Photosynthetic assimilation of atmospheric carbon dioxide by land plants offers the underpinnings for terrestrial carbon (C) sequestration. A proportion of the C captured in plant biomass is partitioned to roots, where it enters the pools of soil organic C and soil inorganic C and can be sequestered for millennia. Bioenergy crops serve the dual role of providing biofuel that offsets fossil-fuel greenhouse gas (GHG) emissions and sequestering C in the soil through extensive root systems. Carbon captured in plant biomass can also contribute to C sequestration through the deliberate addition of biochar to soil, wood burial, or the use of durable plant products. Increasing our understanding of plant, microbial, and soil biology, and harnessing the benefits of traditional genetics and genetic engineering, will help us fully realize the GHG mitigation potential of phytosequestration.

Reversing forest losses through restoration, improvement, and conservation is a critical goal for greenhouse gas mitigation. Here, we examine some ecological, demographic, and economic opportunities and constraints on forest-loss mitigation activities. Reduced deforestation and forest degradation could cut global deforestation rates in half by 2030, preserving 1.5 billion to 3 billion metric tons of carbon dioxide-equivalent (tCO₂e) emissions yearly. Our new economic modeling for the United States suggests that greenhouse gas payments of up to $50 per tCO₂e could reduce greenhouse gas emissions by more than 700 million tCO₂e per year through afforestation, forest management, and bioelectricity generation. However, simulated carbon payments also imply the reduction of agricultural land area in the United States by 10% or more, decreasing agricultural exports and raising commodity food prices, imports, and leakage. Using novel transgenic eucalypts as our example, we predict selective breeding and genetic engineering can improve productivity per area, but maximizing productivity and biomass could make maintaining water supply, biodiversity, and other ecosystem services a challenge in a carbon-constrained world.

Soil carbon (C) is a dynamic and integral part of the global C cycle. It has been a source of atmospheric carbon dioxide (CO₂) since the dawn of settled agriculture, depleting more than 320 billion metric tons (Pg) from the terrestrial pool, 78±12 Pg of which comes from soil. In comparison, approximately 292 Pg C have been emitted through fossil-fuel combustion since about 1750. However, terrestrial pools can act as a sink for as much as 50 parts per million of atmospheric CO₂ for 100 to 150 years. The technical sink capacity of US soils is 0.288 Pg C per year; Earth's terrestrial biosphere can act as a sink for up to 3.8 Pg C per year. The economic potential of C storage depends on its costs and cobenefits, such as global food security, water quality, and soil biodiversity. Therefore, optimally managing the soil C pool must be the bash of any strategy to improve and sustain agronomic production, especially in developing countries.

There is growing recognition that microalgae are among the most productive biological systems for generating biomass and capturing carbon. Further efficiencies are gained by harvesting 100% of the biomass, much more than is possible in terrestrial biomass production systems. Microalgae's ability to transport bicarbonate into cells makes them well suited to capture carbon. Carbon dioxide— or bicarbonate-capturing efficiencies as high as 90% have been reported in open ponds. The scale of microalgal production facilities necessary to capture carbon-dioxide (CO₂) emissions from stationary point sources such as power stations and cement kilns is also manageable; thus, microalgae can potentially be exploited for CO₂ capture and sequestration. In this article, I discuss possible strategies using microalgae to sequester CO₂ with reduced environmental consequences.

Regulatory restrictions have increased in recent years on organisms produced using recombinant DNA and asexual gene transfer, a process commonly called genetic engineering or genetic modification. Regulatory agencies have raised special concerns and required additional scrutiny for perennial grasses and woody plants of interest for biofuels; these plants have incomplete domestication, invasive capabilities, and the ability to mate with wild or feral relatives. Regulations on these plants require extremely stringent containment through all phases of research and development, regardless of the source of the gene, the novelty of the trait, or the plants' anticipated economic or environmental benefits. We discuss the extent to which these requirements conflict with the realities of practical crop breeding, and prevent meaningful agronomic and environmental studies, thus hampering—and in most cases, precluding—the use of recombinant DNA breeding methods for perennial crop improvement. We propose regulatory reforms to better balance benefit and risk and remove unnecessary barriers to agronomic evaluations and environmental studies.

The rate and magnitude of predicted climate change require that we urgently mitigate emissions or sequester carbon on a substantial scale in order to avoid runaway climate change. Geo- and bioengineering solutions are increasingly proposed as viable and practical strategies for tackling global warming. Biotechnology companies are already developing transgenic “super carbon-absorbing” trees, which are sold as a cost-effective and relatively low-risk means of sequestering carbon. The question posed in this article is, Do super carbon trees provide real benefits or are they merely a fanciful illusion? It remains unclear whether growing these trees makes sense in terms of the carbon cost of production and the actual storage of carbon. In particular, it is widely acknowledged that “carbon-eating” trees fail to sequester as much carbon as they oxidize and return to the atmosphere; moreover, there are concerns about the biodiversity impacts of large-scale monoculture plantations. The potential social and ecological risks and opportunities presented by such controversial solutions warrant a societal dialogue.

Assessing extinction risk is a fundamental issue in conservation biology. However, national and international legislation and the implementing regulations that establish categorization procedures often include vague definitions of analytic time horizons (e.g., the “foreseeable future”). Because there is no single framework for interpreting these vague terms, individual decisions are often made on a case-by-case basis. We examine how the lack of an a priori framework for assessing extinction risk over time can lead to capricious decisionmaking, which can in turn hinder biodiversity conservation and scientific credibility. We give recommendations for making more transparent and consistent categorization decisions with respect to time horizons and extinction risk.