(15)N的AON转化率(R值)最高,为50%~70%,氨基酸态氮^(15)N的略低,为45%~55%,硝态氮^(15)N的仅为27%~31%,但随后,各处理R值差别不大,约为30%;(2)氨态氮、硝态氮和氨基酸态氮的AON-^(15)N转入量差别不大,加权均值分别为9.84 mg/kg、8.40 mg/kg和10.49 mg/kg,但不同处理的AON-^(14)N变化值差别较大,氨态氮的加权均值为-116.71 mg/kg,而硝态氮和氨基酸态氮的却为28.05 mg/kg和69.84 mg/kg;(3)基于结构方程模型的路径分析发现,氨态氮的AN-ASN和ASN-UAN路径被增强,r值均为正值,ASN加速了AN向UAN的转化,但氨基酸态氮中被增强的AN-AAN和ANN-UAN的r值均为负值,AAN抑制了AN向UAN的转化;(4)在1 d时,氨态氮的AON含量略高于其它处理,但随后迅速下降,30 d时显著低于硝态氮和氨基酸态氮处理。【结论】不同外源氮形态的AON-^(15)N转入量无明显差别,但对土壤本身^(14)N的转化过程影响较大。氨基酸态氮和硝态氮可通过增加其它来源^(14)N转入量提高了AON,而氨态氮则更有利于土壤本身AON-^(14)N的转出,不利于AON的累积。

Factors influencing the accuracy of the denitrifier method for determining the oxygen isotopic composition of nitrate

The denitrifier method is widely used as a novel pretreatment method for the determination of nitrogen and oxygen isotope ratios as it can provide quantitative and high-sensitivity measurements. Nevertheless, the method is limited by relatively low measurement accuracy for δ18 O. In this study, we analyzed the factors influencing the accuracy of δ18 O determination, and then systematically investigated the effects of dissolved oxygen concentrations and nitrate sample sizes on estimates of the δ15 N and δ18 O of nitrate reference materials. The δ18 O contraction ratio was used to represent the relationship between the measured difference and true difference between two reference materials. We obtained the following main results:(1) a gas-liquid ratio of 3:10(v/v) in ordinary triangular flasks and a shaking speed of 120 r/min produced an optimal range(1.9 to 2.6 mg/L) in the concentration of dissolved oxygen for accurately determining δ18 O, and(2) the δ18 O contraction ratio decreased as nitrate sample size decreased within a certain range(1.0 to 0.1 μmol). Our results suggested that δ18 O contraction is influenced mainly by dissolved oxygen concentrations in pure culture, and provided a model for improving the accuracy of oxygen isotope analysis.

Distribution and Fate of Anthropogenic Nitrogen in the Calamagrostis angustifolia Wetland Ecosystem of Sanjiang Plain, Northeast China

Wetlands are important for the protection of water quality of rivers and lakes, especially those adjacent to agricultural landscapes, by intercepting and removing nutrients in runoff. In this study, the ^15N tracer technique was applied to study the distribution and fate of anthropogenic nitrogen （^15N-fertilizer） in Calamagrostis angustifolia Kom wetland plant-soil microcosms to identify the main ecological effects of it. ^15NH4^15NO3 solution （14.93 mg N/L, 20.28 at.% ^15N） was added to each microcosm of the first group, which was approximate to the current nitrogen concentration （CNC） of farm drainage, and 29.86 mg NIL ^15NH4^15NO3 solution was added to another group, which was approximate to the double nitrogen concentration （DNC） of farm drainage, while no nitrogen （NN） was added to the third group. The results suggest that the input of anthropogenic nitrogen has positive effects on the biomass and total nitrogen content of plant, and the positive effects will be elevated as the increase of its input amount. The increase of ^15N-fertilizer can also elevate its amounts and proportions in plant nitrogen. Soil nitrogen is still the main source of plant nitrogen, but its proportion will be reduced as the increase of ^15 N-fertilizer. The study of the fate of ^15N-fertilizer indicates that, in CNC treatment, only a small proportion is water-dissolved （0,13 ± 0.20%）, a considerable proportion is soil-immobilized （17.02 ± 8.62%）, or plant-assimilated （23.70 ± 0.92%）, and most is lost by gaseous forms （59.15 ± 8.35%）. While in DNC treatment, about 0.09 ± 0.15% is water-dissolved, 15.33 ± 7.46% is soil-immobilized, 23.55±2.86% is plant-assimilated, and 61.01±5.59% is lost by gaseous forms. The double input of anthropogenic nitrogen can not elevate the proportions of plant-assimilation, soil-immobilization and water-dissolution, but it can enhance the gaseous losses.

Nitrapyrin effectiveness in reducing nitrous oxide emissions decreases at low doses of urea in an Andosol

In the tropics,frequent nitrogen(N)fertilization of grazing areas can potentially increase nitrous oxide(N_(2)O)emissions.The application of nitrification inhibitors has been reported as an effective management practice for potentially reducing N loss from the soil-plant system and improving N use efficiency(NUE).The aim of this study was to determine the effect of the co-application of nitrapyrin(a nitrification inhibitor,NI)and urea in a tropical Andosol on the behavior of N and the emissions of N_(2)O from autotrophic and heterotrophic nitrification.A greenhouse experiment was performed using a soil(pH 5.9,organic matter content 78 g kg^(-1),and N 5.6 g kg^(-1))sown with Cynodon nlemfuensis at 60%water-filled pore space to quantify total N_(2)O emissions,N_(2)O derived from fertilizer,soil ammonium(NH_(4)^(+))and nitrate(NO_(3)^(-)),and NUE.The study included treatments that received deionized water only(control,NI).No significant differences were observed in soil NH_(4)^(+)content between the UR and UR+NI treatments,probably because of soil mineralization and NO_(3)^(-)produced by heterotrophic nitrification,which is not effectively inhibited by nitrapyrin.After 56 d,N_(2)O emissions in UR(0.51±0.12 mg N_(2)O-N concluded that the soil organic N mineralization and heterotrophic nitrification are the main processes of NH_(4)^(+)and NO_(3)^(-)production.Additionally,it was found that N_(2)O emissions were partially a consequence of the direct oxidation of the soil's organic N via heterotrophic nitrification coupled to denitrification.Finally,the results suggest that nitrapyrin would likely exert significant mitigation on N_(2)O emissions only if a substantial N surplus exists in soils with high organic matter content.