Decomposition of <Superscript>15</Superscript>N-labelled beech litter and fate of nitrogen derived from litter in a beech forest

The decomposition and the fate of ¹⁵N- labelled beech litter was monitored in a beech forest (Vosges mountains, France) over 3 years. Circular plots around beech trees were isolated from neighbouring tree roots by soil trenching. After removal of the litter layer, ¹⁵N-labelled litter was distributed on the soil. Samples [labelled litter, soil (0–15 cm depths], fine roots, mycorrhizal root tips, leaves) were collected during the subsequent vegetation periods and analysed for total N and ¹⁵N concentration. Mass loss of the ¹⁵N-labelled litter was estimated using mass loss data from a litterbag experiment set up at the field site. An initial and rapid release of soluble N from the decomposing litter was balanced by the incorporation of exogenous N into the litter. Fungal N accounted for approximately 35% of the N incorporation. Over 2 years, litter N was continuously released and rates of N and mass loss were equivalent, while litter N was preferentially lost during the 3rd year. Released ¹⁵N accumulated essentially at the soil surface. ¹⁵N from the decomposing litter was rapidly (i.e. in 6 months) detected in roots and beech leaves and its level increased regularly and linearly over the course of the labelling experiment. After 3 years, about 2% of the original litter N had accumulated in the trees. ¹⁵N budgets indicated that soluble N was the main source for soil microbial biomass. Nitrogen accumulated in storage compounds was the main source of leaf N, while soil organic N was the main source of mycorrhizal N. Use of ¹⁵N-labelled beech litter as decomposing substrate allowed assessment of the fate of litter N in the soil and tree N pools in a beech forest on different time scales. Received: 3 May 1999 / Accepted: 3 January 2000     

Connecting litter quality,microbial community and nitrogen transfer mechanisms in decomposing litter mixtures

Synergistic effects on decomposition in litter mixtures have been suggested to be due to the transfer of nitrogen from N‐rich to N‐poor species. However, the dominant pathway and the underlying mechanisms remain to be elucidated. We conducted an experiment to investigate and quantify the control mechanisms for nitrogen transfer between two litter species of contrasting nitrogen status (¹⁵N labeled and unlabeled Fagus sylvatica and Fraxinus excelsior) in presence and absence of micro‐arthropods. We found that ¹⁵N was predominantly transferred actively aboveground by saprotrophic fungi, rather than belowground or passively by leaching. However, litter decomposition remained unaffected by N‐dynamics and was poorly affected by micro‐arthropods, suggesting that synergistic effects in litter mixtures depend on complex environmental interrelationships. Remarkably, more ¹⁵N was transferred from N‐poor beech than N‐rich ash litter. Moreover, the low transfer of ¹⁵N from ash litter was insensitive to destination species whereas the transfer of ¹⁵N from labeled beech litter to unlabeled beech was significantly greater than the amount of ¹⁵N transferred to unlabeled ash suggesting that processes of nitrogen transfer fundamentally differ between litter species of different nitrogen status. Microbial analyses suggest that nitrogen of N‐rich litter is entirely controlled by bacteria that hamper nitrogen capture of microbes in the environment supporting the source‐theory. In contrast, nitrogen of N‐poor fungal dominated litter is less protected and transferable depending on the nitrogen status and the transfer capacity of the microbial community of the co‐occurring litter species supporting the gradient‐theory. Thus, our results challenge the traditional view regarding the role of N‐rich litter in decomposing litter mixtures. We rather suggest that N‐rich litter is only a poor nitrogen source, whereas N‐poor litter, can act as an important nitrogen source in litter mixtures. Consequently both absolute and relative differences in initial litter C/N ratios of co‐occurring litter species need to be considered for understanding nitrogen dynamics in decomposing litter mixtures.     

Decomposition nitrogen is better retained than simulated deposition from mineral amendments in a temperate forest

Nitrogen (N) deposition (N_DEP) drives forest carbon (C) sequestration but the size of this effect is still uncertain. In the field, an estimate of these effects can be obtained by applying mineral N fertilizers over the soil or forest canopy. A ¹⁵N label in the fertilizer can be then used to trace the movement of the added N into ecosystem pools and deduce a C effect. However, N recycling via litter decomposition provides most of the nutrition for trees, even under heavy N_DEP inputs. If this recycled litter nitrogen is retained in ecosystem pools differently to added mineral N, then estimates of the effects of N_DEP on the relative change in C (?C/?N) based on short‐term isotope‐labelled mineral fertilizer additions should be questioned. We used ¹⁵N labelled litter to track decomposed N in the soil system (litter, soils, microbes, and roots) over 18 months in a Sitka spruce plantation and directly compared the fate of this ¹⁵N to an equivalent amount in simulated N_DEP treatments. By the end of the experiment, three times as much ¹⁵N was retained in the O and A soil layers when N was derived from litter decomposition than from mineral N additions (60% and 20%, respectively), primarily because of increased recovery in the O layer. Roots expressed slightly more ¹⁵N tracer from litter decomposition than from simulated mineral N_DEP (7.5% and 4.5%) and compared to soil recovery, expressed proportionally more ¹⁵N in the A layer than the O layer, potentially indicating uptake of organic N from decomposition. These results suggest effects of N_DEP on forest ?C/?N may not be apparent from mineral ¹⁵N tracer experiments alone. Given the importance of N recycling, an important but underestimated effect of N_DEP is its influence on the rate of N release from litter.     

Nitrogen accumulation and distribution in Danthonia richardsonii swards in response to CO2 and nitrogen supply over four years of growth

Nitrogen‐stressed microcosms of the C3 grass Danthonia richardsonii gained nitrogen from the environment when grown under ambient or enriched (359, ‘amb’ or 719 μL L^{? 1}‘enr’, respectively) atmospheric CO₂ concentrations over a 4‐y period. This gain was apparent at all rates of supplied mineral N (2.2, 6.7 or 19.8 g N m^{? 2} y^{? 1}– low‐N, mid‐N or high‐N), although it was small at high‐N. Small losses of N occurred from the microcosm as leachate, while gaseous losses of N were estimated to be between 10% and 25% of applied mineral N. Losses of applied mineral N were slightly lower under CO₂ enrichment only at the highest rate of mineral N supply. Levels of ¹⁵N natural abundance in green leaf (δ¹⁵Ν) of ? 2‰ (amb low‐N) and of below ? 4‰ (enr low‐ & mid‐N) suggest that absorption of atmospheric NH₃ may have been a source of some of the extra N in the low and mid‐N treatments. Biological N₂ fixation, of up to 2 g m^{? 2} y^{? 1} was hypothesized to form the remainder of the environmental N source. Microcosm C:N ratio was higher under CO₂ enrichment. Nitrogen productivity of microcosm carbon gain (g C accumulated g^{? 1} leaf N day^{? 1}) was increased (up to 100%) by CO₂ enrichment at all rates of mineral N supply. Green leaf %N was reduced by CO₂ enrichment, and there was less nitrogen in the green leaf pool under CO₂ enrichment. Less, or the same amount of nitrogen was present in senesced leaf, surface litter and root under CO₂ enrichment while more nitrogen was present in the soil in organic forms, and as NH₄ + at the highest rate of mineral N supply.     

Acclimation to ozone affects host/pathogen interaction and competitiveness for nitrogen in juvenile Fagus sylvatica and Picea abies trees infected with Phytophthora citricola

In a two-year phytotron study, juvenile trees of European beech (Fagus sylvatica) and Norway spruce (Picea abies) were grown in mixture under ambient and twice ambient ozone (O3) and infected with the root pathogen Phytophthora citricola. We investigated the influence of O3 on the trees' susceptibility to the root pathogen and assessed, through a 15N-labelling experiment, the impact of both treatments (O3 exposure and infection) on belowground competitiveness. The hypotheses tested were that: (1) both P. citricola and O3 reduce the belowground competitiveness (in view of N acquisition), and (2) that susceptibility to P. citricola infection is reduced through acclimation to enhanced O3 exposure. Belowground competitiveness was quantified via cost/benefit relationships, i.e., the ratio of structural investment in roots relative to their uptake of 15N. Beech had a lower biomass acquisition and captured less 15N under enhanced O3 and P. citricola infection alone than spruce, whereas the latter species appeared to profit from the lower resource acquisition of beech in these treatments. Nevertheless, in the combined treatment, susceptibility to P. citricola of spruce was increased, while beech growth and 15N uptake were not further reduced below the levels found under the single treatments. Potential trade-offs between stress defence, growth performance, and associated nitrogen status are discussed for trees affected through O3 and/or pathogen infection. With respect to growth performance, it is concluded that O3 enhances susceptibility to the pathogen in spruce, but apparently raises the defence capacity in beech..     

Does canopy nitrogen uptake enhance carbon sequestration by trees?

Temperate forest ¹⁵N isotope trace experiments find nitrogen (N) addition‐driven carbon (C) uptake is modest as little additional N is acquired by trees; however, several correlations of ambient N deposition against forest productivity imply a greater effect of atmospheric nitrogen deposition than these studies. We asked whether N deposition experiments adequately represent all processes found in ambient conditions. In particular, experiments typically apply ¹⁵N to directly to forest floors, assuming uptake of nitrogen intercepted by canopies (CNU) is minimal. Additionally, conventional ¹⁵N additions typically trace mineral ¹⁵N additions rather than litter N recycling and may increase total N inputs above ambient levels. To test the importance of CNU and recycled N to tree nutrition, we conducted a mesocosm experiment, applying 54 g N/¹⁵N ha^?1 yr^?1 to Sitka spruce saplings. We compared tree and soil ¹⁵N recovery among treatments where enrichment was due to either (1) a ¹⁵N‐enriched litter layer, or mineral ¹⁵N additions to (2) the soil or (3) the canopy. We found that 60% of ¹⁵N applied to the canopy was recovered above ground (in needles, stem and branches) while only 21% of ¹⁵N applied to the soil was found in these pools. ¹⁵N recovery from litter was low and highly variable. ¹⁵N partitioning among biomass pools and age classes also differed among treatments, with twice as much ¹⁵N found in woody biomass when deposited on the canopy than soil. Stoichiometrically calculated N effect on C uptake from ¹⁵N applied to the soil, scaled to real‐world conditions, was 43 kg C kg N^?1, similar to manipulation studies. The effect from the canopy treatment was 114 kg C kg N^?1. Canopy treatments may be critical to accurately represent N deposition in the field and may address the discrepancy between manipulative and correlative studies.     

Timing of nitrogen uptake affects winter storage and spring remobilisation of nitrogen in nectarine (Prunus persica var. nectarina) trees

Two-year old nectarine trees (Prunus persica, Batsch, var. nectarina, cv. Starkredgold on GF305 rootstock) planted in pots each received five applications of 1.0 g ¹⁵N labelled urea either from mid May to mid July (early uptake) or from mid August to the beginning of October (late uptake). All trees were supplied with a corresponding amount of unlabelled urea when they did not receive the labelled N. In autumn, all abscised leaves were collected and during winter randomly selected trees were harvested and divided into main organs. The remaining trees were transplanted into similar pots filled with sand; they received no N fertiliser and were harvested in May to evaluate the remobilisation of N. Total N and ¹⁵N abundance were determined in each organ. Nectarine trees took up similar amounts of N in the 'early' and in the 'late' period; however, more labelled nitrogen was recovered in the perennial organs during the winter when trees received the labelled N in the 'late' than in the 'early' period. Some 73–80% of the N present in the dormant trees was stored in the roots, which contained almost twice the amount of labelled N taken up 'late' than that absorbed 'early'. Nitrogen for spring growth was remobilised predominantly from the roots and accounted for some 43–49% of the labelled N recovered in the tree during winter. Results suggest that the nitrogen taken up 'late' in the season is preferentially stored in roots and used by peach trees to sustain new growth the following spring. This revised version was published online in June 2006 with corrections to the Cover Date.     

降水变化和氮沉降对荒漠草原两种多年生禾草凋落物分解的影响

凋落物分解是陆地生态系统养分循环的重要过程,在生物地球化学循环过程中发挥着重要作用。全球变化是影响凋落物分解的重要因子,其对生态系统养分循环的影响存在诸多不确定性。研究荒漠草原凋落物分解对氮沉降和降水变化及其二者交互作用的响应,是揭示这些不确定性、保护草原生态系统结构和功能的科学基础。以内蒙古四子王旗短花针茅荒漠草原为研究对象,选取建群种短花针茅和优势种无芒隐子草两种植物凋落物,开展为期4年的长期分解实验,探究两种植物凋落物分解特征及养分释放规律。实验采用裂区设计,主区为自然降水（C）、增雨30%（W）和减雨30%（R）3个水分梯度,副区为0（N0）、30（N30）、50（N50）和100（N100） kg hm^-2 a^-1 4个氮素梯度。结果表明：（1）增雨和氮沉降促进荒漠草原凋落物分解,减雨反之,降水对两种凋落物影响具有差异,而氮沉降的作用不依赖于物种;（2）氮沉降缩短凋落物分解周期5.12%-14.82%,增雨与氮沉降交互缩短凋落物分解周期3.69%-28.75%;（3）降水始终有利于凋落物中碳、纤维素和木质素释放,而分解后期氮沉降对其影响不显著,凋落物分解后期主要受木质素分解速率控制。综上所述,影响荒漠草原凋落物分解的主要因素为降水,其次是氮素,二者对凋落物分解具有协同作用。     

Influence of Tree Species on Forest Nitrogen Retention in the Catskill Mountains, New York, USA

This study examines the effect of four tree species on nitrogen (N) retention within forested catchments of the Catskill Mountains, New York (NY). We conducted a 300-day ¹⁵N field tracer experiment to determine how N moves through soil, microbial, and plant pools under different tree species and fertilization regimes. Samples were collected from single-species plots of American beech (Fagus grandifolia Ehrh.), eastern hemlock (Tsuga canadensis L.), red oak (Quercus rubra L.), and sugar maple (Acer saccharum Marsh). Using paired plots we compared the effects of ambient levels of N inputs (11 kg N/ha/y) to additions of 50 kg N/ha/y that began 1.5 years prior to and continued throughout this experiment. Total plot ¹⁵N recovery (litter layer, organic and mineral soil to 12 cm, fine roots, and aboveground biomass) did not vary significantly among tree species, but the distribution of sinks for ¹⁵N within the forest ecosystem did vary. Recovery in the forest floor was significantly lower in sugar maple stands compared to the other species. ¹⁵Nitrogen recovery was 22% lower in the fertilized plots compared to the ambient plots and red oak stands had the largest drop in ¹⁵N recovery as a result of N fertilization. Aboveground biomass became a significantly greater ¹⁵N sink with fertilization, although it retained less than 1% of the tracer addition. These results indicate that different forest types vary in the amount of N retention in the forest floor, and that forest N retention may change depending upon N inputs.     

Nitrogen addition alters mineralization dynamics of 13C‐depleted leaf and twig litter and reduces leaching of older DOC from mineral soil

Recent reviews indicate that N deposition increases soil organic matter (SOM) storage in forests but the undelying processes are poorly understood. Our aim was to quantify the impacts of increased N inputs on soil C fluxes such as C mineralization and leaching of dissolved organic carbon (DOC) from different litter materials and native SOM. We added 5.5 g N m^?2 yr^?1 as NH₄NO₃ over 1 year to two beech forest stands on calcareous soils in the Swiss Jura. We replaced the native litter layer with ¹³C‐depleted twigs and leaves (δ¹³C: ?38.4 and ?40.8‰) in late fall and measured N effects on litter‐ and SOM‐derived C fluxes. Nitrogen addition did not significantly affect annual C losses through mineralization, but altered the temporal dynamics in litter mineralization: increased N inputs stimulated initial mineralization during winter (leaves: +25%; twigs: +22%), but suppressed rates in the subsequent summer. The switch from a positive to a negative response occurred earlier and more strongly for leaves than for twigs (?21% vs. 0%). Nitrogen addition did not influence microbial respiration from the nonlabeled calcareous mineral soil below the litter which contrasts with recent meta‐analysis primarily based on acidic soils. Leaching of DOC from the litter layer was not affected by NH₄NO₃ additions, but DOC fluxes from the mineral soils at 5 and 10 cm depth were significantly reduced by 17%. The ¹³C tracking indicated that litter‐derived C contributed less than 15% of the DOC flux from the mineral soil, with N additions not affecting this fraction. Hence, the suppressed DOC fluxes from the mineral soil at higher N inputs can be attributed to reduced mobilization of nonlitter derived ‘older’ DOC. We relate this decline to an altered solute chemistry by NH₄NO₃ additions, an increased ionic strength and acidification resulting from nitrification, rather than to a change in microbial decomposition.     

Multiyear fate of a 15N tracer in a mixed deciduous forest: retention,redistribution, and differences by mycorrhizal association

The impact of atmospheric nitrogen deposition on forest ecosystems depends in large part on its fate. Past tracer studies show that litter and soils dominate the short‐term fate of added ¹⁵N, yet few have examined its longer term dynamics or differences among forest types. This study examined the fate of a ¹⁵N‐ tracer over 5–6 years in a mixed deciduous stand that was evenly composed of trees with ectomycorrhizal and arbuscular mycorrhizal associations. The tracer was expected to slowly mineralize from its main initial fate in litter and surface soil, with some ¹⁵N moving to trees, some to deeper soil, and some net losses. Recovery of added ¹⁵N in trees and litterfall totaled 11.3% both 1 and 5–6 years after the tracer addition, as ¹⁵N redistributed from fine and especially coarse roots into cumulative litterfall and small accumulations in woody tissues. Estimates of potential carbon sequestration from tree ¹⁵N recovery amounted to 12–14 kg C per kg of N deposition. Tree ¹⁵N acquisition occurred within the first year after the tracer addition, with no subsequent additional net transfer of ¹⁵N from detrital to plant pools. In both years, ectomycorrhizal trees gained 50% more of the tracer than did trees with arbuscular mycorrhizae. Much of the ¹⁵N recovered in wood occurred in tree rings formed prior to the ¹⁵N addition, demonstrating the mobility of N in wood. Tracer recovery rapidly decreased over time in surface litter material and accumulated in both shallow and deep soil, perhaps through mixing by earthworms. Overall, results showed redistribution of tracer ¹⁵N through trees and surface soils without any losses, as whole‐ecosystem recovery remained constant between 1 and 5–6 years at 70% of the ¹⁵N addition. These results demonstrate the persistent ecosystem retention of N deposition even as it redistributes, without additional plant uptake over this timescale.     

Does elevated atmospheric carbon dioxide affect internal nitrogen allocation in the temperate trees Alnus glutinosa and Pinus sylvestris?

Nitrogen‐fixing plant species growing in elevated atmospheric carbon dioxide concentration ([CO₂]) should be able to maintain a high nutrient supply and thus grow better than other species. This could in turn engender changes in internal storage of nitrogen (N) and remobilisation during periods of growth. In order to investigate this one‐year‐old‐seedlings of Alnus glutinosa (L.) Gaertn and Pinus sylvestris (L.) were exposed to ambient [CO₂] (350 µ mol mol ^{? 1}) and elevated [CO₂] (700 µ mol mol ^{? 1}) in open top chambers (OTCs). This constituted a main comparison between a nitrogen‐fixing tree and a nonfixer, but also between an evergreen and a deciduous species. The trees were supplied with a full nutrient solution and in July 1994, the trees were given a pulse of ¹⁵N‐labelled fertiliser. The allocation of labelled N to different tissues (root, leaves, shoots) was followed from September 1994 to June 1995. While N allocation in P. sylvestris (Scots pine) showed no response to elevated [CO₂], A. glutinosa (common alder) responded in several ways. During the main nutrient uptake period of June–August, trees grown in elevated [CO₂] had a higher percentage of N derived from labelled fertiliser than trees grown in ambient [CO₂]. Remobilisation of labelled N for spring growth was significantly higher in A. glutinosa grown in elevated [CO₂] (9.09% contribution in ambient vs. 29.93% in elevated [CO₂] leaves). Exposure to elevated [CO₂] increased N allocation to shoots in the winter of 1994–1995 (12.66 mg in ambient vs. 43.42 mg in elevated 1993 shoots; 4.81 mg in ambient vs. 40.00 mg in elevated 1994 shoots). Subsequently significantly more labelled N was found in new leaves in April 1995. These significant increases in movement of labelled N between tissues could not be explained by associated increases in tissue biomass, and there was a significant shift in C‐biomass allocation away from the leaves towards the shoots (all above‐ground material except leaves) in A. glutinosa. This experiment provides the first evidence that not only are shifts in C allocation affected by elevated [CO₂], but also internal N resource utilisation in an N₂‐fixing tree.     

Combined effects of nitrogen addition and litter manipulation on nutrient resorption of Leymus chinensis in a semi‐arid grassland of northern China

Plant growth in semi‐arid ecosystems is usually severely limited by soil nutrient availability. Alleviation of these resource stresses by fertiliser application and aboveground litter input may affect plant internal nutrient cycling in such regions. We conducted a 4‐year field experiment to investigate the effects of nitrogen (N) addition (10 g N·m^?2·year^?1) and plant litter manipulation on nutrient resorption of Leymus chinensis, the dominant native grass in a semi‐arid grassland in northern China. Although N addition had no clear effects on N and phosphorus (P) resorption efficiencies in leaves and culms, N fertilisation generally decreased leaf N resorption proficiency by 54%, culm N resorption proficiency by 65%. Moreover, N fertilisation increased leaf P resorption proficiency by 13%, culm P resorption proficiency by 20%. Under ambient or enriched N conditions, litter addition reduced N and P resorption proficiencies in both leaves and culms. The response of P resorption proficiency to litter manipulation was more sensitive than N resorption proficiency: P resorption proficiency in leaves and culms decreased strongly with increasing litter amount under both ambient and enriched N conditions. In contrast, N resorption proficiency was not significantly affected by litter addition, except for leaf N resorption proficiency under ambient N conditions. Furthermore, although litter addition caused a general decrease of leaf and culm nutrient resorption efficiencies under both ambient and enriched N conditions, litter addition effects on nutrient resorption efficiency were much weaker than the effects of litter addition on nutrient resorption proficiency. Taken together, our results show that leaf and non‐leaf organs of L. chinensis respond consistently to altered soil N availability. Our study confirms the strong effects of N addition on plant nutrient resorption processes and the potential role of aboveground litter, the most important natural fertiliser in terrestrial ecosystems, in influencing plant internal nutrient cycling.     

Role of six European tree species and land‐use legacy for nitrogen and water budgets in forests

Water and nutrient fluxes for single stands of different tree species have been reported in numerous studies, but comparative studies of nutrient and hydrological budgets of common European deciduous tree species are rare. Annual fluxes of water and inorganic nitrogen (N) were established in a 30‐year‐old common garden design with stands of common ash (Fraxinus excelsior), European beech (Fagus sylvatica L.), pedunculate oak (Quercus robur), small‐leaved lime (Tilia cordata Mill.), sycamore maple (Acer pseudoplatanus) and Norway spruce (Picea abies [L.] Karst.) replicated at two sites in Denmark, Mattrup and Vallø during 2 years. Mean annual percolation below the root zone (mm yr^?1±SE, n=4) ranked in the following order: maple (351±38)>lime (284±32), oak (271±25), beech (257±30), ash (307±69)? spruce (75±24). There were few significant tree species effects on N fluxes. However, the annual mean N throughfall flux (kg N ha^?1 yr^?1±SE, n=4) for spruce (28±2) was significantly larger than for maple (12±1), beech (11±1) and oak (9±1) stands but not different from that of lime (15±3). Ash had a low mean annual inorganic N throughfall deposition of 9.1 kg ha^?1, but was only present at Mattrup. Annual mean of inorganic N leaching (kg ha^?1 yr^?1±SE, n=4) did not differ significantly between species despite of contrasting tree species mean values; beech (25±9)>oak (16±10), spruce (15±8), lime (14±8)? maple (1.9±1), ash (2.0±1). The two sites had similar throughfall N fluxes, whereas the annual leaching of N was significantly higher at Mattrup than at Vallø. Accordingly, the sites differed in soil properties in relation to rates and dynamics of N cycling. We conclude that tree species affect the N cycle differently but the legacy of land use exerted a dominant control on the N cycle within the short‐term perspective (30 years) of these stands.     

Impact of Elevated PCO2 on Mass Flow of Reduced Nitrogen in Trees

To analyze the effects of elevated carbon dioxide concentration （PCO2） on the mass flow of reduced nitro- gen （N） in the phloem and xylem of trees, juvenile beech （Fagus sylvatica L.） and spruce （Picea abies （L.） Karst.） were grown in phytotrons and exposed to ambient and elevated PCO2 （plus 687.5 mg/m^3 CO2） for three growing seasons. Elevated PCO2 significantly decreased the mass flow of N from the shoot to roots of beech by significantly reducing the concentration of soluble amino compounds in the phloem, even if the area of conductive phloem of cross-sectional bark tissue was significantly increased, because of less callus deposition in the sieve elements. In spruce, the downward mass flow of reduced N also tended to be decreased, similar to that in beech. Resembling findings in the phloem, N mass flow from roots to shoot in both tree species was significantly diminished owing to significantly reduced concentrations of amino compounds in the xylem and a lower transpiration rate. Therefore, the mass flow of reduced N between shoots and roots of trees was mainly governed by the concentrations of soluble amino compounds in the phloem and xylem in relation to the loading of reduced N in both long-distance transport pathways.     

长白山苔原带凋落物生态化学计量特征及其对模拟氮沉降的响应

长白山苔原是我国乃至欧亚大陆东部独有的高山苔原,根据前人调查植被以灌木苔原为主要类型。在全球变暖背景下,近30年来,长白山岳桦林下的草本植物侵入苔原带,原生灌木苔原分化为灌木苔原、灌草苔原和草本苔原,形成了灌木、灌草混合和草本3种不同类型的凋落物,凋落物数量和质量发生显著改变。与此同时长白山苔原氮沉降量也在逐年增加,导致了土壤中氮的累积,势必影响凋落物的分解。凋落物作为连接植物和土壤的纽带,其分解过程中碳（C）、氮（N）、磷（P）等化学组分和化学计量比的变化直接和间接影响着土壤养分有效性和植物养分利用策略。为揭示氮沉降增加对长白山苔原带不同类型凋落物化学组分及生态化学计量特征早期变化的影响,开展了为期8个月的模拟氮沉降室内凋落物分解实验。在苔原带采集灌木优势种牛皮杜鹃和草本优势种小叶章的凋落物带回实验室,模拟灌木牛皮杜鹃群落、灌草混合的牛皮杜鹃-小叶章群落和草本小叶章群落的3种不同类型凋落物,设置三个施氮处理：对照（CK,0 g N m^-2 a^-1）、低氮（LN,10 g N m^-2 a^-1）、高氮（HN,20 g N m^-2 a^-1）。研究表明：（1）不施氮处理时,3种凋落物的C、P均呈释放状态,木质素（Li）呈先累积再略有降解趋势;牛皮杜鹃凋落物的N元素富集而其余两种凋落物N元素呈释放状态;灌草混合和草本凋落物比原生的灌木凋落物C和N元素释放快、Li累积少;而灌木凋落物的P释放略快于灌草和草本凋落物。3种植被类型凋落物的C/N、C/P、Li/N大小表现为：牛皮杜鹃凋落物>牛皮杜鹃-小叶章混生群落凋落物>小叶章凋落物;N/P表现为：小叶章凋落物>牛皮杜鹃凋落物>牛皮杜鹃-小叶章混生群落凋落物。（2）氮沉降促进3种类型凋落物分解过程中C、N和P化学组分的释放,且氮浓度越高促进作用越显著。在牛皮杜鹃凋落物分解过程中,氮素添加到达某一阈值后,其C/N、C/P、N/P、Li/N的降幅最大,后续若再增加氮素,其对化学计量比的影响均会减弱;本实验中的氮素添加量增加促进了小叶章凋落物的C/N、Li/N下降。（3）草本植物入侵引起凋落物类型的变化带来凋落物分解加快,将导致长白山苔原带养分循环的变化;氮沉降增加对小叶章凋落物化学组分的释放及C/N、Li/N的下降更为促进,小叶章凋落物内难分解化合物减少,分解受到促进。高氮沉降加快了小叶章凋落物与土壤、草本植物之间的养分循环。因此,随着未来苔原带氮沉降量的增加,将更有利于小叶章在与牛皮杜鹃的竞争中获胜,使苔原带呈现草甸化趋势。     

Segregation of nitrogen use between ammonium and nitrate of ectomycorrhizas and beech trees

Here, we characterized nitrogen (N) uptake of beech (Fagus sylvatica) and their associated ectomycorrhizal (EM) communities from NH₄⁺ and NO₃^?. We hypothesized that a proportional fraction of ectomycorrhizal N uptake is transferred to the host, thereby resulting in the same uptake patterns of plants and their associated mycorrhizal communities. ¹⁵N uptake was studied under various field conditions after short‐term and long‐term exposure to a pulse of equimolar NH₄⁺ and NO₃^? concentrations, where one compound was replaced by ¹⁵N. In native EM assemblages, long‐term and short‐term ¹⁵N uptake from NH₄⁺ was higher than that from NO₃^?, regardless of season, water availability and site exposure, whereas in beech long‐term ¹⁵N uptake from NO₃^? was higher than that from NH₄⁺. The transfer rates from the EM to beech were lower for ¹⁵N from NH₄⁺ than from NO₃^?. ¹⁵N content in EM was correlated with ¹⁵N uptake of the host for ¹⁵NH₄⁺, but not for ¹⁵NO₃^?‐derived N. These findings suggest stronger control of the EM assemblage on N provision to the host from NH₄⁺ than from NO₃^?. Different host and EM accumulation patterns for inorganic N will result in complementary resource use, which might be advantageous in forest ecosystems with limited N availability.     

Long-term effects of experimental nitrogen additions on foliar litter decay and humus formation in forest ecosystems

Decomposition rates and N dynamics of foliar litter from 4 tree species were measured over a 72 month period on the Chronic Nitrogen Addition plots at the Harvard Forest, Petersham MA, beginning in November 1988. Plots received nitrogen additions of 0, 5 and 15 g N m^-2yr^-1 in two different stand types: red pine and mixed hardwood. Bags were collected in August and November of each year and litter analysed for mass remaining, nitrogen, cellulose and lignin content. Mass remaining was significantly greater for litter in nitrogen treated plots than in control plots after 48 months. Lignin content of litter was significantly higher with nitrogen treatments but there was little effect of treatment on cellulose content. N concentration was similar between treatments, but greater mass remaining in treated plots resulted in a higher total amount of N in humus produced in the high N plot. This mechanism could be a sink for up to 1.5 g N·m^-2yr^-1 of the 1.5 g N·m^-2yr^-1 added annually to the high N plots. Reduced decomposition rates in conjunction with increased lignin accumulation could impact global carbon sequestration as well.     

C and N storage in living trees within Finland since 1950s

Living biomass contains 45 to 60% carbon and 0.05 to 3% nitrogen, in dry weight. Like throughout Europe, the amount of living biomass in Finnish forests has increased on average over the last decades, largely because of changes in forest management. The storage of organic C and N in biomass has also increased.Changes in biomass vary between regions. Data were analysed on changes in the last 30–40 years in C and N storage in living trees in Finland, subdivided into 20 regions. Tree biomass increased in 17 regions, and decreased in 3 regions. The storage rate varied between -170 and +480 kg C ha^-1 a^-1, and between –0.5 and +1.2 kg N ha^-1 a^-1.Nitrogen accumulation in trees was less than 15% of atmospheric N deposition in all regions. Although the eventual increase of the nitrogen concentration in tree tissues was omitted, it is not possible that living biomass has been the major sink for atmospheric N deposition to forests. A hypothesis is presented that the main sink is litter layer and organic soil. Carbon can also be accumulating in soils essentially faster than hitherto estimated in analyses of carbon budgets of European forests.Died on September 2, 1994.     

模拟氮沉降对华西雨屏区慈竹林凋落物分解的影响

试验设对照(CK,0 kg·hm^-2·a^-1)、低氮(LN,50 kg·hm^-2·a^-1)、中氮(MN,150 kg·hm^-2·a^-1)和高氮(HN,300 kg·hm^-2·a^-1)4个施氮水平,通过原位试验,研究了模拟N沉降对华西雨屏区慈竹(Neosinocalamus affinis)林凋落物分解的影响.结果表明：不同组分凋落物分解过程中,慈竹叶片分解速率最快,其次是箨,枝最慢,分解15个月时,叶片、箨、枝的质量残留率分别为26.38%、46.18%和54.54%,三者差异极显著(P<0.01);叶片在凋落后第1～2月和7～10月分解较快,而箨和枝则在第5～8月分解较快;凋落叶片分解95%需要的时间(2.573年)分别比箨和枝短1.686年和3.319年.凋落叶分解15个月时,各N沉降处理间分解率差异不显著;凋落箨分解95%需要2.679～4.259年,其中MN分解率最高,CK最低;凋落枝经过15个月的分解,各处理分解率大小顺序为MN>HN>LN>CK,MN与LN处理间差异达显著水平(P<0.05).说明N沉降对3种凋落物分解均有明显的促进作用,且对凋落箨促进作用最强;但随着N沉降浓度的增加和时间的延长,其促进作用减缓.