Organic matter and nitrogen accumulation and nitrogen fixation during early ecosystem development in Hawaii

We used a chronosequence comprised of 10 y, 52 y and 142 yold `a'a lava flows on Mauna Loa, Hawaii, to determine theaccumulation of organic matter and nitrogen and rates of nitrogenfixation through time. The mass of organic matter (live and deadbiomass and soil organic matter) on the 1984, 1942 and 1852 lavaflows was 0.6, 2.2 and 7.6 kg m^{– 2}, respectively, while total N was 4.8, 10.9 and 85.7 g m^{– 2}.We estimated the total rates of nitrogen fixation for thethree different aged ecosystems using an acetylene reduction assaycalibrated with ¹⁵N incubations. While mean rates of total N fixation remained largely constant across the three sites – between2.0 and 3.1 kg ha^{– 1} y^{– 1} – the most important sources of N fixation changed. On the 10 y flow, the most important fixer was the pioneering cyanolichen, Stereocaulon vulcani. After 52 years ofecosystem development, the most important N fixer was a cyanoalga,while after 142 years, the predominant N fixers were heterotrophicbacteria associated with leaf litter, twigs and detritus. The totalamount of N accumulated after 52 years of ecosystem development wasequivalent to cumulative inputs through biological N fixation. After 142 years, however, cumulative inputs from N fixation couldonly account for between 27–59% of the total nitrogen accrued.We used fertilizer additions of all essential nutrients otherthan N to test whether the availability of lithophilic nutrientsregulated rates of N fixation in early ecosystem development. Ratesof nitrogen fixation by the lichen, S. vulcani, approximately doubled when fertilized on the 1984 and 1942 flows. Rates of N-fixation by heterotrophic nitrogen fixing bacteria on leaf litter ofMetrosideros polymorpha also increased significantly when fertilized with lithophilic nutrients. These findings suggest that weathering rates of lava in part regulate rates of nitrogen fixation in these young ecosystems.     

松嫩平原玉米带农田土壤氮密度时空格局

基于1980年吉林省第二次全国土壤普查剖面资料和2003—2006年的实测数据,估算了松嫩平原玉米带不同土壤类型农田表层(0—20 cm)土壤氮密度和储量,分析了该地区土壤氮密度的25a时空变化特征及其原因。结果表明,两个时期松嫩平原玉米带农田土壤氮密度的空间分布格局基本一致,中部高、边缘低,平均土壤氮密度变化不大,均为0.31 kg/m2,但25 a间不同土壤类型和土地利用方式的土壤氮密度变化趋势存在差异,暗棕壤、水稻土和沼泽土的氮密度上升,其它类型土壤的氮密度不变或下降,旱田的氮密度不变,水田的氮密度明显下降,25 a间研究区内的农田土壤总氮储量每年减少7.6×105kg。25 a间土壤氮密度的变化与1980年的初始值负相关,土壤氮密度的新稳定状态值为0.32 kg/m2。如保持1980年的土地利用方式和栽培耕作措施不变,该地区农田土壤总固氮潜力为5.18×106kg/a。但实际上,与固氮潜力相比,2005年该区农田土壤总氮储量偏低了1.20×108kg。因此,今后该区应多注重肥料的合理施用,加强农田管理,尤其是旱田改水田的管理。     

Sustainable forestry: A model of the effects of nitrogen removals in wood harvesting and fire on the nitrogen balance of regrowth eucalypt stands

Abstract Sustainable forest use is an integral part of Australia's recently adopted National Forest Policy; consequently, there is an urgent need for quantitative, ecologically based measures of sustainability. One process that may affect ecosystem sustainability is the removal of nutrients through forest harvesting and fire. This paper presents a model-based analysis of the combined consequences of harvesting and fire management practices for the nitrogen (N) budgets of managed forest ecosystems. The model, called N-BAL, evaluates the balance between N removals due to harvesting and fire (prescribed and regeneration burns), and N inputs (both natural and as added fertilizer), and leads to a criterion for the maintenance of site N reserves. That criterion can be used to estimate the accretion (or depletion) of site N over a single forest rotation, or to predict sustainable stem productivity for given N inputs and management practices. The analysis is applied to managed stands of karri (Eucalyptus diversicolor F. Muell.) in southwestern Australia to investigate whether natural N inputs are sufficient to maintain site N capital under current harvesting and fire practices. Model predictions for stands harvested at age 100 years with slash burns and regular prescribed burns range from a rotation-averaged depletion rate of 22 kg ha^?1 year^?1 to an annual accretion of 14 kg ha^?1 year^?1, depending on assumed N inputs and fire frequency and intensity. The mean annual N balance is highly sensitive to rates of natural N inputs, fire intensity and inter-fire period, and less sensitive to rotation length. These results are tentative and highlight the need for further research to improve estimates of several key model parameters and relationships.     

Dynamics of nitrogen and phosphorus retention during wetland ecosystem succession

We compared the mechanisms of nitrogen (N) and phosphorus (P) removal in four young (<15 years old) constructed estuarine marshes with paired mature natural marshes to determine how nutrient retention changes during wetland ecosystem succession. In constructed wetlands, N retention begins as soon as emergent vegetation becomes established and soil organic matter starts to accumulate, which is usually within the first 1–3 years. Accumulation of organic carbon in the soil sets the stage for denitrification which, after 5–10 years, removes approximately the same amount of N as accumulating organic matter, 5–10 g/m²/yr each, under conditions of low N loadings. Under high N loadings, the amount of N stored in accumulating organic matter doubles while N removal from denitrification may increase by an order of magnitude or more. Both organic N accumulation and denitrification provide for long-term reliable N removal regardless of N loading rates. Phosphorus removal, on the other hand, is greatest during the first 1–3 years of succession when sediment deposition and sorption/precipitation of P are greatest. During this time, constructed marshes may retain from 3 g P/m²/yr under low P loadings to as much as 30 g P/m²/yr under high loadings. However, as sedimentation decreases and sorption sites become saturated, P retention decreases to levels supported by organic P accumulation (1–2 g P/m²/yr) and sorption/precipitation with incoming aqueous and particulate Fe, Al and Ca. Phosphorus cycling in wetlands differs from forest and other terrestrial ecosystems in that conservation of P is greatest during the early years of succession, not during the middle or late stages. Conservation of P by wetlands is largely regulated by geochemical processes (sorption, precipitation) which operate independently of succession. In contrast, the conservation of N is controlled by biological processes (organic matter accumulation, denitrification) that change as succession proceeds.     

Regional gains and losses of nitrogen in the Amazon basin

In order to better understand the relative importance of different ecosystems and nitrogen cycling processes within the Amazon basin to the nitrogen economy of this region, we constructed a generalized nitrogen budget for the region based on data for hydrologic losses of nitrogen and nitrogen fixation in Amazon forests. Data included information available for nitrogen in water entering and leaving both the entire basin and watersheds on oxisol and ultisol soils near Manaus, Brazil, in addition to biological nitrogen fixation in forests on ultisol, oxisol and entisol (‘varzea’) soils in Central Amazonia. Available data indicate that 4–6 kg N ha^?1 yr^?1 are lost via the River Amazonas, and that a similar amount enters in rainfall. Root-associated biological nitrogen fixation contributesca. 2 kg N ha^?1 yr^?1 to forests on oxisols, 20 kg N ha^?1 yr^?1 to forests on utisols, and 200 kg N ha^?1 yr^?1 to forests on fertile varzea soils. There is 5–10 fold more NH₄ ⁺?N than NO₃?N in rain and stream water entering and leaving the waterbasin near Manaus. Calculations based on these data plus certain assumption yield the following regional nitrogen balance estimate: inputs through bulk deposition of 36×10⁸ kg N yr^?1 and through biological nitrogen fixation of 120×10⁸ kg N yr^?1, and outputsvia the River Amazonas of 36×10⁸ kg N yr^?1 andvia denitrification and volatization (by difference) of 120×10⁸ kg N yr^?1.     

Nutrient levels within leaves,stems, and roots of the xeric species Reaumuria soongorica in relation to geographical,climatic, and soil conditions

Besides water relations, nutrient allocation, and stoichiometric traits are fundamental feature of shrubs. Knowledge concerning the nutrient stoichiometry of xerophytes is essential to predicting the biogeochemical cycling in desert ecosystems as well as to understanding the homoeostasis and variability of nutrient traits in desert plants. Here, we focused on the temperate desert species Reaumuria soongorica and collected samples from plant organs and soil over 28 different locations that covered a wide distributional gradient of this species. Carbon (C), nitrogen (N), and phosphorus (P) concentrations and their stoichiometry were determined and subsequently compared with geographic, climatic, and edaphic factors. The mean leaf C, N, and P concentrations and C/N, C/P, and N/P ratios were 371.6 mg g⁻¹, 10.6 mg g⁻¹, 0.73 mg g⁻¹, and 59.7, 837.9, 15.7, respectively. Stem and root C concentrations were higher than leaf C, while leaf N was higher than stem and root N. Phosphorus concentration and N/P did not differ among plant organs. Significant differences were found between root C/N and leaf C/N as well as between root C/P and leaf C/P. Leaf nutrient traits respond to geographic and climatic factors, while nutrient concentrations of stems and roots are mostly affected by soil P and pH. We show that stoichiometric patterns in different plant organs had different responses to environmental variables. Studies of species-specific nutrient stoichiometry can help clarify plant–environment relationships and nutrient cycling patterns in desert ecosystems.     

模拟氮沉降对杉木幼苗养分平衡的影响

为研究氮沉降对植物养分平衡的影响,对1a生杉木（Cunninghamia lanceolata（Lamb.）Hook.）幼苗进行了室内模拟试验。以NH4NO3作为外加氮源,设计了N0（0 g N m-2?a-1）、N1（6 g N m-2?a-1）、N2（12 g N m-2?a-1）、N3（24 g N m-2?a-1）和N4（48g N m-2?a-1）等5种氮沉降水平,每处理重复6次。通过1a的试验发现,杉木幼苗叶、茎、粗根和细根中的N、K、Mg含量随氮处理水平的增加而上升,但Ca在各器官中的含量则呈下降趋势;中低氮（N1,N2）对叶、茎和粗根中P的含量表现为促进作用,而高氮（N3,N4）则表现为抑制作用。幼苗各器官中的N与其他养分元素的比值随氮处理水平的增加而普遍升高,但粗根中的N/K、N/Mg则表现为下降。与对照（N0）相比,在N1、N2、N3、N4处理中,幼苗对外加氮素的表观利用率分别为60.7%、57.9%、43.3%和27.9%,随氮处理水平增加,利用率呈明显下降趋势。随着氮处理水平的增加,幼苗体内的氮分配到叶和细根中的比例增加,而分配到茎和粗根中的比例下降。因此,氮沉降明显增加了杉木幼苗各器官的氮含量,影响了幼苗的养分平衡。     

A Migratory Ungulate Facilitates Cross-Boundary Nitrogen Transport in Forested Landscapes

Forest ungulates impact ecosystems in a number of ways. Most studies have focused on consumptive effects that may cascade to other components of the ecosystem, and tend to be motivated by harvest management or the mitigation of undesired effects on vegetation. In this study, we demonstrate that white-tailed deer (Odocoileus virginianus), a common forest ungulate in eastern North America, may directly increase availability and heterogeneity of nitrogen due to excretion of nitrogenous wastes. We conducted fecal pellet counts in 39 winter cover habitat patches, ranging in area from 0.04 to 59.6 ha, each spring for eight consecutive years. Pellet counts were used to develop allometric models of annual deer-associated nitrogen inputs at both whole-stand and fine (<10 m²) spatial scales. Deer-associated nitrogen estimates were in the range of 1–4 kg[N] ha^?1 in persistently used patches when estimated at the stand scale. Fine-scale estimates in areas of consistent aggregation were equivalent to up to 20 kg[N] ha^?1. These areas, such as bedding sites and trails, experienced greater N inputs compared to the surrounding forest matrix. Annual deer use and associated nitrogen inputs were highly variable over space, and spatial patterns in use were consistent over time at both stand and fine spatial scales. Deer-associated nitrogen likely represents a cross-boundary nitrogen flux into patches of conifer cover because deer accumulate nutrients in other habitat types during the warm season, and lose body mass during winter. Nitrogen hotspots and heterogeneity derived from the nitrogenous wastes of forest ungulates may be a generally overlooked phenomenon in forest ecology, with impacts varying according to ecological context.     

火干扰对小兴安岭白桦沼泽和落叶松-苔草沼泽凋落物和土壤碳储量的影响

火干扰在湿地生态系统中起着重要的作用,尽管湿地占全球陆地生态系统很小一部分,却是陆地生态系统一个重要的碳汇。然而关于火干扰对我国小兴安岭森林沼泽生态系统土壤碳库影响的研究鲜有报道。因此选取两种森林沼泽典型地段进行土壤取样,研究火干扰对小兴安岭白桦(Betula platyphylla)沼泽和落叶松(Larix gmelinii)-苔草(Carex schmidtii)沼泽地表凋落物和土壤碳储量(0—50 cm)的影响。研究结果表明:①重度火烧使得白桦沼泽地表凋落物量和碳储量降低了36.36%(0.50 kg/m2)和35.52%(0.23 kg C/m2),而轻度火烧无显著影响;轻度火烧和重度火烧落叶松-苔草沼泽地表凋落物量和碳储量分别减少了45.32%(0.99 kg/m2)和44.66%(0.42 kg C/m2)、50.42%(1.10 kg/m2)和49.71%(0.47 kg C/m2);②白桦沼泽和落叶松-苔草沼泽两者对照样地、轻度火烧样地、重度火烧样地的土壤碳储量(0—50 cm)分别为(23.55±6.34)kg C/m2、(18.50±8.16)kg C/m2、(32.50±7.22)kg C/m2和(20.89±2.59)kg C/m2、(23.52±16.03)kg C/m2、(21.75±6.60)kg C/m2,然而火干扰对两种森林沼泽土壤碳储量(0—50 cm)影响不显著。研究结果可为我国东北开展森林湿地计划火烧和碳管理提供理论依据。     

库布齐东段不同植被恢复阶段荒漠生态系统碳氮储量及分配格局

为科学评价植被恢复促进沙漠化逆转对碳氮储量的影响,以流动沙地、半固定沙地、油蒿固定沙地、柠条固定沙地、沙柳固定沙地5个阶段荒漠生态系统为研究对象,采用时空替代法分析植被恢复过程中荒漠生态系统碳氮储量及分配格局。结果表明:不同恢复阶段碳氮储量均表现为:流动沙地(3320.97 kg C/hm~2、346.69 kg N/hm~2)半固定沙地(4371.46 kg C/hm~2、435.95 kg N/hm~2)油蒿固定沙地(6096.50 kg C/hm~2、513.76 kg N/hm~2)柠条固定沙地(9556.80 kg C/hm~2、926.31 kg N/hm~2)沙柳固定沙地(19488.54 kg C/hm~2、982.11 kg N/hm~2)。植被层碳氮储量均呈现随植被恢复逐渐增加的趋势,除流动沙地外,其他阶段碳氮储量均以灌木层为主,占比分别为66.65%—91.41%和52.94%—93.39%,草本和凋落物占比较小。灌木各器官生物量及碳储量分配均为:茎根叶,氮储量分配无明显规律,草本各器官生物量及碳氮储量分配均为地上部分高于地下部分。土壤层是荒漠生态系统碳氮储量的主体,碳储量占比为68.64%—99.62%,氮储量占比为89.26%—99.89%,同样呈现随植被恢复逐渐增加的趋势。碳氮储量随土层加深逐渐降低,具有明显的表层富集特征,且随植被恢复过程富集性显著加强。这说明人工建植促进植被演替实现沙漠化逆转可以显著增强荒漠生态系统的碳氮固存能力。