Lyla L. Taylor

Fire and fire‐adapted vegetation promoted C4 expansion in the late Miocene

Large proportions of the Earths land surface are covered by biomes dominated by C(4) grasses. These C(4)-dominated biomes originated during the late Miocene, 3-8 million years ago (Ma), but there is evidence that C(4) grasses evolved some 20 Ma earlier during the early Miocene/Oligocene. Explanations for this lag between evolution and expansion invoke changes in atmospheric CO(2), seasonality of climate and fire. However, there is still no consensus about which of these factors triggered C(4) grassland expansion. We use a vegetation model, the adaptive dynamic global vegetation model (aDGVM), to test how CO(2), temperature, precipitation, fire and the tolerance of vegetation to fire influence C(4) grassland expansion. Simulations are forced with late Miocene climates generated with the Hadley Centre coupled ocean-atmosphere-vegetation general circulation model. We show that physiological differences between the C(3) and C(4) photosynthetic pathways cannot explain C(4) grass invasion into forests, but that fire is a crucial driver. Fire-promoting plant traits serve to expand the climate space in which C(4)-dominated biomes can persist. We propose that three mechanisms were involved in C(4) expansion: the physiological advantage of C(4) grasses under low atmospheric CO(2) allowed them to invade C(3) grasslands; fire allowed grasses to invade forests; and the evolution of fire-resistant savanna trees expanded the climate space that savannas can invade.

Evaluating the effects of terrestrial ecosystems, climate and carbon dioxide on weathering over geological time: a global-scale process-based approach

Global weathering of calcium and magnesium silicate rocks provides the long-term sink for atmospheric carbon dioxide (CO2) on a timescale of millions of years by causing precipitation of calcium carbonates on the seafloor. Catchment-scale field studies consistently indicate that vegetation increases silicate rock weathering, but incorporating the effects of trees and fungal symbionts into geochemical carbon cycle models has relied upon simple empirical scaling functions. Here, we describe the development and application of a process-based approach to deriving quantitative estimates of weathering by plant roots, associated symbiotic mycorrhizal fungi and climate. Our approach accounts for the influence of terrestrial primary productivity via nutrient uptake on soil chemistry and mineral weathering, driven by simulations using a dynamic global vegetation model coupled to an ocean–atmosphere general circulation model of the Earths climate. The strategy is successfully validated against observations of weathering in watersheds around the world, indicating that it may have some utility when extrapolated into the past. When applied to a suite of six global simulations from 215 to 50 Ma, we find significantly larger effects over the past 220 Myr relative to the present day. Vegetation and mycorrhizal fungi enhanced climate-driven weathering by a factor of up to 2. Overall, we demonstrate a more realistic process-based treatment of plant fungal–geosphere interactions at the global scale, which constitutes a first step towards developing ‘next-generation’ geochemical models.

Modeling the evolutionary rise of ectomycorrhiza on sub-surface weathering environments and the geochemical carbon cycle

For the past two decades, the spread of angiosperm plants in the Cretaceous and Paleogene has been thought to have enhanced silicate weathering fluxes of Ca and Mg to the oceans, thereby drawing down atmospheric CO2 and ultimately sequestering it in marine carbonate sediments. However, the rise of angiosperm trees in the Cretaceous was coincident with the evolution of ectomycorrhizal fungal associations in angiosperm and gymnosperm trees that have increasingly supplanted trees with the ancestral arbuscular-mycorrhizal associations. This represents the most profound alteration in root functioning to occur in plant evolutionary history, with far-reaching implications for weathering and soil biogeochemistry because the fine roots are enveloped with a fungal sheath. Ectomycorrhizal fungi provide the main nutrient and water-absorbing interface with soil, and the pathway through which organic acids and protons are actively secreted at the scale of individual mineral grains. Here, we test the hypothesis that the rise of ectomycorrhizal trees was a major contributor to the drawdown of atmospheric CO2 over the past 120 Ma through enhanced silicate weathering. We developed a process-based soil chemistry model incorporating the effects of plants with ancestral arbuscular mycorrhizas, and more recently evolved ectomycorrhizas on soil chemistry via its effects on the biological proton cycle, and integrated it into a leading model of the long-term carbon cycle (GEOCARBSULF). Our mechanistic, process-based modeling reveals that the rise of ectomycorrhizal trees can explain the CO2 drawdown previously attributed empirically to the spread of angiosperms. We suggest, therefore, that the evolutionary rise of ectomycorrhizas represents an important driving force of the long-term carbon cycle by enhancing chemical weathering and draw-down of atmospheric CO2 into marine carbonates.

The role of forest trees and their mycorrhizal fungi in carbonate rock weathering and its significance for global carbon cycling

On million-year timescales, carbonate rock weathering exerts no net effect on atmospheric CO2 concentration. However, on timescales of decades-to-centuries, it can contribute to sequestration of anthropogenic CO2 and increase land-ocean alkalinity flux, counteracting ocean acidification. Historical evidence indicates this flux is sensitive to land use change, and recent experimental evidence suggests that trees and their associated soil microbial communities are major drivers of continental mineral weathering. Here, we review key physical and chemical mechanisms by which the symbiotic mycorrhizal fungi of forest tree roots potentially enhance carbonate rock weathering. Evidence from our ongoing field study at the UKs national pinetum confirms increased weathering of carbonate rocks by a wide range of gymnosperm and angiosperm tree species that form arbuscular (AM) or ectomycorrhizal (EM) fungal partnerships. We demonstrate that calcite-containing rock grains under EM tree species weather significantly faster than those under AM trees, an effect linked to greater soil acidification by EM trees. Weathering and corresponding alkalinity export are likely to increase with rising atmospheric CO2 and associated climate change. Our analyses suggest that strategic planting of fast-growing EM angiosperm taxa on calcite- and dolomite-rich terrain might accelerate the transient sink for atmospheric CO2 and slow rates of ocean acidification.

Constraining the role of early land plants in Palaeozoic weathering and global cooling.

How the colonization of terrestrial environments by early land plants over 400 Ma influenced rock weathering, the biogeochemical cycling of carbon and phosphorus, and climate in the Palaeozoic is uncertain. Here we show experimentally that mineral weathering by liverworts—an extant lineage of early land plants—partnering arbuscular mycorrhizal (AM) fungi, like those in 410 Ma-old early land plant fossils, amplified calcium weathering from basalt grains threefold to sevenfold, relative to plant-free controls. Phosphate weathering by mycorrhizal liverworts was amplified 9–13-fold over plant-free controls, compared with fivefold to sevenfold amplification by liverworts lacking fungal symbionts. Etching and trenching of phyllosilicate minerals increased with AM fungal network size and atmospheric CO2 concentration. Integration of grain-scale weathering rates over the depths of liverwort rhizoids and mycelia (0.1 m), or tree roots and mycelia (0.75 m), indicate early land plants with shallow anchorage systems were probably at least 10-fold less effective at enhancing the total weathering flux than later-evolving trees. This work challenges the suggestion that early land plants significantly enhanced total weathering and land-to-ocean fluxes of calcium and phosphorus, which have been proposed as a trigger for transient dramatic atmospheric CO2 sequestration and glaciations in the Ordovician.

Montane forest root growth and soil organic layer depth as potential factors stabilizing Cenozoic global change

Tree roots and their symbiotic fungal partners are believed to play a major role in regulating long-term global climate, but feedbacks between global temperature and biotic weathering have not yet been explored in detail. In situ field data from a 3000 m altitudinal transect in Peru show fine root growth decreases and organic layer depth increases with the cooler temperatures that prevail at increased altitude. We hypothesize that this observation suggests a negative feedback: as global temperatures rise, the soil organic layer will shrink, and more roots will grow in the mineral layer, thereby accelerating weathering and reducing atmospheric CO2. We examine this mechanism with a process-based biological weathering model and demonstrate that this negative feedback could have contributed to moderating long-term global Cenozoic climate during major Cenozoic CO2 changes linked to volcanic degassing and tectonic uplift events.

Simulating carbon capture by enhanced weathering with croplands: an overview of key processes highlighting areas of future model development

Enhanced weathering (EW) aims to amplify a natural sink for CO2 by incorporating powdered silicate rock with high reactive surface area into agricultural soils. The goal is to achieve rapid dissolution of minerals and release of alkalinity with accompanying dissolution of CO2 into soils and drainage waters. EW could counteract phosphorus limitation and greenhouse gas (GHG) emissions in tropical soils, and soil acidification, a common agricultural problem studied with numerical process models over several decades. Here, we review the processes leading to soil acidification in croplands and how the soil weathering CO2 sink is represented in models. Mathematical models capturing the dominant processes and human interventions governing cropland soil chemistry and GHG emissions neglect weathering, while most weathering models neglect agricultural processes. We discuss current approaches to modelling EW and highlight several classes of model having the potential to simulate EW in croplands. Finally, we argue for further integration of process knowledge in mathematical models to capture feedbacks affecting both longer-term CO2 consumption and crop growth and yields.

Farming with crops and rocks to address global climate, food and soil security

The magnitude of future climate change could be moderated by immediately reducing the amount of CO2 entering the atmosphere as a result of energy generation and by adopting strategies that actively remove CO2 from it. Biogeochemical improvement of soils by adding crushed, fast-reacting silicate rocks to croplands is one such CO2-removal strategy. This approach has the potential to improve crop production, increase protection from pests and diseases, and restore soil fertility and structure. Managed croplands worldwide are already equipped for frequent rock dust additions to soils, making rapid adoption at scale feasible, and the potential benefits could generate financial incentives for widespread adoption in the agricultural sector. However, there are still obstacles to be surmounted. Audited field-scale assessments of the efficacy of CO2 capture are urgently required together with detailed environmental monitoring. A cost-effective way to meet the rock requirements for CO2 removal must be found, possibly involving the recycling of silicate waste materials. Finally, issues of public perception, trust and acceptance must also be addressed.To reduce climate warming we must stop adding CO2 to the atmopshere, and develop approaches for removing it. Adding crushed, fast-reacting silicate rocks to croplands could improve productivity, restore soil quality and reduce atmospheric CO2.

Using criterion-referenced assessment and ‘preflights’ to enhance education in practical assignments

Abstract This paper looks at the reasons students often complain of ‘not enough time’ in practical assignments. One reason is that they come across ‘sticking points’ at a late stage and cannot deal with them satisfactorily. The use of criterion-referenced assessment is one way to identify sticking points before a task is set. A ‘preflight’ (using Just-in-Time teaching ideas) is a short assignment which helps students identify difficulties in advance and get started on the main task even before it is set. The following identifies ways of building ‘preflights’ and criterion-referenced assessment into experiential education.

Publisher Correction: Farming with crops and rocks to address global climate, food and soil security

In the version of this Perspective originally published, ‘acidification’ was incorrectly spelt as ‘adification’ in Fig. 4. This has now been corrected.