矿质养分输入对森林生物固氮的影响

生物固氮是森林生态系统重要的氮素来源,并且在全球氮循环中占有重要的地位。近代以来,因人类活动加剧而导致氮沉降的增加以及其它矿质养分元素(如磷、钼、铁等)输入的改变已成为影响森林生态系统生物固氮的重要因素之一,并引起了学术界的普遍关注。综述了国内外关于森林生物固氮对矿质养分输入的响应及机理。主要内容包括:(1)森林生物固氮的概念及主要的测定方法;(2)矿质养分输入对森林生物固氮的影响。整体上讲,氮素输入抑制了森林生物固氮,磷和其他营养元素输入则表现为促进作用。氮和磷、磷和微量元素同时添加均提高了森林的固氮量;(3)矿质养分改变森林生物固氮的机理。包括生物作用机制(如改变地表层固氮菌的数量或群落丰度、改变结瘤植物的根瘤生物量和附生植物的丰度或盖度)和环境作用机制(如引起土壤酸化、改变碳源物质的含量);(4)探讨了矿质养分输入对森林生物固氮影响研究中所存在的问题,并对未来该领域的研究提出建议。     

The natural history of nitrogen fixation

In recent years, our understanding of biological nitrogen fixation has been bolstered by a diverse array of scientific techniques. Still, the origin and extant distribution of nitrogen fixation has been perplexing from a phylogenetic perspective, largely because of factors that confound molecular phylogeny such as sequence divergence, paralogy, and horizontal gene transfer. Here, we make use of 110 publicly available complete genome sequences to understand how the core components of nitrogenase, including NifH, NifD, NifK, NifE, and NifN proteins, have evolved. These genes are universal in nitrogen fixing organisms-typically found within highly conserved operons-and, overall, have remarkably congruent phylogenetic histories. Additional clues to the early origins of this system are available from two distinct clades of nitrogenase paralogs: a group composed of genes essential to photosynthetic pigment biosynthesis and a group of uncharacterized genes present in methanogens and in some photosynthetic bacteria. We explore the complex genetic history of the nitrogenase family, which is replete with gene duplication, recruitment, fusion, and horizontal gene transfer and discuss these events in light of the hypothesized presence of nitrogenase in the last common ancestor of modern organisms, as well as the additional possibility that nitrogen fixation might have evolved later, perhaps in methanogenic archaea, and was subsequently transferred into the bacterial domain.     

Bioavailability of mineral-associated trace metals as cofactors for nitrogen fixation by Azotobacter vinelandii

Life on Earth depends on N₂-fixing microbes to make ammonia from atmospheric N₂ gas by the nitrogenase enzyme. Most nitrogenases use Mo as a cofactor; however, V and Fe are also possible. N₂ fixation was once believed to have evolved during the Archean-Proterozoic times using Fe as a cofactor. However, δ¹⁵N values of paleo-ocean sediments suggest Mo and V cofactors despite their low concentrations in the paleo-oceans. This apparent paradox is based on an untested assumption that only soluble metals are bioavailable. In this study, laboratory experiments were performed to test the bioavailability of mineral-associated trace metals to a model N₂-fixing bacterium Azotobacter vinelandii. N₂ fixation was observed when Mo in molybdenite, V in cavansite, and Fe in ferrihydrite were used as the sole sources of cofactors, but the rate of N₂ fixation was greatly reduced. A physical separation between minerals and cells further reduced the rate of N₂ fixation. Biochemical assays detected five siderophores, including aminochelin, azotochelin, azotobactin, protochelin, and vibrioferrin, as possible chelators to extract metals from minerals. The results of this study demonstrate that mineral-associated trace metals are bioavailable as cofactors of nitrogenases to support N₂ fixation in those environments that lack soluble trace metals and may offer a partial answer to the paradox.     

Fertilizers and biological nitrogen fixation as sources of plant nutrients: Perspectives for future agriculture

Biological nitrogen fixation (BNF) has an assured place in agriculture, mainly as a source of nitrogen for legumes. Legumes are currently grown mostly as a source of vegetable oil and as food for humans and animals, but not as nitrogen source.Other crops with BNF capability may be eventually be developed eventually. Such crops will also need mineral fertilizers to maintain a good status of soil nutrients, but their possible effects to the environment is also a concern. Fertilizers, however, will remain a necessary and sustainable input to agriculture to feed the present and increasing human population. It is not a case of whether BNF is better or worse than mineral fertilizers because both plays an important role in agriculture.     

Trends in biological nitrogen fixation research and application

In the world each year 17.2×10⁷ tons of N are biologically fixed. Biological nitrogen fixation (BNF) contributes to plant production in arable lands and in natural ecosystems. Research to improve BNF is progressing through the breeding of efficient N-fixing organisms and host plants, selection of the best combinations of host plant and microsymbiont, and by the improvement of inoculation techniques and field management. Biotechnology is useful for the creation of promising N₂-fixing organisms. However, to increase plant production through enhanced BNF the constraints in establishing effective N₂-fixing systems in the field should be understood and eliminated.     

Partitioning of nitrogen from biological fixation and fertilizer in Phaseolus vulgaris

Nitrogen uptake, distribution and remobilization in the vegetative and reproductive parts of the plant were studied in bean (Phaseolus vulgaris L.) cultivars Negro Argel and Rio Tibagi inoculated with either Rhizobium strain C05 or 127 K-17. Greenhouse grown plants were supplied with 2.5 mg N (plant)⁻¹ day⁻¹ as KNO₃ or K¹⁵NO₃ and the relative contribution to total plant nitrogen of mineral and symbiotically fixed nitrogen was determined. Control plants included those entirely dependent on fixed nitrogen as well as uninoculated plants supplied with 10 mg N (plant)⁻¹ day⁻¹. No differences were observed between inoculated treatments in total nitrate reductase activity and in the amount of mineral nitrogen absorbed, but there were considerable differences in the contribution of fixed nitrogen. Nitrogen fixation supplied from 58 to 72% of the total nitrogen assimilated during the bean growth cycle and the symbiotic combinations fixed most of their nitrogen (66 to 78% of total nitrogen) after flowering. Maximum uptake of mineral nitrogen was in the 15-day-period between flowering and mid-podfill (47 to 58% of total mineral nitrogen). Nitrogen partitioning varied with Rhizobium strains, and inoculation with strain C05 increased the nitrogen harvest index of both cultivars. Applied mineral nitrogen had a variable effect and in cv. Negro Argel was more beneficial to vegetative growth, resulting in smaller nitrogen harvest indices. Seed yield was not increased by heavy nitrogen fertilization. In contrast, cv. Rio Tibagi always benefited from nitrogen applications. Among the various nitrogen sources supplying the grain, the most important one was the fixed nitrogen translocated directly from nodules or after a rapid transfer through leaves, representing from 60 to 64% of the total nitrogen incorporated into the seeds.     

Oxygen and the regulation of nitrogen fixation in legume nodules

In N₂-fixing legume nodules, O₂ is required in large amounts for aerobic respiration, yet nitrogenase, the bacterial enzyme that fixes N₂, is O₂ labile. A high rate of O₂ consumptition and a cortical barrier to gas diffusion work together to maintain a low, non-inhibitory O₂ concentration in the central, infected zone of the nodule. At this low O₂ concentration, cytosolic leghemoglobin is required to facilitate the diffusion of O₂ through the infected cell to the bacteria. The resistance of the cortical diffusion barrier is variable and is used by legume nodules to regulate the O₂ concentration in the infected cells such that it limits aerobic respiration and N₂ fixation at all times. The resistance of the diffusion barrier and therefore the degree of O₂ limitation seems to be regulated in response to changes in the O₂ concentration of the central infected zone, the supply of phloem sap to the nodule, and the rate of N assimilation into the end products of fixation.     

Microbiological strategies for enhancing biological nitrogen fixation in nonlegumes

In an agro-ecosystem, industrially produced nitrogenous fertilizers are the principal sources of nitrogen for plant growth; unfortunately these also serve as the leading sources of pollution. Hence, it becomes imperative to find pollution-free methods of providing nitrogen to crop plants. A diverse group of free-living, plant associative and symbiotic prokaryotes are able to perform biological nitrogen fixation (BNF). BNF is a two component process involving the nitrogen fixing diazotrophs and the host plant. Symbiotic nitrogen fixation is most efficient as it can fix nitrogen inside the nodule formed on the roots of the plant; delivering nitrogen directly to the host. However, most of the important crop plants are nonleguminous and are unable to form symbiotic associations. In this context, the plant associative and endophytic diazotrophs assume importance. BNF in nonlegumes can be encouraged either through the transfer of BNF traits from legumes or by elevating the nitrogen fixing capacity of the associative and endophytic diazotrophs. In this review we discuss mainly the microbiological strategies which may be used in nonleguminous crops for enhancement of BNF.     

湿润持续时间对生物土壤结皮固氮活性的影响

土壤可利用氮是干旱半干旱区生态系统中除水分之外的关键限制因子,研究湿润持续时间和温度变化对温性荒漠藻类结皮和藓类结皮固氮活性的影响,对于深入认识和准确评价全球变化大背景下生物土壤结皮对区域生态系统的氮贡献至关重要。通过野外调查采样,在一次较大降水事件发生后,利用开顶式生长室,采用乙炔还原法连续测定了沙坡头地区人工植被区和天然植被区两类典型生物土壤结皮固氮活性的变化,分析了湿润持续时间和模拟增温对其固氮活性的影响。研究结果表明:在经历31d持续干旱,降水发生后第4天两类结皮的固氮活性达到最大,此后随样品水分含量下降,至第10天其固氮活性将至最低;结皮固氮活性与水分含量之间呈显著的二次函数关系,其固氮活性随水分含量的增加呈先上升后下降的趋势,藻类结皮的固氮活性显著高于藓类结皮;短期模拟增温并不能显著提高其固氮活性,增温主要通过加速结皮水分散失来影响其固氮活性。上述结果反映了水分是控制生物土壤结皮固氮活性的关键因子,而实验前样品所经历的环境条件则决定了降水发生后其到达最大固氮速率的时间,野外长期观测结合控制严格的室内实验才能准确评价生物土壤结皮对区域生态系统的氮贡献。     

腾格里沙漠东南缘生物土壤结皮的固氮潜力

以腾格里沙漠东南缘广泛分布的3类典型生物土壤结皮(藻类结皮、地衣结皮和藓类结皮)为研究对象,在野外环境下连续一年(2010年6月至2011年5月)测定了不同类型结皮的固氮潜力、季节变化,及其对环境因子的响应特征.结果表明：整个试验期间,藻类、地衣和藓类结皮的固氮活性分别为14～133、20～101和4～28 mol·m^-2·h^-1,差异显著,这种差异主要是由结皮种类组成的差异所致.3类结皮固氮活性对环境因子的响应特征基本一致.3类结皮固氮活性与降水量关系不显著,但与试验前3天小于3 mm的降水量均呈显著的正相关关系,说明该地区3类结皮在3 mm降水条件下即可达到最大固氮速率.3类结皮固氮活性与试验期温度均呈显著的二次函数关系,随气温升高均呈先上升后下降的趋势,藻类和地衣结皮的固氮活性在超过30 ℃后即迅速下降,而藓类结皮的固氮活性在超过25 ℃后即开始下降,说明不同类型结皮具有不同的固氮适宜温度区间.3类结皮固氮活性的季节变化均表现为秋季>春季>夏季>冬季,夏季高温和冬季低温抑制了结皮固氮酶活性,而春末秋初适宜的水热条件促进了其固氮活性的提高,结皮固氮活性的季节变化主要受水热因子的共同调控.温带荒漠区生物土壤结皮在湿润条件下全年均具有固氮能力,环境因子对其氮固定的控制作用层次分明,水分是影响其固氮速率和持续时间的关键因子,在水分和碳源充足的条件下,温度则是制约其固氮能力的主要因子.     

Towards an ecological understanding of biological nitrogen fixation

Biogeochemistry - N limitation to primary production and other ecosystem processes is widespread. To understand the causes and distribution of N limitation, we must understand the controls of...     

Breding common bean for improved biological nitrogen fixation

Common bean (Phaseolus vulgaris L.), which is an important food crop in the Americas, Africa and Asia, usually is thought to fix only small amounts of atmospheric nitrogen. However, field data indicate considerable genetic variability for total N₂ fixation and traits associated with fixation. Studies have shown that selection to increase N₂ fixation will be successful if: (1) discriminating traits (selection criteria) are measured precisely, (2) variability in germplasm is heritable, (3) selected parents are also agronomically suitable, (4) units of selection facilitate quantification of selection criteria, and (5) a breeding procedure that allows maximum genetic gain for N₂ fixation and recombination with essential agronomic traits is chosen. Breeding lines capable of fixing enough atmospheric N₂ to support seed yields of 1000–2000 kg ha^–1 have been identified and new cultivars with high N₂ fixation potential are being released.     

内蒙古黑岱沟露天煤矿植被恢复区生物土壤结皮的固氮潜力

氮素限制在陆地生态系统中普遍存在,在干旱受损生态系统中表现得尤为明显.生物土壤结皮是干旱、半干旱区受损生态系统植被恢复过程中的重要组成部分和氮源贡献者.以黑岱沟露天煤矿植被恢复区广泛分布的两类典型生物土壤结皮(藻类结皮和藓类结皮)为研究对象,通过野外调查采集样品,在实验室条件下测定了两类结皮的固氮活性,分析了其固氮活性对水热因子的响应特征及其与草本、结皮盖度的关系.结果表明：不同演替阶段人工植被及相邻撂荒地和天然植被下生物土壤结皮的固氮活性在9～150 μmol C₂H₄·m^-2·h^-1,藻类结皮(平均为77 μmol C₂H₄·m^-2·h^-1)显著高于藓类结皮(17 μmol C₂H₄·m^-2·h^-1).人工植被区3种常见植被类型下藻类和藓类结皮固氮活性均表现为“灌-草型”显著高于“乔-灌型”和“乔-灌-草型”.藻类和藓类结皮的固氮活性与样品相对含水量(10%～100%)和培养温度(5～45 ℃)均呈显著的二次函数关系,其固氮活性随水分、温度升高均呈先上升后下降的趋势,分别在60%和80%相对含水量时达到最大固氮速率,其最适固氮温度均为25 ℃.藻类结皮固氮活性与草本盖度呈显著的二次函数关系,草本盖度超过20%时固氮活性开始降低,藓类结皮固氮活性与草本植物盖度呈显著负相关.两类结皮固氮活性与其盖度均呈显著的正相关关系,随结皮盖度增加其固氮活性显著升高.露天煤矿植被恢复区两类生物土壤结皮固氮活性差异主要由结皮组成生物体即隐花植物的差异所致,不同植被类型下的水热差异及不同植被演替阶段草本、结皮盖度的差异是影响生物土壤结皮氮固定的关键因子,生物土壤结皮在人工植被区的拓殖发育及其氮输入是系统健康发展的重要标志.     

The reallocation of carbon in P deficient lupins affects biological nitrogen fixation

It is not known how phosphate (P) deficiency affects the allocation of carbon (C) to biological nitrogen fixation (BNF) in legumes. The alteration of the respiratory and photosynthetic C costs of BNF was investigated under P deficiency. Although BNF can impose considerable sink stimulation on host respiratory and photosynthetic C, it is not known how the change in the C and energy allocation during P deficiency may affect BNF. Nodulated Lupinus luteus plants were grown in sand culture, using a modified Long Ashton nutrient solution containing no nitrogen (N) for ca. four weeks, after which one set was exposed to a P-deficient nutrient medium, while the other set continued growing on a P-sufficient nutrient medium. Phosphorus stress was measured at 20 days after onset of P-starvation. During P stress the decline in nodular P levels was associated with lower BNF and nodule growth. There was also a shift in the balance of photosynthetic and respiratory C toward a loss of C during P stress. Below-ground respiration declined under limiting P conditions. However, during this decline there was also a shift in the proportion of respiratory energy from maintenance toward growth respiration. Under P stress, there was an increased allocation of C toward root growth, thereby decreasing the amount of C available for maintenance respiration. It is therefore possible that the decline in BNF under P deficiency may be due to this change in resource allocation away from respiration associated with direct nutrient uptake, but rather toward a long term nutrient acquisition strategy of increased root growth.     

Methods for measuring biological nitrogen fixation in grain legumes

To assure proper management and fully realize the benefits of the legume-Rhizobium symbiosis it is necessary to be able to quantify the amount of nitrogen fixed. Having measured the effectiveness of atmospheric N₂ fixation the macro- or micro-symbionts as well as agronomic factors can be manipulated with the objective to maximize biological nitrogen fixation. A suitable method to quantify nitrogen fixation is therefore necessary in any programme aiming at increasing N₂ fixation, like the one being reported in this volume. There are several methods available to quantify nitrogen fixation and most of the commonly used ones are described in the present paper listing their advantages and disadvantages.     

生物固氮研究中的几个热点问题

氮素化肥在农业生产中一直发挥重要作用,为了发展持续生态农业,全世界的研究者都在进行着长期不懈的努力,不断优化和拓展生物固氮系统。介绍固氮研究中的4个热点问题：⑴联合固氮;⑵根际微生物量氮及微生物活度;⑶通过豆科植物凝集素基因转化扩大根瘤菌宿主范围;⑷结瘤固与“类根瘤”固氮。     

Effect of Rhizobium sp. BARIRGm901 inoculation on nodulation,nitrogen fixation and yield of soybean (Glycine max) genotypes in gray terrace soil

Soybean plants require high amounts of nitrogen, which are mainly obtained from biological nitrogen fixation. A field experiment was conducted by soybean (Glycine max) genotypes, growing two varieties (Shohag and BARI Soybean6) and two advanced lines (MTD10 and BGM02026) of soybean with or without Rhizobium sp. BARIRGm901 inoculation. Soybean plants of all genotypes inoculated with Rhizobium sp. BARIRGm901 produced greater nodule numbers, nodule weight, shoot and root biomass, and plant height than non-inoculated plants. Similarly, inoculated plants showed enhanced activity of nitrogenase (NA) enzyme, contributing to higher nitrogen fixation and assimilation, compared to non-inoculated soybean plants in both years. Plants inoculated with Rhizobium sp. BARIRGm901 also showed higher pod, stover, and seed yield than non-inoculated plants. Therefore, Rhizobium sp. BARIRGm901 established an effective symbiotic relationship with a range of soybean genotypes and thus increased the nodulation, growth, and yield of soybean grown in gray terrace soils in Bangladesh.