Changes in nitrogen resorption of trembling aspen (Populus tremuloides) with stand development

Variation in plant N resorption may change with stand development because plants tend to adjust their ecophysiological traits with aging. In addition, changes in soil nitrogen (N) pools associated with stand development may also affect plant N resorption. Here, we examined green- and senesced-leaf N concentrations and resorption of trembling aspen ( Populus tremuloides Michx.) in boreal forest stands of different ages (7, 25, 85 and 139 years, respectively). All sampled stands originated from wildfires and established on similar parent materials (glacial tills) and had similar climates. N concentrations in both green and senesced leaves increased between 27% and 54% along the stand age chronosequence. Resorption efficiency (percentage difference of N between green and senesced leaves) and proficiency (N concentration in senesced leaves) were higher for leaves in younger stands than in older stands. An analysis of covariance indicated that the patterns of leaf N concentration and resorption were affected significantly by stand age, but not by available soil N concentration. Our results indicate that at an intra-specific level, plants could adjust their N resorption efficiency and proficiency with stand development.     

Resorption of nitrogen,phosphorus and potassium from leaves of lucerne stands of different ages

Background and aims

Nutrient resorption from the senesced to the green leaves can help a plant re-use elements, thus improving adaptability and persistence. How the resorption of nitrogen (N), phosphorus (P) and potassium (K) varies among differently aged lucerne (Medicago sativa) stands and how they correlate to their stoichiometry in the leaves and soil remain uncertain. This study aimed to analyze the resorption efficiencies (REs) of N, P and K and their possible correlations with stoichiometric ratios in the plant and soil.

Methods

The concentrations of plant N, P and K and soil N, P, K and carbon (C) were measured under lucerne stands established in different years, and stoichiometric ratios and REs were calculated. The relationships of REs with stoichiometric ratios were analyzed.

Results

The nitrogen resorption efficiency (NRE) was quite variable among the different stands and tended to rise and then drop with stand age, ranging from 4.6 to 33.7 % with an average of 16.2 %. The phosphorus resorption efficiency (PRE) tended to increase with stand age, ranging from 11.1 to 38.3 % with an average of 27.3 %. The potassium resorption efficiency (KRE) increased with stand age, ranging from 21.0 to 49.8 % with an average of 36.9 %. The KRE was generally highest, followed by the PRE, and the NRE was lowest. Leaf N:P and N:K generally decreased and then increased with stand age, while the K:P increased and then decreased. In the green leaves, total N concentration increased significantly with NRE and PRE, and total P concentration rose significantly with PRE, while in the senesced leaves, total N concentration decreased significantly with NRE and KRE. The N:P in the green leaves decreased significantly with PRE and the K:P in the senesced leaves dropped with NRE. Furthermore, the REs decreased with total soil nutrition status if there was any correlation. The REs increased significantly with soil ammonium N concentration, while the NRE decreased significantly with soil nitrate N concentration. In addition, soil available P concentration at most depths led to significant increases in NRE and KRE. However, the REs were rarely influenced by stoichiometric ratios of soil N, P, K and C.

Conclusions

The NRE rose and then dropped, and the PRE and KRE both increased with stand age. Leaf N:P and N:K generally decreased and then increased with stand age, while K:P increased and then decreased. The concentrations of N, P and K increased in the green leaves and decreased in the senesced leaves with REs if there was any correlation. The REs decreased with total soil nutrition status if there was any correlation. However, the REs hardly changed with stoichiometric ratios in the leaves and soil under differently aged lucerne stands. There appear to be no correlations between REs and element stoichiometries.     

羌塘高原降水梯度带紫花针茅叶片氮回收特征及影响因素

植物回收衰老叶片的氮是植物重要的养分保持和环境适应机制,在寒旱贫瘠的生境更是如此。为了理解降水梯度上植物对高寒贫瘠环境的养分适应特征,研究了羌塘高寒草原优势物种紫花针茅叶片氮回收策略及其与环境因子的关系。结果表明,降水梯度带上紫花针茅叶片具有较高的叶氮水平和氮回收能力。生长季盛期紫花针茅绿叶平均氮含量为(23.87±3.92)g/kg,高于中国草地平均水平(20.9 g/kg)及全球平均值(20.1 g/kg);绿叶氮含量与年降水量(MAP)呈显著负相关,干旱端(西部)绿叶中氮含量明显高于湿润端(东部)。枯叶养分回收后的氮水平(NRP)很低,平均为(6.76±1.42)g/kg,叶片平均氮回收效率(NRE)为(71.25±6.46)%,明显高于中国温带草原和全球的平均水平(46.9%—58.5%)。枯叶中氮回收水平对叶片氮回收效率起决定作用,是维持高养分回收效率的物质基础。NRE与MAP、土壤全氮(TN)和土壤无机氮呈显著负相关;NRP与TN相关性不显著,但与土壤无机氮显著负相关。尽管NRE与NRP呈显著负相关,但二者与绿叶氮含量均没有显著相关性。年均气温、海拔对NRE和NRP影响均不显著。因此,紫花针茅叶片极高的NRE和低NRP反映了它对极端干旱贫瘠环境的养分保持能力,通过内部氮循环来降低养分流失。土壤氮的有效性是影响紫花针茅叶片氮回收能力的关键因子,降水通过影响土壤氮的有效性以及绿叶中氮含量间接影响紫花针茅叶片氮回收效率。     

Nitrogen economy of alpine plants on the north Tibetan Plateau: Nitrogen conservation by resorption rather than open sources through biological symbiotic fixation

Nitrogen (N) is one of the most important factors limiting plant productivity, and N fixation by legume species is an important source of N input into ecosystems. Meanwhile, N resorption from senescent plant tissues conserves nutrients taken up in the current season, which may alleviate ecosystem N limitation. N fixation was assessed by the ¹⁵N dilution technique in four types of alpine grasslands along the precipitation and soil nutrient gradients. The N resorption efficiency (NRE) was also measured in these alpine grasslands. The aboveground biomass in the alpine meadow was 4–6 times higher than in the alpine meadow steppe, alpine steppe, and alpine desert steppe. However, the proportion of legume species to community biomass in the alpine steppe and the alpine desert steppe was significantly higher than the proportion in the alpine meadow. N fixation by the legume plants in the alpine meadow was 0.236 g N/m², which was significantly higher than N fixation in other alpine grasslands (0.041 to 0.089 g N/m²). The NRE in the alpine meadows was lower than in the other three alpine grasslands. Both the aboveground biomass and N fixation of the legume plants showed decreasing trends with the decline of precipitation and soil N gradients from east to west, while the NRE of alpine plants showed increasing trends along the gradients, which indicates that alpine plants enhance the NRE to adapt to the increasing droughts and nutrient‐poor environments. The opposite trends of N fixation and NRE along the precipitation and soil nutrient gradients indicate that alpine plants adapt to precipitation and soil nutrient limitation by promoting NRE (conservative nutrient use by alpine plants) rather than biological N fixation (open sources by legume plants) on the north Tibetan Plateau.     

Nitrogen dynamics of a boreal black spruce wildfire chronosequence

This study examined the nitrogen (N) dynamics of a black spruce (Picea mariana (Mill.) BSP)-dominated chronosequence in Manitoba, Canada. The seven sites studied each contained separate well- and poorly drained stands, originated from stand-killing wildfires, and were between 3 and 151 years old. Our goals were to (i) measure total N concentration ([N]) of all biomass components and major soil horizons; (ii) compare N content and select vegetation N cycle processes among the stands; and (iii) examine relationships between ecosystem C and N cycling for these stands. Vegetation [N] varied significantly by tissue type, species, soil drainage, and stand age; woody debris [N] increased with decay state and decreased with debris size. Soil [N] declined with horizon depth but did not vary with stand age. Total (live + dead) biomass N content ranged from 18.4 to 99.7 g N m⁻² in the well-drained stands and 37.8–154.6 g N m⁻² in the poorly drained stands. Mean soil N content (380.6 g N m⁻²) was unaffected by stand age. Annual vegetation N requirement (5.9 and 8.4 g N m⁻² yr⁻¹ in the middle-aged well- and poorly drained stands, respectively) was dominated by trees and fine roots in the well-drained stands, and bryophytes in the poorly drained stands. Fraction N retranslocated was significantly higher in deciduous than evergreen tree species, and in older than younger stands. Nitrogen use efficiency (NUE) was significantly lower in bryophytes than in trees, and in deciduous than in evergreen trees. Tree NUE increased with stand age, but overall stand NUE was roughly constant (∼ ∼150 g g⁻¹ N) across the entire chronosequence.     

模拟N沉降下不同林龄马尾松林凋落叶分解-土壤C、N化学计量特征

模拟N沉降对森林生态系统的影响是当今全球变化生态学研究的一个热点问题,土壤碳库对N沉降比较敏感,N沉降增加了凋落叶分解过程中外源N含量,间接影响凋落叶分解的化学过程并改变凋落叶分解速率,因此,研究模拟N沉降下凋落叶分解-土壤C-N关系对预测森林C吸存有重要意义。利用原位分解袋法研究了模拟N沉降下三峡库区不同林龄马尾松林(Pinus massoniana)凋落叶分解过程中凋落叶-土壤C、N化学计量响应及其关系;N沉降水平分对照(CK,0 g m~(-2)a~(-1))、低氮(LN,5 g m~(-2)a~(-1))、中氮(MN,10 g m~(-2)a~(-1))和高氮(HN,15 g m~(-2)a~(-1))。结果表明:分解540 d后,N沉降促进20年生和30年生马尾松林凋落叶分解,46年生马尾松林中仅低氮处理促进凋落叶分解,4种处理均是30年生分解最快,说明同一树种起始N含量低的凋落叶对N沉降呈正响应,N沉降处理促进起始N含量低的凋落叶分解,起始N含量高的凋落叶分解过程中易达到"N饱和"。N沉降抑制20年生和46年生凋落叶C释放(低于对照0.62%—6.69%),促进30年生C释放(高于对照0.28%—5.55%);30年生和46年生林分N固持量均高于对照(高于对照0.15%—21.34%),20年生则低于对照(5.70%—13.87%),说明模拟N沉降处理促进起始C含量低的凋落叶C释放和起始N含量低的凋落叶N固持。N沉降处理下仅30年生马尾松林土壤有机碳较对照增加,且土壤有机质与凋落叶C、N和分解速率呈正相关,与凋落叶C/N比呈显著负相关;土壤总氮与凋落叶分解速率、凋落叶N含量呈正相关,土壤有机碳/总氮比与凋落叶C、N含量呈正相关;对照处理中凋落叶分解指标对土壤养分影响顺序是分解速率凋落物C含量凋落物C/N比凋落物N含量,低、中、高氮处理中则是凋落物C含量分解速率凋落物N含量凋落物C/N比。研究表明低土壤养分含量马尾松林对N沉降呈正响应,N沉降促进低土壤养分马尾松林凋落叶分解并提高土壤肥力;凋落叶质量和土壤养分含量低的生态系统土壤C对N沉降响应更显著。     

N2 fixing alder (Alnus viridis spp. fruticosa) effects on soil properties across a secondary successional chronosequence in interior Alaska

Green alder (Alnus viridis ssp. fruticosa) is a dominant understory shrub during secondary successional development of upland forests throughout interior Alaska, where it contributes substantially to the nitrogen (N) economy through atmospheric N₂ fixation. Across a replicated 200+ year old vegetation chronosequence, we tested the hypotheses that green alder has strong effects on soil chemical properties, and that ecosystem-level N inputs via N₂ fixation decrease with secondary successional stand development. Across early-, mid-, and late-successional stands, alder created islands of elevated soil N and carbon (C), depleted soil phosphorus (P), and more acidic soils. These effects translated to the stand-level in response to alder stem density. Although neither N₂ fixation nor nodule biomass differed among stand types, increases in alder densities with successional time translated to increasing N inputs. Estimates of annual N inputs by A. viridis averaged across the upland chronosequence (6.6 ± 1.2 kg N ha^?1 year^?1) are substantially less than inputs during early succession by Alnus tenuifolia growing along Alaskan floodplains. However, late-succession upland forests, where densities of A. viridis are highest, may persist for centuries, depending on fire return interval. This pattern of prolonged N inputs to late successional forests contradicts established theory predicting declines in N₂-fixation rates and N₂-fixer abundance as stands age.     

Effects of Nitrogen Addition on Nitrogen Resorption in Temperate Shrublands in Northern China

Nutrient resorption from senescing leaves is a key mechanism of nutrient conservation for plants. The nutrient resorption efficiency is highly dependent on leaf nutrient status, species identity and soil nutrient availability. Nitrogen is a limiting nutrient in most ecosystems, it is widely reported that nitrogen resorption efficiency (NRE) was highly dependent on the soil nitrogen availability and vary with N deposition. The effects of nitrogen deposition on NRE and nitrogen concentration in green and senescing leaves have been well established for forests and grasslands; in contrast, little is known on how plants in shrublands respond to nitrogen deposition across the world. In this study, we conducted a two-year nitrogen addition manipulation experiment to explore the responses of nitrogen concentration in green and senescing leaves, and NRE of seven dominant species, namely, Vitex negundo, Wikstroemia chamaedaphne, Carex rigescens and Cleistogenes chinensis from the Vitex negundo community, and Spirea trilobata, Armeniaca sibirica, V. negundo, C. rigescens and Spodiopogon sibiricus from the Spirea trilobata community, to nitrogen deposition in two typical shrub communities of Mt. Dongling in northern China. Results showed that NRE varied remarkably among different life forms, which was lowest in shrubs, highest in grasses, and intermediate in forbs, implying that shrubs may be most capable of obtaining nitrogen from soil, grasses may conserve more nitrogen by absorption from senescing leaves, whereas forbs may adopt both mechanisms to compete for limited nitrogen supply from the habitats. As the N addition rate increases, N concentration in senescing leaves ([N]_s) increased consistent from all species from both communities, that in green leaves ([N]_g) increased for all species from the Vitex negundo community, while no significant responses were found for all species from the Spirea trilobata community; NRE decreased for all species except A. sibirica from the Vitex community and W. chamaedaphn from the Spirea community. Given the substantial interspecific variations in nutrient concentration, resorption and the potentially changing community composition, and the increased soil nutrient availability due to fertilization may indirectly impact nutrient cycling in this region.     

Ectomycorrhiza communities of red oak (<Emphasis Type="BoldItalic">Quercus rubra</Emphasis> L.) of different age in the Lusatian lignite mining district,East Germany

Ectomycorrhizal (ECM) communities were assessed on a 720 m² plot along a chronosequence of red oak (Quercus rubra) stands on a forest reclamation site with disturbed soil in the lignite mining area of Lower Lusatia (Brandenburg, Germany). Adjacent to the mining area, a red oak reference stand with undisturbed soil was investigated reflecting mycorrhiza diversity of the intact landscape. Aboveground, sporocarp surveys were carried out during the fruiting season in a 2-week interval in the years 2002 and 2003. Belowground, ECM morphotypes were identified by comparing sequences of the internal transcribed spacer regions from nuclear rDNA with sequences from the GenBank database. Fifteen ECM fungal species were identified as sporocarps and 61 belowground as determined by morphological/anatomical and molecular analysis of their ectomycorrhizas. The number of ECM morphotypes increased with stand age along the chronosequence. However, the number of morphotypes was lower in stands with disturbed soil than with undisturbed soil. All stands showed site-specific ECM communities with low similarity between the chronosequence stands. The dominant ECM species in nearly all stands was Cenococcum geophilum, which reached an abundance approaching 80% in the 21-year-old chronosequence stand. Colonization rate of red oak was high (>95%) at all stands besides the youngest chronosequence stand where colonization rate was only 15%. This supports our idea that artificial inoculation with site-adapted mycorrhizal fungi would enhance colonization rate of red oak and thus plant growth and survival in the first years after outplanting.     

Nitrogen and phosphorus economy of the riparian shrub <Emphasis Type="Italic">Salix gracilistyla</Emphasis> in western Japan

Salix gracilistyla is one of the dominant plants in the riparian vegetation of the upper-middle reaches of rivers in western Japan. This species colonizes mainly sandy habitats, where soil nutrient levels are low, but shows high potential for production. We hypothesized that S.␣gracilistyla uses nutrients conservatively within stands, showing a high resorption efficiency during leaf senescence. To test this hypothesis, we examined seasonal changes in nitrogen (N) and phosphorus (P) concentrations in aboveground organs of S. gracilistyla stands on a fluvial bar in the Ohtagawa River, western Japan. The concentrations in leaves decreased from April to May as leaves expanded. Thereafter, the concentrations showed little fluctuation until September. They declined considerably in autumn, possibly owing to nutrient resorption. We converted the nutrient concentrations in each organ to nutrient amounts per stand area on the basis of the biomass of each organ. The resorption efficiency of N and P in leaves during senescence were estimated to be 44 and 46%, respectively. Annual net increments of N and P in aboveground organs, calculated by adding the amounts in inflorescences and leaf litter to the annual increments in perennial organs, were estimated to be 9.9 g and 0.83 g m⁻² year⁻¹, respectively. The amounts released in leaf litter were 6.7 g N and 0.44 g P m⁻². These values are comparable to or larger than those of other deciduous trees. We conclude that S. gracilistyla stands acquire large amounts of nutrients and release a large proportion in leaf litter.     

Nitrogen efficiency and leaf nitrogen remobilisation of oilseed rape lines and hybrids

Improved nitrogen (N) efficiency of oilseed rape is crucial for reducing environmental N surpluses. In this study, a 2‐year field experiment as well as a hydroponic experiment were performed with four hybrids and their corresponding lines. Seed yield and N efficiency parameters of oilseed rape cultivars grown at low N (0 kg N ha^?1) and high N (150 kg N ha^?1) supply were investigated in the field experiments. Hybrids showed higher seed yield than lines, especially at low N supply, because of a superior N uptake. Moreover, hybrids showed higher N harvest index (NHI) across N rates, which also contributed to higher seed yields. Results from the hydroponic experiment showed significant genotypic variation in leaf N remobilisation efficiency (NRE), but no hybrid‐versus‐line difference was found. Cultivars differed significantly in specific N content in senescent leaves, and leaf NRE was negatively correlated with specific N content in senescent leaves. When linking the hydroponic results with the field results, no relationship was found between leaf NRE and NHI. In conclusion, hybrids were superior to their corresponding lines in N efficiency because of higher N uptake and NHI. The higher NHI was, however, not related to genotypic variation in leaf NRE.     

不同林龄杉木人工林的水分利用效率与叶片养分浓度

以亚热带不同林龄(3、8、14、21、46年生)杉木人工林为研究对象,探索不同叶龄(当年生、1年生、2年生和3年生)叶片氮、磷养分状况和水分利用效率的差异及其相互关系.结果表明: 不同叶龄水分利用效率差异显著,总体趋势为当年生>1年生>2年生>3年生,而林龄对水分利用效率影响不显著.叶片N/P为11.4～19.6,其中,幼龄林和老龄林叶片N/P高于速生期林分叶片N/P,氮、磷浓度随叶龄的变化趋势一致,为当年生>1年生>2年生>3年生.水分利用效率随林龄变化不显著,可能是因为叶片光合速率和气孔导度同时随林龄下降.水分利用效率与叶片氮浓度相关不显著,而与叶片磷浓度呈显著正相关,与N/P呈显著负相关,表明在氮沉降增加的背景下,亚热带森林中植物磷含量逐渐成为影响水分利用效率的重要因子.     

广西猫儿山不同海拔常绿和落叶树种的营养再吸收模式

土壤养分供给性大小是否影响植物氮和磷再吸收效率仍存在争议。调查了广西猫儿山不同海拔常绿和落叶树种成熟和衰老叶片的氮和磷含量,探讨营养再吸收是否受到叶片习性和海拔的影响。所有树种氮和磷再吸收效率的平均值分别为56.5%和52.1%。常绿树种比落叶树种有显著较高的氮再吸收效率(P0.001)和磷再吸收效率(P0.01),这与前者有较低的衰老叶片氮和磷含量密切相关。随着海拔的上升,氮再吸收效率显著下降(P0.01),磷再吸收效率显著提高(P0.05)。氮再吸收效率与土壤氮:磷比(r=-0.41,P0.05)和成熟叶片氮:磷比(r=-0.37,P0.05)负相关,磷再吸收效率与土壤氮:磷比(r=0.44,P0.05)和成熟叶片氮:磷比(r=0.47,P0.01)正相关,表明了树种对低海拔氮限制的适应逐渐转变为对高海拔磷限制的适应。此外,氮再吸收效率与年均温正相关(r=0.43,P0.05)而磷再吸收效率与年均温负相关(r=-0.45,P0.01),这表明气温也是调节树木营养再吸收格局的重要影响因素。不同海拔树种氮和磷再吸收模式的差异可能是引起广西猫儿山常绿树种沿海拔形成双峰分布的原因之一。     

Effects of drought and shade on nitrogen cycling in the leaves and canopy of Mediterranean Quercus seedlings

Nitrogen (N) withdrawn from leaves before abscission can help to supply N requirements in plants of nutrient poor habitats. Besides N shortage, Mediterranean Quercus seedlings must face water and light stresses. However, there is little information on the influence of these stresses in the nitrogen resorption efficiency (NRE) at leaf level, and none at canopy level. We tested in two separated experiments how changes in water and light availability affect NRE and its components at both levels in seedlings of two evergreen oaks [Quercus coccifera L. and Quercus ilex subsp. ballota (Desf.) Samp] and in a semi-deciduous one (Quercus faginea Lam.). In the summer drought experiment seedlings were left to dehydrate to ?2.5 and to ?0.5 MPa (water stress and control, respectively) before watering. In the light experiment seedlings were grown at 100, 20 and 5% of full sunlight. The leaf abscission pattern was monitored and N content, N loss and NRE were calculated in the two peaks of leaf abscission (spring and late summer). After one year of treatments summer drought had little effect on N resorption and its components at both leaf and canopy levels. Moderate shade increased NRE at leaf level in Q. faginea but this response vanished at canopy level. N loss at the leaf level was unaffected. Deep shade decreased N lost at the canopy level in spring but increased it in late summer. N resorption and N losses at the canopy level were lower at late summer than in spring, due to fewer leaves falling. This study highlights the importance of the scale on the study of nitrogen dynamics (leaf vs. whole canopy), as the scaling factor (amount of leaf shedding) also responds to environmental factors, either enhancing or reversing the effects found at leaf level.     

氮添加对黄河三角洲滨海湿地芦苇养分再吸收效率的影响

大气氮沉降增加能改变土壤养分可利用性,影响滨海湿地植物的养分再吸收。目前研究多关注氮沉降量对养分再吸收过程的影响,且研究集中于叶片,鲜有研究区分不同形态氮素对植物不同器官养分再吸收过程的影响。通过两年的野外控制实验,研究硝态氮、铵态氮添加对黄河三角洲滨海湿地芦苇(Phragmites australis)叶、茎养分再吸收效率的影响。结果表明：两类氮添加均显著增加叶、茎的氮、磷含量(P<0.001),增幅达32.74%—43.22%(氮)、30.91%—36.51%(磷)。叶片氮的再吸收效率为54.14%—67.66%,茎氮的再吸收效率为50.60%—62.85%。叶片磷的再吸收效率为56.80%—70.38%,茎磷的再吸收效率为77.43%—84.95%。两类氮添加均显著降低氮、磷的再吸收效率(P<0.001),但两类氮添加处理下的养分再吸收效率无差异。叶、茎氮的再吸收效率无差异,但茎磷的再吸收效率明显高于叶(P<0.01)。总之,氮添加降低芦苇对氮、磷的再吸收效率,且茎对养分的再吸收也具有不可忽略的贡献。     

Changes in soil carbon and nitrogen cycling along a 72-year wildfire chronosequence in Michigan jack pine forests

We investigated the changes in soil processes following wildfire in Michigan jack pine (Pinus banksiana) forests using a chronosequence of 11 wildfire-regenerated stands spanning 72 years. The objective of this study was to characterize patterns of soil nutrients, soil respiration and N mineralization with stand development, as well as to determine the mechanisms driving those patterns. We measured in situ N mineralization and soil respiration monthly during the 2002 growing season and used multiple regression analysis to determine the important factors controlling these processes. Growing-season soil respiration rates ranged from a low of 156 g C/m² in the 7-year-old stand to a high of 254 g C/m² in the 22-year-old stand, but exhibited no clear pattern with stand age. In general, soil respiration rates peaked during the months of July and August when soil temperatures were highest. We used a modified gamma function to model a temporal trend in total N mineralization (total N mineralization = 1.853−0.276 × age × e ^{−0.814 × age}; R ² = 0.381; P = 0.002). Total N mineralization decreased from 2.8 g N/m² in the 1-year-old stand to a minimum value of 0.5 g N/m² in the 14-year-old stand, and then increased to about 1.5 g N/m² in mature stands. Changes in total N mineralization were driven by a transient spike in N turnover in the mineral soil immediately after wildfire, followed by a gradual accrual of a slow-cycling pool of N in surface organic horizons as stands matured. Thus, in Michigan jack pine forests, the accumulation of surface organic matter appears to regulate N availability following stand-replacing wildfire.     

不同林龄短枝木麻黄小枝单宁含量及养分再吸收动态

通过对福建省惠安县不同林龄短枝木麻黄人工林不同发育阶段小枝单宁含量及氮磷再吸收率的研究,探讨了短枝木麻黄林分发育过程中的营养保存策略。结果表明,幼龄林(5年生)成熟小枝中的总酚(TP)、可溶性缩合单宁(ECT)总缩合单宁(TCT)含量及蛋白质结合能力(PPC)显著高于成熟林(21年生)和衰老林(38年生)。随着林分发育,成熟小枝中N含量显著升高,而P含量呈降低趋势。不同发育阶段林分成熟小枝中N:P比均大于20,且随着林龄的增加而升高。磷再吸收率(PRE)显著高于氮再吸收率(NRE),均以成熟林分最高。这表明短枝木麻黄小枝单宁含量与养分再吸收受林龄影响,其养分保存机制会随着林分生长发育的变化而进行调节。     

N limitation increases along a temperate forest succession: evidences from leaf stoichiometry and nutrient resorption

温带森林演替加剧了氮限制：来自叶片化学计量和养分重吸收的证据 森林生产力和碳汇功能在很大程度上取决于土壤氮和磷的有效性。然而,迄今为止,养分限制随森林演替的时间变化仍存在争议。叶片化学计量和养分重吸收是预测植物生长养分限制的重要指标。基于此,本研究测定了温带森林4个演替阶段所有木本植物叶片和凋落叶中氮和磷的含量,并分析了演替过程中非生物因子和生物因子如何影响叶片化学计量和养分重吸收。研究结果表明,在个体尺度上,叶片氮磷含量在演替末期显著增加,而叶片氮磷比无显著变化;氮的重吸收效率随演替显著增加,然而磷的重吸收效率先增加后减少;氮重吸收效率与磷重吸收效率的比值仅在演替末期显著增加。此外,植物氮素循环对土壤养分的响应比磷素循环更弱。在群落尺度上,叶片氮磷含量随森林演替呈现先降低后升高的趋势,主要受香农-维纳多样性指数和物种丰富度的影响;叶片氮磷比随演替而显著变化,主要由胸径的群落加权平均值决定;氮的重吸收效率增加,主要受物种丰富度和胸径的影响,而磷的重吸收效率相对稳定。因此,氮重吸收效率与磷重吸收效率的比值显著增加,表明随着温带森林演替,氮限制加剧。这些结果可能反映了较高生物多样性群落中物种间对有限资源的激烈竞争,强调了生物因子在驱动森林生态系统养分循环中的重要性,为中国温带和北方森林可持续经营的施肥管理提供了参考。     

The Role of Wood Ants (Formica rufa group) in Carbon and Nutrient Dynamics of a Boreal Norway Spruce Forest Ecosystem

Wood ants (Formica rufa group) are regarded as keystone species in boreal and mountain forests of Europe and Asia by their effect on ecosystem carbon (C) and nutrient pools and fluxes. To quantify the impact of their activity on boreal forest ecosystems, C, nitrogen (N), phosphorus (P), potassium (K) and calcium (Ca) pools and fluxes in wood ant nests (WAN), and soil were assessed along a 5-, 30-, 60-, and 100-year-old Norway spruce (Picea abies L. Karsten) dominated successional gradient in eastern Finland. Amounts of C and nutrients in WAN increased with stand age, but contained less than 1% of total C and nutrient pools in these stands. The CO₂-efflux from nests was also insignificant, as compared to CO₂-efflux from the forest floor. Annually, the amount of C brought by wood ants into their nests as honeydew, prey and nest-building materials ranged from 2.7 to 49.3 kg ha^?1 C, but this is only 0.1–0.7% of the combined net primary production of trees and understorey in boreal forests. The difference between wood ant nest C inputs and outputs was very small in the younger-aged stands, and increased in the older stands. Carbon accumulation rates in nests over a 100 year period are estimated to be less than 10 kg ha^?1 a^?1. In contrast to C, annual inputs of N, P, and K are larger compared to wood ant nest nutrient pool size, ranging from 3 to 6% of the annual tree stand and understorey uptake. This indicates a more rapid turnover and transport of N, P, and K out of WAN, and suggests that wood ants increase the cycling rate of these nutrients in boreal forests.     

刨花楠叶片碳氮磷化学计量比与个体大小的关系

以9和13年生刨花楠为对象,分析叶片碳氮磷含量与林木胸径的关系,研究林木叶片碳氮磷化学计量特征在个体大小及林龄间的变化规律.结果表明： 利用实生苗所营造的刨花楠人工林,个体分化随年龄增大而明显;两刨花楠人工林林木叶片C、N、P含量及C∶N均存在显著差异,但二者叶片C∶P及N∶P差异不显著;9 年生林木叶片平均C、N、P含量及N∶P均低于13年生林木,但叶片C∶N、C∶P高于13年生林木;同一林龄中,不同个体大小之间叶片的N、P含量及化学计量特征均存在显著差异;9年生刨花楠不同个体叶片N、P含量及其化学计量特征与胸径呈显著线性相关,13年生不同个体林木叶片N、P含量及C∶P、N∶P与胸径呈显著抛物线关系,而C∶N与胸径呈显著线性相关;9年生刨花楠叶片N、P转移率高于13年生,叶片N、P在生长季与非生长季的养分转移策略因不同生长阶段和生长环境而产生差异.