A review of rhizosphere carbon flow modelling

Rhizosphere processes play a key role in nutrient cycling in terrestrial ecosystems. Plant rhizodeposits supply low-molecular weight carbon substrates to the soil microbial community, resulting in elevated levels of activity surrounding the root. Mechanistic compartmental models that aim to model carbon flux through the rhizosphere have been reviewed and areas of future research necessary to better calibrate model parameters have been identified. Incorporating the effect of variation in bacterial biomass physiology on carbon flux presents a considerable challenge to experimentalists and modellers alike due to the difficulties associated with differentiating dead from dormant cells. A number of molecular techniques that may help to distinguish between metabolic states of bacterial cells are presented. The calibration of growth, death and maintenance parameters in rhizosphere models is also discussed. A simple model of rhizosphere carbon flow has been constructed and a sensitivity analysis was carried out on the model to highlight which parameters were most influential when simulating carbon flux. It was observed that the parameters that most heavily influenced long-term carbon compartmentalisation in the rhizosphere were exudation rate and biomass yield. It was concluded that future efforts to simulate carbon flow in the rhizosphere should aim to increase ecological realism in model structure. This revised version was published online in June 2006 with corrections to the Cover Date.     

Boomerang ecosystem fluxes: organic carbon inputs from land to lakes are returned to terrestrial food webs via aquatic insects

Many ecosystems are linked to their adjacent ecosystems by movements of organisms. For instance, aquatic and terrestrial ecosystems are linked via emerging aquatic insects that serve as prey for terrestrial consumers. However, the role of these organisms in returning recycled carbon to the ecosystem from which it originated is not well known. This is due to the fact that values of carbon isotope signatures from terrestrial leaves and aquatic resources are usually similar and hence results of isotope mixing models need to be considered with caution. We overcame this problem by adding isotopically distinct terrestrial particulate organic carbon (tPOC) as a tracer to the experimental sides of two lakes that were divided in two equal halves with plastic curtains. We focused on aquatic insect larvae (Chironomidae) that fed on maize Zea mays leaves experimentally added to the lakes, and subsequently became prey for terrestrial predators (spiders) after emergence. The carbon isotope values of Chironomidae and spiders were significantly elevated in the lake treatment sides as compared to reference sides, whereas the values of all autochthonous resources were not affected by maize additions. Estimates from stable isotope mixing models indicated a low but demonstrable contribution of maize leaves to the diet of Chironomidae. Overlap between the isotope values of alder leaves, the major natural tPOC source, and autochthonous resources prevented a reliable quantification of allochthony of Chironomidae. However, we qualitatively demonstrated the flow of terrestrial particulate organic carbon to lakes, as leaf fall, and back to terrestrial surroundings via emerging insects. This ‘boomerang’ carbon flux between land and lakes blurs the distinction between autochthonous and allochthonous carbon sources.     

气候变暖对陆地生态系统碳循环的影响

作为全球变化的主要表现之一,气候变暖对全球陆地生态系统碳循环的影响巨大,揭示这一作用对于精确理解碳循环的过程和相关政策的制定具有重要的指导意义。该文综述了此领域近十几年来的主要研究工作,总结了陆地生态系统碳循环对气候变暖响应的主要内部机制及其过程,简述了相关模型的发展及其主要应用,并指出以往研究中存在的主要问题以及未来研究的主要方向。在气候变暖条件下,陆地生态系统碳循环的变化主要体现在以下几个方面:1)低纬度地区生态系统NPP一般表现为降低,而在中高纬度地区通常表现为增加,而在全球尺度上表现为NPP增加;2)土壤呼吸作用增强,但经过一段时间后表现出一定的适应性;3)高纬度地区的生态系统植被碳库表现为增加趋势,低纬度地区生态系统植被碳库变化不大,或略微降低,在全球尺度上表现为植被碳库增加;4)地表凋落物的产量和分解速率增加;5)土壤有机碳分解加速,进而减少土壤碳储存,同时植被碳库向土壤碳库的流动增加从而增加土壤碳库,这两种作用在不同生态系统的比重不同,在全球尺度上表现为土壤碳库的减少;6)尽管不同生态系统表现各异,总体上全球陆地生态系统表现为一个弱碳源。生物物理模型、生物地理模型和生物地球化学模型陆续被开发出来用于研究工作,并取得了一定的成果,但是研究结果仍然存在很大的不确定性。在未来的数年甚至是数十年间,气候变暖与全球变化的其它表现间的协同影响将是下一步的研究重点,气候变暖和陆地生态系统间的双向反馈作用机制是进行更准确研究的理论基础,生态系统结构和功能对气候变化的适应性是准确理解和预测未来气候情景下陆地生态系统碳循环的前提。     

土地利用/覆盖变化对陆地生态系统碳循环的影响

土地利用/覆盖变化是学术界最为关注的环境变化问题之一,它能够影响陆地生态系统的生物多样性、水、碳和养分循环、能量平衡,引起温室气体释放增加等其它环境问题。不同类型的土地利用/覆盖变化对生态系统碳循环的作用不同,由高生物量的森林转化为低生物量的草地、农田或城市后,大量的CO₂将释放到大气中。全球土地利用/覆盖变化具有很强的空间变异性,对生态系统碳循环的影响同样具有明显的空间差异:热带地区的土地利用/覆盖变化造成大量的碳释放,而中高纬度地区土地利用/覆盖变化则表现为碳汇。目前,土地利用/覆盖变化引起的生态系统碳循环变化主要是通过模型模拟来估算的。尽管土地利用/覆盖变化及其相关过程与生态系统碳循环的关系已经比较清楚,但是,由于土地利用/覆盖变化过程复杂且影响广泛,对于如何量化两者之间的关系还存在很多不确定性。目前的量化过程主要是利用经验数据来实现的,机理性不强,使得对土地利用/覆盖变化造成的陆地生态系统CO₂释放量的估测差异很大。除了进一步加强长期定位研究以获得土地利用/覆盖变化与生态系统碳循环过程的定量关系外,土地利用/覆盖变化模型与植被动态模型、生态系统过程模型的耦合也是今后模型发展的主要方向之一。采用合理的管理措施能够大量增加土地利用/覆盖变化过程中的碳储存量,降低碳释放量,因此在模型中耦合管理措施来研究土地利用/覆盖变化过程对生态系统碳循环的影响是未来几年的工作重点。     

Altered Composition and Microbial versus UV-Mediated Degradation of Dissolved Organic Matter in Boreal Soils Following Wildfire

Production, transport, and degradation of terrestrial dissolved organic matter (DOM) influence carbon (C) and nutrient cycling in both soils and downstream aquatic ecosystems. Here, we assessed the impacts of wildfire on DOM production, composition, and reactivity (biodegradation versus UV degradation) from soils of upland forest and peatland ecosystems. Soil C solubility was lowest for upland char samples, highest from surface soils in unburned spruce stands and decreased with a higher degree of peat humification regardless of fire history. Soil nitrogen (N) became relatively more soluble in both upland and peat soils post-fire, as leachate C/N decreased. Biodegradability was lower for DOM leachates from burned than unburned soils, both in upland and peatland sites. Several DOM composition indices were related to biodegradability; with the strongest relationship for specific UV absorbance at 254 nm (indicator of aromaticity). Parallel factor analysis revealed distinctive characteristics of leachates from burned soils and char that were related to low biodegradability and high UV-mediated losses. Relative to dark incubations, incubation under UV conditions led to greater C losses for highly aromatic leachates, but reduced losses for leachates with low aromaticity. This suggests that UV-mediated degradation could provide a pathway for highly stable terrestrial organic matter, including char, to become rapidly mineralized and released to the atmosphere once it reaches aquatic ecosystems in dissolved form. Together our results demonstrate that wildfire can potentially alter both turnover of DOM in terrestrial soils and linkages between terrestrial and aquatic C cycling through its influence on terrestrial DOM production and composition.     

THE IMPORTANCE OF INTEGRATING ALGAL SYSTEMS INTO A BALANCE MANAGEMENT APPROACH TO AQUACULTURE

McVey, J. P. Program Director, National Sea Grant College Program, 1315 East West Highway, Silver Spring, Maryland, 20910, USA The National Oceanic and Atmospheric Administration (NOAA), through its National Marine Fisheries Service (NMFS) and National Sea Grant College Program (NSGCP) has developed a vision for how seafood will be produced in the USA which includes the proper management of natural fisheries, aquaculture and the management and involvement by human coastal communities. The concept of balanced ecosystem management is not only being talked about at national levels but active research programs are being planned and supported. The recent $5 million National Marine Aquaculture Initiative (NMAI) specifically called for proposals that study the “trophic level consequences of marine aquaculture and marine species enhancement”. Recent workshops at the World Aquaculture Society meeting in Orlando and at the regional aquaculture meeting held at Boston on the topic of “Aquaculture and the Environment” have focused on a balanced approach to both aquaculture and fisheries management. All of the workshops focused on the important role of plants in the aquatic community. The basic premise about a balanced ecosystem approach is to incorporate the biological functions of a diverse group of plants and animals into a unified system that maintains the natural interactions of species and allows an ecosystem to function. Models are useful in understanding the energy and nutrient flow within an ecosystem; as are GIS technologies that allow us to map biological and ecological regimes. Macroalgae and phytoplankton both convert nutrients to plant material and transform carbon dioxide to oxygen. In contrast, animals derive much of their nutrition from plants, in one way or another and transform oxygen to carbon dioxide. This presentation will discuss the need to incorporate the use of plants in ecosystem maintenance such that there is balance between the animal, including humans, and plant communities in coastal areas. This will all be related to new NOAA programs and funding opportunities for research support in this area.     

Revisiting the ‘bank of microscopic forms’ in macroalgal‐dominated ecosystems

Theoretical ecological models, such as succession and facilitation, were defined in terrestrial habitats, and subsequently applied to marine and freshwater habitats in intertidal and then subtidal realms. One such model is the soil seed bank, defined as all viable seeds (or fruits) found near the soil surface that facilitate community restoration/recovery. “Banks of microscopic forms” have been hypothesized in aquatic habitats and recent work from aquaculture has highlighted dormancy in algal life cycle stages. To reinvigorate the discussions about these algal banks, we discuss differences in life cycles, dispersal, and summarize research on banks of macroalgal stages in aquatic ecosystems that may be easier to explore with modern advances in molecular technology. With focus on seminal work in global kelp forest ecosystems, we present a pilot study in northern California as proof of concept that Nereocystis luetkeana and Alaria marginata stages can be detected within kelp forests in the biofilm of rocks and bedrock using targeted primers long after zoospore release. Considering the increased interest in algae as an economic resource, [blue] carbon sink, and as ecosystem engineers, the potential for “banking” macroalgal forms could be a mechanism of resilience and recovery in aquatic populations that have complex life cycles and environmental cues for reproduction. Molecular barcoding is becoming an important tool for identifying banks of macroalgal forms in marine communities. Understanding banks of macroalgal stages, especially in deforested habitats with intense disturbance and grazer pressure, will allow researchers and marine resource managers to facilitate this natural process in recovery of the aquatic system.     

陆地生态系统地下碳输入与输出过程研究进展

生态系统地下碳输入与输出过程是陆地生态系统碳分配和转化的核心,并直接影响着全球碳循环。陆地生态系统凋落物、根系周转、根系分泌物、土壤有机碳、土壤微生物和土壤呼吸是地下碳输入与输出过程中的重要组成部分。由于这些组分非常复杂且其研究技术和方法受到限制,目前人们对陆地生态系统地下碳输入与输出过程尚缺乏全面的认识,故在陆地生态系统碳循环研究中存在诸多的不确定性。该文概述了凋落物、根系周转、根系分泌物、土壤有机碳、土壤微生物和土壤呼吸的研究方法,以及它们对气候变化的响应,探讨了陆地生态系统地下碳输入与输出过程中的研究难点,并对未来需要深入探究的一些领域进行了展望。     

陆地生态系统类型转变与碳循环

 土地利用变化引起的陆地生态系统类型转变对于全球碳循环有着极其重要的作用。　通过总结国内外有关森林砍伐以及森林、草地转变成农田对于碳循环的影响,阐述了可能引起全球“未知汇”现象的重要原因,强调未来中国陆地生态系统碳循环研究应充分重视陆地生态系统类型转变对于全球碳循环的影响研究,包括研究陆地生态系统的不同发展阶段(自然与退化生态系统)、利用方式的改变(森林转化为人工林或农田,草地转化为农田、退耕还林草等)所引起的碳库类型转换的增汇机理及其对全球变化响应,并指出了建立统一观测方法与规范的陆地生态系统碳通量观测网     

长白山二道白河森林流域溪流倒木调查研究

溪流倒木是森林生态系统对水生态系统最重要、最直观的输入和干扰之一,也是两系统之间的主要联结,对于溪流生态系统的稳定、水生生物多样性、河槽形态及其变化过程有着重要的作用。重点对长白山北坡溪流倒木现存量进行了调查和研究,在调查的红松阔叶林植被带内４５００ｍ长河道内,共发现溪流倒木４２５株．分属于１７个树种;其中ｌ、ｗ级腐烂占相当大的比重,与林地倒木Ｉ、ｌ级腐烂占忧有所不同,其原因可能与分解环境的不同有关。所有溪流倒木的总材积为７７．９８ｍ＾２,故溪流倒木的现存量为１．７３３ｍ＾３／１００ｍ和１０．８３ｍ＾３／ｈｍ＾２。溪流倒木的树种组成和不同树种的材积与河岸带植被密切相关,但存在差异。研究表明林分形成倒木并进入河流在时间上可能是均匀或随机的,但不同树种间,其形成倒木并进入河流时的树木材积或生长年龄存在较大差异。溪流倒木和林地活立木的个体数量的径级分布基本上为反Ｊ型,而它们材积的径级分布均为典型的Ｊ型。     

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.     

Toxicological Considerations of Contaminants in the Terrestrial Environment for Ecological Risk Assessment

The terrestrial environment acts as a “sink” for contaminants that have been purposely or accidentally released into the environment. Science and policy that support protective measures for terrestrial ecosystems have run behind those of aquatic toxicology and water quality concerns. As a result ecological risk assessment (ERA) involving terrestrial environments tends to be conducted at a simplistic level, relying on numeric targets (soil quality criteria) as a basis for decision-making. However, soil criteria for ecological receptors are somewhat deficient in terms of the numbers available and the data that supports these numbers. Direct toxicity assessments (DTA) for terrestrial environments, such as those used for water quality evaluations, can provide additional useful information about the toxicity and bioavailability of mixtures of contaminants present in soils. This article outlines the approaches used for assessing the toxicity of soil contaminants in terrestrial environments and critiques their advantages and pitfalls.     

Challenges to incorporating spatially and temporally explicit phenomena (hotspots and hot moments) in denitrification models

Denitrification, the anaerobic reduction of nitrogen oxides to nitrogenous gases, is an extremely challenging process to measure and model. Much of this challenge arises from the fact that small areas (hotspots) and brief periods (hot moments) frequently account for a high percentage of the denitrification activity that occurs in both terrestrial and aquatic ecosystems. In this paper, we describe the prospects for incorporating hotspot and hot moment phenomena into denitrification models in terrestrial soils, the interface between terrestrial and aquatic ecosystems, and in aquatic ecosystems. Our analysis suggests that while our data needs are strongest for hot moments, the greatest modeling challenges are for hotspots. Given the increasing availability of high temporal frequency climate data, models are promising tools for evaluating the importance of hot moments such as freeze-thaw cycles and drying/rewetting events. Spatial hotspots are less tractable due to our inability to get high resolution spatial approximations of denitrification drivers such as carbon substrate. Investigators need to consider the types of hotspots and hot moments that might be occurring at small, medium, and large spatial scales in the particular ecosystem type they are working in before starting a study or developing a new model. New experimental design and heterogeneity quantification tools can then be applied from the outset and will result in better quantification and more robust and widely applicable denitrification models.     

Beaver activity increases aquatic subsidies to terrestrial consumers

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Weak Response of Animal Allochthony and Production to Enhanced Supply of Terrestrial Leaf Litter in Nutrient-Rich Lakes

Ecosystems are generally linked via fluxes of nutrients and energy across their boundaries. For example, freshwater ecosystems in temperate regions may receive significant inputs of terrestrially derived carbon via autumnal leaf litter. This terrestrial particulate organic carbon (POC) is hypothesized to subsidize animal production in lakes, but direct evidence is still lacking. We divided two small eutrophic lakes each into two sections and added isotopically distinct maize litter to the treatment sections to simulate increased terrestrial POC inputs via leaf litter in autumn. We quantified the reliance of aquatic consumers on terrestrial resources (allochthony) in the year subsequent to POC additions by applying mixing models of stable isotopes. We also estimated lake-wide carbon (C) balances to calculate the C flow to the production of the major aquatic consumer groups: benthic macroinvertebrates, crustacean zooplankton, and fish. The sum of secondary production of crustaceans and benthic macroinvertebrates supported by terrestrial POC was higher in the treatment sections of both lakes. In contrast, total secondary and tertiary production (supported by both autochthonous and allochthonous C) was higher in the reference than in the treatment sections of both lakes. Average aquatic consumer allochthony per lake section was 27–40%, although terrestrial POC contributed less than about 10% to total organic C supply to the lakes. The production of aquatic consumers incorporated less than 5% of the total organic C supply in both lakes, indicating a low ecological efficiency. We suggest that the consumption of terrestrial POC by aquatic consumers facilitates a strong coupling with the terrestrial environment. However, the high autochthonous production and the large pool of autochthonous detritus in these nutrient-rich lakes make terrestrial POC quantitatively unimportant for the C flows within food webs.     

Role of first-order consumers in ecosystem carbon flow

Herbivores and detritus consumers (i.e. microbial decomposers and invertebrate and vertebrate detritivores) are pivotal components of trophic food webs and thus play a paramount role in the trophic transference and turnover of producer‐fixed carbon. Hence, elucidating patterns in carbon flux through these first‐order consumers is important to understand the nature and controls of carbon flow in ecosystems. Here, using the largest literature compilation to date, I show that, in contrast with the current belief, aquatic herbivores accumulate on average three times as much biomass as do terrestrial herbivores for a given level of primary production and, as a consequence, turn over the ingested carbon only slightly faster than do terrestrial herbivores. Conversely, aquatic detritus consumers generally accumulate a much lower biomass (i.e. over ten times lower) than their terrestrial counterparts for a given level of primary production and, thus, they turn over the ingested carbon much more quickly (i.e. over ten times faster). Because the detrital pathway generally dominates the trophic flow of carbon in both aquatic and terrestrial ecosystems, carbon also tends to flow through the total compartment of first order‐consumers (both herbivores and detritus consumers) at a much faster rate in aquatic than in terrestrial ecosystems. Thus, aquatic systems, because of faster carbon recycling rates through their basal and first‐order levels of the food chain, appear to have a lower capacity than do terrestrial systems for retaining carbon under natural or anthropogenic increases in photosynthetic fixation.     

From Transient to Steady-state Response of Ecosystems to Atmospheric CO2-enrichment and Global Climate Change: Conceptual Challenges and Need for an Integrated Approach

Evidence continues to accumulate that humans are significantly increasing atmospheric CO₂ concentrations, resulting in unprecedented changes in the global climate system. Experimental manipulations of terrestrial ecosystems and their components have greatly increased our understanding of short-term responses to these global perturbations and have provided valuable input to ecosystem, dynamic vegetation, and global scale models. However, concerns exist that these initial experimental responses may be transitory, thereby limiting our ability to extrapolate short-term experimental responses to infer longer-term effects. To do these extrapolations, it will be necessary to understand changes in response patterns over time, including alterations in the magnitude, direction, and rate of change of the responses. These issues represent one of our largest challenges in accurately predicting longer-term changes in ecosystems and associated feedbacks to the climate system. Key issues that need to be considered when designing future experiments or refining models include: linear vs. non-linear responses, direct vs. indirect effects, lags in response, acclimation, resource limitation, homeostasis, buffers, thresholds, ecosystem stoichiometry, turnover rates and times, and alterations in species composition. Although experimental and landscape evidence for these response patterns exist, extrapolating longer-term response patterns from short-term experiments will ultimately require a unified multidisciplinary approach, including better communication and collaboration between theoreticists, experimentalists and modelers.     

Dissolved Organic Carbon in Terrestrial Ecosystems: Synthesis and a Model

The movement of dissolved organic carbon (DOC) through soils is an important process for the transport of carbon within ecosystems and the formation of soil organic matter. In some cases, DOC fluxes may also contribute to the carbon balance of terrestrial ecosystems; in most ecosystems, they are an important source of energy, carbon, and nutrient transfers from terrestrial to aquatic ecosystems. Despite their importance for terrestrial and aquatic biogeochemistry, these fluxes are rarely represented in conceptual or numerical models of terrestrial biogeochemistry. In part, this is due to the lack of a comprehensive understanding of the suite of processes that control DOC dynamics in soils. In this article, we synthesize information on the geochemical and biological factors that control DOC fluxes through soils. We focus on conceptual issues and quantitative evaluations of key process rates to present a general numerical model of DOC dynamics. We then test the sensitivity of the model to variation in the controlling parameters to highlight both the significance of DOC fluxes to terrestrial carbon processes and the key uncertainties that require additional experiments and data. Simulation model results indicate the importance of representing both root carbon inputs and soluble carbon fluxes to predict the quantity and distribution of soil carbon in soil layers. For a test case in a temperate forest, DOC contributed 25% of the total soil profile carbon, whereas roots provided the remainder. The analysis also shows that physical factors—most notably, sorption dynamics and hydrology—play the dominant role in regulating DOC losses from terrestrial ecosystems but that interactions between hydrology and microbial–DOC relationships are important in regulating the fluxes of DOC in the litter and surface soil horizons. The model also indicates that DOC fluxes to deeper soil layers can support a large fraction (up to 30%) of microbial activity below 40 cm. Received 14 January 2000; accepted 6 September 2000