Oceanic uptake of anthropogenic carbon dioxide (CO2) is altering the seawater chemistry of the world’s oceans with consequences for marine biota. Elevated partial pressure of CO2 (pCO2) is causing the calcium carbonate saturation horizon to shoal in many regions, particularly in high latitudes and regions that intersect with pronounced hypoxic zones. The ability of marine animals, most importantly pteropod molluscs, foraminifera, and some benthic invertebrates, to produce calcareous skeletal structures is directly affected by seawater CO2 chemistry. CO2influences the physiology of marine organisms as well through acid-base imbalance and reduced oxygen transport capacity. The few studies at relevant pCO2 levels impede our ability to predict future impacts on foodweb dynamics and other ecosystem processes. Here we present new observations, review available data, and identify priorities for future research, based on regions, ecosystems, taxa, and physiological processes believed to be most vulnerable to ocean acidification. We conclude that ocean acidification and the synergistic impacts of other anthropogenic stressors provide great potential for widespread changes to marine ecosystems.
Terrestrial vascular plants obtain their major constituent – carbon (C) – from atmospheric carbon dioxide (CO2), but draw all other chemical elements largely from the soil. Concentrations of these elements, however, do not change in unison with steadily increasing concentrations of CO2 [CO2]. Thus, relative to pre-industrial times, modern plants are experiencing a global elemental imbalance. Could this imbalance affect the elemental composition of plants, the most important food source on Earth? Apart from an overall decline in nitrogen concentration, very little is known about the effects of high [CO2] on other chemical elements, such as iron, iodine and zinc, which are already deficient in the diets of the half of human population. Here, I apply stoichiometric theory to argue that high [CO2], as a rule, should alter the elemental composition of plants, thus affecting the quality of human nutrition. The first compilation, to my knowledge, of published data supports the claim and shows an overall decline of the (essential elements):C ratio. Therefore, high [CO2] could intensify the already acute problem of micronutrient malnutrition.
Nitrogen (N) is central to living systems, and its addition to agricultural cropping systems is an essential facet of modern crop management and one of the major reasons that crop production has kept pace with human population growth. The benefits of N added to cropping systems come, however, at well-documented environmental costs: Increased coastal hypoxia, atmospheric nitrous oxide (N2O), reactive N gases in the troposphere, and N deposition onto forests and other natural areas are some of the consequences of our inability to keep fertilizer N from leaving cropped ecosystems via unmanaged pathways. The N cycle is complex, and solutions require a thorough understanding of both the biogeochemical pathways of N in agricultural systems and the consequences of different management practices. Despite the complexity of this challenge, however, a number of technologies are available today to reduce N loss. These include adding rotational complexity to cropping systems to improve N capture by crops, providing farmers with decision support tools for better predicting crop fertilizer N requirements, improving methods for optimizing fertilizer timing and placement, and developing watershed-level strategies to recapture N lost from fields. Solutions to the problem of agricultural N loss will require a portfolio approach in which different technologies are used in different combinations to address site-specific challenges. Solutions will also require incentives that promote their adoption.
The influence of elevated CO2 concentration ([CO2]) during plant growth on the carbon:nutrient ratios of tissues depends in part on the time and space scales considered. Most evidence relates to individual plants examined over weeks to just a few years. The C:N ratio of live tissues is found to increase, decrease or remain the same under elevated [CO2]. On average it increases by about 15% under a doubled [CO2]. A testable hypothesis is proposed to explain why it increases in some situations and decreases in others. It includes the notion that only in the intermediate range of N-availability will C:N of live tissues increase under elevated [CO2]. Five hypotheses to explain the mechanism of such increase in C:N are discussed; none of these options explains all the published results. Where elevated [CO2] did increase the C:N of green leaves, that response was not necessarily expressed as a higher C:N of senesced leaves. An hypothesis is explored to explain the observed range in the degree of propogation of a CO2 effect on live tissues through to the litter derived from them. Data on C:P ratios under elevated [CO2] are sparse and also variable. They do not yet suggest a generalising-hypothesis of responses. Although, unlike for C:N, there is no theoretical expectation that C:P of plants would increase under elevated [CO2], the average trend in the data is of such an increase. The processes determining the C:P response to elevated [CO2] seem to be largely independent of those for C:N. Research to advance the topic should be structured to examine the components of the hypotheses to explain effects on C:N. This involves experiments in which plants are grown over the full range of N and of P availability from extreme limitation to beyond saturation. Measurements need to: distinguish structural from non-structural dry matter; organic from inorganic forms of the nutrient in the tissues; involve all parts of the plant to evaluate nutrient and C allocation changes with treatments; determine resorption factors during tissue senescence; and be made with cognisance of the temporal and spatial aspects of the phenomena involved.
Plants grown under elevated atmospheric [CO2] typically have decreased tissue concentrations of N compared with plants grown under current ambient [CO2]. The physiological mechanisms responsible for this phenomenon have not been definitely established, although a considerable number of hypotheses have been advanced to account for it. In this review we discuss and critically evaluate these hypotheses. One contributing factor to the decreases in tissue N concentrations clearly is dilution of N by increased photosynthetic assimilation of C. In addition, studies on intact plants show strong evidence for a general decrease in the specific uptake rates (uptake per unit mass or length of root) of N by roots under elevated CO2. This decreased root uptake appears likely to be the result both of decreased N demand by shoots and of decreased ability of the soil-root system to supply N. The best-supported mechanism for decreased N supply is a decrease in transpiration-driven mass flow of N in soils due to decreased stomatal conductance at elevated CO2, although some evidence suggests that altered root system architecture may also play a role. There is also limited evidence suggesting that under elevated CO2, plants may exhibit increased rates of N loss through volatilization and/or root exudation, further contributing to lowering tissue N concentrations.