Elevated pCO2 increases sperm limitation and risk of polyspermy in the red sea urchin Strongylocentrotus franciscanus

Anthropogenic carbon dioxide (CO₂) emissions and the resultant acidification of surface ocean waters are predicted to have far‐reaching consequences for biological processes in the marine environment. For example, because changes in pH and pCO₂ can alter sperm performance, ocean acidification may be accompanied by reductions in the success of fertilization in marine broadcast spawners. Several studies have attempted to determine the effects of elevated pCO₂ on marine invertebrate fertilization success, albeit with differing results. These conflicts may stem from the use of inappropriate sperm–egg contact times and, in several cases, the lack of measurements over a range of sperm concentrations extending from sperm‐limited conditions to polyspermy scenarios. In our study, we used biologically realistic sperm–egg contact times and a full range of sperm concentrations to assess the effect of elevated pCO₂ on fertilization in the broadcast spawning sea urchin, Strongylocentrotus franciscanus. Fertilization experiments were carried out in seawater bubbled with CO₂ to 400 (control), 800, and 1800 ppm. Using a fertilization kinetics model, we estimate that elevated pCO₂ levels both increased sperm limitation and reduced the efficiency of fast blocks to polyspermy. Thus, elevated pCO₂ decreased the range of sperm concentrations over which high fertilization success was likely. Given the inherent difficulties in achieving high fertilization success in broadcast spawners, raised pCO₂ levels are likely to exacerbate low fertilization success in low‐density populations or in areas with high water turbulence.     

Does climate directly influence NPP globally?

The need for rigorous analyses of climate impacts has never been more crucial. Current textbooks state that climate directly influences ecosystem annual net primary productivity (NPP), emphasizing the urgent need to monitor the impacts of climate change. A recent paper challenged this consensus, arguing, based on an analysis of NPP for 1247 woody plant communities across global climate gradients, that temperature and precipitation have negligible direct effects on NPP and only perhaps have indirect effects by constraining total stand biomass (M_tot) and stand age (a). The authors of that study concluded that the length of the growing season (l_gs) might have a minor influence on NPP, an effect they considered not to be directly related to climate. In this article, we describe flaws that affected that study's conclusions and present novel analyses to disentangle the effects of stand variables and climate in determining NPP. We re‐analyzed the same database to partition the direct and indirect effects of climate on NPP, using three approaches: maximum‐likelihood model selection, independent‐effects analysis, and structural equation modeling. These new analyses showed that about half of the global variation in NPP could be explained by M_tot combined with climate variables and supported strong and direct influences of climate independently of M_tot, both for NPP and for net biomass change averaged across the known lifetime of the stands (ABC = average biomass change). We show that l_gs is an important climate variable, intrinsically correlated with, and contributing to mean annual temperature and precipitation (T_ann and P_ann), all important climatic drivers of NPP. Our analyses provide guidance for statistical and mechanistic analyses of climate drivers of ecosystem processes for predictive modeling and provide novel evidence supporting the strong, direct role of climate in determining vegetation productivity at the global scale.     

铁施肥促进海洋浮游植物和生物碳泵的不确定性

铁作为浮游植物所必需的微量元素,限制了全球超过三分之一海域的初级生产力,尤其是在高营养盐、低叶绿素海域(high nutrient low chlorophyll,HNLC)。长期以来海洋铁施肥被认为是一项可以降低大气二氧化碳含量的地球工程策略。然而通过13次海洋人工铁施肥(artificial ocean iron fertilization,aOIF)实验发现,铁的额外添加对海洋深层碳输出量的促进作用要显著低于预期。本文简要地总结了碳在海洋和大气中的循环过程,回顾了人工铁施肥实验对生物碳泵和碳通量等的影响,分析了从海洋铁施肥到海洋碳汇关键生物地球化学过程的影响因素。综上分析发现,科学界对生物碳泵过程及其调控机制的认识仍十分浅薄,考虑到海洋铁施肥还会对海洋生态系统带来一定的负面作用,铁施肥能否作为降低大气中CO₂的有效手段,以达到碳中和并缓解温室效应仍需进一步研究。     

Antarctic ecosystems in transition – life between stresses and opportunities

Important findings from the second decade of the 21st century on the impact of environmental change on biological processes in the Antarctic were synthesised by 26 international experts. Ten key messages emerged that have stakeholder-relevance and/or a high impact for the scientific community. They address (i) altered biogeochemical cycles, (ii) ocean acidification, (iii) climate change hotspots, (iv) unexpected dynamism in seabed-dwelling populations, (v) spatial range shifts, (vi) adaptation and thermal resilience, (vii) sea ice related biological fluctuations, (viii) pollution, (ix) endangered terrestrial endemism and (x) the discovery of unknown habitats. Most Antarctic biotas are exposed to multiple stresses and considered vulnerable to environmental change due to narrow tolerance ranges, rapid change, projected circumpolar impacts, low potential for timely genetic adaptation, and migration barriers. Important ecosystem functions, such as primary production and energy transfer between trophic levels, have already changed, and biodiversity patterns have shifted. A confidence assessment of the degree of ‘scientific understanding’ revealed an intermediate level for most of the more detailed sub-messages, indicating that process-oriented research has been successful in the past decade. Additional efforts are necessary, however, to achieve the level of robustness in scientific knowledge that is required to inform protection measures of the unique Antarctic terrestrial and marine ecosystems, and their contributions to global biodiversity and ecosystem services.     

Effects of large-scale Amazon forest degradation on climate and air quality through fluxes of carbon dioxide, water, energy, mineral dust and isoprene

Loss of large areas of Amazonian forest, through either direct human impact or climate change, could exert a number of influences on the regional and global climates. In the Met Office Hadley Centre coupled climate-carbon cycle model, a severe drying of this region initiates forest loss that exerts a number of feedbacks on global and regional climates, which magnify the drying and the forest degradation. This paper provides an overview of the multiple feedback process in the Hadley Centre model and discusses the implications of the results for the case of direct human-induced deforestation. It also examines additional potential effects of forest loss through changes in the emissions of mineral dust and biogenic volatile organic compounds. The implications of ecosystem-climate feedbacks for climate change mitigation and adaptation policies are also discussed.     

Roles of food web and heterotrophic microbial processes in upper ocean biogeochemistry: Global patterns and processes

The growth and dynamics of plankton in the ocean vary with natural cycles, global climate change and the long-term evolution of ecosystems. The ocean is a large reservoir for CO₂ and the food webs in the upper ocean play critical roles in regulating the global carbon cycle, changes in atmospheric CO₂ and associated global warming. Microheterotrophs are a key component of the upper ocean food webs. Here, we report on the results of an analysis of the distribution of bacteria and related properties in the World Ocean. We found that, for the data set as a whole, there is a significant latitudinal gradient in all field-measured and computed bacterial properties, except growth rate. Gradients were, for the most part, driven by an equator-ward increase in the Southern Hemisphere. The biomass, rates of production and respiration and dissolved organic carbon concentrations were significantly higher in the Northern than the Southern hemispheres. In contrast, growth rates were the same in the two hemispheres. We conclude that the lower biomass and production in the Southern Hemisphere reflects greater top-down control by microbial grazers, which would be due to a lower abundance or activity of omnivorous zooplankton in the Southern than Northern Hemispheres. These large spatial differences in dynamics, structure and activity of the bacterial community and the microbial food web will be reflected in different patterns of carbon cycling, export and air–sea exchange of CO₂ and the potential ability of the ocean to sequester carbon.     

Terrestrial carbon‐cycle feedback to climate warming: experimental evidence on plant regulation and impacts of biofuel feedstock harvest

Feedback between global carbon (C) cycles and climate change is one of the major uncertainties in projecting future global warming. Coupled carbon–climate models all demonstrated a positive feedback between terrestrial C cycle and climate warming. The positive feedback results from decreased net primary production (NPP) in most models and increased respiratory C release by all the models under climate warming. Those modeling results present interesting hypotheses of future states of ecosystems and climate, which are yet to be tested against experimental results. In this study, we examined ecosystem C balance and its major components in a warming and clipping experiment in a North America tallgrass prairie. Infrared heaters have been used to elevate soil temperature by approximately 2 °C continuously since November 1999. Clipping once a year was to mimic hay or biofuel feedstock harvest. On average of data over 6 years from 2000 to 2005, estimated NPP under warming increased by 14% without clipping (P<0.05) and 26% with clipping (P<0.05) in comparison with that under control. Warming did not result in instantaneous increases in soil respiration in 1999 and 2000 but significantly increased it by approximately 8% without clipping (P<0.05) from 2001 to 2005. Soil respiration under warming increased by 15% with clipping (P<0.05) from 2000 to 2005. Warming‐stimulated plant biomass production, due to enhanced C₄ dominance, extended growing seasons, and increased nitrogen uptake and use efficiency, offset increased soil respiration, leading to no change in soil C storage at our site. However, biofuel feedstock harvest by biomass removal resulted in significant soil C loss in the clipping and control plots but was carbon negative in the clipping and warming plots largely because of positive interactions of warming and clipping in stimulating root growth. Our results demonstrate that plant production processes play a critical role in regulation of ecosystem carbon‐cycle feedback to climate change in both the current ambient and future warmed world.     

农田生态系统碳汇研究进展

农田生态系统碳汇包括农作物生物量碳汇和农田土壤碳汇两个方面,中国农田生态系统面积大,碳储量高,是全球生态系统碳循环的重要组成部分。厘清中国农作物生物量和土壤有机碳含量、变化率和影响因素对于解析全球碳循环和维系粮食安全具有重要意义。梳理农田生态系统碳汇相关概念的基础上,比较农田生态系统碳汇研究方法的适用性及存在问题,通过以往研究和SoilGrids250数据研究中国农田生态系统碳库时空分布,并分析农田生态系统碳汇的影响因素及固碳方法。结果表明,中国近30年来农作物生物量呈现增加趋势,农田土壤有机碳含量普遍较低且空间分布不均,0-5cm土壤有机碳含量平均值在16.7 g/kg到86.5 g/kg之间,增加农田土壤有机碳含量是未来中国农田生态系统碳汇的重要方向。肥料和有机残留管理、保护性耕作、种植模式、灌溉等管理措施是增加土壤有机碳汇的主要措施,但农田生态系统碳汇潜力估算仍存在不确定性。最后,从农田生态系统碳汇潜力估算、影响因素厘定和增汇技术研发3个方面提出未来研究方向。研究结果有助于推动农田生态系统碳汇科学研究和技术推广,为实现农田生态系统助力\"碳中和\"寻求重要路径。     

Carbon balance in the tundra, boreal forest and humid tropical forest during climate change: scaling up from leaf physiology and soil carbon dynamics

Carbon exchange by the terrestrial biosphere is thought to have changed since pre-industrial times in response to increasing concentrations of atmospheric CO₂ and variations (anomalies) in inter-annual air temperatures. However, the magnitude of this response, particularly that of various ecosystem types (biomes), is uncertain. Terrestrial carbon models can be used to estimate the direction and size of the terrestrial responses expected, providing that these models have a reasonable theoretical base. We formulated a general model of ecosystem carbon fluxes by linking a process-based canopy photosynthesis model to the Rothamsted soil carbon model for biomes that are not significantly affected by water limitation. The difference between net primary production (NPP) and heterotrophic soil respiration (R_h) represents net ecosystem production (NEP). The model includes (i) multiple compartments for carbon storage in vegetation and soil organic matter, (ii) the effects of seasonal changes in environmental parameters on annual NEP, and (iii) the effects of inter-annual temperature variations on annual NEP. Past, present and projected changes in atmospheric CO₂ concentration and surface air temperature (at different latitudes) were analysed for their effects on annual NEP in tundra, boreal forest and humid tropical forest biomes. In all three biomes, annual NEP was predicted to increase with CO₂ concentration but to decrease with warming. As CO₂ concentrations and temperatures rise, the positive carbon gains through increased NPP are often outweighed by losses through increased R_h, particularly at high latitudes where global warming has been (and is expected to be) most severe. We calculated that, several times during the past 140 years, both the tundra and boreal forest biomes have switched between being carbon sources (annual NEP negative) and being carbon sinks (annual NEP positive). Most recently, significant warming at high latitudes during 1988 and 1990 caused the tundra and boreal forests to be net carbon sources. Humid tropical forests generally have been a carbon sink since 1960. These modelled responses of the various biomes are in agreement with other estimates from either field measurements or geochemical models. Under projected CO₂ and temperature increases, the tundra and boreal forests will emit increasingly more carbon to the atmosphere while the humid tropical forest will continue to store carbon. Our analyses also indicate that the relative increase in the seasonal amplitude of the accumulated NEP within a year is about 0–14% year^?1 for boreal forests and 0–23% year^?1 in the tundra between 1960 and 1990.     

Modeled interactive effects of precipitation, temperature, and [CO2] on ecosystem carbon and water dynamics in different climatic zones

Interactive effects of multiple global change factors on ecosystem processes are complex. It is relatively expensive to explore those interactions in manipulative experiments. We conducted a modeling analysis to identify potentially important interactions and to stimulate hypothesis formulation for experimental research. Four models were used to quantify interactive effects of climate warming (T), altered precipitation amounts [doubled (DP) and halved (HP)] and seasonality (SP, moving precipitation in July and August to January and February to create summer drought), and elevated [CO₂] (C) on net primary production (NPP), heterotrophic respiration (R_h), net ecosystem production (NEP), transpiration, and runoff. We examined those responses in seven ecosystems, including forests, grasslands, and heathlands in different climate zones. The modeling analysis showed that none of the three‐way interactions among T, C, and altered precipitation was substantial for either carbon or water processes, nor consistent among the seven ecosystems. However, two‐way interactive effects on NPP, R_h, and NEP were generally positive (i.e. amplification of one factor's effect by the other factor) between T and C or between T and DP. A negative interaction (i.e. depression of one factor's effect by the other factor) occurred for simulated NPP between T and HP. The interactive effects on runoff were positive between T and HP. Four pairs of two‐way interactive effects on plant transpiration were positive and two pairs negative. In addition, wet sites generally had smaller relative changes in NPP, R_h, runoff, and transpiration but larger absolute changes in NEP than dry sites in response to the treatments. The modeling results suggest new hypotheses to be tested in multifactor global change experiments. Likewise, more experimental evidence is needed for the further improvement of ecosystem models in order to adequately simulate complex interactive processes.     

Changing Global Climate: Historical Carbon and Nitrogen Budgets and Projected Responses of Ohio’s Cropland Ecosystems

As the evidence of global climate change continues to mount, its consequences for cropland productivity assume particular significance. Against the backdrop of past agricultural practices, simulation models offer a glimpse into the future, showing the effect of temperature changes on crop production. In this study, we first quantified the carbon (C) and nitrogen (N) budgets of Ohios cropland ecosystems using inventory yield data of corn for grain, oat, and all wheat for the period 1866–1996 and soybean for the period 1924–96. Then we explored the responses of Ohios continuous soybean croplands to changes in temperature, carbon dioxide (CO₂) concentration, initial soil organic C and N (SOC-N) pools, soil texture, and management practices by developing a simple cropland ecosystem model (CEM) and performing a long-term sensitivity analysis. Finally, CEM simulations were evaluated against independent observations of SOC values (0–19 cm) averaged over 470 northwest Ohio sites between 1954 and 1987 under conventional tillage and rotations of corn–soybean–winter wheat by using the historical yield data (r ² = 0.8). The C contents per hectare of crop harvests increased by 178% for oats, 300% for corn for grain, and 652% for all wheat between 1866 and 1996 and by 305% for soybean between 1924 and 1996. Ohio croplands acted as C–N sources, releasing average net ecosystem emissions (NEE), including the removal of harvested C–N, of 4,598 kg CO₂ ha^–1 and 141 kg N ha^–1 in 1886 and 205 kg CO₂ ha^–1 (except for the corn-for-grain cropland) and 39 kg N ha^–1 in 1996. The continuous corn croplands continued to become a C sink, sequestering 255 kg C ha^–1 in 1996. Results of the sensitivity analysis for Ohios continuous soybean croplands revealed that the SOC pool increased by 6.9% and decreased by 7.5% in response to a doubled CO₂ concentration and a temperature increase of 2.8°C over 100 years, respectively. The sequestration potential of the SOC pool increased by 6.5% at a rate of 24.6 kg C ha^–1 y^–1 for the same period with finer soil texture (loam to silty clay loam). The shift from conventional to conservation residue practice led to an 11% increase in the steady-state SOC storage at a rate of 42 kg C ha^–1 y^–1 for 100 years.