Seasonal and within‐canopy variation in shoot‐scale resource‐use efficiency trade‐offs in a Norway spruce stand

Previous leaf‐scale studies of carbon assimilation describe short‐term resource‐use efficiency (RUE) trade‐offs where high use efficiency of one resource requires low RUE of another. However, varying resource availabilities may cause long‐term RUE trade‐offs to differ from the short‐term patterns. This may have important implications for understanding canopy‐scale resource use and allocation. We used continuous gas exchange measurements collected at five levels within a Norway spruce, Picea abies (L.) karst., canopy over 3 years to assess seasonal differences in the interactions between shoot‐scale resource availability (light, water and nitrogen), net photosynthesis (A_n) and the use efficiencies of light (LUE), water (WUE) and nitrogen (NUE) for carbon assimilation. The continuous data set was used to develop and evaluate multiple regression models for predicting monthly shoot‐scale A_n. These models showed that shoot‐scale A_n was strongly dependent on light availability and was generally well described with simple one‐ or two‐parameter models. WUE peaked in spring, NUE in summer and LUE in autumn. However, the relative importance of LUE for carbon assimilation increased with canopy depth at all times. Our results suggest that accounting for seasonal and within‐canopy trade‐offs may be important for RUE‐based modelling of canopy carbon uptake.     

Feedback of carbon and nitrogen cycles enhances carbon sequestration in the terrestrial biosphere

The efforts to explain the ‘missing sink’ for anthropogenic carbon dioxide (CO₂) have included in recent years the role of nitrogen as an important constraint for biospheric carbon fluxes. We used the Nitrogen Carbon Interaction Model (NCIM) to investigate patterns of carbon and nitrogen storage in different compartments of the terrestrial biosphere as a consequence of a rising atmospheric CO₂ concentration, in combination with varying levels of nitrogen availability. This model has separate but closely coupled carbon and nitrogen cycles with a focus on soil processes and soil–plant interactions, including an active compartment of soil microorganisms decomposing litter residues and competing with plants for available nitrogen. Biological nitrogen fixation is represented as a function of vegetation nitrogen demand. The model was validated against several global datasets of soil and vegetation carbon and nitrogen pools. Five model experiments were carried out for the modeling periods 1860–2002 and 2002–2100. In these experiments we varied the nitrogen availability using different combinations of biological nitrogen fixation, denitrification, leaching of soluble nitrogen compounds with constant or rising atmospheric CO₂ concentrations. Oversupply with nitrogen, in an experiment with nitrogen fixation, but no nitrogen losses, together with constant atmospheric CO₂, led to some carbon sequestration in organismic pools, which was nearly compensated by losses of C from soil organic carbon pools. Rising atmospheric CO₂ always led to carbon sequestration in the biosphere. Considering an open nitrogen cycle including dynamic nitrogen fixation, and nitrogen losses from denitrification and leaching, the carbon sequestration in the biosphere is of a magnitude comparable to current observation based estimates of the ‘missing sink.’ A fertilization feedback between the carbon and nitrogen cycles occurred in this experiment, which was much stronger than the sum of separate influences of high nitrogen supply and rising atmospheric CO₂. The demand‐driven biological nitrogen fixation was mainly responsible for this result. For the modeling period 2002–2100, NCIM predicts continued carbon sequestration in the low range of previously published estimates, combined with a plausible rate of CO₂‐driven biological nitrogen fixation and substantial redistribution of nitrogen from soil to plant pools.     

植物与微生物碳利用效率及影响因子研究进展

陆地生态系统碳循环是生物地球化学循环的关键过程之一。碳利用效率（carbon use efficiency,CUE）是描述生物用于形成生物量的碳占其所吸收总碳比例的一个定量指标,反映了生物的碳同化能力和固碳潜力,是研究生态系统碳循环中碳通量和碳分配模式的重要参数,能有效预测生物与周围环境之间的碳流通和碳反馈。目前,关于CUE的研究还不充分,尤其是对CUE及其影响因子的系统性综合论述还较少。为此,本文综述了国内外有关碳利用效率（植物碳利用效率（CUE_a）和微生物碳利用效率（CUE_h））的研究方法和研究进展,分析了CUE_a和CUE_h的异同、内在联系及作用机理。基于分析对今后的研究提出几点展望：（1）优化测量手段和计算方法,适当地调整参数,将模型方法与实测数据结合,使CUE的定量描述结果更准确;（2）结合不同尺度的研究结果,探究个体、种群、群落、生态系统等不同空间尺度和时间尺度上CUE的联系及变化规律,为碳循环和碳流通的时空变化规律提供新证据;（3）研究CUE对全球变化（如高温、干旱、CO₂浓度增加等）的响应,探讨CUE对未来气候情景的响应和适应机制;（4）开展有关物种丰富度或生物多样性的梯度变化对CUE的影响研究,阐释物种多样性减少或物种灭绝等现象对碳循环过程的影响,将生态系统物种多样性与生态系统功能相联系;（5）加强对CUE_h的研究,定量探究其与CUE_a的异同,并将二者结合起来,更全面地解释地上-地下生态系统碳的分配特征。同时适当开展动物CUE的研究,目前该类研究还缺乏系统性。     

Microbial carbon use efficiency in different ecosystems: A meta-analysis based on a biogeochemical equilibrium model

Soil microbial carbon use efficiency (CUE) is a crucial parameter that can be used to evaluate the partitioning of soil carbon (C) between microbial growth and respiration. However, general patterns of microbial CUE among terrestrial ecosystems (e.g., farmland, grassland, and forest) remain controversial. To address this knowledge gap, data from 41 study sites (n = 197 soil samples) including 58 farmlands, 95 forests, and 44 grasslands were collected and analyzed to estimate microbial CUEs using a biogeochemical equilibrium model. We also evaluated the metabolic limitations of microbial growth using an enzyme vector model and the drivers of CUE across different ecosystems. The CUEs obtained from soils of farmland, forest, and grassland ecosystems were significantly different with means of 0.39, 0.33, and 0.42, respectively, illustrating that grassland soils exhibited higher microbial C sequestration potentials (p < .05). Microbial metabolic limitations were also distinct in these ecosystems, and carbon limitation was dominant exhibiting strong negative effects on CUE. Exoenzyme stoichiometry played a greater role in impacting CUE values than soil elemental stoichiometry within each ecosystem. Specifically, soil exoenzymatic ratios of C:phosphorus (P) acquisition activities (EEA_C:P) and the exoenzymatic ratio of C:nitrogen (N) acquisition activities (EEA_C:N) imparted strong negative effects on soil microbial CUE in grassland and forest ecosystems, respectively. But in farmland soils, EEA_C:P exhibited greater positive effects, showing that resource constraints could regulate microbial resource allocation with discriminating patterns across terrestrial ecosystems. Furthermore, mean annual temperature (MAT) rather than mean annual precipitation (MAP) was a critical climate factor affecting CUE, and soil pH as a major factor remained positive to drive the changes in microbial CUE within ecosystems. This research illustrates a conceptual framework of microbial CUEs in terrestrial ecosystems and provides the theoretical evidence to improve soil microbial C sequestration capacity in response to global change.     

土壤微生物碳素利用效率研究进展

土壤微生物碳素利用效率(CUE)是指微生物将吸收的碳(C)转化为自身生物量C的效率,也称为微生物的生长效率。土壤微生物CUE是生态系统C循环中的重要生理生态学参数,影响着生态系统的C固持、周转、土壤矿化以及温室气体排放等过程。在全球环境变化背景下,认识土壤微生物CUE的变异及其影响机制,对于更好的认识生态系统C循环过程及其对全球变化的响应具有重要意义。概述了CUE的定义及其测定方法,重点综述和分析土壤微生物CUE的变异及影响因素取得的研究进展。基于现有研究的分析得出,土壤微生物CUE通常表示为微生物的生长与吸收的比值,分为基于微生物生长速率、微生物生物量、底物吸收速率和底物浓度变化等方法进行测定。土壤微生物CUE在0.2—0.8的范围内变化,这种变异主要受到来自热力学、生态环境因子、底物养分质量和有效性、化学计量平衡以及微生物群落组成的影响。今后土壤微生物CUE的研究应加强对微量代谢组分的定量分析,生物和环境要素交互影响的调控机理解析,以及微生物动态生理响应过程的碳循环模型优化。     

植被类型变化对土壤微生物碳利用效率的影响研究进展

植被类型变化强烈影响着土壤碳循环。土壤微生物碳利用效率（CUE）是微生物将从环境中获取的碳分配给自身生长的比例,是土壤碳循环的综合指标。研究植被类型变化对CUE的影响有助于从微生物视角理解该过程中的土壤碳动态,可以为评估植被类型变化对土壤质量及生态系统碳循环的影响提供基础,具有重要的理论及实际价值。通过系统查阅相关文献,综述了植被类型变化导致的CUE变化情况,以及该过程中影响CUE的因子与机制。目前,相关研究主要涉及以林地、草地和农业用地为起点或终点的植被变化类型。天然林（原生林、次生林）变化为人工林、林地变化为草地后CUE普遍下降,随终点植被的发展CUE可能恢复至起点水平。植被成熟度越高,发生转变时CUE变化越剧烈。植被类型变化以农业用地为起点或终点时,CUE变化方向的不确定性及幅度的变异性均增加。植被类型变化导致的CUE变化主要受到植被、土壤、微生物因子及其交互作用的驱动,指示CUE的指标、采样季节和土层也会一定程度上影响CUE的变化。今后相关研究应采用直接的CUE测定方法,拓宽研究气候区及植被变化类型,关注植被变化过程中CUE变化的土层差异及动态监测,深入对植被类型变化导致的生态环境因子变化与CUE的关系及作用机制的研究。     

Rhizosphere processes are quantitatively important components of terrestrial carbon and nutrient cycles

While there is an emerging view that roots and their associated microbes actively alter resource availability and soil organic matter (SOM) decomposition, the ecosystem consequences of such rhizosphere effects have rarely been quantified. Using a meta‐analysis, we show that multiple indices of microbially mediated C and nitrogen (N) cycling, including SOM decomposition, are significantly enhanced in the rhizospheres of diverse vegetation types. Then, using a numerical model that combines rhizosphere effect sizes with fine root morphology and depth distributions, we show that root‐accelerated mineralization and priming can account for up to one‐third of the total C and N mineralized in temperate forest soils. Finally, using a stoichiometrically constrained microbial decomposition model, we show that these effects can be induced by relatively modest fluxes of root‐derived C, on the order of 4% and 6% of gross and net primary production, respectively. Collectively, our results indicate that rhizosphere processes are a widespread, quantitatively important driver of SOM decomposition and nutrient release at the ecosystem scale, with potential consequences for global C stocks and vegetation feedbacks to climate.     

微生物介导的碳氮循环过程对全球气候变化的响应

土壤是地球表层最为重要的碳库也是温室气体的源或汇。自工业革命以来,对土壤温室气体的容量、收支平衡和通量等已有较多研究和估算,但对关键过程及其源/汇的研究却十分有限。微生物是土壤碳氮转化的主要驱动者, 在生态系统碳氮循环过程中扮演重要的角色,对全球气候变化有着响应的响应、适应及反馈,然而其个体数量,群落结构和多样性如何与气候扰动相互关联、进而怎样影响生态系统过程的问题仍有待进一步探索。从微生物介导的碳氮循环过程入手,重点讨论微生物对气候变化包括温室气体(CO₂,CH₄,N₂O)增加、全球变暖、大气氮沉降等的响应和反馈,并由此提出削减温室气体排放的可能途径和今后发展的方向。     

氮添加土壤微生物群落结构影响微生物碳利用效率

尽管近年来中国氮(N)沉降水平逐渐趋于稳定,但中国东南地区N沉降相比于其他地区仍处于较高水平。N沉降对陆地生态系统碳循环过程的影响不容忽视。微生物碳利用效率(CUE)是指微生物将吸收的碳转化为生物量碳的效率,高微生物CUE意味着高土壤有机碳存储潜力。因此,探究N沉降背景下微生物CUE的变化将有助于进一步认识陆地生态系统土壤碳存储的变化。然而,目前关于N沉降下微生物群落结构的变化如何影响微生物CUE鲜有报道。在福建省泉州市戴云山国家级自然保护区的罗浮栲林通过N添加模拟N沉降。实验共包括三个N添加处理：对照(CT,+0 kg hm^-2 a^-1)、低氮(LN,+40 kg hm^-2 a^-1)和高氮(HN,+80 kg hm^-2 a^-1)。测定不同处理土壤基本理化性质、微生物生物量、酶活性和CUE,并使用高通量测序对微生物群落结构和多样性进行测定。结果表明,N添加显著影响微生物CUE,随着N添加水平的增加,CUE逐渐增加;相反,土壤pH、可提取有机碳(EOC)和微...     

CO2 emissions from land‐use change affected more by nitrogen cycle,than by the choice of land‐cover data

The high uncertainty in land‐based CO₂ fluxes estimates is thought to be mainly due to uncertainty in not only quantifying historical changes among forests, croplands, and grassland, but also due to different processes included in calculation methods. Inclusion of a nitrogen (N) cycle in models is fairly recent and strongly affects carbon (C) fluxes. In this study, for the first time, we use a model with C and N dynamics with three distinct historical reconstructions of land‐use and land‐use change (LULUC) to quantify LULUC emissions and uncertainty that includes the integrated effects of not only climate and CO₂ but also N. The modeled global average emissions including N dynamics for the 1980s, 1990s, and 2000–2005 were 1.8 ± 0.2, 1.7 ± 0.2, and 1.4 ± 0.2 GtC yr^?1, respectively, (mean and range across LULUC data sets). The emissions from tropics were 0.8 ± 0.2, 0.8 ± 0.2, and 0.7 ± 0.3 GtC yr^?1, and the non tropics were 1.1 ± 0.5, 0.9 ± 0.2, and 0.7 ± 0.1 GtC yr^?1. Compared to previous studies that did not include N dynamics, modeled net LULUC emissions were higher, particularly in the non tropics. In the model, N limitation reduces regrowth rates of vegetation in temperate areas resulting in higher net emissions. Our results indicate that exclusion of N dynamics leads to an underestimation of LULUC emissions by around 70% in the non tropics, 10% in the tropics, and 40% globally in the 1990s. The differences due to inclusion/exclusion of the N cycle of 0.1 GtC yr^?1 in the tropics, 0.6 GtC yr^?1 in the non tropics, and 0.7 GtC yr^?1 globally (mean across land‐cover data sets) in the 1990s were greater than differences due to the land‐cover data in the non tropics and globally (0.2 GtC yr^?1). While land‐cover information is improving with satellite and inventory data, this study indicates the importance of accounting for different processes, in particular the N cycle.     

Grain yield and kernel weight of two maize genotypes differing in nitrogen use efficiency at various levels of nitrogen and carbohydrate availability during flowering and grain filling

Grain yield per plant (GYP) and mean kernel weight (KW) of maize (Zea mays L.) are sensitive to changes in the environment during the lag phase of kernel growth (the time after pollination in which the potential kernel size is determined), and during the phase of linear kernel growth. The aim of this study was to assess genotypic differences in the response to environmental stresses associated with N and/or carbohydrate shortage at different phases during plant development. The rate and timing of N and carbohydrate supply were modified by application of fertilizer, shading, and varying the plant density at sowing, at silking or at 14 d after silking. The effects of these treatments on the photosynthetic capacity, grain yield and mean kernel weight were investigated in two hybrids differing in N use efficiency. The total above-ground biomass and grain yield per plant of the efficient hybrid responded little to altered environmental conditions such as suboptimal N supply, enhanced inter-plant competition, and shading for 14 d during flowering, when compared to the less efficient genotype. We conclude that grain yields in the efficient genotype are less sensitive not only to N stress, but also to carbohydrate shortage before grain filling. Shading of N deficient plants from 14 d after silking to maturity did not significantly reduce grain yield in the non-efficient genotype, indicating complete sink limitation of grain yield during grain filling. In the efficient genotype, in contrast, grain yield of N-deficient plants was significantly reduced by shading during grain filling. The rate of photosynthesis declined with decreasing foliar N content. No genotypic differences in photosynthesis were observed at high or low foliar N contents. However, at high plant density and low N supply, the leaf chlorophyll content after flowering in the efficient genotype was higher than that in the non-efficient genotype. Obviously, the higher source capacity of the efficient genotype was not due to higher photosynthetic N use efficiency but due to maintenance of high chlorophyll contents under stressful conditions. In the efficient genotype, the harvest index was not significantly affected by N fertilization, plant density, or shading before the grain filling period. In contrast, in the non-efficient genotype the harvest index was diminished by N deficiency and shading during flowering. We conclude that the high yielding ability of the efficient genotype under stressful conditions was associated with formation of a high sink capacity of the grains under conditions of low carbohydrate and N availability during flowering and with maintenance of high source strength during grain filling under conditions of high plant density and low N availability.     

Gas exchange and water‐use efficiency in plant canopies

In this review, I first address the basics of gas exchange, water‐use efficiency and carbon isotope discrimination in C₃ plant canopies. I then present a case study of water‐use efficiency in northern Australian tree species. In general, C₃ plants face a trade‐off whereby increasing stomatal conductance for a given set of conditions will result in a higher CO₂ assimilation rate, but a lower photosynthetic water‐use efficiency. A common garden experiment suggested that tree species which are able to establish and grow in drier parts of northern Australia have a capacity to use water rapidly when it is available through high stomatal conductance, but that they do so at the expense of low water‐use efficiency. This may explain why community‐level carbon isotope discrimination does not decrease as steeply with decreasing rainfall on the North Australian Tropical Transect as has been observed on some other precipitation gradients. Next, I discuss changes in water‐use efficiency that take place during leaf expansion in C₃ plant leaves. Leaf phenology has recently been recognised as a significant driver of canopy gas exchange in evergreen forest canopies, and leaf expansion involves changes in both photosynthetic capacity and water‐use efficiency. Following this, I discuss the role of woody tissue respiration in canopy gas exchange and how photosynthetic refixation of respired CO₂ can increase whole‐plant water‐use efficiency. Finally, I discuss the role of water‐use efficiency in driving terrestrial plant responses to global change, especially the rising concentration of atmospheric CO₂. In coming decades, increases in plant water‐use efficiency caused by rising CO₂ are likely to partially mitigate impacts on plants of drought stress caused by global warming.     

氮输入对东北土壤碳蓄积氮素利用效率的影响

由于人类活动影响,通过沉降和施肥方式进入生态系统的活性氮显著增加,其对土壤有机碳库产生重要影响。氮素利用效率(NUE)作为深入理解陆地生态系统碳氮耦合关系的重要参数,对NUE时空规律的研究不仅可以评估目前氮输入对陆地生态系统碳汇增加的贡献,同时也有助于预测未来氮输入情况下陆地生态系统的碳平衡。利用生态系统过程模型——CEVSA2模型的模拟结果,分析了东北地区氮输入情况下,土壤碳的氮素利用效率(SNUE)的时空变化规律及其影响因素,结果表明:(1)1961—2010年,氮输入的显著增加促进了土壤碳的蓄积,但SNUE显著下降;(2)森林的平均SNUE最高,农田最低;灌丛的下降速率最大,森林的SNUE变化趋势最不显著;(3)三江平原和长白山地区以及大小兴安岭的部分地区SNUE最大,其次是辽河平原、松嫩平原地区;内蒙古高原、呼伦贝尔高原地区以及大、小兴安岭的部分地区SNUE出现负值,说明在这些地区,外援氮输入抑制了土壤碳的蓄积;(4)氮输入的空间分异和不同生态系统响应氮输入的差异共同决定了SNUE及其变化的空间格局。该研究结果可为进一步分析不同区域氮促汇潜力和预测未来氮输入情景下的区域碳平衡提供参考。     

Cereal area and nitrogen use efficiency are drivers of future nitrogen fertilizer consumption

At a global scale, cereal yields and fertilizer N consumption have increased in a near-linear fashion during the past 40 years and are highly correlated with one another. However,large differences exist in historical trends of N fertilizer usage and nitrogen use efficiency (NUE)among regions, countries, and crops. The reasons for these differences must be understood to estimate future N fertilizer requirements. Global nitrogen needs will depend on: (i) changes in cropped cereal area and the associated yield increases required to meet increasing cereal demand from population and income growth, and (ii) changes in NUE at the farm level. Our analysis indicates that the anticipated 38% increase in global cereal demand by 2025 can be met by a 30% increase in N use on cereals, provided that the steady decline in cereal harvest area is halted and the yield response to applied N can be increased by 20%. If losses of cereal cropping area continue at the rate of the past 20 years (-0.33% per year) and NUE cannot be increased substantially, a 60% increase in global N use on cereals would be required to meet cereal demand. Interventions to increase NUE and reduce N losses to the environment must be accomplished at the farm- or field-scale through a combination of improved technologies and carefully crafted local policies that contribute to the adoption of improved N management; uniform regional or national directives are unlikey to be effective at both sustaining yield increases and improving NUE. Examples from several countries show that increases in NUE at rates of 1% per year or more can be achieved if adequate investments are made in research and extension. Failure to arrest the decrease in cereal crop area and to improve NUE in the world's most important agricultural systems will likely cause severe damage to environmental services at local, regional, and global scales due to a large increase in reactive N load in the environment.     

The sensitivity of soil respiration to soil temperature,moisture, and carbon supply at the global scale

Soil respiration (Rs) is a major pathway by which fixed carbon in the biosphere is returned to the atmosphere, yet there are limits to our ability to predict respiration rates using environmental drivers at the global scale. While temperature, moisture, carbon supply, and other site characteristics are known to regulate soil respiration rates at plot scales within certain biomes, quantitative frameworks for evaluating the relative importance of these factors across different biomes and at the global scale require tests of the relationships between field estimates and global climatic data. This study evaluates the factors driving Rs at the global scale by linking global datasets of soil moisture, soil temperature, primary productivity, and soil carbon estimates with observations of annual Rs from the Global Soil Respiration Database (SRDB). We find that calibrating models with parabolic soil moisture functions can improve predictive power over similar models with asymptotic functions of mean annual precipitation. Soil temperature is comparable with previously reported air temperature observations used in predicting Rs and is the dominant driver of Rs in global models; however, within certain biomes soil moisture and soil carbon emerge as dominant predictors of Rs. We identify regions where typical temperature‐driven responses are further mediated by soil moisture, precipitation, and carbon supply and regions in which environmental controls on high Rs values are difficult to ascertain due to limited field data. Because soil moisture integrates temperature and precipitation dynamics, it can more directly constrain the heterotrophic component of Rs, but global‐scale models tend to smooth its spatial heterogeneity by aggregating factors that increase moisture variability within and across biomes. We compare statistical and mechanistic models that provide independent estimates of global Rs ranging from 83 to 108 Pg yr^?1, but also highlight regions of uncertainty where more observations are required or environmental controls are hard to constrain.     

Herbivory alters plant carbon assimilation,patterns of biomass allocation and nitrogen use efficiency

Herbivory can trigger physiological processes resulting in leaf and whole plant functional changes. The effects of chronic infestation by an insect on leaf traits related to carbon and nitrogen economy in three Prunus avium cultivars were assessed. Leaves from non-infested trees (control) and damaged leaves from infested trees were selected. The insect larvae produce skeletonization of the leaves leaving relatively intact the vein network of the eaten leaves and the abaxial epidermal tissue. At the leaf level, nitrogen content per mass (N_mass) and per area (N_area), net photosynthesis per mass (A_mass) and per area (A_area), photosynthetic nitrogen-use efficiency (PNUE), leaf mass per area (LMA) and total leaf phenols content were measured in the three cultivars. All cultivars responded to herbivory in a similar fashion. The N_mass, A_mass, and PNUE decreased, while LMA and total content of phenols increased in partially damaged leaves. Increases in herbivore pressure resulted in lower leaf size and total leaf area per plant across cultivars. Despite this, stem cumulative growth tended to increase in infected plants suggesting a change in the patterns of biomass allocation and in resources sequestration elicited by herbivory. A larger N investment in defenses instead of photosynthetic structures may explain the lower PNUE and A_mass observed in damaged leaves. Some physiological changes due to herbivory partially compensate for the cost of leaf removal buffering the carbon economy at the whole plant level.     

The impact of Indonesian peatland degradation on downstream marine ecosystems and the global carbon cycle

Tropical peatlands are among the most space‐efficient stores of carbon on Earth containing approximately 89 Gt C. Of this, 57 Gt (65%) are stored in Indonesian peatlands. Large‐scale exploitation of land, including deforestation and drainage for the establishment of oil palm plantations, is changing the carbon balance of Indonesian peatlands, turning them from a natural sink to a source via outgassing of CO₂ to the atmosphere and leakage of dissolved organic carbon (DOC) into the coastal ocean. The impacts of this perturbation to the coastal environment and at the global scale are largely unknown. Here, we evaluate the downstream effects of released Indonesian peat carbon on coastal ecosystems and on the global carbon cycle. We use a biogeochemical box model in combination with novel and literature observations to investigate the impact of different carbon emission scenarios on the combined ocean–atmosphere system. The release of all carbon stored in the Indonesian peat pool, considered as a worst‐case scenario, will increase atmospheric pCO₂ by 8 ppm to 15 ppm within the next 200 years. The expected impact on the Java Sea ecosystems is most significant on the short term (over a few hundred years) and is characterized by an increase of 3.3% in phytoplankton, 32% in seagrass biomass, and 5% decrease in coral biomass. On the long term, however, the coastal ecosystems will recover to reach near pre‐excursion conditions. Our results suggest that the ultimate fate of the peat carbon is in the deep ocean with 69% of it landing in the deep DIC pool after 1000 years, but the effects on the global ocean carbonate chemistry will be marginal.     

The greening of the Northern Great Plains and its biogeochemical precursors

Vegetation greenness has increased across much of the global land surface over recent decades. This trend is projected to continue—particularly in northern latitudes—but future greening may be constrained by nutrient availability needed for plant carbon (C) assimilation in response to CO₂ enrichment (eCO₂). eCO₂ impacts foliar chemistry and function, yet the relative strengths of these effects versus climate in driving patterns of vegetative greening remain uncertain. Here we combine satellite measurements of greening with a 135 year record of plant C and nitrogen (N) concentrations and stable isotope ratios (δ¹³C and δ¹⁵N) in the Northern Great Plains (NGP) of North America to examine N constraints on greening. We document significant greening over the past two decades with the highest proportional increases in net greening occurring in the dries and warmest areas. In contrast to the climate dependency of greening, we find spatially uniform increases in leaf‐level intercellular CO₂ and intrinsic water use efficiency that track rising atmospheric CO₂. Despite large spatial variation in greening, we find sustained and climate‐independent declines in foliar N over the last century. Parallel declines in foliar δ¹⁵N and increases in C:N ratios point to diminished N availability as the likely cause. The simultaneous increase in greening and decline in foliar N across our study area points to increased N use efficiency (NUE) over the last two decades. However, our results suggest that plant NUE responses are likely insufficient to sustain observed greening trends in NGP grasslands in the future.     

Land use change and the global carbon cycle: the role of tropical soils

Millions of hectares of tropical forest are cleared annually for agriculture, pasture, shifting cultivation and timber. One result of these changes in land use is the release of CO₂ from the cleared vegetation and soils. Although there is uncertainty as to the size of this release, it appears to be a major source of atmospheric CO₂, second only to the release from the combustion of fossil fuels. This study estimates the release of CO₂ from tropical soils using a computer model that simulates land use change in the tropics and data on (1) the carbon content of forest soils before clearing; (2) the changes in the carbon content under the various types of land use; and (3) the area of forest converted to each use. It appears that the clearing and use of tropical soils affects their carbon content to a depth of about 40 cm. Soils of tropical closed forests contain approximately 6.7 kg C · m^-2; soils of tropical open forests contain approximately 5.2 kg C · m^-2 to this depth. The cultivation of tropical soils reduces their carbon content by 40% 5 yr after clearing; the use of these soils for pasture reduces it by about 20%. Logging in tropical forests appears to have little effect on soil carbon. The carbon content of soils used by shifting cultivators returns to the level found under primary forest about 35 yr after abandonment. The estimated net release of carbon from tropical soils due to land use change was 0.11–0.26 × 10¹⁵ g in 1980.