Tracing nitrate sources using the isotopic composition of skeletal-bound organic matter from the calcareous green algae Halimeda

Coral Reefs - The nitrogen (N) isotopic composition (δ15N) of water column nitrate (NO3−) reveals important information about the source(s) of N, and/or the transformation processes...     

Gradient-dominated ecosystems: sources and regulatory functions of dissolved organic matter in freshwater ecosystems

The emergent wetland and littoral components of the land-water zone are functionally coupled by the amounts and types of dissolved organic matter that are released, processed, transported to, and then further processed within the recipient waters. Operational couplings and integrations in freshwater ecosystems occur along physical and metabolic gradients of a number of scales from micrometer to kilometer dimensions. The operation and turnover of the microbial communities, largely associated with surfaces, generate the metabolic foundations for material fluxes along larger-scale gradients. Because of the predominance of small, shallow freshwater bodies, most dissolved organic carbon (DOC) of lacustrine and riverine ecosystems is derived from photosynthesis of higher plants and microflora associated with detritus, including sediments, and is only augmented by photosynthesis of phytoplankton. As the dissolved organic compounds generated in the wetland and littoral interface regions move toward the open-water regions of the ecosystems, partial utilization effects a selective increase in organic recalcitrance. Even though DOC from allochthonous and from interface sources is more recalcitrant than that produced by planktonic microflora, decomposition of the much larger interface quantities imported to the pelagic zone dominates ecosystem decomposition. The observed high sustained productivity of the land-water interface zone results from extensive recycling of essential resources (nutrients, inorganic carbon) and conservation mechanisms. On the average in lakes and streams, greater than 90 percent of the decomposition in the ecosystem is by bacteria utilizing DOM from non-pelagic sources of primary productivity. In addition to direct mineralization of DOC from non-pelagic sources, many of the organic compounds function indirectly to influence metabolism. New evidence is presented to demonstrate formation of complexes between humic and fulvic organic acids and extracellular enzymes. These complexes inhibit enzyme activity and can be transported within the ecosystem. The complex can be decoupled by mild ultraviolet photolysis with regeneration of enzyme activity in displaced locations.     

Complementary pathways of dissolved organic carbon removal pathways in clear-water Amazonian ecosystems: photochemical degradation and bacterial uptake

Dissolved organic carbon (DOC) photochemical reactions establish important links between DOC and planktonic bacteria. We hypothesize that seasonal changes in DOC quality, related to the flood pulse, drive the effects of light-DOC interactions on uptake by planktonic bacteria uptake in clear-water Amazonian ecosystems. Water samples from two ecosystems (one lake and one stream) were incubated in sunlight during different hydrological periods and were then exposed to bacterial degradation. Photochemical and bacterial degradation were driven by seasonal DOC inputs. Bacterial mineralization was the main degradation pathway of autochthonous DOC in the lake, while allochthonous DOC was more available for photochemical oxidation. We suggest that sunlight enhances the bacterial uptake of refractory DOC but does not alter uptake of labile forms. We also observed a positive relationship between sunlight and bacterial degradation of DOC, instead of competition. We conclude that photochemical reactions and bacteria complementarily degrade the different sources of DOC during the flood pulse in Amazonian clear-water aquatic ecosystems.     

Tannins in nutrient dynamics of forest ecosystems - a review

Tannins make up a significant portion of forest carbon pools and foliage and bark may contain up to 40% tannin. Like many other plant secondary compounds, tannins were believed to function primarily as herbivore deterrents. However, recent evidence casts doubts on their universal effectiveness against herbivory. Alternatively, tannins may play an important role in plant–plant and plant–litter–soil interactions. The convergent evolution of tannin-rich plant communities on highly acidic and infertile soils throughout the world, and the intraspecific variation in tannin concentrations along edaphic gradients suggests that tannins can affect nutrient cycles. This paper reviews nutrient dynamics in forest ecosystems in relation to tannins. Tannins comprise a complex class of organic compounds whose concentration and chemistry differ greatly both among and within plant species. Because the function and reactivity of tannins are strongly controlled by their chemical structure, the effects of tannins on forest ecosystem processes are expected to vary widely. Tannins can affect nutrient cycling by hindering decomposition rates, complexing proteins, inducing toxicity to microbial populations and inhibiting enzyme activities. As a result, tannins may reduce nutrient losses in infertile ecosystems and may alter N cycling to enhance the level of organic versus mineral N forms. The ecological consequences of elevated tannin levels may include allelopathic responses, changes in soil quality and reduced ecosystem productivity. These effects may alter or control successional pathways. While a great deal of research has addressed tannins and their role in nutrient dynamics, there are many facets of tannin biogeochemistry that are not known. This lack of information hinders a complete synthesis of tannin effects on forest ecosystem processes and nutrient cycling. Areas of study that would help clarify the role of tannins in forest ecosystems include improved characterization and quantification techniques, enhanced understanding of structure-activity relationships, investigation of the fate of tannins in soil, further determination of the influence of environmental factors on plant tannin production and decomposition, and additional information on the effects of tannins on soil organisms.     

Tracing estuarine organic matter sources into the southern North Sea using C and N isotopic signatures

Sources and distribution of particulate organic matter in surface waters of the Humber and Thames estuaries and in the East Anglian plume in the southern North Sea were investigated in winter 2006/2007. Carbon (C) and nitrogen (N) stable isotopes provided evidence for the presence of three particulate organic matter sources; riverine plankton (δ¹³C ?30 ‰ and δ¹⁵N 7.9 ‰) identified in the Thames estuary only, marine plankton (average δ¹³C ?21.4 ‰ and δ¹⁵N 4.5 ‰) and a third source with an enriched ¹³C signature (>?16.7 ‰) and elevated C:N ratio (>12.7). Particulate organic matter with enriched ¹³C values were observed throughout the Humber estuary and at the marine end-member of the Thames estuary. While bacterial cycling of organic carbon undoubtedly takes place within these estuaries, these processes on their own are unlikely to account for the isotopic signatures seen. The ¹³C enriched organic matter source is suggested to be due to particulate organic matter input from marsh plants and seagrasses such as Spartina spp. and Zostera on the adjacent salt marshes and mudflats and/or macroalgae along the banks of the estuaries. This ¹³C enriched signal was also identified approximately 50 km offshore within the southern North Sea, in the East Anglian plume, which transports UK riverine water off-shore in a discrete plume. This plume therefore provides a mechanism to transport this estuarine derived organic matter pool offshore out of the estuaries. These results indicate that estuarine derived organic matter from marsh plants, seagrasses and/or macroalgae contributes to the southern North Sea organic matter pool and is therefore likely to contribute to winter-time shelf sea carbon and nitrogen cycles.     

森林生态系统DOM的来源、特性及流动

可溶性有机物质（Dissolved Organic Matter)是森林生态系统主要的可移动碳库及重要的养分库。系统综述了森林生态系统DOM的来源,组成,性质,季节动态;DOM释放与存留机制及影响因素,森林生态系统DOM的流动及干扰对DOM动态影响等,已有研究表明DOM的森林生态系统C、N、P循环,成土作用,污染物迁移等方面起着重要作用。今后森林生态系统DOM的研究应集中于以几方面：（1）确定森林生态系统中DOM源和汇;（2）评价森林水文条件对DOM释放与存留的调节作用;（3）探讨全球气候变化对森林生态系统DOM的影响;（4）可溶性有机氮（Dissolved Organic Nitrogen),可溶性有机磷（Dissolved Organic Phosphorus)动态与可溶性有碳（Dissolved Organic Carbon）动态的差别。     

Seasonal patterns of ammonium and nitrate uptake in nine temperate forest ecosystems

Summary Seasonal patterns of net N mineralization and nitrification in the 0–10 cm mineral soil of 9 temperate forest sites were analyzed using approximately monthlyin situ soil incubations. Measured nitrification rates in incubated soils were found to be good estimates of nitrification in surrounding forest soils. Monthly net N mineralization rates and pools of ammonium-N in soil fluctuated during the growing season at all sites. Nitrate-N pools in soil were generally smaller than ammonium-N pools and monthly nitrification rates were less variable than net N mineralization rates. Nitrate supplied most of the N taken up annually by vegetation at 8 of the 9 sites. Furthermore, despite the large fluctuations in ammonium-N pools and monthly net N mineralization, nitrate was taken up at relatively uniform rates during the growing season at most sites.     

土壤溶解性有机碳在陆地生态系统碳循环中的作用

土壤溶解性有机碳(DOC)是有机碳库的活跃组分,在陆地生态系统碳循环中发挥重要作用.本文从碳循环重要性着手,综述了土壤DOC在土壤碳固持与温室气体排放中的作用;结合我国的现实情况(如土壤酸化、气候变暖等),探讨了土壤DOC的相关影响因素如土壤性质、环境因素、人为活动对土壤DOC的影响及作用机制,对进一步理解土壤DOC在陆地生态系统碳循环与温室气体减排中的作用具有重要意义.     

Coarse woody debris in Australian forest ecosystems: A review

Abstract Coarse woody debris (CWD) is the standing and fallen dead wood in a forest and serves an important role in ecosystem functioning. There have been several studies that include estimates of CWD in Australian forests but little synthesis of these results. This paper presents findings from a literature review of CWD and fine litter quantities. Estimates of forest‐floor CWD, snags and litter from the literature are presented for woodland, rainforest, open forest and tall open forest, pine plantation and native hardwood plantation. Mean mass of forest floor CWD in Australian native forests ranged from 19 t ha^?1 in woodland to 134 t ha^?1 in tall open forest. These values were generally within the range of those observed for similar ecosystems in other parts of the world. Quantities in tall open forests were found to be considerably higher than those observed for hardwood forests in North America, and more similar to the amounts reported for coniferous forests with large sized trees on the west coast of the USA and Canada. Mean proportion of total above‐ground biomass as forest floor CWD was approximately 18% in open forests, 16% in tall open forests, 13% in rainforests, and 4% in eucalypt plantations. CWD can be high in exotic pine plantations when there are considerable quantities of residue from previous native forest stands. Mean snag biomass in Australian forests was generally lower than the US mean for snags in conifer forests and higher than hardwood forest. These results are of value for studies of carbon and nutrient stocks and dynamics, habitat values and fire hazards.     

Experimental warming studies on tree species and forest ecosystems: a literature review

Temperature affects a cascade of ecological processes and functions of forests. With future higher global temperatures being inevitable it is critical to understand and predict how forest ecosystems and tree species will respond. This paper reviews experimental warming studies in boreal and temperate forests or tree species beyond the direct effects of higher temperature on plant ecophysiology by scaling up to forest level responses and considering the indirect effects of higher temperature. In direct response to higher temperature (1) leaves emerged earlier and senesced later, resulting in a longer growing season (2) the abundance of herbivorous insects increased and their performance was enhanced and (3) soil nitrogen mineralization and leaf litter decomposition were accelerated. Besides these generalizations across species, plant ecophysiological traits were highly species-specific. Moreover, we showed that the effect of temperature on photosynthesis is strongly dependent on the position of the leaf or plant within the forest (canopy or understory) and the time of the year. Indirect effects of higher temperature included among others higher carbon storage in trees due to increased soil nitrogen availability and changes in insect performance due to alterations in plant ecophysiological traits. Unfortunately only a few studies extrapolated results to forest ecosystem level and considered the indirect effects of higher temperature. Thus more intensive, long-term studies are needed to further confirm the emerging trends shown in this review. Experimental warming studies provide us with a useful tool to examine the cascade of ecological processes in forest ecosystems that will change with future higher temperature.     

森林生态系统碳循环的基本概念和野外测定方法评述

全球气候变化与森林生态系统碳循环息息相关,定量评估森林碳收支是生态系统与全球变化研究的重要任务。30年来森林生态系统碳循环研究已经取得了长足的进展,但全球和区域森林碳收支仍然存在很大的不确定性。这一方面与森林生态系统本身的复杂性有关,另一方面也与具体研究方法有关。评述了森林生态系统碳循环的基本概念和主要野外测定方法,为我国森林生态系统碳循环研究提供可参考的方法论。从生态系统碳浓度、密度、通量、分配和周转5个方面回顾了碳循环相关概念,指出碳浓度和碳储量是对碳库的静态描述,而碳通量和碳周转是对碳库的动态描述。净初级生产力是测量最普遍的碳通量组分,但大多数情况下因忽略了一些细节而被系统低估。普遍使用的净生态系统生产力,由于没有包含非CO2形式的水文、气象和干扰过程产生的碳通量,通常情况下高于生态系统净碳累积速率。在详细介绍碳通量组分的基础上,改进了森林生态系统碳循环的概念模型。重点讨论了碳通量的3种地面实测方法:测树学方法、箱法和涡度协方差法,并指出了其注意事项和不确定性来源。针对当前碳循环研究的突出问题,建议从4个方面减小碳循环测定的不确定性:(1)恰当运用生物量方程估算乔木生物量;(2)尽可能全面测定生态系统碳组分;(3)给出碳通量估算值的不确定性;(4)多种途径交互验证。     

Correlations between nitrate and sulfate in the soil solution of disturbed forest ecosystems

Based on data from three German forest ecosystems severely disturbed by windthrow events, correlation patterns between NO₃ ⁻ and SO₄ ²⁻ dynamics in the soil solution are described. Each of the correlation types was related to site-specific SO₄ ²⁻ retention processes. The relative importance of SO₄ ²⁻ adsorption/desorption and precipitation/dissolution was found to be different for the studied soils depending on their contents of SO₄ ²⁻ -adsorbing sesquioxides and of A1 hydroxy sulfate minerals. Enhanced NO₃ ⁻ concentrations in the soil solution resulting from excess nitrification promoted either SO₄ ²⁻ adsorption or the dissolution of Al hydroxy sulfates. Both processes result in different ecological consequences: Whereas the former reaction reduces anion concentration peaks in the soil solution, the latter increases them. Thus, a prediction of cation export from soil ecosystems subjected to excess nitrification has to regard site-specific interrelationships between NO₃ ⁻ and SO₄ ²⁻. As a third type of conelation the independence of NO₃ ⁻ and SO₄ ²⁻ concentrations in the soil solution is presented. This type is suggested to be typical for soils with low SO₄ ²⁻. adsorption capacity and absence of A1 hydroxy sulfates.     

林火干扰对森林生态系统土壤有机碳的影响研究进展

林火干扰是森林生态系统特殊而重要的生态因子,可改变生态系统的养分循环与能量传递。研究林火干扰对森林生态系统土壤有机碳的影响,有助于理解森林生态系统中土壤碳固持和碳循环过程,为制定科学合理的旨在减缓全球变化的林火管理策略具有重要意义。从4个方面阐述了林火干扰对森林生态系统土壤有机碳的影响及内在机制:分别从大尺度和小尺度两个方面阐述了林火干扰对土壤有机碳的影响及对森林生态系统碳循环与碳平衡的作用机制;探讨了不同林火干扰类型和林火干扰强度下,土壤活性有机碳对林火干扰的响应机制;阐明了林火干扰对土壤惰性有机碳的影响及作用机制;论述了林火干扰主要通过改变土壤有机碳的输入和输出过程进而影响土壤有机碳的稳定性及内在机制。最后提出了提高林火干扰对森林生态系统土壤有机碳影响定量化研究的4种路径选择:(1)全面比较研究不同林火干扰类型对土壤有机碳循环及其碳素再分配过程的功能特征;(2)进一步阐明林火干扰通过改变植被结构进而影响土壤生物群落结构,剖析土壤碳库循环的内在机制;(3)完善不同时空尺度下林火干扰对森林生态系统土壤碳库周转过程的定量化研究;(4)加强不同林火干扰类型土壤碳库稳定性差异的研究。     

A mechanism of abiotic immobilization of nitrate in forest ecosystems: the ferrous wheel hypothesis

Forest soils, rather than woody biomass, are the dominant long‐term sink for N in forest fertilization studies and, by inference, for N from atmospheric deposition. Recent evidence of significant abiotic immobilization of inorganic‐N in forest humus layers challenges a previously widely held view that microbial processes are the dominant pathways for N immobilization in soil. Understanding the plant, microbial, and abiotic mechanisms of N immobilization in forest soils has important implications for understanding current and future carbon budgets. Abiotic immobilization of nitrate is particularly perplexing because the thermodynamics of nitrate reduction in soils are not generally favorable under oxic conditions. Here we present preliminary evidence for a testable hypothesis that explains abiotic immobilization of nitrate in forest soils. Because iron (and perhaps manganese) plays a key role as a catalyst, with Fe(II) reducing nitrate and reduced forms of carbon then regenerating Fe(II), we call this ‘the ferrous wheel hypothesis’. After nitrate is reduced to nitrite, we hypothesize that nitrite reacts with dissolved organic matter through nitration and nitrosation of aromatic ring structures, thus producing dissolved organic nitrogen (DON). In addition to ignorance about mechanisms of DON production, little is known about DON dynamics in soil and its fate within ecosystems. Evidence from leaching and watershed studies suggests that DON production and consumption may be largely uncoupled from seasonal biological processes, although biological processes ultimately produce the DOC and reducing power that affect DON formation and the entire N cycle. The ferrous wheel hypothesis includes both biological and abiological processes, but the reducing power of plant‐derived organic matter may build up over seasons and years while the abiotic reduction of nitrate and reaction of organic matter with nitrite may occur in a matter of seconds after nitrate enters the soil solution.     

Tracing origins and migration of wildlife using stable isotopes: a review

To understand the ecology of migratory animals it is important to link geographic regions used by individuals including breeding, wintering, and intermediate stopover sites. Previous conventional approaches used to track animal movements have relied on extrinsic markers and typically the subsequent recovery of individuals. This approach has generally been inappropriate for most small, or non-game animals. The use of intrinsic markers such as fatty acid profiles, molecular DNA analyses, and the measurement of naturally occurring stable isotopes in animal tissues offer alternative approaches. This paper reviews the use of stable isotope analyses (primarily δ¹³C, δ¹⁵N, δ³⁴S, δD, δ⁸⁷Sr) to trace nutritional origin and migration in animals. This approach relies on the fact that foodweb isotopic signatures are reflected in the tissues of organisms and that such signatures can vary spatially based on a variety of biogeochemical processes. Organisms moving between isotopically distinct foodwebs can carry with them information on the location of previous feeding. Such an approach has been used to track animal use of inshore versus offshore, marine versus freshwater, terrestrial C₃ versus marine, terrestrial mesic versus xeric, and C₃ versus C₄ or Crassulacean acid metabolism foodwebs. More recently, the use of stable hydrogen isotope analyses (δD) to link organisms to broad geographic origin in North America is based on large-scale isotopic contours of growing-season average δD values in precipitation. This technique, especially when combined with the assay of other stable isotopes, will be extremely useful in helping to track migration and movement of a wide range of animals from insects to birds and mammals. Future research to refine our understanding of natural and anthropogenic-induced isotopic gradients in nature, and to explore the use of stable isotopes of other elements, is recommended. Received: 1 July 1998 / Accepted: 9 December 1998     

氮沉降对森林土壤磷循环的影响

磷是生物体必需的大量元素之一,也是许多生态系统的主要限制因子。近年来,大气氮沉降日益加剧,已对森林生态系统磷循环产生了不可忽视的影响。关于氮沉降对生态系统磷循环的影响已开展了一系列的研究,然而尚缺少对其整体的认识。因此,通过收集国内外已发表的相关文章,从以下五个方面综述氮沉降对森林生态系统土壤磷循环的影响及其机理:1)阐述了森林生态系统土壤磷循环的概念;2)介绍了氮沉降对森林土壤磷循环影响的研究方法,包括长期定位模拟氮沉降法、自然氮沉降梯度法和同位素示踪法等;3)概述了氮沉降对森林生态系统土壤磷循环的影响。目前的研究结论趋向于认为长期氮沉降使森林土壤磷循环速率加快。长期氮输入易于使土壤中可溶性磷向非活性磷酸盐库迁移而难以被利用。因此,为了满足需求,土壤磷酸酶活性将增加以加速有机磷的矿化,从而加速磷素在土壤-植物-微生物之间的周转。4)探讨了氮沉降影响森林土壤磷循环的机制。氮沉降可通过改变土壤有机质的性质、微生物群落组成、磷酸酶活性以及阳离子的流动性等途径影响森林土壤磷循环;5)指出了当前研究存在的问题及未来的研究方向。     

Seasonal and diel relationships between the isotopic compositions of dissolved and particulate organic matter in freshwater ecosystems

A study of the isotopic composition of organic matter was conducted in a freshwater marsh over seasonal and diel time scales to determine the sources of dissolved organic matter (DOM) and the processes leading to its formation. Bulk C and N isotopic compositions of the bacterial fraction (0.2–0.7 m) and particulate organic matter (POM; 0.7–10 m) were compared on a seasonal basis with the change in ¹³C of DOM. The bulk isotopic data support the idea that DOM was, in part, derived from the breakdown of larger organic matter fractions. The bacterial fraction and POM were compositionally similar throughout the year, based on a comparison of the ¹³C of individual amino acids in each fraction. Annual variation in the ¹³C of amino acids in DOM was greater relative to the variation in larger fractions indicating that microbial reworking was an important factor determining the proteinaceous component of DOM. The ¹³C enrichment of serine and leucine in each organic matter fraction suggested microbial reworking was an important factor determining organic matter composition during the most productive times of year. Changes in the bulk ¹³C of DOM were more significant over daily, relative to seasonal, time scales where values ranged by 6 and followed changes in chlorophyll a concentrations. Although bulk ¹³C values for POM ranged only from –29 to –28 during the same diel period, the ¹³C of alanine in POM ranged from –30 to –22. Alanine is directly synthesized from pyruvate and is therefore a good metabolic indicator. The ¹³C of individual amino acids in DOM revealed the diel change in the importance of autotrophic versus heterotrophic activity in influencing DOM composition. Diel changes in the ¹³C of phenylalanine, synthesized by common pathways in phytoplankton and bacteria, were similar in both DOM and POM. The diel change in ¹³C of isoleucine and valine, synthesized through different pathways in phytoplankton and bacteria, were distinctly different in DOM versus POM. This disparity indicated a decoupling of the POM and DOM pools, which suggests a greater source of bacterial-derived organic matter at night. The results of this study demonstrate the use of the isotopic composition of individual amino acids in determining the importance of microbial reworking and autotrophic versus heterotrophic contributions to DOM over both diel and seasonal time scales.     

Influence of land use history on seed banks in European temperate forest ecosystems: a review

This study summarises European research on seed banks in temperate forest systems and analyses for differences in seed bank composition between geographically scattered forests with a different land use history. Special attention is given to seed bank characteristics of ancient forest species. Results of Detrended Correspondence Analysis suggest that historical land use is a key factor in determining the seed bank composition. Particularly seed banks of forests on former heathland sites differ from seed banks of ancient forest due to a high contribution of early successional species. The effect of former land use decreases after 50 yr, due to seed senescence. Total seed density decreases with recent forest age. Seed bank composition of eastern European forests is different from northern or western European forests, a difference which is mainly caused by species with a higher Ellenberg indicator value for continentality, temperature and reaction. In general, ancient forest species are poorly represented. Only a limited number is mentioned to have a persistent seed bank, and their densities are relatively low, which means that restoration of typical ancient forest vegetation can not rely on the seed bank. However, there is still a considerable lack of knowledge concerning seed bank and germination characteristics of forest species.     

Visible and invisible effects of hurricanes on forest ecosystems: an international review

Abstract Hurricanes have visible and invisible effects on forests. The visible effects are dramatic, noticeable over the short‐term and relatively well documented in the literature. Invisible effects are less understood as they require well‐focused research both in the short‐ and long‐term time scales. This review of the literature on hurricane effects focuses on the Neotropics and the temperate zone of North America. The material is organized according to a heuristic model that distinguishes between immediate effects (0 to 3 years), immediate responses (0 to 20 years), trajectories of responses (0 to 100 years) and long‐term legacies (>100 years). It is suggested that the ecological role of hurricanes involves six principal effects: 1. they change the ecological space available to organisms; 2. they set organisms in motion; 3. they increase the heterogeneity of the landscape and the variability in ecosystem processes; 4. they rejuvenate the landscape and its ecosystems and redirect succession; 5. they shape forest structure, influence their species composition and diversity and regulate their function; and 6. they induce evolutionary change through natural selection and ecological creativity through self‐organization. A new approach to hurricane research will study hurricanes at the same scale at which they operate (i.e., across latitudes and longitudes and over disturbed and undisturbed landscapes). This research will require networks of observation platforms located along expected hurricane paths to facilitate forest structure and functioning observations across gradients of hurricane frequency and intensity. This research will also require use of remote sensing and automated wireless technology, hardened to survive hurricane‐strength winds and floods to assure real time measurements of the characteristics of hurricanes and ecosystem responses. No progress will be forthcoming in the understanding of hurricane effects if we do not learn to quantify objectively the energy dissipation of hurricanes on the full grid of affected forests as the hurricane passes over a landscape.