Responses of exchangeable base cations to continuously increasing nitrogen addition in alpine steppe: A case study of Stipa purpurea steppe

土壤交换性盐基离子(Ca ²⁺、Mg ²⁺、K ⁺、Na ⁺)在维持土壤养分与缓冲土壤酸化中起着重要作用, 了解其对氮添加的响应有助于准确评估氮沉降背景下生态系统结构与功能的动态变化。然而, 目前关于土壤交换性盐基离子对氮添加响应的相关研究主要集中在酸性土中。鉴于目前在碱性土中研究相对较少的现状, 该研究以青藏高原高寒草原为研究对象, 依托氮添加控制实验平台, 通过连续3年(2014-2016)的测定, 考察了8个不同施氮水平(0、1、2、4、8、16、24、32 g·m ^-2·a ^-1)下土壤交换性盐基离子含量变化及其可能原因。结果显示: 随着施氮量的增加, 土壤交换性盐基离子, 尤其是Mg ²⁺与Na ⁺含量显著降低。并且, 盐基离子含量与植物地上生物量显著负相关(p < 0.05), 说明氮添加通过促进植物生长, 加速植物对盐基离子的吸收, 进而导致土壤中盐基离子含量降低。此外, 盐基离子含量也与土壤无机氮含量呈显著负相关(p < 0.05)关系, 说明施氮还通过提高土壤中无机氮含量进而导致更多NH₄ ⁺与土壤吸附的盐基离子交换, 同时加剧NO₃ ^-淋溶, 带走等电荷阳离子。需要指出的是, 虽然连续施氮导致土壤pH值下降, 但该土壤目前仍处于碳酸盐缓冲阶段, 说明通常在酸性土中报道的“因缓冲土壤酸化引起的盐基离子损失机制”在碱性土中并不成立。这些结果意味着持续的氮输入会造成碱性土中盐基离子损失, 进而影响土壤缓冲能力与植被生产力, 未来草原生态系统管理中应重视这一问题。     

Uptake of SO2 in shoots of Scots pine: field measurements of net flux of sulphur in relation to stomatal conductance

Abstract. Net fluxes of gaseous sulphur compounds, water vapour and carbon dioxide to current- and one-year-old shoots of Scots pine were simultaneously measured in the field. The shoots were fumigated in temperature-controlled assimilation chambers with sulphur dioxide at concentrations ranging from 40 to 250 μg m⁻³ (15–95 ppb). The hypothesis that stomatal opening regulates the uptake of sulphur dioxide was tested. The following conclusions were reached concerning the dry deposition of sulphur dioxide to a dry coniferous shoot.
There was a marked diurnal variation in the uptake rate of sulphur dioxide. The net deposition velocity of sulphur was about three times higher during the day than during the night during July-October. Stomatal opening was not the primary factor controlling the dry deposition rate of sulphur dioxide. The net dry deposition rate was significantly smaller than the calculated rate based on stomatal conductances for water vapour.
Part of this deviation could be explained by re-emission of reduced sulphur compound(s) from the needles. The re-emission of sulphur was a light-dependent process and might be of great significance for the sulphur balance calculations.
A variable deposition velocity should be used in atmospheric transport models to account for the diurnal and seasonal variation in the surface resistance of a dry canopy.     

Effect of different nitrogen nutrients on the viability, protein synthesis and tannin production of Scots pine callus

The effect of two different media on the growth, metabolism and viability of Scots pine ( Pinus sylvestris ) callus cultures was studied. Inorganic nitrogen in the culture media (modified MS) was in the form of either KNO₃ or NH₄NO₃. The cultures were started from buds of mature Scots pine. Growth was poor on the medium with KNO₃, but this compound had a noticeable effect on the metabolism of the callus, which was reflected in alterations in protein and polyphenol synthesis and the pH of the culture medium. Although the fresh mass, water content and viability of the callus decreased when KNO₃ was the exclusive inorganic nitrogen nutrient, protein synthesis was more abundant. Electrophoretic analyses indicated alterations in the patterns of soluble proteins and purified glycoproteins. Phenylalanine ammonia-lyase (EC 4.3.1.5) activities were high in all the calluses, and concentrations of condensed tannins and their precursors, catechins, were higher than in intact buds. The role of inorganic nitrogen nutrition in the deterioration of tissues is discussed on the basis of the effect of ammonium on the metabolism of pine callus.     

The simulation of soil organic matter and nitrogen accumulation in Scots pine plantations on bare parent material using the combined forest model EFIMOD

The individual-based combined forest model EFIMOD including the soil-sub model SOMM has been used for the simulation of Scots pine stand growth and soil organic matter (SOM) accumulation on a humus-free bare mineral surface. The growth of Scots pine plantation, with an initial density of 10 000 trees ha⁻¹ and average tree biomass of 0.01 kg was simulated for 50 yr under Central European climatic conditions (i) with varying atmospheric nitrogen inputs and (ii) different rates of initial application of raw undecomposed organic material or compost, on humus-free parent material. The accumulation of typical raw humus was simulated in all cases. The accumulation was most intensive in the simulation of high atmospheric nitrogen input. The humus pool in the mineral topsoil was small but achieved its maximum value with compost application. SOM nitrogen accumulation was scant in all cases, except the compost applications with low atmospheric nitrogen input. No statistically significant differences of SOM and stand parameters were found between variants without organic matter and those with low input of organic manure. However, the maximum relative rate of SOM and nitrogen accumulation was found in the scenario without organic manure, under slowly growing unstable Scots pine plantation. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date.     

重庆缙云山两种林分土壤呼吸对模拟氮沉降的季节响应差异性

氮沉降对土壤呼吸的影响仍然存在着争论,需要进一步研究。选择重庆缙云山的马尾松林和柑橘林开展了氮添加实验,分别设置3个氮添加水平(低氮T_5:20 g N m~(-2)a~(-1),中氮T_(10):40 g N m~(-2)a~(-1)和高氮T_(15):60 g N m~(-2)a~(-1))和对照(T_0:0 g N m~(-2)a~(-1))共4个水平的处理,各林分每个处理各9次重复,每个处理量分4次,在每个季度开始各施1次。采用ACE(Automated Soil CO_2 Exchange Station,UK)自动土壤呼吸监测系统测定两林分土壤表层(0—10 cm)的呼吸、温度和湿度,分别在当年的7月、9月、11月、第2年的1月、2月、3月、5月、6月各连续测定4d,每天(8:00—18:00)4次,以揭示两种林分土壤呼吸对模拟氮沉降的季节动态响应及其差异性。结果表明:(1)柑橘林与马尾松林林下土壤表层呼吸表现出一致的季节变化动态趋势:夏季春季秋季冬季,但柑橘林土壤呼吸显著高于马尾松林(P0.05)。(2)总体上氮沉降抑制了2种林分土壤表层呼吸,而N沉降量大抑制程度越高。只在冬季土壤湿度低的马尾松林下氮沉降促进了土壤呼吸。(3)土壤温度与土壤呼吸有极显著的正相关指数关系(P0.01),而土壤水分与土壤呼吸有显著的二次模型拟合关系,但均受到氮沉降量处理的影响。综合分析表明,在亚热带山区2类森林下的典型案例结果支持氮沉降抑制土壤呼吸的认识。     

Atmospheric deposition and watershed nitrogen export along an elevational gradient in the Catskill Mountains, New York

Cumulative effects of atmospheric N deposition mayincrease N export from watersheds and contribute tothe acidification of surface waters, but naturalfactors (such as forest productivity and soildrainage) that affect forest N cycling can alsocontrol watershed N export. To identify factors thatare related to stream-water export of N, elevationalgradients in atmospheric deposition and naturalprocesses were evaluated in a steep, first-orderwatershed in the Catskill Mountains of New York, from1991 to 1994.Atmospheric deposition of SO ₄ ^2– , andprobably N, increased with increasing elevation withinthis watershed. Stream-water concentrations ofSO ₄ ^2– increased with increasing elevationthroughout the year, whereas stream-waterconcentrations of NO ₃ ^– decreased withincreasing elevation during the winter and springsnowmelt period, and showed no relation with elevationduring the growing season or the fall. Annual exportof N in stream water for the overall watershed equaled12% to 17% of the total atmospheric input on thebasis of two methods of estimation. This percentagedecreased with increasing elevation, from about 25%in the lowest subwatershed to 7% in the highestsubwatershed; a probable result of an upslope increasein the thickness of the surface organic horizon,attributable to an elevational gradient in temperaturethat slows decomposition rates at upper elevations. Balsam fir stands, more prevalent at upper elevationsthan lower elevations, may also affect the gradient ofsubwatershed N export by altering nitrification ratesin the soil. Variations in climate and vegetationmust be considered to determine how future trends inatmospheric deposition will effect watershed export ofnitrogen.     

Major changes in forest carbon and nitrogen cycling caused by declining sulphur deposition

Sulphur (S) and nitrogen (N) deposition are important drivers of the terrestrial carbon (C) and N cycling. We analyzed changes in C and N pools in soil and tree biomass at a highly acidified spruce site in the Czech Republic during a 15 year period. Total S deposition decreased from 5 to 1.1 g m^?2 yr^?1 between 1995 and 2009, whereas bulk N deposition did not change. Over the same period, C and N pools in the Oa horizon declined by 116 g C and 4.2 g N m^?2 yr^?1, a total decrease of 47% and 42%, respectively. This loss of C and N probably originated from organic matter (OM) that had accumulated during the period of high acid deposition when litter decomposition was suppressed. The loss of OM from the Oa horizon coincided with a substantial leaching (1.3 g N m^?2 yr^?1 at 90 cm) in the 1990s to almost no leaching (<0.02 g N m^?2 yr^?1) since 2006. Forest floor net N mineralization also decreased. This had consequences for spruce needle N concentration (from 17.1 to 11.4 mg kg^?1 in current needles), an increase in litterfall C/N ratio (from 51 to 63), and a significant increase in the Oi + Oe horizon C/N ratio (from 23.4 to 27.3) between 1994 and 2009/2010. Higher forest growth and lower canopy defoliation was observed in the 2000s compared to the 1990s. Our results demonstrate that reducing S deposition has had a profound impact on forest organic matter cycling, leading to a reversal of historic ecosystem N enrichment, cessation of nitrate leaching, and a major loss of accumulated organic soil C and N stocks. These results have major implications for our understanding of the controls on both N saturation and C sequestration in forests, and other ecosystems, subjected to current or historic S deposition.     

Effect of atmospheric ammonia on the nitrogen metabolism of Scots pine (Pinus sylvestris) needles

Four-year-old seedlings of Scots pine ( Pinus sylvestris L.) were exposed to filtered air (FA), and to FA supplemented with NH₃ (60 and 240 μg m⁻³) in controlled-environment chambers for 14 weeks. Exposure to the higher NH₃ concentration resulted in an increased activity of glutamine synthetase (GS, EC 6.3.1.2), and an increase in the concentrations of soluble proteins, total nitrogen, free amino acids and leaf pigments in the needles. The GS activity (μmol g⁻¹ fresh weight h⁻¹) in the needle extract increased to levels 69% higher than in FA and the soluble protein concentration to levels 22% higher. Total nitrogen concentration in the needles was 42% higher than in FA, while the free amino acid concentration was 300% higher, which was caused by an increase in arginine, glutamate, aspartate and glutamine. Chlorophyll a , chlorophyll b and carotenoid concentrations were 29, 38 and 11% higher, respectively. Neither the glutamate dehydrogenase (GDH, EC 1.4.1.2) activity nor the concentrations of free NH₄⁺ and glucose in the needles were affected by exposure to NH₃. After NH₃ fumigation at 240 μg m⁻³ the starch concentration decreased by 39% relative to the FA. The results indicate that the metabolism of Scots pine acclimates to concentrations of NH₃ which are 3 to 10 times higher than the average concentration in areas with intensive stock farming. The possible mechanisms underlying acclimation to NH₃ are discussed.     

Effect of ectomycorrhizal fungi on nitrogen mineralisation and the growth of Scots pine seedlings in natural peat

The aim of this study was to investigate the potential of different isolates of ectomycorrhizal (EM) fungi to enhance the growth of Pinus sylvestris seedlings in five natural peat substrates with different nitrogen concentrations, and the effect of the Scots pine seedlings and fungal inoculum on the formation of dissolved inorganic and organic nitrogen in peat. Utilization of different organic nitrogen compounds by microbial community in the peat was also investigated using Biolog MT MicroPlates. Inoculation of the seedlings with EM fungi enhanced seedling growth. Piloderma croceum increased root growth especially, whereas Lactarius rufus increased needle growth and Suillus variegatus I, II and III improved both root and needle growth. All the EM fungi also significantly affected stem growth. Nitrogen concentration of the peat did not affect seedling growth as much as the EM fungi. At the lowest peat N concentration (1.17%) NH ₄ ⁺ mineralisation was lower and DON (dissolved organic nitrogen) accumulation higher than at higher peat N concentrations. The EM fungal isolates had different effects on NH ₄ ⁺ and DON accumulation/degradation in peat. The EM fungal isolates significantly increased NH ₄ ⁺ formation in the peat, whereas L. rufus and P. croceum had an opposite effect on DON accumulation. S. variegatus I significantly decreased the DON concentrations during peat incubation. The N concentration of the peat slightly affected the utilization of amino acids and polyamines by the microbial community, whereas inoculation with S. variegatus I, II or III had no effect.     

Light microscopic structure of needles of Scots pine (Pinus sylvestris L.) in relation to air pollution and needle element concentrations in S.E. Finland and the Karelian Isthmus, N.W. Russia

 The mesophyll structure of current and second year needles of mature Scots pines (Pinus sylvestris L.), growing on a survey line from the Karelian Isthmus (N.W. Leningrad region, Russia) to S.E. Finland, was studied at the light microscopic level in 1992 and 1994. The area is affected by sulphur dioxide and nitrogen oxide emissions from Russia (200  000–285 000 ton/year SO₂ and about 100  000 ton/year NO_X), Estonia (140  000–320  000 ton/year SO₂ and 30  000–50  000 ton/year NO_X) and Finland (6000–9000 ton/year SO₂ and 23 000–25 000 ton/year NO_X) and by unknown amounts of neutralizing compounds from Estonia. Mean concentrations of SO₂ were between 1.0 and 1.5 μg m^–3 and those of ozone between 52 and 61 μg m^–3 during growing seasons of 1991–1994. The concentrations tended to be lower in 1993/94 than in 1991/92. The results showed cellular changes related to exposure to sulphur dioxide, ozone and acid wet deposition. The percentage of cells with injuries related to acid deposition was higher in 1994 than in 1992, possibly due to higher reduction of neutralizing than of acidifying compounds in the main emission areas, while the percentage of cells with injuries related to sulphur dioxide correlated positively with needle sulphur concentrations and showed a declining trend between Karelian Isthmus and S.E. Finland in the needles sampled in 1992. These changes and those related to ozone were considerably less numerous in 1994 than in 1992, possibly on account of the lower concentrations of SO₂ and ozone in 1993/94 than in 1991/92, especially in early summer, the time of the most sensitive phase of needle growth. In addition, periods of lower wet deposition in summer 1992, and considerably lower temperatures in autumn/winter 1992 than in 1994 may have enhanced the injuries in the needles collected in October/December 1992, relative to those collected in 1994. The results of the microscopic work agreed well with the simultaneous chemical measurements, pointing to the usefulness of cellular analysis for diagnostic work in the field. Received: 26 September 1997 / Accepted: 24 November 1997     

温带两种林型对氮沉降的再分配及其生长季动态与影响因子

氮(N)沉降对森林生态系统的结构与功能具有重要的影响,而森林对到达林地的N沉降量及其分配格局的影响尚不清楚。量化了2012—2013年5—10月两个生长季蒙古栎林和杂木林的林内树干径流和穿透雨及其林外大气降雨总氮(TN)、可溶性氮(DN)和颗粒态氮(PN)沉降通量的季节动态,旨在比较两种林型对N沉降的再分配格局及其季节变化,分析影响其变异的主要因子。结果表明:林外大气降雨、蒙古栎林林内、杂木林林内(树干径流+穿透雨)TN沉降平均值分别为:8.49、15.97、13.39 kg hm~(-2)a~(-1),其中DN分别占其TN的76.35%、82.79%和75.02%,PN分别占其TN的26.35%、17.21%和24.98%,蒙古栎林和杂木林林内穿透雨TN沉降量分别占其TN的95.5%和94.5%。蒙古栎林和杂木林冠层淋溶TN沉降量分别为7.48kg hm~(-2)a~(-1)和4.90 kg hm~(2)a~(-1);其中,前者的DN高于后者,但PN呈相反趋势。两种林型的N沉降组分具有明显的季节动态:沉降量均集中在生长季中期(6—8月),生长季前期和末期较低。林外降雨量分别与林外大气降雨、蒙古栎林和杂木林林内的树干径流和穿透雨中的TN、DN浓度呈显著负指数函数关系(P0.001)。连续降雨天数对蒙古栎林、杂木林林内TN、DN浓度的影响表现为连续降雨2 d以内为富集作用,之后为稀释作用。研究表明林冠对大气氮沉降有显著富集作用,其富集强度及时间动态与森林类型和降雨特征有关,建议氮沉降试验应考虑林冠的富集效应。     

Assimilation of nitrate and ammonium by the Scots pine (Pinus sylvestris) seedling under conditions of high nitrogen supply

Seedlings of Scots pine ( Pinus sylvestris L.) were grown on perlite for 21 days under controlled conditions. Apart from the water control, KNO₃ (15 m M ), (NH₄)₂SO₄ (7.5 m M ), and NH₄NO₃ (15 m M ) were offered to study the effects of a high nitrogen supply on nitrogen assimilation. In some experiments 1.3 m M potassium was added to the basic ammonium solutions. In labelling studies nitrate and ammonium were 2.3 atom%¹⁵N-enriched. It was found that over the 21-day period approximately three times more ammonium-N was taken up than nitrate-N. However, nitrate and ammonium, applied simultaneously, were taken up to the same extent as if they were applied separately (additivity). The presence of K⁺ in the medium did not affect N-uptake. Among the soluble N-containing compounds nitrate, ammonium and 8 amino acids were quantified. It was found that assimilation of nitrate can cope with the uptake of NO⁻₃ under all circumstances. Neither free nitrate nor ammonium or amino acids accumulated to an extent exceeding the values of water-grown seedlings. On the other hand, in case of high ammonium supply considerably more nitrogen was taken up than could be incorporated into nonsoluble N-containing substance ('protein'). The remaining nitrogen was found to accumulate in intermediary storage pools (free NH₄⁺, glutamine, asparagine, arginine). Part of this accumulated N could be incorporated into protein when potassium was offered in the nutrient solution. It is concluded that potassium is a requirement for a high rate of protein synthesis not only in crop plants but also in conifers.     

Attack dynamics of the pine shoot beetle, Tomicus piniperda (Col.; Scolytinae) in Scots pine stands defoliated by Bupalus piniaria (Lep.; Geometridae)

Abstract 1 After a 1‐year, extensive pine looper (Bupalus piniaria) outbreak, plots were laid out to study tree susceptibility to attack, and performance of Tomicus piniperda in pine trees suffering from varying levels of defoliation. 2 Tomicus piniperda was the dominating stem‐attacking species among the primary stem colonizers, and 82% of all trees that died had been colonized by T. piniperda. 3 Beetle attacks primarily struck severely defoliated trees, i.e. trees that suffered from 90% to 100% defoliation. 4 Beetle attacks peaked in the second year after cessation of the outbreak, and suppressed trees were both more frequently attacked and more susceptible to beetle attack than intermediate and dominant trees. 5 Trees surviving beetle attacks carried more foliage than trees that did not survive the attacks. 6 A single year of severe defoliation is enough to render pine trees susceptible to secondary pests, such as T. piniperda.     

模拟氮沉降对温带典型森林土壤有效氮形态和含量的影响*

通过室内模拟氮沉降试验,研究了氮沉降对温带典型森林土壤有效氮的影响.结果表明:试验期间,与对照相比,经过氮沉降处理的土壤铵态氮、硝态氮和有效氮均呈增长的趋势,增加的程度取决于森林类型、土层、氮处理类型和氮处理的持续时间.氮沉降对不同林型土壤有效氮形态和含量的影响不同,氮沉降对混交林的影响弱于阔叶林,强于针叶人工纯林;土壤A层对氮沉降的敏感程度大于土壤B层;铵态氮形态沉降对土壤铵态氮含量的影响比对土壤硝态氮含量的影响大,而硝态氮形态沉降对土壤硝态氮含量的影响比对土壤铵态氮含量的影响大,混合形态的氮沉降对二者均有促进作用,且增加幅度更高;氮沉降对土壤有效氮的影响存在累加效应.     

The effects of SO2 and O3 fumigation on acid deposition and foliar leaching in the Liphook forest fumigation experiment

The ambient pollution climate at the Liphook forest fumigation site, where coniferous trees were fumigated with SO₂ and O₃, for 4 years under field conditions, was characteristic of the fringes of the areas where pollutant effects are a problem. Experimental treatments increased SO₂ concentrations to levels more characteristic of Eastern Europe, and summer O₃ concentrations by 30%. Deposition of SO₂ to the soil between the trees (inferred from shallow lysimeters) was significant, the deposition velocity being 2–1 mms^?1. Deposition to Scots pine and Sitka spruce canopies was greater, deposition velocities being 8.5 and 9.4 mm s^?1, respectively. These high values may perhaps be explained by co-deposition with NH₃. Calculations assume that dry deposition was the sole source of SO₄^2? gain in throughfall, and that there was no significant retention by the trees. There was a trend for O₃ to enhance SO₂ deposition to both soil and trees. Fumigation with SO₂ led to a significant increase in leaching of cations from foliage. Each species neutralized about 63% of the dry-deposited SO₂, predominantly by ion exchange for Ca and K. Equations are provided which allow calculation of foliar leaching given SO₂ concentrations or SO₄^2? deposition. Fumigation increased the rate of nutrient cycling considerably, without affecting foliar concentrations or damaging the trees. Ozone treatments did not enhance foliar leaching, calling into question some suggested mechanisms for the causes of forest decline.     

The response of soil solution chemistry in European forests to decreasing acid deposition

Acid deposition arising from sulphur (S) and nitrogen (N) emissions from fossil fuel combustion and agriculture has contributed to the acidification of terrestrial ecosystems in many regions globally. However, in Europe and North America, S deposition has greatly decreased in recent decades due to emissions controls. In this study, we assessed the response of soil solution chemistry in mineral horizons of European forests to these changes. Trends in pH, acid neutralizing capacity (ANC), major ions, total aluminium (Al_tot) and dissolved organic carbon were determined for the period 1995–2012. Plots with at least 10 years of observations from the ICP Forests monitoring network were used. Trends were assessed for the upper mineral soil (10–20 cm, 104 plots) and subsoil (40–80 cm, 162 plots). There was a large decrease in the concentration of sulphate () in soil solution; over a 10‐year period (2000–2010), decreased by 52% at 10–20 cm and 40% at 40–80 cm. Nitrate was unchanged at 10–20 cm but decreased at 40–80 cm. The decrease in acid anions was accompanied by a large and significant decrease in the concentration of the nutrient base cations: calcium, magnesium and potassium (Bc = Ca²⁺ + Mg²⁺ + K⁺) and Al_tot over the entire dataset. The response of soil solution acidity was nonuniform. At 10–20 cm, ANC increased in acid‐sensitive soils (base saturation ≤10%) indicating a recovery, but ANC decreased in soils with base saturation >10%. At 40–80 cm, ANC remained unchanged in acid‐sensitive soils (base saturation ≤20%,  ≤ 4.5) and decreased in better‐buffered soils (base saturation >20%,  > 4.5). In addition, the molar ratio of Bc to Al_tot either did not change or decreased. The results suggest a long‐time lag between emission abatement and changes in soil solution acidity and underline the importance of long‐term monitoring in evaluating ecosystem response to decreases in deposition.     

Interactions of elevated CO2, NH3 and O3 on mycorrhizal infection,gas exchange and N metabolism in saplings of Scots pine

Four-year-old saplings of Scots pine (Pinus sylvestris) (L.) were exposed for 11 weeks in controlled-environment chambers to charcoad-filtered air, or to charcoal-filtered air supplemented with NH₃ (40 g m^–3), O₃ (110 g m^–3 during day/ 40 g m^–3 during night) or NH₃+O₃. All treatments were carried out at ambient (259 L L^–1) and at elevated CO₂ concentration (700 L L^–1). Total tree biomass, mycorrhizal infection, net CO₂ assimilation (P_n), stomatal conductance (g_s), transpiration of the shoots and NH₃ metabolization of the needles were measured. In ambient CO₂ (1) gaseous NH₃ decreased mycorrhizal infection, without significantly affecting tree biomass or N concentration and it enhanced the activity of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in one-year-old needles; (2) ozone decreased mycorrhizal infection and the acitivity of GS in the needles, while it increased the activity og GDH; (3) exposure to NH₃+O₃ lessened the effects of single exposures to NH₃ and O₃ on reduction of mycorrhizal infection and on increase in GDH activity. Similar lessing effects on mycorrhizal infection as observed in trees exposed to NH₃+O₃ at ambient CO₂, were measured in trees exposed to NH₃+O₃ at elevated CO₂. Exposure to elevated CO₂ without pollutants did not significantly affect any of the parameters studied, except for a decrease in the concentration of soluble proteins in the needles. Elevated CO₂ _NH₃ strongly decreased root branching and mycorrhizal infection and temporarily stimulated P_n and g_s. The exposure to elevated CO₂+NH₃+O₃ also transiently stimulated P_n. The possible mechanisms underlying and integrating these effects are discussed. Elevated CO₂ clearly did not alleviate the negative effects of NH₃ and O₃ mycoorhiral infection. The significant reduction of mycorrhizal infection after exposure to NH₃ or O₃, observed before significant changes in gas exchange or growth occurred, suggest the use of mycorrhizal infection as an early indicator for NH₃ and O₃ induced stress.Abbreviations DW dry weight - FA filtered air - FA_a filtered air at ambient CO₂ - FA_e filtered air at elevated CO₂ - FW fresh weight - GDH glutamate dehydrogenase - GS glutamine synthetase - g_s stomatal conductance - P_n net CO₂ assimilation - RWR root weight ratio - SRL specific root length     

Persistent effects of low concentrations of SO2 and NO2 on photosynthesis in Scots pine (Pinus sylvestris) needles

In an open-field experiment, 50-year-old trees of Scots pine (Pinus sylvestris L.) were fumigated with low concentrations of SO₂ and NO₂ (10–15 nl I^?1) during the growing season in four consecutive years (1988 to 1991). Results from the autumn and early winter of 1991 and 1992 are presented. The maximum photochemical efficiency of photosystem II (PSII), as indicated by the ratio of variable to maximum fluorescence (F_v/F_M) was assessed in current and one-year-old needles from the top and the bottom of the canopy. Furthermore, simultaneous measurements of photosynthetic O₂ evolution and chlorophyll fluorescence were made in current-year needles at 20°C. In general, the F_v/F_M ratio as well as the gross rate of O₂ evolution in needles of fumigated trees was not significantly different from that in needles of control trees during the fumigation period. However, both current and one-year-old needles sampled in November and December 1991 from the top of the canopy of fumigated trees had significantly lower F_v/F_M values than corresponding needles of control trees. Similar differences in F_v/F_M correlated with the treatments were observed in needles from the bottom of the canopy, indicating that the depression of F_v/F_M in needles of fumigated trees was not due to an increased susceptibility to photoinhibition. In 1992, when no fumigation occurred, differences in F_v/F_M between the treatments were not significant during autumn and early winter. The gross rate of O₂ evolution at high irradiances was significantly lower in current-year needles of fumigated trees sampled in November and December 1991 than in those of control trees. Furthermore, a nearly identical linear relationship between the quantum yield of PSII electron transport determined from chlorophyll fluorescence and the quantum yield of O₂ evolution (gross rate of O₂ evolution/PPFD) was found during autumn and early winter. This appeared to be largely a result of changes in the thermal energy dissipation within PSII. The observed differences in photosynthetic characteristics correlated with the different treatments after the fumigation period is suggested to be mainly caused by increased sensitivity of the needles of fumigated trees to low and subfreezing temperatures. However, current-year needles of fumigated trees tended to have a lower N content than those of control trees, which may partly explain the differences in gross photosynthesis between fumigated and control trees.