铵硝比对亚热带树种幼苗氮吸收偏好及吸收根属性的影响

【目的】明确不同铵硝比对亚热带针、阔叶树苗的氮素吸收偏好及吸收根属性的影响，为阐明人工林养分获取策略与土壤氮循环提供理论依据。【方法】以亚热带常见树种湿地松、马尾松、深山含笑和枫香的2年生幼苗为对象，设置3种铵硝比(15∶1、8∶8和1∶15)进行沙培试验，利用15N同位素示踪技术测定植物的无机氮吸收速率，分析其与吸收根形态、构型属性的相关性。【结果】1) 4个树种幼苗的无机氮吸收速率具有环境可塑性，当铵氮(NH4+-N)占优势时，4个树种均偏好吸收铵氮；当硝氮(NO₃^--N)占优势时则偏好吸收硝氮。当铵硝比为8∶8时，则表现出树种对不同形态N吸收的差异：针叶树种的NO₃^--N吸收速率是NH₄⁺-N的2倍多，但阔叶树种之间没有显著差异。2)铵硝比对植物吸收根的比根长和比表面积有显著影响，对直径、组织密度、分支比和分支呈强度无显著影响。3) 4种树种幼苗的无机氮吸收速率与根分支呈显著正相关，与各器官生物量呈显著负相关。【结论】4个树种幼苗对不同形态N的吸收依...     

异质性供氮环境下杉木、马尾松、木荷氮素吸收偏好及其根系觅氮策略

【目的】林木可吸收利用土壤中的NH_4~+-N和NO_3~--N存在很大的空间异质性,不同树种具有不同的N素吸收偏好和根系觅N策略。研究亚热带主要针叶树种杉木、马尾松和阔叶树种木荷的N素吸收偏好及异质N环境中的根系形态特征和苗木生长,为揭示3个树种的N素营养性状遗传特性提供参考,对营造针阔混交林、提高林地N素利用效率和生产力具有重要理论价值。【方法】以杉木、马尾松和木荷幼苗为研究对象,采用自主研发的异质养分环境根箱培养系统,构建不同N素形态(NH_4~+-N和NO_3~--N)和浓度配比的4个供N处理(NH_4~+∶NO_3~-为10∶0和0∶10的高异质性、8∶2和2∶8的中异质性、6∶4和4∶6的低异质性、5∶5和5∶5的同质性),分析3个树种根系生长与形态特征、苗木生长和生物量。【结果】1)异质性供N处理下,杉木在NO_3~--N斑块中的根长、根表面积和根生物量均显著大于NH_4~+-N斑块中的值,而马尾松和木荷则是NH_4~+-N斑块中的值大于NO_3~--N斑块中的值;杉木根系平均直径NO_3~--N斑块中的小于NH_4~+-N斑块中的值,而马尾松和木荷根系平均直径为NO_3~--N大于NH_4~+-N斑块中的值,3个树种根系特征参数在同质供N处理下的2个斑块间无显著差异。总根长、根表面积和根生物量均表现为:同质低异质中异质高异质,高异质性供N处理下根系生物量比其他3个处理分别高7.96%～20.15%、3.47%～19.07%、4.49%～9.08%。2)3个树种苗高、地径、总生物量表现为:低异质同质中异质高异质,生物量在低异质性供N处理下比其他3个处理分别高5.40%～33.67%、7.61%～31.24%和11.32%～36.61%,根冠比在高异质供N处理下相比同质供N处理下的提高58.47%～92.68%。【结论】在同质供N的理想环境中,杉木、马尾松和木荷的根系生长无显著的N素吸收偏好性。但在NO_3~--N和NH_4~+-N的异质性供N环境下,杉木偏向于在NO_3~--N的斑块中生长更多更细长的根系,而马尾松和木荷则偏向于在NH_4~+-N斑块中生长更多更细长的根系。3个树种根长、表面积和生物量随着异质供N性的程度提高而增大,但根系平均直径越小,且N异质性程度越大越不利于苗木生长和生物量的累积。     

丛枝菌根真菌和不同形态氮对杉木幼苗根际土壤氮磷养分含量及其相关酶化学计量比的影响

[目的]探究丛枝菌根真菌(AMF)和不同形态氮(NH₄⁺-N、NO₃^--N)对杉木根际土壤氮(N)与磷(P)养分含量、相关酶活性及其化学计量比的影响,为杉木人工林的可持续经营管理提供理论依据与数据支持。[方法]本研究以1年生杉木幼苗为研究对象,采用盆栽实验,研究杉木幼苗在接种摩西球囊霉(G.mosseae,Gm)和添加不同形态N(NH₄⁺-N、NO₃^--N)后根际土壤N、P养分含量及其相关酶活性与化学计量比的变化。[结果](1)AMF接种提高了土壤有效P含量,降低了土壤中硝态氮、铵态氮、可溶性有机氮以及全P含量,与NH₄⁺-N处理相比,NO₃^--N处理下AMF对土壤N、P养分的调节作用更显著(p<0.05);(2) AMF和不同形态N添加提高了土壤中酸性磷酸酶(AP)、脲酶(URE)、N-乙酰-β-D葡萄糖苷酶(NAG)...     

Effects of Different Nitrogen Sources on Transformation and Availability of Inorganic Phosphorus in Root-Soil Interface of Pinus thunbergii Seedlings

The calcareous fluvo-aquic soil was collected and a microcosm study was carried out with root-mat and frozenslicing method in laboratory. The pH in the root-soil interface with the control treatment was just slightly lower than in the bulk soil.However,the addition of NH₄^- -N significantly decreased the pH value in the root-soil interface and the addition of No₃ -N slightly increased the pH value in the root-soil interface.The magnitude of pH changes in the root-soil interface depended upon the concentrations of the nitrogen sources added.The contents of Ca₂-P,Fe-P and Al-P in the root-soil interface were much lower after treated with NH₄ -N and slightly higher after treated with No₃ -N compared with control treatment.After treated with 100,200 and 400 mg·kg^-1NH₄⁺ -N,the deficiency rates of Ca₂-P in the area 0-1 mm from the root plane were 37.1%,45.9%and 57.7%,respectively,the deficiency rates of Fe-P were 23.4%,29.1%and 38.2%,respectively,and the deficiency rates of Al-P were 25.1%,28.0%and 33.2%,respectively. Compared with the control the deficiency rates of Ca₈-P in NH₄⁺ -N and No₃-N treatments decreased and increased,respectively,but the differences were not obvious.The contents of Ca₁₀ -P and O-P in the root-soil interface did not significantly change after treated with NH₄⁺ -N or No₃ -N,suggesting that Ca₁₀- P and O-P were remarkably difficult to be mobilized even at the presence of high concentration of NH₄⁺ -N.The lowered pH in the root-soil interface induced by the addition of the NH₄ -N promoted the transformation of phosphates in the root-soil interface,enhanced the mobilization and bioavailability of phosphates,and thereby remarkably increased the absorption of phosphorus by roots.     

湖南省森林植物园景观质量影响原因研究

以湖南省森林植物园为研究对象,借鉴SBE法对视觉景观质量进行评价,借鉴SD法对景观特征进行评价。通过相关分析建立有关城市森林公园景观质量的评价模型。景观质量评价结果得出植被、水体及路面或广场对视觉景观质量影响较大;景观特征分析了形态丰富度、美感等8个因子对景观质量的影响,建立模型为SBE=-0.58+0.101X₁+0.139X₂+0.257X₃+0.162X₄+0.113X₅+0.181X₆-0.092X₇-0.108X₈,其中,对景观质量影响较大的因子有形态丰富度、美感和生命力,而空间感和变化性与景观质量呈负相关。综合分析结果得出多样化、植被覆盖度高的景观评价较高,人们更为向往自然、幽静的森林景观,森林公园中杂乱无章的植物对景观质量产生消极影响。     

氮素形态及配比对铁核桃苗生长及营养吸收的影响

采用盆栽试验方法,以铵态氮、硝态氮和尿素[CO(NH2)2]为氮源,研究氮素形态及配比对铁核桃苗植株大小、生物量、根系形态特征和苗木元素含量及积累量的影响。结果表明:氮素形态及配比对铁核桃苗木的高度、基径、根冠比值、地上部和根系的生物量、根系形态特性、苗木中元素含量及积累量有明显的影响。以50%NH+4-N+50%NO-3-N为氮源的苗高、基径、根冠比值、整株及地上部和根系的生物量、根系的总表面积及平均直径、根尖数、苗木中元素含量及积累量都最大;单一供应CO(NH2)2的苗木根系总长度和总体积最大,苗木整株和地上部及根系的生物量、苗木中的元素含量及积累量仅低于50%NH+4-N+50%NO-3-N的处理;单一供应NO-3-N的苗木矮小,整株、地上部及根系的生物量和根系的形态及生长指标都最低,各种元素的含量及积累量也最少;单一供应NH+4-N时,在夏季叶片出现大量灼烧状坏死,其症状随NH+4-N的比例增加而加重,苗木也纤细,组织不充实,须根短,坏死较多,植株的生物量和根冠比及根系总长度、表面积、体积、平均直径和根尖数都较小,营养元素的积累量也较低。铁核桃对NH+4-N或NO-3-N没有明显的偏好。在NH+4-N与NO-3-N的比例各占50%时能够更好地促进铁核桃苗木的生长及营养的吸收,尿素也有较好的效果。     

盐胁迫下施氮对木麻黄幼苗生长生理的影响

本试验以木麻黄幼苗为试验材料,采用水培法以NH₄NO₃作为氮肥,分析了盐胁迫下不同浓度施氮处理木麻黄幼苗生长生理指标的变化。结果表明：与无盐害正常生长的对照处理相比,在2～10 g·L^-1 NaCl处理下幼苗遭受了不同程度盐害,盐害率为4.3%～100%,耐盐阈值6 g·L^-1。在6 g·L^-1 NaCl处理下,添加0.25～1 g·L¹ NH₄NO₃显著降低了幼苗盐害率,其中以0.75 g·L¹ NH₄NO₃处理下盐害率最低,较未施氮处理降低了52.3%。NH₄NO₃明显降低了叶片中Na⁺浓度,而Cl^-浓度变化趋势不明显,同时叶片净光合速率(Pn)、水分利用效率(WUE)、胞间二氧化碳浓度(Ci)增大,蒸腾速率(Tr)、气孔导度(Gs)减小,相对电导率和丙二醛...     

不同氮形态处理条件下杨树根尖差异表达基因的特征分析

[目的]利用高通量转录组测序技术，在硝态氮或铵态氮处理条件下，对杨树根尖差异表达基因进行了筛选和研究，同时分析和描述了差异表达基因对杨树根尖生长发育的影响，为后续开发高氮素吸收利用效率的杨树新种质提供科学依据。[方法]以灰杨幼苗根尖为材料，用0.5mmol·L^-1硝态氮（NO₃^-）和0.5mmol·L^-1铵态氮（NH₄⁺）对幼苗处理10 d，并对植株根尖进行转录组测序以及生物信息学分析。[结果]硝态氮处理下的主根长度几乎是铵态氮处理条件下的一倍。从两种不同氮形态处理杨树根尖转录组文库中，筛选到2 207个差异表达基因。通过差异基因GO和KEGG功能聚类分析，分别获得50个GO功能聚类和20条KEGG通路。进一步利用MapMan分析，筛选出36个氮代谢通路过程、各类氨基酸的生物合成以及代谢过程相关的差异表达基因。对这些差异基因互作调控网络分析发现，硝酸还原酶（Potri.005G172400）基因通过响应不同氮形态，在影响杨树根尖生长发育过程中发挥了重要作用...     

影响省藤弹性的参数和弹性方程式的确定

本文评估了省藤结构(基本薄壁组织和微管束分布)、物理特征(微纤维角度、结晶度和非晶区)和细胞壁化学成分(纤维素、半纤维素和木质素)对压缩应力的影响,以确定一个能测定省藤弹性的数学模式。进行了图象分析,碘染色,x光衍射技术和标准化学分析。最后,用热机械分析仪测定了压缩应力。回归方程表明,在不同参数中,基本薄壁组织比例、微纤维角度、非晶区大小和半纤维含量与压缩应力呈正相关。多元回归的数学方程为Y=0.012x₂-0.0001x₁+0.016x₃+0.017x₄-1.861,其中Y=压缩应力,x₂=微纤维角度,x₁=基本薄壁组织,x₃=非晶区,x₄=半纤维含量。在这些参数中,当P=0.050,半纤维含量的影响最大。     

模拟氮沉降对山区水库消落带湿地土壤细菌群落结构的影响

为了解氮沉降背景下水库消落带湿地土壤微生物群落结构对氮沉降浓度差异的响应，以浙江长潭水库消落带落羽杉Taxodium distichum种植区的湿地土壤为研究对象，设置了3个氮水平即低氮(NH₄NO₃ 30 kg·hm^-2·a^-1)、高氮(NH₄NO₃ 60 kg·hm^-2·a^-1)和对照(不施氮肥)的模拟氮沉降实验，采用高通量测序技术，分析水库消落带湿地土壤细菌群落对氮沉降浓度差异的响应。结果表明：在氮沉降增加条件下，水库消落带湿地土壤细菌的Shannon指数和Chao1指数在低氮条件下都是呈增加趋势，比对照分别增加了5.54%和13.29%，而在高氮条件下，其值呈降低趋势，分别比对照降低了4.41%和6.64%。氮沉降增加明显改变了消落带湿地土壤细菌群落结构，在门水平上，土壤酸杆菌门Acidobacteria相对丰度随着氮沉降的增加呈一直增加的趋势，土壤放线菌门Actinobacteria的相对丰度随着氮沉降...     

Nitrogen form and concentration interact to affect the performance of two ecologically distinct Mediterranean forest trees

Most studies examining inorganic N form effects on growth and nutrition of forest trees have been conducted on single species from boreal or temperate environments, while comparative studies with species from other biomes are scarce. We evaluated the response of two Mediterranean trees of contrasting ecology, Quercus ilex L. and Pinus halepensis Mill., to cultivation with distinct inorganic N forms. Seedlings were fertilized with different NH₄ ⁺/NO₃ ^? proportion at either 1 or 10 mM N. In both species, N forms had small effects at low N concentration, but at high N concentration they markedly affected the plant performance. A greater proportion of NH₄ ⁺ in the fertilizer at high N caused toxicity as it reduced growth and caused seedling death, with the effect being greater in Q. ilex than in P. halepensis. An increase in the proportion of NO₃ ^? at high N strongly enhanced growth relative to low N plants in P. halepensis but had minor effects in Q. ilex. Relatively more NH₄ ⁺ in the fertilizer enhanced plant P concentration but reduced K concentration in both species, while the opposite effect occurred with NO₃ ^?, and these effects were enhanced under high N concentration. We conclude that species responses to inorganic N forms were related to their ecology. P. halepensis, a pioneer tree, had improved performance with NO₃ ^? at high N concentration and showed strong plasticity to changes in N supply. Q. ilex, a late successional tree, had low responsiveness to N form or concentration.     

Post-fire soil nitrogen content and vegetation composition in Sub-Boreal spruce forests of British Columbia's central interior,Canada

Forest fires are known to influence nutrient cycling, particularly soil nitrogen (N), as well as plant succession in northern forest ecosystems. However, few studies have addressed the dynamics of soil N and its relationship to vegetation composition after fire in these forests. To investigate soil N content and vegetation establishment after wildfire, 13 sites of varying age class were selected in the Sub-Boreal spruce zone of the central interior of British Columbia, Canada. Sites varied in time since the last forest fire and were grouped into three seral age classes: (a) early-seral (<14 years), (b) mid-seral (50–80 years) and (c) late-seral (>140 years). At each site, we estimated the percent cover occupied by trees, shrubs, herbs and mosses. In addition, the soil samples collected from the forest floor and mineral horizons were analyzed for the concentrations of total N, mineralizable N, available NO₃⁻-N and available NH₄⁺-N. Results indicated that soil N in both the forest floor and mineral horizons varied between the three seral age classes following wildfire. Significant differences in mineralizable N, available NO₃⁻-N and available NH₄⁺-N levels with respect to time indicated that available soil N content changes after forest fire. Percent tree and shrub cover was significantly correlated to the amount of available NH₄⁺-N and mineralizable N contents in the forest floor. In the mineral horizons, percent tree cover was significantly correlated to the available NH₄⁺-N, while herb cover was significantly correlated with available NO₃⁻-N. Moss cover was significantly correlated with total N, available NO₃⁻-N and mineralizable N in the forest floor and available NO₃⁻-N in the mineral horizons. We identified several unique species of shrubs and herbs for each seral age class and suggest that plant species are most likely influencing the soil N levels by their contributions to the chemical composition and physical characteristics of the organic matter.     

Biomass and nutrients allocation in pot cultured beech seedlings： influence of nitrogen fertilizer

Allocation of biomass and nutrient elements including Nitrogen to above and belowground compartments of beech seedlings (Fagus sylvatica L.) treated by labeled nitrogen fertilizer in the form of ¹⁵NH₄ and ¹⁵NO₃ were investigated at the end of two successive growing seasons. Pot cultured beech seedlings were grown at a green house on intact soil cores sampled from three adjacent stands including beech, Norway spruce and mixed beech-spruce cultures of Solling forest, Germany. Comparing biomass allocation and nutrients concentrations of the seedlings between the control and ¹⁵N-fertilized treatments revealed no significant effect of N fertilization on nutrients uptake by seedlings over the experiment. The form of N input influenced its movement into plant pools. It was demonstrated that beech seedlings take up nitrogen mainly in the form of nitrate, which is then reduced in the leaves, although the differences between the retention of NO₃ ^?-N and NH₄ ⁺-N in plants were not statistically significant. Percent recoveries of ¹⁵N in trees were typically greater after ¹⁵NO₃ than after ¹⁵NH₄ additions. It was indicated that immobilization of 15N tracer in fine roots was a slower process comparing other plant compartments such as stem and coarse roots, but a powerful sink for N during the course of study.     

Seasonal dynamics of mineral N pools and N-mineralization in soils under homegarden trees in South Andaman,India

Agroforestry trees are now well known to play a central role in the build up of nutrients pools and their transformations similar to that of forest ecosystem, however, information on the potential of homegarden trees accumulating and releasing nitrogen (mineralization) is lacking. The present study reports seasonal variations in pool sizes of mineral N (NH₄⁺-N and NO₃⁻-N), and net N-mineralization rate in relation to rainfall and temperature under coconut (Cocos nucifera L.), clove (Eugenia caryophyllata Thunb) and nutmeg (Myristica fragrans Houtt. Nees) trees in a coconut-spice trees plantation for two annual cycles in the equatorial humid climate of South Andaman Island of India. Concentration of NH₄⁺-N was the highest during wet season (May–October) and the lowest during post-wet season (November–January) under all the tree species. On the contrary, concentration of NO₃⁻-N was the lowest in the wet season and the highest during the post-wet season. However, concentrations of the mineral N were the highest under the nutmeg and the lowest under the coconut trees. Like the pool sizes, mean annual mineralization was the highest under the nutmeg (561 mg kg⁻¹ yr⁻¹) and the lowest under the coconut trees (393 mg kg⁻¹ yr⁻¹). Rate of mineralization was the highest during the post-wet season and the lowest during the dry season (February–April) under all the tree species. High rainfall during the wet season, however, reduced the rate of nitrification under all the tree species. The mean annual mineralization was logarithmically related with rainfall amount and mean monthly temperature.     

Soil nitrogen dynamics in alley cropping and no-till systems on ultisols of the Georgia Piedmont, USA

On highly-weathered Ultisols of the Georgia (USA) Piedmont, a combination of no-till agriculture and alley cropping presents an option for rapidly increasing soil nitrogen availability while restoring long-term soil fertility. Three years after the establishment of Albizia julibrissin hedgerows and no-till agriculture trials, we measured inorganic soil nitrogen (NO₃ ^-–N and NH₄ ^-–N) and net nitrogen mineralization during a 4-month field study and a 14-day laboratory study . We also measured the influence of tree leaf amendments on grain sorghum production and N uptake. Soil nitrate increased four-fold within two weeks of adding Albizia leaf mulch. Soil ammonium did not increase as rapidly nor to the same extent after tree mulch addition. Averaged over the 4-month study, soil nitrate and ammonium were 2.8 and 1.4 times higher in the alley-cropped than in the treeless no-till plots. Net nitrification and mineralization were no higher in the alley cropping plots, during either field or laboratory incubations. Tree mulch additions enhanced crop biomass production and N uptake 2 to 3.5 times under both high and low soil moisture conditions. Our study demonstrates the dramatic short-term impacts of Albizia mulch addition on plant available nitrogen. Combined with no-till practices, alley cropping with Albizia hedges offers Piedmont farmers an option for reducing reliance upon chemical N fertilizer while improving soil organic matter levels. This revised version was published online in August 2006 with corrections to the Cover Date.     

Tracing the fate of mineral N compounds under high ambient N deposition in a Norway spruce forest at Solling/Germany

The fate of high and equally distributed ammonium and nitrate deposition was followed in a 72-year-old roofed Norway spruce forest at Solling in central Germany by separately adding ¹⁵NH₄⁺ and ¹⁵NO₃⁻ to throughfall water since November 2001. The objective was to quantify the retention of atmospheric ammonium and nitrate in different ecosystem compartments as well as the leaching loss from the forest ecosystem. δ¹⁵N excess in tree tissues (needles, twigs, branches and bole woods) decreased with increased tissue age. Clear ¹⁵N signals in old tree tissues indicated that the added ¹⁵N was not only assimilated to newly produced tree tissues but also retranslocated to old ones. During a period of over 3-year ¹⁵N addition, 30% of ¹⁵NH₄⁺ and 36% of ¹⁵NO₃⁻ were found in tree compartments. For both ¹⁵N tracers, 15% of added ¹⁵N was found in needles, followed by woody tissues (twigs, branches and boles, 7–13%) and live fine roots (7%). The recovery of ¹⁵NH₄⁺ and ¹⁵NO₃⁻ in the live fine roots differed with soil depth. The recovery of ¹⁵NH₄⁺ tended to be higher in the live fine roots in the organic layer than in the upper mineral soil. In the live fine roots in deeper soil, the recovery of ¹⁵NO₃⁻ tended to be higher than that of ¹⁵NH₄⁺. Soil retained the largest proportion of ¹⁵N, accounting for 71% of ¹⁵NH₄⁺ and 42% of ¹⁵NO₃⁻. Most of ¹⁵NH₄⁺ was recovered in the organic layer (65%) and the recovery decreased with soil depth. Conversely, only 8% of ¹⁵NO₃⁻ was found in the organic layer and 34% of ¹⁵NO₃⁻ was evenly distributed throughout the mineral soil layers. Nitrate leaching accounted for 3% of ¹⁵NH₄⁺ and 19% of ¹⁵NO₃⁻. Only less than 1% of the both added ¹⁵N was leached as DON. These results suggested that trees had a high contribution to the retention of atmospheric N and soil retention capacity determined the loss of atmospheric N by nitrate leaching.     

重庆酸雨区缙云山典型林分冠层酸雨淋洗特征

选取重庆缙云山的针阔混交林、常绿阔叶林、毛竹林、灌木林4种典型林分,观测酸性降水过程中林外雨、穿透雨及干流等林内水分转换分量中的主要离子含量变化,分析林分冠层对雨水化学组成的影响,结果表明:(1)降雨中的离子当量浓度大小依次是SO42-＞Ca2+＞ NH4+＞Mg2+＞K+＞Na+＞NO3-;(2)降雨经过林冠层后pH值降低,干流的酸化程度增加最大;(3)降雨经林冠层后离子浓度明显增加(除灌木林),穿透雨中通量增加最大的阴离子和阳离子分别为SO42-(2.19×103～6.47×103 eq·hm-2)和Ca2+(1.41×103～3.39×103 eq-hm-2),离子来源主要为大气沉降和植物分泌物或淋出;(4)同一离子在不同林分的干流和穿透雨中的通量变化不同,反映出不同林分冠层的离子交换性差异.在针阔混交林中,林下降雨净淋溶量大小顺序为SO42-＞Ca2+＞ NO3-＞K+＞NH4+＞Mg2+＞ Na+;常绿阔叶林为SO42-＞ Ca2+＞ K+＞NO3-＞ NH4+＞ Mg2+ ＞Na+;毛竹林为Ca2+＞ SO42-＞ K+＞NO3-＞ NH4+＞Na+＞Mg2+;灌木林为Ca2+＞ NO3-＞ K+＞ Na+＞Mg2+＞ NH4+＞ SO42-.     

Adaptation to exploit nitrate in surface soils predisposes yellow-cedar to climate-induced decline while enhancing the survival of western redcedar: A new hypothesis

Yellow-cedar (Chamaecyparis nootkatensis (D. Don) Spach) and western redcedar (Thuja plicata Donn), two valuable tree species of Pacific Northwest forests, are competitive in low productivity forests on wet, nearly saturated soils with low nitrogen (N) availability and turnover. We propose a mechanism where cedar trees survive in marginal conditions through exploiting a coupled Ca–NO₃⁻ nutrient cycle where trees assimilate N as nitrate (NO₃⁻), but must accumulate a counter-ion to NO₃⁻ such as calcium (Ca⁺²) to control their internal cell pH and provide electrochemical balance. The availability of NO₃⁻ in cedar forests is favored by increased microbial activity and shifts in microbial community composition that is conducive to N mineralization and nitrification at higher pH. Cedars influence the soils under their canopy by enriching the forest floor with calcium compounds leading to increases in pH. Cedars are also prone to precocious dehardening in the spring when N is released from freeze–thaw events in the soils and conditions appear to favor nitrifying microbial communities. Cedars must concentrate fine-root biomass near the soil surface to access Ca and NO₃⁻, but this beneficial physiological adaptation also creates a vulnerability to periodic root freezing injury that is leading to the decline and mortality of at least one of them—yellow-cedar.     

Nitrogen cycling in Pinus sylvestris stands exposed to different nitrogen inputs

The objective of this study was to quantify the effects of high nitrogen (N) inputs on N cycling in a 35–45-yr-old Scots pine (Pinus sylvestris L.) forest. Nitrogen was added annually (single doses) as NH₄NO₃ in doses of 0 (N0), 30 (N1) and 90 (N2) kg N ha^?1 yr^?1. The only N input to the N0 plots was atmospheric deposition of 10 kg N ha^?1 yr^?1. The N cycle in these plots was tight, with almost complete retention of the incoming N. In the N1 plots the N retention was 83% after 9 yrs of N addition. The trees were the major sink, but the soil also contributed to the N retention. In the N2 plots the N retention was 63%, being mainly accounted for by accumulation in the soil. The leaching of N from the N2 stands was as high as 35 kg N ha^?1 yr^?1. The N2 system was N saturated.     

Soil NH4 +/NO3 − nitrogen characteristics in primary forests and the adaptability of some coniferous species

In terrestrial ecosystems, soil nutrient regimes at a plant’s living site generally represent the plant’s “nutrition habitat”. Plant species frequently well adapt to their original “nutrition habitat” during a long process of evolution, and the apparent preference for ammonium or nitrate nitrogen source (NH₄ ⁺ or NO₃ ⁻) might be an important aspect of the adaptation. Plants typically favor the nitrogen form most abundant in their natural habitats. Nitrate has been recognized as the dominant mineral nitrogen form in most agricultural soils and the main nitrogen source for crops, but it is not usually the case in forest ecosystems. A large number of studies show that the “nutrition habitats” associated with primary forest soils are typically dominated by NH₄ ⁺ rather than NO₃ ⁻, generally with NO₃ ⁻ content much lower than NH₄ ⁺. Low levels of NO₃ ⁻ in these forest soils generally correspond to low net rates of nitrification. The probable reasons for this phenomenon include: 1) nitrification limitations and/or inhibitions caused by lower pH, lower NH₄ ⁺ availability (autotrophic nitrifiers cannot successfully compete for NH₄ ⁺ with heterotrophic organisms and plants), or allelopathic inhibitors (tannins or higher-molecular-weight proanthocyanidins) in the soil; or 2) substantial microbial acquisition of nitrate in the soils, which makes net nitrification rates substantially less than gross nitrification rates even though the latter are relatively high. Many coniferous species (especially such late successional tree species as Tsuga heterophylla, Pinus banksiana, Picea glauca, Pseudotsuga meziesii, Picea abies, etc.) fully adapt to their original NH₄ ⁺-dominated “nutrition habitats” so that their capacities of absorbing and using non-reduced forms of nitrogen (e.g., NO₃ ⁻) substantially decrease. These conifers typically show distinct preference to NH₄ ⁺ and reduced growth due to nitrogen-metabolism disorder when NO₃ ⁻ is the main nitrogen source. The physiological and biochemical mechanisms that account for the adaptation to NH₄ ⁺-dominated systems (or limited ability to use NO₃ ⁻) for the coniferous species include: i) distribution and activity of enzymes for catalyzing nitrogen reduction and assimilation, generally characterized by lower nitrate reductase (NR); ii) greater tolerance to NH₄ ⁺ or rapid detoxification of ammonium nitrogen in the roots; iii) lower capacity of absorption to NO₃ ⁻ by roots that might be controlled by feedback regulations of certain N-transport compounds, such as glutamine; iv) relations and balance between nitrogen and other elements (such as Ca²⁺, Mg²⁺, and Zn²⁺ etc.). Some NH₄ ⁺-preferred conifers might be more adapted (tolerant) to lower base cation conditions; v) NO₃ ⁻ nutrition, rather than NH₄ ⁺, that may lead to the loss of considerable quantities of organic and inorganic carbon to the surrounding media and mycorrhizal symbiont and probably contribute to slower growth; and vi) the metabolic cost of reducing NO₃ ⁻ to NH₄ ⁺ that may make shade-tolerant conifers favor the uptake of reduced nitrogen (NH₄ ⁺). The adaptation of late successional conifers to NH₄ ⁺-dominated habitats has profound ecological implications. First, it might be an important prerequisite for the climax forest communities dominated by these conifers to maintain long-term stability. Second, primary coniferous or coniferous-broadleaved forests have been widely perturbed because of commercial exploitation, where the soil ammonium nitrogen pool tends to be largely transformed to nitrate after disturbance. In such a situation, the coniferous species that were dominant in undisturbed ecosystems may become poor competitors for nitrogen, and the site will be occupied by early successional (pioneer) plants better adapted to nitrate utilization. In other words, the implicit adaptation of many conifers dominant in undisturbed communities to ammonium nitrogen will cause difficulties in their regeneration on disturbed sites, which must be taken into account in the practical restoration of degraded temperate forest ecosystems. __________ Translated from Acta Ecologica Sinica, 2005, 25(11): 3,082–3,092 [译自: 生态学报]