A review of transformative strategies for climate mitigation by grasslands

Grasslands can significantly contribute to climate mitigation. However, recent trends indicate that human activities have switched their net cooling effect to a warming effect due to management intensification and land conversion. This indicates an urgent need for strategies directed to mitigate climate warming while enhancing productivity and efficiency in the use of land and natural (nutrients, water) resources. Here, we examine the potential of four innovative strategies to slow climate change including: 1) Adaptive multi-paddock grazing that consists of mimicking how ancestral herds roamed the Earth; 2) Agrivoltaics that consists of simultaneously producing food and energy from solar panels on the same land area; 3) Agroforestry with a reverse phenology tree species, Faidherbia (Acacia) albida, that has the unique trait of being photosynthetically active when intercropped herbaceous plants are dormant; and, 4) Enhanced Weathering, a negative emission technology that removes atmospheric CO2 from the atmosphere. Further, we speculate about potential unknown consequences of these different management strategies and identify gaps in knowledge. We find that all these strategies could promote at least some of the following benefits of grasslands: CO2 sequestration, non-CO2 GHG mitigation, productivity, resilience to climate change, and an efficient use of natural resources. However, there are obstacles to be overcome. Mechanistic assessment of the ecological, environmental, and socio-economic consequences of adopting these strategies at large scale are urgently needed to fully assess the potential of grasslands to provide food, energy and environmental security

Impacts of Changes in Winter Precipitation on C Stocks and Fluxes in Arctic Tussock Tundra

The alteration of winter precipitation patterns in Arctic regions represents a potentially important climate forcing agent. However, climate/carbon (C) cycle forcing feedbacks from Arctic regions remain largely unresolved due to uncertainties in the strength, form (CO2 and CH4), direction and timing of ecosystem C fluxes under future precipitation scenarios. I combined C flux measurements and soil organic carbon (SOC) inventories with stable isotope and radioisotope methods in a multi-year, multi-level snow manipulation experiment in Arctic tundra to investigate: i) the rate at which permafrost C will become available for decomposition and will be released relative to ecosystem C inputs under future precipitation scenarios, ii) the magnitude, form and direction of derived climate/C-cycle feedbacks, and iii) the mechanisms driving long-term impacts of changes in winter precipitation on Arctic tundra C budget and fluxes. Results indicated the potential of Arctic tundra to become a transient C source through accelerated soil organic carbon (SOC) mineralization rates under future precipitation scenarios, but also to act as an additional long-term C sink with persistent increases in winter precipitation, as recently thawed SOC may remain largely immobilized over decades under thaw-induced near-water saturated conditions. This additional C sink however, may come at the cost of a substantial positive feedback on climate derived from increases in the net CH4 source strength of Arctic tundra, as warmer and wetter active layer stimulate CH4 production above CH4 oxidation further subsidized by enhanced plant-mediated transport associated to transitions in supported vegetation over the course of progressive permafrost degradation. Results suggested that much of current divergence among model predicted Arctic climate/C-cycle feedbacks may stem from inaccurate representations of the sensitivity of both physical and biological processes to changes in winter precipitation over time. Findings presented here indicate that projected precipitation scenarios will drive the Arctic tundra C budget and shape the radiative forcing from Arctic regions, critically affecting future climate

Hidden Challenges in Ecosystem Responses to Climate Change

Terrestrial ecosystems exchange vast amounts of C with the atmosphere between the processes of gross primary photosynthesis (GPP) and ecosystem respiration. As such, land surface processes that affect the balance between photosynthesis and respiration should affect the atmospheric concentration of CO2. Because atmospheric CO2 concentrations have been stable over millennia during the Holocene, it can be hypothesized that any process that has affected one biospheric C flux component has been compensated by changes in the other component. However, human activities are causing a net release of CO2 into the atmosphere, which is altering the C flux balance between global GPP and terrestrial ecosystem respiration. Reliable predictions of direct effects of CO2 and related climate forcing factors on vegetation and their feedbacks on the climate system depend deeply on our understanding of this global photosynthesis-ecosystem respiration balance. Tremendous progress has been made on understanding the photosynthetic flux of the terrestrial biosphere, but our understanding of the respiration flux and its components has advanced at a much slower pace [1]. As the majority of the ecosystem respiration flux originates from soils, understanding plant and soil biota interactions in terrestrial ecosystems represent a major challenge for climate predictions. Belowground processes are complex and govern major feedbacks between the terrestrial biosphere and climate. Here, we identified two major belowground biogeochemical processes that have been elusive to ecosystem scientists

Changes in Respiratory Mitochondrial Machinery and Cytochrome and Alternative Pathway Activities in Response to Energy Demand Underlie the Acclimation of Respiration to Elevated CO2 in the Invasive Opuntia ficus-indica1[OA]

Studies on long-term effects of plants grown at elevated CO2 are scarce and mechanisms of such responses are largely unknown. To gain mechanistic understanding on respiratory acclimation to elevated CO2, the Crassulacean acid metabolism Mediterranean invasive Opuntia ficus-indica Miller was grown at various CO2 concentrations. Respiration rates, maximum activity of cytochrome c oxidase, and active mitochondrial number consistently decreased in plants grown at elevated CO2 during the 9 months of the study when compared to ambient plants. Plant growth at elevated CO2 also reduced cytochrome pathway activity, but increased the activity of the alternative pathway. Despite all these effects seen in plants grown at high CO2, the specific oxygen uptake rate per unit of active mitochondria was the same for plants grown at ambient and elevated CO2. Although decreases in photorespiration activity have been pointed out as a factor contributing to the long-term acclimation of plant respiration to growth at elevated CO2, the homeostatic maintenance of specific respiratory rate per unit of mitochondria in response to high CO2 suggests that photorespiratory activity may play a small role on the long-term acclimation of respiration to elevated CO2. However, despite growth enhancement and as a result of the inhibition in cytochrome pathway activity by elevated CO2, total mitochondrial ATP production was decreased by plant growth at elevated CO2 when compared to ambient-grown plants. Because plant growth at elevated CO2 increased biomass but reduced respiratory machinery, activity, and ATP yields while maintaining O2 consumption rates per unit of mitochondria, we suggest that acclimation to elevated CO2 results from physiological adjustment of respiration to tissue ATP demand, which may not be entirely driven by nitrogen metabolism as previously suggested

Deeper snow increases the net soil organic carbon accrual rate in moist acidic tussock tundra:²¹⁰Pb evidence from Arctic Alaska

Abstract The net change in the carbon inventory of arctic tundra remains uncertain as global warming leads to shifts in arctic water and carbon cycles. To better understand the response of arctic tundra carbon to changes in winter precipitation amount, we investigated soil depth profiles of carbon concentration and radionuclide activities (⁷Be, ¹³⁷Cs, ²¹⁰Pb, and ²⁴¹Am) in the active layer of a twenty-two-year winter snow depth manipulation experiment in moist acidic tussock tundra at Toolik Lake, Alaska. Depth correlations of cumulative carbon dry mass (g cm⁻²) vs. unsupported ²¹⁰Pb activity (mBq g⁻¹) were examined using a modified constant rate of supply (CRS) model. Results were best fit by two-slope CRS models indicating an apparent step temporal increase in the accumulation rate of soil organic carbon. Most of the best-fit model chronologies indicated that the increase in carbon accumulation rate apparently began and persisted after snow fence construction in 1994. The inhomogeneous nature of permafrost soils and their relatively low net carbon accumulation rates make it challenging to establish robust chronologic records. Nonetheless, the data obtained in this study support a decadal-scale increase in net soil organic carbon accumulation rate in the active layer of arctic moist acidic tussock tundra under conditions of increased winter precipitation

Accelerating the development of a sustainable bioenergy portfolio through stable isotopes

Abstract Bioenergy could help limit global warming to 2°C above pre‐industrial levels while supplying almost a fourth of the world's renewable energy needs by 2050. However, the deployment of bioenergy raises concerns that adoption at meaningful scales may lead to unintended negative environmental consequences. Meanwhile, the full consolidation of a bioenergy industry is currently challenged by a sufficient, resilient, and resource‐efficient biomass supply and an effective conversion process. Here, we provide a comprehensive analysis of how stable isotope approaches have accelerated the development of a robust bioeconomy by advancing knowledge about environmental sustainability, feedstock development, and biological conversion. We show that advances in stable isotope research have generated crucial information to (1) gain mechanistic insight into the potential of bioenergy crops to mitigate climate change as well as their impact on water and nutrient cycling; (2) develop high‐yielding, resilient feedstocks that produce high‐value bioproducts in planta; and (3) engineer microbes to enhance feedstock conversion to bioenergy products. Further, we highlight knowledge gaps that could benefit from future research facilitated by stable isotope approaches. We conclude that advances in mechanistic knowledge and innovations within the field of stable isotopes in cross‐disciplinary research actions will greatly contribute to breaking down the barriers to establishing a robust bioeconomy

Bioenergy Underground: Challenges and opportunities for phenotyping roots and the microbiome for sustainable bioenergy crop production

Abstract Bioenergy production often focuses on the aboveground feedstock production for conversion to fuel and other materials. However, the belowground component is crucial for soil carbon sequestration, greenhouse gas fluxes, and ecosystem function. Roots maximize feedstock production on marginal lands by acquiring soil resources and mediating soil ecosystem processes through interactions with the microbial community. This belowground world is challenging to observe and quantify; however, there are unprecedented opportunities using current methodologies to bring roots, microbes, and soil into focus. These opportunities allow not only breeding for increased feedstock production but breeding for increased soil health and carbon sequestration as well. A recent workshop hosted by the USDOE Bioenergy Research Centers highlighted these challenges and opportunities while creating a roadmap for increased collaboration and data interoperability through standardization of methodologies and data using F.A.I.R. principles. This article provides a background on the need for belowground research in bioenergy cropping systems, a primer on root system properties of major U.S. bioenergy crops, and an overview of the roles of root chemistry, exudation, and microbial interactions on sustainability. Crucially, we outline how to adopt standardized measures and databases to meet the most pressing methodological needs to accelerate root, soil, and microbial research to meet the pressing societal challenges of the century