火烧对内蒙古草原中坚韧胶衣固氮活性的影响

 坚韧胶衣(Collema tenax)是干旱和半干旱草原中常见的一种固氮地衣, 是草原生态系统中生物土壤结皮(Biological soil crust)的 主要组成部分, 对生态系统氮循环具有重要的影响。火烧作为一种干扰因子, 是草原生态系统结构和功能维持的重要因素之一。该文采用乙炔 还原法(Acetylene reduction assay), 研究了火烧对内蒙古草原生态系统中坚韧胶衣固氮活性的短期影响。结果表明, 在个体尺度上, 与对照 相比, 火烧区中地衣体烧损的坚韧胶衣固氮活性降低了42.3%, 而无烧损的个体固氮活性则升高了28.4%。这表明火烧对坚韧胶衣的固氮功能在 个体尺度上具有双重影响: 1)通过烧损地衣体、恶化地表温度和水分条件, 而抑制个体的固氮活性; 2)通过改善光照条件, 使表土养分呈现脉 冲式增高, 而促进未烧损个体的固氮活性。在种群尺度上, 火烧与对照之间固氮活性并无显著差异, 这可能是由于火烧在个体尺度上对坚韧胶 衣的固氮活性的双重影响相互抵消所致。     

不同水分条件下坚韧胶衣固氮活性对冻融的响应

坚韧胶衣Collema tenax是干旱和半干旱生态系统中生物土壤结皮的重要组分,其固氮作用对生态系统氮素循环具有重要影响。该地衣生长于温度和水分条件变化剧烈的土壤表面,但在我国北方其固氮活性对冻融的响应尚不清楚。采用乙炔还原法研究了坚韧胶衣固氮活性在不同水分条件下(湿冻组:地衣体含水量200%干重;干冻组:地衣体含水量20%干重)和4次冻融处理之间的差异。坚韧胶衣固氮活性为2,371.0-8,701.8nmolC2H4/m2·h,湿冻组固氮活性低于干冻组,对照组固氮活性最高,湿冻组和干冻组的固氮活性(对照的百分比)均与冻融次数呈显著的线性负相关关系(干冻组:R2=0.916,p0.001;湿冻组:R2=0.965,p0.001),但湿冻组的斜率绝对值(25.05)明显高于干冻组斜率绝对值(7.60)。结果表明低温对坚韧胶衣固氮活性具有显著抑制作用,干燥条件下坚韧胶衣固氮活性对于低温胁迫的敏感性比地衣体湿润条件下更低,这种较低的敏感性可能是该地衣对我国北方干燥和冷热变化剧烈的气候特点的生理适应策略。     

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。     

十二种地卷目地衣固氮活性的种间差异

地卷目地衣是对许多生态系统的氮素循环具有重要影响的固氮类群,但其固氮活性的种间差异研究甚少。将不同产地和不同馆藏时间的12种地卷目地衣于相同条件下培养,即：亚石胶衣Collema subflaccidum、亚黑胶衣C. subnigrescens、坚韧胶衣C. tenax、裸果猫耳衣Leptogium hildenbrandii、猫耳衣L. menziesii、柄猫耳衣L. pedicellatum、土星猫耳衣L. saturninum、黑猫耳衣L. trichophorum、犬地卷Peltigera canina、分指地卷P. didactyla、裂芽地卷P. praetextata和地卷P. rufescens。以乙炔还原法（acetylene reduction assay）于培养1d、10d和15d测定其固氮活性,结果显示所有地衣的固氮活性在培养10d与15d之间差异不显著,表明经过至多10d的培养后地衣固氮活性即可完全恢复;培养10d和15d的固氮活性平均值表现出显著的种间差异（P＜0.001）：黑猫耳衣Leptogium trichophorum固氮活性最高[（4.532±0.368）μmol C2H4/gdw·h],约为犬地卷Peltigera canina的2倍[（2.349±0.223）μmol C2H4/gdw·h];其他10种地衣固氮活性大致相近。显示地衣固氮作用的可塑性较大,是地衣具有较强适应能力的又一例证。     

城市景观水体中固氮微生物多样性及固氮活性检测

为探究城市景观水体中固氮微生物群落结构、多样性及固氮活性, 揭示水体中固氮蓝藻的固氮贡献, 研究选取新乡市牧野湖和人民公园水体两个小型水体进行研究。通过理化指标测定, 发现两个水体均处于富营养化状态, 借助高通量测序, 对两水体中微生物的16S rDNA和固氮酶nifH基因进行测序, 并利用乙炔还原法测定水体中固氮微生物的固氮速率。结果表明: 牧野湖水体中的原核生物类群共检测出32个门, 275个属; 固氮微生物共检测出9个门, 66个属。人民公园水体中的原核生物类群共检测出31个门, 238个属; 固氮微生物共检测出4个门, 13个属。固氮蓝藻在两个水体固氮微生物类群中分别占有3%和9.3%, 牧野湖固氮微生物丰富度相对较高, 与人民公园水体固氮微生物多样性差异较大。乙炔还原法测固氮速率结果显示, 两水体均未检测到固氮活性, 推测在富营养化的水体中固氮活性可能被抑制。     

沙坡头地区生物土壤结皮的固氮活性及其对水热因子的响应

氮是除水分之外影响干旱区生态系统生物活性的关键因子。生物土壤结皮是干旱半干旱荒漠地表景观的重要组成部分, 也是荒漠生态系统氮素的主要贡献者。通过野外调查采样, 利用开顶式生长室, 模拟不同降水梯度, 采用乙炔还原法连续测定了沙坡头地区典型生物土壤结皮(藻类结皮、地衣结皮和藓类结皮)在其主要固氮活跃期(6-10月, 湿润期)的固氮活性, 及其对水热因子的响应特征。结果表明, 试验期三类生物土壤结皮的固氮活性介于2.5 × 10³-6.2 × 10⁴ nmol C₂H₄·m^-2·h^-1之间, 其中藻类结皮的最高(平均达2.8 × 10⁴ nmol C₂H₄·m^-2·h^-1), 地衣结皮的次之(2.4 × 10⁴ nmol C₂H₄ ·m^-2·h^-1), 藓类结皮的最低(1.4 × 10⁴ nmol C₂H₄·m^-2·h^-1), 差异显著(p < 0.001)。在模拟降水3 mm时, 三类结皮均可达到最大固氮速率, 当发生> 3 mm的降水事件时, 它们的固氮速率无显著增加; 不同结皮的固氮活性与温度均呈显著的负相关关系(r_藻类结皮 = -0.711, r_地衣结皮 = -0.732, r_藓类结皮 = -0.755, p < 0.001), 藻类和藓类结皮的固氮活性的最适温度区间为25-30 ℃, 地衣结皮为20-30 ℃。三类结皮之间的这种固氮差异主要归因于结皮组成生物体即隐花植物的差异, 藻类结皮主要成分为大量的蓝细菌和一些绿藻, 地衣结皮也由大量的固氮藻和真菌共生形成, 而藓类结皮的主要组成部分苔藓植物并不具有固氮作用, 其微弱的固氮量是结皮中混生的少量蓝细菌或地衣所致。     

黄土高原不同侵蚀类型区生物结皮固氮活性及对水热因子的响应

在野外调查的基础上,采集不同侵蚀类型区内发育至稳定阶段的生物结皮,分析水分和温度变化对生物结皮固氮活性的影响.结果表明:水蚀区、水蚀风蚀交错区、风蚀区生物结皮固氮活性表现为水蚀区(127.7μmol.m-2.h-1)>水蚀风蚀交错区(34.6μmol.m-2.h-1)>风蚀区(6.0μmol.m-2.h-1);3个侵蚀类型区生物结皮固氮的最适温度分别为35、25和15℃.在最适温度条件下,水蚀区及水蚀风蚀交错区生物结皮固氮活性在100%～40%田间持水量时差异不显著;风蚀区生物结皮固氮活性对水分变化较为敏感,当含水量降至80%田间持水量时固氮活性开始显著降低,降至20%田间持水量时,生物结皮固氮作用停止.3个侵蚀类型区生物结皮固氮活性及其对水分与温度变化响应的差异与不同侵蚀类型区的气候、环境及生物结皮物种组成有关.     

温度对三种满江红光合及固氮活性的影响

本文报道满江红(Azolla imbricata)、卡洲满江红(A. caroliniana)和蕨状满江红(A. filiculoides)的光合与固氮(乙炔还原)活性对温度的反应差异。根区培养液温度在7—40℃范围内满江红与卡洲满江红的光合活性在30℃达到峰值,高于或低于30℃均急剧下降。而蕨状满江红在7—25℃的根区温度范围内光合活性差异不大,高于25℃则下降(图2)。0℃24小时或40℃2小时暗预处理对三种满江红光合活性与乙炔还原活性表现出不同影响(图3,4)。本文报道的方法可用于满江红生理生态参量的比较研究。     

温度对三种满江红光合及固氮活性的影响

本文报道满江红(Azolla imbricata)、卡洲满江红(A.caroliniana)和蕨状满江红(A.filiculoides)的光合与固氮(乙炔还原)活性对温度的反应差异。根区培养液温度在7—40℃范围内满江红与卡洲满江红的光合活性在30℃达到峰值,高于或低于30℃均急剧下降。而蕨状满江红在7—25℃的根区温度范围内光合活性差异不大,高于25℃则下降(图2)。0℃24小时或40℃2小时暗预处理对三种满江红光合活性与乙炔还原活性表现出不同影响(图3,4)。本文报道的方法可用于满江红生理生态参量的比较研究。     

毛乌素沙地生物土壤结皮对油蒿水分吸收的影响模拟

植物可利用水分是决定沙生灌木生长的主要因子,生物土壤结皮(简称生物结皮)在降雨期影响降水入渗,而在干旱期改变土壤蒸发,从而影响土壤水分分布,最终可能影响灌木水分吸收。然而,关于不同降水条件下生物结皮对灌木水分吸收和水分胁迫的影响机制认识不清。以油蒿为研究对象,基于试验数据和1990—2019年气象数据,采用数学模拟,定量研究了毛乌素沙地不同降水条件下生物结皮对土壤水分分布和油蒿水分吸收的影响,评价干旱期生物结皮对油蒿水分胁迫的影响。结果表明：与无结皮处理相比,生物结皮处理的土壤蒸发降低了5.1%;生物结皮改善了干旱期的土壤水分条件;生物结皮降低了植物水分胁迫的比例,平均降低比例为8.1%;生物结皮提高了植物水分吸收,平均增加比例为12.8%;生物结皮和对照植物水分吸收的比值随季节降水量的增加而降低,均值为1.13。综上,生物结皮的出现并未消极地影响沙生灌木的水分吸收。研究结果有助于理解生物结皮与灌木的共生或竞争关系。     

Nitrogen fixation associated with Suillus tomentosus tuberculate ectomycorrhizae on Pinus contorta var. latifolia

BACKGROUND AND AIMS: Tuberculate ectomycorrhizae are a unique form of ectomycorrhiza where densely packed clusters of mycorrhizal root tips are enveloped by a thick hyphal sheath to form a tubercle. The functional significance of such a unique structure has not previously been established. The purpose of the present study was to investigate and measure the potential nitrogenase activity associated with Suillus tomentosus/Pinus contorta tuberculate ectomycorrhizae in two stand ages, young and old, and across a range of nitrogen-poor soil conditions. METHODS: Short roots were compared with other mycorrhizae and non-mycorrhizal secondary roots using tuberculate ectomycorrhizae. Assessment of nitrogenase activity was determined and quantitative measurements were taken on tuberculate ectomycorrhizae in situ in a variety of different circumstances, by using an adaptation of the acetylene reduction assay. KEY RESULTS: Significant nitrogenase activity was measured associated with S. tomentosus/P. contorta tuberculate ectomycorrhizae whereas no nitrogenase activity was measured with non-tuberculate mycorrhizae or secondary roots without mycorrhizae. Average nitrogenase activity ranged from undetectable to 5696.7 nmol C2H4 g(-1) tubercle 24 h(-1). Maximum nitrogenase activity was 25,098.8 nmol C2H4 g(-1) tubercle 24 h(-1). Nitrogenase activity was significantly higher in young stands than in old stands of P. contorta. Season or some covariate also seemed to affect nitrogenase activity and there was suggestion of a site effect. CONCLUSIONS: Suillus tomentosus/P. contorta tuberculate ectomycorrhizae are sites of significant nitrogenase activity. The nitrogenase activity measured could be an important contribution to the nitrogen budget of P. contorta stands. Season and stand age affect levels of nitrogenase activity.     

Nitrogen fixation in the stag beetle, Dorcus (Macrodorcus) rectus (Motschulsky) (Col., Lucanidae)

Abstract:  Stag beetles are xylophagous insects that feed mainly on dead wood. They play an important role in the decomposition of dead wood in forest ecosystems. Most dead wood contains 1% nitrogen at most. It is suspected that stag beetles can utilize atmospheric nitrogen. We show that the larvae of Dorcus ( Macrodorcus ) rectus exposed to nitrogen reduce acetylene to ethylene in a time-dependent fashion. No reaction was detected with the dead wood or autoclaved larvae, suggesting that living larvae use the reaction for fixing nitrogen. Acetylene reduction to ethylene by larvae increased with incubation time. This effect was not seen using decayed wood only, autoclaved wood only or autoclaved larvae. Acetylene reduction by the larva proceeded at 1.25 ± 0.37 nmol acetylene/h/g (fresh wt), corresponding to the fixation of 0.25  μ g nitrogen per day per larva.     

Fixation of molecular nitrogen by Methanosarcina barkeri

Abstract Methanosarcina barkeri cells were observed in ammonia-free anaerobic acetate enrichments for sulfate-reducing bacteria. The capacity of Methanosarcina to grow diazotrophically was proved with a pure culture in mineral media with methanol. The cell yields with N₂ or NH₄⁺ ions as nitrogen source were 2.2 g and 6.1 g dry weight, respectively, per mol of methanol. Growth experiments with ¹⁵N₂ revealed that 84% of the cell nitrogen was derived from N₂. Acetylene was highly toxic to Methanosarcina and only reduced at concentrations lower than 100 μmol dissolved per 1 of medium. Assimilation of N₂ and reduction of acetylene were inhibited by NH₄⁺ ions. The experiments show that N₂ fixation occurs not only in eubacteria but also in archaebacteria. The ecological significance of diazotrophic growth of Methanosarcina is discussed.     

Effects of the herbicides Lindex and Simazine on chloroplast and nodule development,nodule activity,and grain yield inLupinus albus L.

Field and laboratory investigations were carried out to determine the influence of two commercial herbicides Lindex and Simazine on symbiotic N₂ fixation, the photosynthetic apparatus, percentage of proteins, and grain yield of lupins (Lupinus albus L. cv. Multolupa). The herbicides were added (3 kg commercial product in 9001 per ha) two weeks after sowing. The nodulated roots were tested for nitrogenase activity by the acetylene reduction assay (ARA) at the first and second flowering. The ARA values decreased with herbicide application. The decrease was smaller in the Simazine treatments with inoculated seeds. The ARA for plants treated with Lindex did not increase with inoculation. Nitrogenase activity was greater in the non-inoculated plants growing together with weeds. The effect of Simazine on the photosynthetic apparatus proved to be more toxic than that of Lindex, not only considerably reducing the size of the chloroplasts but also affecting the grana structure. The latter appeared as an electron-dense compact mass. There were no visible alterations in the photosynthetic apparatus of plants treated with Lindex. However starch, which was not present in the control leaves, accumulated. No ultrastructural changes were observed in the nodule cells treated with Lindex. Simazine altered nodule cells by causing vesicle formation, degeneration of bacteria, and by decreasing the number of N₂-fixing bacteroids. The grain yield decreased with both herbicide treatments, the decrease being lower in the inoculated plants, especially in the Simazine treatment. Our results suggest that each herbicide had a different effect on N₂ fixation, the photosynthetic apparatus and grain yield. Laboratory studies indicated that cyanazine and simazine did not adversely affect the growth of Bradyrhizobium.     

Nitrogen Fixation by Termites in Tropical Forests, Thailand

Nitrogen (N) fixed by termites was evaluated as a N input to decomposition processes in two tropical forests, a dry deciduous forest (DDF) and the neighboring dry evergreen forest (DEF), Thailand. A diverse group of termite species were assayed by acetylene reduction method and only the wood/litter-feeding termites were found to fix N. More intensive samplings of two abundant species, Microcerotermes crassus and Globitermes sulphureus, were done across several seasons, suggesting N fixation rates of 0.21 and 0.28 kg ha⁻¹ y⁻¹ by termites in the DDF and DEF, respectively. Also, estimates of asymbiotic N fixation rates were 0.75 and 3.95 kg ha⁻¹ y⁻¹. N fixed by termites and by asymbiotic fixers is directly supplied to decomposers breaking down dead plant material and could be a major source of their N. N fixed by termites was 7–22% of that fixed by termites and asymbiotic fixers. Although N fixed by termites is a small input compared to other inputs, this N is likely important for decomposition processes.