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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Serie: RGS-IBG Book Series
- Sprache: Englisch
The first systematic examination of the role of geomorphological processes in the cycling of carbon through the terrestrial system.
- Argues that knowledge of geomorphological processes is fundamental to understanding the ways in which carbon is stored and recycled in the terrestrial environment
- Integrates classical geomorphological theory with understanding of microbial processes controlling the decomposition of organic matter
- Develops an interdisciplinary research agenda for the analysis of the terrestrial carbon cycle
- Informed by work in ecology, microbiology and biogeochemistry, in order to analyse spatial and temporal patterns of terrestrial carbon cycling at the landscape scale
- Considers the ways in which, as Humanity enters the Anthropocene, the application of this science has the potential to manage the terrestrial carbon cycle to limit increases in atmospheric carbon
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This edition first published 2022
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Library of Congress Cataloging-in-Publication Data
Names: Evans, Martin, 1970- author. Title: Geomorphology and the carbon cycle / Martin Evans. Description: Hoboken, NJ : John Wiley & Sons, 2022. | Series: RGS-IBG book series | Includes bibliographical references and index. | Summary: “As global atmospheric carbon concentrations continue to rise, there has been an increasing focus in the 21st century on understanding terrestrial components of the carbon cycle. This has been a major interdisciplinary research agenda and advances in remote sensing and modelling of vegetation systems have developed increasingly detailed understanding of above ground carbon cycling (Fatichi et al. 2019; Lees et al. 2018). Similarly, the storage of carbon in soils below ground has been the focus of extensive and detailed research (Wiesmeier et al. 2019). However, arguably understanding of soil carbon processes lags behind analysis of above ground systems. For example, it is notable that, in the paper cited at the top of this chapter (Bloom et al. 2016), the terrestrial carbon model that the paper applies includes significant detail around the cycling of carbon through biomass, modelling carbon in leaves, roots and wood separately, whilst soil carbon represents a single store. Where more detailed models of soil carbon cycling are applied that consider multiple solid carbon pools (e.g. Abramoff et al., 2018), a notable absence is consideration of lateral transfers of organic carbon in the soil and sediment system. Over the last ten years however, there has been an increasing recognition of the importance of lateral carbon fluxes within the landscape as a key part of understanding carbon dynamics at the large scale (e.g. Battin et al. 2008). Figure 1.1 is the 5th Intergovernmental Panel on Climate Change (IPCC) representation of the terrestrial carbon cycle (IPCC 2013). Flux from the land to the oceans is represented by the fluvial carbon flux. Whilst the IPCC estimates distinguish pre-industrial and post-industrial fluxes for many of the key elements of the cycle, human impacts are not quantified for the fluvial system. Clearly, a more detailed picture of the fluvial system is required. The fluvial carbon flux is relatively small compared to the magnitude of terrestrial carbon storage, but is simply the residual of carbon transformation which occurs as organic matter is transported from headwaters to the oceanic sink. Much of the uncertainty about the relative importance of lateral carbon fluxes in the terrestrial carbon budget stems from a lack of knowledge about how large this residual is as a proportion of the total amount of organic carbon which is transported and delivered from hillslopes”-- Provided by publisher. Identifiers: LCCN 2021028996 (print) | LCCN 2021028997 (ebook) | ISBN 9781119393214 (hardback) | ISBN 9781119393252 (paperback) | ISBN 9781119393283 (pdf) | ISBN 9781119393245 (epub) | ISBN 9781119393290 (ebook) Subjects: LCSH: Carbon cycle (Biogeochemistry) | Geomorphology. Classification: LCC QH344 .E83 2022 (print) | LCC QH344 (ebook) | DDC 577/.144--dc23 LC record available at https://lccn.loc.gov/2021028996LC ebook record available at https://lccn.loc.gov/2021028997
Cover image: Courtesy of Martin Evans
Cover design by Wiley
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The information, practices and views in this book are those of the author and do not necessarily reflect the opinion of the Royal Geographical Society (with IBG).
Contents
Cover
Series page
Title page
Copyright
Series Editors’ Preface
Acknowledgements
Part I The Terrestrial Carbon Cycle and Geomorphological Theory
1 Geomorphology and the Terrestrial Carbon Cycle
2 Geomorphology and the Fast Carbon Cycle
3 Geomorphology and the Geological Carbon Cycle
4 Geomorphological Theory and Practice: Material Fluxes in the Terrestrial Carbon Cycle
Part II Geomorphology and Carbon Cycling Across the Sediment Cascade
5 Carbon Cycling in Headwater Catchments
6 Hillslope Soil Erosion and Terrestrial Carbon Cycling
7 The Role of Floodplains in Terrestrial Carbon Cycling
8 Geomorphology and Carbon Cycling in the Coastal Ecotone
Part III A Geomorphological Approach to the Carbon Cycle
9 Geomorphology and Carbon Cycling in the Anthropocene
10 Towards a Geomorphologically Informed Model of Terrestrial Carbon Cycling
References
Index
End User License Agreement
List of Illustrations
Chapter 1
Figure 1.1 A simplified schematic...
Figure 1.2 The sediment cascade...
Figure 1.3 A model of the upland...
Chapter 2
Figure 2.1 The fast carbon cycle.
Figure 2.2 Mean global CO2 concentration 2012–2017....
Figure 2.3 (a) Global photosynthesis from 17 global models...
Figure 2.4 (a) and (b) Relation between...
Figure 2.5 (a) Relation between air...
Figure 2.6 Controls on soil organic matter decomposition.
Figure 2.7 Controls on temperature sensitivity...
Figure 2.8 SOM decomposition...
Figure 2.9 Summary of controls on...
Figure 2.10 Dissolved organic carbon flux to the oceans.
Figure 2.11 Global patterns of carbon flux to the oceans.
Figure 2.12 Changing microbial niches...
Figure 2.13 Global dataset...
Chapter 3
Figure 3.1 The geological...
Figure 3.2 Fluxes and timescales of...
Figure 3.3 Global atmospheric CO2 concentrations for the past...
Figure 3.4 The role of geomorphology as a...
Figure 3.5 Time and space scales for weathering...
Figure 3.6 Geomorphological controls on weathering...
Figure 3.7 A cold environments landsystem. Denis Mercier.
Chapter 4
Figure 4.1 Conceptual representation of the sediment cascade...
Figure 4.2 Conceptual sediment budget model...
Figure 4.3 Example of a quantified...
Figure 4.4 Stable and unstable landscape...
Figure 4.5 The combined impact of...
Figure 4.6 Classification of types of geomorphological equilibrium.
Figure 4.7 Landslide risk has a...
Figure 4.8 (a) Conceptual descriptions...
Figure 4.9 (a) A landsystem model for eroding peatlands...
Figure 4.10 General modelling results fo ecosystem...
Figure 4.11 A conceptual diagram of a...
Chapter 5
Figure 5.1 Changes in sediment...
Figure 5.2 Characteristic patterns of...
Figure 5.3 High POC yields in steeplands...
Figure 5.4 Altitudinal zonation of upland...
Figure 5.5 Headwater peatlands. (a) Blanket....
Figure 5.6 Hydro-topographical...
Figure 5.7 The Peatland carbon cycle.
Figure 5.8 Empirical relation beetween peatland...
Figure 5.9 Gully erosion in the south Pennines UK...
Figure 5.10 Patterns of DOC and POC loss from eroding blanket...
Figure 5.11 Altitudinal distribution of...
Figure 5.12 Characteristic carbon landsystems of headwaters...
Chapter 6
Figure 6.1 (a) Erosion on arable hillslope in East Sussex....
Figure 6.2 Global organic carbon flux to the ocean from major...
Figure 6.3 Estimated lateral carbon flux based....
Figure 6.4 This plot positions key papers in...
Figure 6.5 Topographic controls on soil carbon stock and the...
Figure 6.6 Variation of specific sediment yield with catchment area.
Figure 6.7 Significant variation in sediment...
Figure 6.8 Key carbon transformation processes on hillslope...
Figure 6.9 Scaling of hillslope and floodplain...
Figure 6.10 A hillslope carbon landsystem. 1)...
Chapter 7
Figure 7.1 Floodplain of the River...
Figure 7.2 The floodplain carbon budget.
Figure 7.3 Relation of floodplain sediment residence time...
Figure 7.4 Holocene history of alluvial aggradation and ...
Figure 7.5 The fluvial carbon landsystem.
Figure 7.6 Microbial responses to floodplain inundation.
Figure 7.7 The role of inundation and organic matter quality...
Figure 7.8 Reported magnitudes of organic...
Figure 7.9 Classification of conditions...
Figure 7.10 (a) Modelled carbon flux from a...
Figure 7.11 (a) Downstream change in the...
Figure 7.12 The River Continuum Concept...
Figure 7.13 A conceptual model of...
Chapter 8
Figure 8.1 Carbon fluxes through the coastal system.
Figure 8.2 Estuary classification based...
Figure 8.4 Classification of mangrove forest...
Figure 8.3 (a) Global distribution...
Figure 8.5 Characteristic timescales...
Figure 8.6 Map of global salt marsh abundance.
Figure 8.7 Morphological settings for coastal salt marsh.
Figure 8.8 Carbon fluxes at the land ocean interface associated...
Figure 8.9 Carbon sequestration losses due to...
Figure 8.10 The coastal carbon landsystem...
Figure 8.11 Contributions of coastal systems...
Chapter 9
Figure 9.1 Flows of sand and gravel in the UK in 1997.
Figure 9.2 River system changes in western European rivers...
Figure 9.3 Human impacts on elements of fluvial carbon cycling.
Figure 9.4 Landscape change, organic carbon flux and channel...
Figure 9.5 Before (a) and after (b) images...
Figure 9.6 Impact of re-vegetation on particulate...
Figure 9.7 (a) Plastic piling gully blocks. Source: Martin Evans....
Figure 9.8 Links between Geomorphological and Biotic components...
Figure 9.9 Estimated carbon storage in Leicester, UK.
Chapter 10
Figure 10.1 Controls on the terrestrial carbon cycle.
Figure 10.2 Key stores and fluxes of carbon in the sediment...
Figure 10.3 Timescales and drivers of change in geomorphological...
Figure 10.4 Hypothetical relationship between...
Figure 10.5 Controls on carbon flux through...
List of Tables
Chapter 2
Table 2.1 Factors affecting the temperature...
Table 2.2 Soil carbon pools as characterised...
Table 2.3 Estimates of global fluvial carbon...
Chapter 3
Table 3.1 Summary chemistry of the main...
Table 3.2 Summarising key points from Goudie and Viles 2012
Chapter 4
Table 4.1 Ecological impacts on...
Chapter 5
Table 5.1 Interactions of geomorphological...
Table 5.2 Typical values of headwater...
Table 5.3 Characteristics of headwater...
Chapter 6
Table 6.1 Processes of carbon transformation on hillslopes underpinning the carbon landsystem model presented in Figure 6.10.
Chapter 7
Table 7.1 Rates of overbank deposition...
Table 7.2 Reported floodplain sediment...
Table 7.3 Carbon sequestration...
Table 7.4 Based on a model channel with assumed floodplain...
Chapter 9
Table 9.1 Relative contributions...
Guide
Cover
Series page
Title page
Copyright
Series Editors’ Preface
Acknowledgements
Table of Contents
Begin Reading
References
Index
End User License Agreement
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Series Editors’ Preface
The RGS-IBG Book Series only publishes work of the highest international standing. Its emphasis is on distinctive new developments in human and physical geography, although it is also open to contributions from cognate disciplines whose interests overlap with those of geographers. The Series places strong emphasis on theoretically-informed and empirically-strong texts. Reflecting the vibrant and diverse theoretical and empirical agendas that characterize the contemporary discipline, contributions are expected to inform, challenge and stimulate the reader. Overall, the RGS-IBG Book Series seeks to promote scholarly publications that leave an intellectual mark and change the way readers think about particular issues, methods or theories.
For details on how to submit a proposal please visit: www.rgsbookseries.com
Ruth Craggs, King’s College London, UKChih Yuan Woon, National University of SingaporeRGS-IBG Book Series Editors
David FeatherstoneUniversity of Glasgow, UKRGS‐IBG Book Series Editor (2015–2019)
Acknowledgements
This book has forced me to engage with a broad sweep of geomorphology. As a geographically trained geomorphologist I have been grateful, as I have carried out the research for this book, for the breadth of that education. I would like to acknowledge the influence of a number of remarkable geographers and geomorphologists (and one soil scientist!) that I have studied with, including Richard Crabtree, Tim Burt, June Ryder, Mike Church, Les Lavkulich and Olav Slaymaker. I think that their influence on my thinking is written through this book.
The ideas in this book have also been influenced by conversations in offices, seminar rooms, pubs, coffee shops and in the field with many of my colleagues over the years. I would particularly like to acknowledge the influence in various ways of Yvonne Martin, Steve Rice, Jeff Warburton, Fred Worrall and Tim Allott.
During the period of writing this book I have been involved in two very stimulating workshop series. Colleagues who contributed to the MadCaP seminars in Manchester and to the NERC supported Peatland Resilience and Microbial Processes workshops have helped to develop my thinking around the key importance of understanding the interrelation of geomorphological and microbial processes as drivers for terrestrial carbon cycling.
One of the joys of an academic career is the opportunity to work with and to learn from brilliant graduate students. I have been very fortunate in this regard and I would like to thank former and current students (several of whom are now colleagues) Juan Yang, James Rothwell, Amer Al-Roichdi, Alan Clarke, Laura Liddaman, Eleanor Teague, Sarah Crowe, Steve Daniels, Richard Pawson, Claire Goulsbra, Emma Shuttleworth, Beth Lowe, Andrew Stimson, Donald Edokpa, Sarah Brown, Jane Mellor, Dylan Zhang, Adam Johnson and Richard Figuera for myriad conversations from which I have learnt an enormous amount.
The figures for this volume have been prepared by Nick Scarle and Graham Bowden in the cartographic unit in the department of geography in Manchester. Their skill and attention to detail in bringing the illustrations to life is very much appreciated.
My research has been supported in many ways over the years by colleagues in the geography laboratories in Manchester whose expertise in the lab and in the field is invaluable. John Moore, Martin Kay, Jonathan Yarwood and Tom Bishop in particular.
My research has been funded by a wide range of bodies including NERC, DEFRA, Environment Agency, United Utilities, Natural England, Moors for the Future, the Royal Society, Manchester University, The British Society for Geomorphology, The Royal Geographical Society and the Leverhulme Trust, and I am very grateful for the support they have provided to explore some of the challenges outlined in this book. I am particularly grateful to the Leverhulme Trust who granted me a one-year fellowship during which a significant amount of the work on this volume was completed.
The most significant acknowledgement for this book is to Danielle Alderson as a PhD student and as a colleague. Discussions with Danielle have significantly shaped my thinking on geomorphology and carbon cycling, particularly in fluvial contexts. She has copy-edited all of the chapters, made useful comments, and helped with many of the practicalities of preparing the manuscript. I am enormously grateful to her for her contribution.
I would also like to thank Bob Hilton and an anonymous reviewer for a really helpful set of comments on the first draft of the manuscript which has significantly improved the final draft.
Finally, I would like to thank my family for putting up with me and ‘the book’ for so long!
Part I The Terrestrial Carbon Cycle and Geomorphological Theory
Chapter One Geomorphology and the Terrestrial Carbon Cycle
The terrestrial carbon cycle is currently the least constrained component of the global carbon budget.
(Bloom et al. 2016: 1285)
Introduction
As global atmospheric carbon concentrations continue to rise, there has been an increasing focus in the twenty-first century on understanding terrestrial components of the carbon cycle. This has been a major interdisciplinary research agenda and advances in remote sensing and modelling of vegetation systems have developed increasingly detailed understanding of above ground carbon cycling (Fatichi et al. 2019; Lees et al. 2018). Similarly, the storage of carbon in soils below ground has been the focus of extensive and detailed research (Wiesmeier et al. 2019). However, arguably, understanding of soil carbon processes lags behind analysis of above ground systems. For example, it is notable that in the paper cited at the top of this chapter (Bloom et al. 2016), the terrestrial carbon model that the paper considers includes significant detail around the cycling of carbon through biomass, modelling carbon in leaves, roots and wood separately, whilst soil carbon represents a single store.
Where more detailed models of soil carbon cycling are applied that consider multiple solid carbon pools (e.g., Abramoff et al. 2018), a notable absence is consideration of lateral transfers of organic carbon in the soil and sediment system. Over the last ten years however, there has been an increasing recognition of the importance of lateral carbon fluxes within the landscape as a key part of understanding carbon dynamics at the large scale (e.g., Battin et al. 2008). Figure 1.1 is the 5th Intergovernmental Panel on Climate Change (IPCC) representation of the terrestrial carbon cycle (Cubasch et al. 2013). Flux from the land to the oceans is represented by the fluvial carbon flux. Whilst the IPCC estimates distinguish pre-industrial and post-industrial fluxes for many of the key elements of the cycle, human impacts are not quantified for the fluvial system. Clearly a more detailed picture of the fluvial system is required. The fluvial carbon flux is relatively small compared to the magnitude of terrestrial carbon storage, but is simply the residual of carbon transformation which occurs as organic matter is transported from headwaters to the oceanic sink. Much of the uncertainty about the relative importance of lateral carbon fluxes in the terrestrial carbon budget stems from a lack of knowledge about how large this residual is as a proportion of the total amount of organic carbon which is transported and delivered from hillslopes.
Figure 1.1 A simplified schematic of the global carbon cycle. Black text indicates pre-industrial stores and fluxes and grey indicates estimated changes post circa 1750. Source: After Ciais et al. 2013. Figure 6.1 in Climate Change2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (p. 471). Reproduced with permission of Cambridge University Press. (https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_Chapter06_FINAL.pdf)
Organic carbon in solid or particulate form is transported from hillslopes to the fluvial system, and can be transformed or mineralised in transit either physically, or through the action of macro and micro biota. Transit of organic matter across hillslope systems is however complex and variable in time and space. The proportion of organic sediment eroded in a given period or event that is delivered to the river system (the sediment delivery ratio Walling 1983) is considerably less than 100%, so that an understanding of hillslope geomorphology is required to determine where eroded carbon is deposited. Once organic matter reaches the river system, timescales for direct transfer of dissolved and suspended material to the ocean are typically hours to days (e.g., Jobson 2001). However, Ferguson (1981) described the fluvial sediment transport as a ‘jerky conveyor belt’ so that a proportion of sediment is redeposited within the fluvial system (on bars or floodplains, or in lakes or reservoirs). Material may be mobilised and redeposited multiple times before reaching the ocean, so that virtual velocities may drop by several orders of magnitude and travel times are consequently measured in centuries rather than days. The transit of organic carbon from hillslope source (involving the processes of carbon fixation by vegetation and transfer of litter to the soil system) to oceanic sink is complex. Along the way, material may be stored in zones of sediment accumulation (depositional landforms), representing long-term carbon sequestration or alternatively may be mineralised and lost to the atmosphere through processes of microbial decomposition and respiration. The interactions of carbon fixed by the terrestrial biosphere within the sedimentary system are significantly more complex than the representation in the IPCC carbon budget.
Initially work on lateral transfers of carbon tended to be focussed on the transformation of organic carbon within freshwater systems, building on concepts such as the River Continuum Concept (Vannote et al. 1980). This concept postulates predictable patterns of downstream change in organic matter quality as it is cycled by in-stream processes. Increasingly however, the role of geomorphological processes in controlling the lateral transfer of carbon on hillslopes and through river systems has been recognised (e.g., M. Evans et al. 2013; Hoffmann et al. 2013a). Agricultural hillslope systems have been a major focus of geomorphological work in this area and anthropogenic modification of these systems constitute a major alteration of the overall terrestrial carbon cycle. However, a focus on agricultural systems has tended to drive a focus on field scale patterns of sediment transfer.
Unpicking the black box of headwater to ocean carbon transfers implied in Figure 1.1 requires consideration of carbon fluxes across the entire sediment cascade. A rapidly expanding body of geomorphological research has begun to explore the role of the sediment system in the terrestrial carbon cycle (e.g., Hoffmann et al. 2009; Kirkels et al. 2014). However, the focus of geomorphological carbon cycling research has predominantly been on characterising the magnitude of carbon storage in major loci of sediment accumulation (depositional landforms). There is increasing recognition that a complete understanding of sedimentary carbon storage also requires analysis of rates of carbon addition and removal from storage, and the processes which control this. This requires an integration of biological and geomorphological analyses. Nevertheless, despite the call by Slaymaker and Spencer (1998) for biogeochemical cycling to become a central concern of physical geography and geomorphology, the wider engagement of geomorphology with an understanding of biogeochemical cycling has been so far limited.
In the context of a rapidly shortening time horizon for effective action to mitigate rising greenhouse gas concentrations in the atmosphere, it is argued that rapid progress in this area is vital. The requirement to deliver a more complete understanding of the terrestrial carbon cycle has two main components. Firstly, a functional understanding of the processes which drive carbon flux through the terrestrial system is needed in order to understand the interaction of the terrestrial biosphere with excess atmospheric carbon derived from fossil fuel use. In particular, a focus on carbon storage and release in the terrestrial system is fundamental to identifying positive feedback mechanisms and threshold conditions that might exacerbate anthropogenically driven rates of change. Secondly, understanding the processes by which carbon is added to major terrestrial carbon stores, such as live biomass and particularly soil and sediment carbon storage, offers the potential to manipulate these natural systems to sequester carbon and therefore potentially provide some mitigation of rising atmospheric carbon levels.
Increasing amounts of academic labour are being focussed on these critical problems, but arguably too much of this work is siloed within traditional disciplinary structures and networks. This book is written from the perspective of a geomorphologist trained in geography departments in the UK and Canada, and is born partly from the conviction that there is much which traditional geomorphological understanding of landscape systems can bring to the grand challenge of understanding and managing terrestrial carbon storage. A multidisciplinary approach is fundamental to meeting this challenge; this book will both explore and explain the ways in which geomorphological understanding can contribute to, and also identify the challenges of integrating this knowledge, with understanding of the biosphere and its role in the fixation and release of atmospheric carbon.
The Aims of This Book
The overall aim of this book is to develop a research agenda for the integration of geomorphological, biological and microbiological understanding into analyses of the terrestrial organic carbon cycle. To achieve this, this book has three main objectives:
To identify challenges and opportunities in the application of geomorphological methods and insights to the analysis of terrestrial carbon cycling.
To synthesise the rapidly expanding understanding of geomorphology and carbon cycling in the academic literature to define the state of the science.
To develop a conceptual framework based on geomorphological theory, and informed by work in ecology, microbiology and biogeochemistry, in order to analyse spatial patterns of terrestrial carbon cycling at the landscape scale.
Achieving the aim of fully integrating geomorphological expertise into a multi-disciplinary approach to the analysis of terrestrial carbon cycling will not happen just because this integration offers answers to key questions, it also requires an understanding across scientific communities of what those questions are. This book is written with three audiences in mind. It is written for geomorphologists, to provide a synthesis for those in the field, but also to persuade the wider geomorphological community that core geomorphological data, skills and understanding are required to untangle the complexities of the terrestrial carbon cycle, and that they hold the key to progress in this area.
This book is also written for biologists and microbiologists whose work drives our understanding of both the fixation of atmospheric carbon and the mineralisation of soil carbon, which underpins carbon sequestration into sedimentary stores. This book will make the case to this audience that the physical movement of carbon substrate across the landscape, and the disturbance of equilibrium communities that are associated with phases of erosion and deposition, are components of carbon cycling which need to be integrated into an understanding of carbon storage at longer timescales. These decadal and longer timescales are critical in the context of anthropogenically driven changes in atmospheric carbon concentrations over similar periods.
Finally, this book aims to engage with the community who manage the terrestrial system. Landowners, planners and policy makers are the people who have the capacity to effect change in the anthropogenically dominated landscape that we live in. Managing erosion through landscape restoration, re-naturalising river courses and modifying agricultural practices are all components of conservation practice which drive change in biological and geomorphological systems, and thus modify the flux of carbon through terrestrial systems. Understanding these changes offers the potential to design such interventions in ways which maximise carbon sequestration into sedimentary stores, and so have potential to mitigate some anthropogenic carbon emissions. By characterising these stores and the processes which control carbon sequestration at a range of timescales, this book will offer the potential to argue for carbon sequestration co-benefits in landscape conservation schemes.
Organisation and Focus of This Book
The main argument of this book is developed in three parts. Part I (Chapters 2–4 of this book) outlines the key elements of the fast (organic) and slow (inorganic) terrestrial carbon cycles (Chapters 2 and 3 respectively), in order to provide the context for the discussion of geomorphological influences on carbon cycling in the subsequent chapters. The main focus in this book is on the fast carbon cycle, but a review of the key elements of the slow cycle is important context for understanding what follows and is also included for completeness since this is an area where geologically trained geomorphologists are driving key elements of the research agenda.
One of the assumptions behind this book is that the processes which drive the fluxes within and the reorganisation of sediment systems are a major influence on carbon cycling. If this is the case, then the developed techniques and gathered process-based knowledge of more than a century of geomorphological research will make a contribution to a fuller explanation of the terrestrial carbon cycle. Therefore, in Chapter 4 a range of key conceptual approaches which underpin modern geomorphological thinking are highlighted and the ways in which they can contribute to understanding of carbon cycling are explored.
Part II (Chapters 5 to 8 of this book) focusses in more detail on the fast carbon cycle and the ways in which geomorphological processes interact with vegetation and soil microbiota to cycle carbon through the terrestrial system. Two organising principles underpin the concept of the sediment cascade and the idea of a carbon landsystem.
The Sediment Cascade
In Figure 1.1, the fluvial system which is the primary conduit for the direct transfer of carbon from the continents to the oceans is represented by a simple line. This is in contrast to the detail on the land surface, which indicates a range of processes driving terrestrial carbon cycling. However, as briefly discussed above, the fluvial system is complex and dynamic, and the previous model of the fluvial system as a pipe, simply transporting carbon from hillslope to ocean cannot be sustained. One of the organisational principles of this book is the sediment cascade (Burt and Allison 2010; Schumm 1977). Figure 1.2, from the paper by Schumm which initially outlined the concept, identifies key features of the cascade and defines a linear cascading system describing the flux of water and sediment through landscape systems, from production on hillslopes to deposition in oceans and estuaries. Figure 1.2 indicates that the key role of hillslopes and headwaters in the sediment system is the production of sediment, and the delivery of this material to downstream fluvial systems. In the context of organic carbon fluxes, the River Continuum Concept makes similar assumptions. However, as outlined in a recent analysis by Joyce et al. (2018), the flux of sediment through the upland sediment cascade involves both production of sediment through upland erosion, but also storage in a range of depositional landforms (Figure 1.3).
Figure 1.2 The sediment cascade. Source: After Schumm 1977. Reproduced with permission of The Blackburn Press.
Figure 1.3 A model of the upland sediment indicating the complexity and continuity of sediment transfer in this sub-component of the wider cascade. Source: After Joyce et al. 2018 (https://doi.org/10.1016/j.geomorph.2018.05.002). Licensed by CCA 4.0.
A full description of the flux of sediment and organic carbon through the landscape therefore requires analysis of all components of the sediment cascade, considering the production, storage and cycling of carbon. Chapters 5–8 review the literature on carbon cycling for four key components of the sediment cascade. These are hillslopes (Chapter 5), headwaters (Chapter 6), the fluvial system (Chapter 7) and estuaries/coasts (Chapter 8). For these specific contexts the chapters explore the interaction of geomorphological processes, vegetation and succession as a control on primary production and decomposition and mineralisation of organic carbon by the microbial system.
The Carbon Landsystem
The review and synthesis of the literature in Chapters 5–8 aims to identify the primary carbon stores within each sub-component of the sediment cascade, and to analyse the key processes which drive translocation and transformation of organic carbon within the system. In the geomorphological literature, the term landsystem is used to describe conceptual models of synthetic landscapes that describe key landforms and the processes that drive material flux through the system and produce these characteristic landforms (e.g., Evans 2014). The analysis in this book builds from the assumption that the geomorphological context is not a boundary condition to the terrestrial carbon cycle, but is a dynamic driver of the flux of carbon through the sediment cascade, so that quantification of carbon flux requires integration of geomorphological, biological and microbiological processes. In this context, it is a short conceptual step to move beyond the landsystem as a description of characteristic geomorphologies to thinking about the carbon landsystem as the characteristic set of interactions between these three process types, which drive fluxes of carbon through depositional landforms at points along the sediment cascade. Chapters 5–8 attempt to characterise the carbon landsystem for key elements of the cascade.
Part III (Chapters 9 and 10 of this book) explores some of the implications of a geomorphological approach to understanding the carbon cycle from the perspective of the management of carbon landsystems, in order to mitigate anthropogenically driven increases in atmospheric carbon content. With the International Chronostratigraphic Commission looking likely to ratify a recommendation that the Anthropocene be recognised as a new geological epoch (Zalasiewicz et al. 2017), there is an emerging consensus that human action is the dominant control on environmental systems. As discussed above, one of the key drivers for developing an understanding of the functioning of carbon landsystems is to have the tools to actively manage these systems. Chapter 9 reviews progress in this direction. Finally, in Chapter 10 the benefits of integrating geomorphological understanding into our analysis of the terrestrial carbon cycle are summarised and conceptual and practical approaches to the concurrent analysis of biological, microbiological and geomorphological components of the carbon landsystem are proposed.
Chapter Two Geomorphology and the Fast Carbon Cycle
Introduction
Carbon is cycled through the terrestrial system at a range of time and space scales via a wide variety of processes. A distinction is commonly made between the slow carbon cycle, which occurs over timescales of hundreds of thousands of years, cycling carbon between the lithosphere, the oceans and the atmosphere, through processes of weathering and sedimentation, and the fast carbon cycle, which operates at timescales of seconds to millennia, and involves the transfer of carbon between soils, vegetation and the atmosphere dominantly controlled by biological processes.
This chapter outlines the key processes which drive the fast carbon cycle and identifies major interactions between geomorphological processes and terrestrial carbon cycling. The fast carbon cycle is effectively the biological carbon cycle. Carbon is removed from the atmosphere photosynthetically by plants, both on land and in the oceans. It is also returned to the atmosphere through plant and animal respiration processes, which mineralise organic carbon and release carbon dioxide and methane to the atmosphere (Figure 2.1).
Figure 2.1 The fast carbon cycle. Source: Martin Evans.
Photosynthesis
Organic carbon is fixed into living vegetation through photosynthesis. Photosynthesis is a critical component of the fast carbon cycle since it removes gaseous carbon from the atmospheric store (carbon dioxide) and fixes it into solid organic matter in plant tissues. Photosynthesis is a reduction reaction facilitated by chlorophyll in green plants. Chlorophyll is a pigment which absorbs light energy to utilise in enzyme mediated reactions, which fix carbon into carbohydrate and releases oxygen, as shown in Equation 2.1:
Equation 2.1: 6CO2+6H2O+photons→C6H12O6+6O2
The primary environmental controls on the photosynthetic reaction are light intensity, carbon dioxide concentration and temperature. Water supply (soil moisture) is also important since it controls the opening and closure of leaf stomata, through which leaves exchange the gaseous photosynthetic reactants and products (CO2 and O2).
The critical control of the biosphere and photosynthesis over the fast carbon cycle is apparent from monthly atmospheric CO2 data (Figure 2.2). The seasonal cycle of CO2 flux is driven by enhanced photosynthetic fixation of carbon and lower atmospheric carbon dioxide concentrations during the northern hemisphere summer (which has a greater impact due to the hemispheric distribution of land masses).
Figure 2.2 Mean global CO2 concentration 2012–2017. Superimposed on the rising trend is a clear seasonal pattern. Source: After NOAA (https://www.esrl.noaa.gov/gmd/ccgg/trends) Public Domain Data.
Globally, Beer et al. (2010) estimate that total fixation of carbon by terrestrial plants is 123 ± 8 PgC per year, with 40% of this uptake occurring in tropical forests. This Gross Primary Productivity (GPP) figure does not represent the net carbon flux from the atmosphere because it does not take account of carbon release from vegetation by autotrophic respiration. The balance of these two fluxes is referred to as Net Primary Productivity (NPP).
Estimates of global terrestrial NPP are derived from a range of modelling approaches. Models typically are driven by light intensity (usually measured as Photosynthetically Active Radiation (PAR) which is defined as wavelengths between 400–700 nm). The ability of plants to photosynthesise carbohydrates from this light energy is controlled by plant physiology. Climatic variables, particularly temperature and precipitation, are often taken as proxies for plant growth in simple models, although more complex approaches have process-based sub-models describing plant function, and allocate material between soils, the atmosphere and vegetation (Cramer et al. 1999). In a major model comparison study, Cramer et al. (1999) ran 17 models spanning 3 main approaches (satellite-based, fixed plant structure models and dynamic vegetation models) (Figures 2.3a and 2.3b). Model outputs predict a global total terrestrial NPP of 44.4–66.3 PgC per year. This is approximately 10% of the IPCC estimate of global carbon storage in vegetation (Cubasch et al. 2013). It is a characteristic of the global carbon cycle that net gaseous carbon fluxes are small residuals from the difference between carbon uptake and release. As both of these fluxes are large, the sensitivity of this residual to proportionally small changes in uptake or release may be substantial. In the oceans, primary productivity is of a similar magnitude to the terrestrial system, but lower rates of NPP are balanced by the larger area of the ocean. Estimates range from 38–65 PgC yr–1 (Buitenhuis et al. 2013).
Figure 2.3 (a) Global photosynthesis from 17 global models; (b) Annual NPP g cm2 a–1 averaged across 17 models. Source: After Cramer et al. 1999. Reproduced with permission of Wiley.
Heterotrophic, Soil and Ecosystem Respiration
Ecosystem respiration is the sum of autotrophic respiration in plants and heterotrophic respiration by decomposer organisms. Net Primary Productivity (NPP) is the balance of Gross Primary Productivity (GPP equivalent to total photosynthetic fixation) and autotrophic respiration. This means that net carbon balance can be conceptualized either as the balance of GPP and ecosystem respiration or as the balance of NPP and heterotrophic respiration. Another term which is widely used is ‘soil respiration’. Soil respiration is measured by release of CO2 to the atmosphere at the soil surface. This represents total below ground respiration or the sum of heterotrophic respiration and plant root respiration (cellular respiration in the roots of vegetation).
Globally, plant root respiration is 30–50% total soil respiration (Bond-Lamberty et al. 2004). Recent data on the ratio of heterotrophic respiration to total soil respiration indicates that the proportion of heterotrophic respiration in soil respiration is increasing over time in response to climate warming with increases from 54% to 63% between 1990 and 2014 (Bond-Lamberty et al. 2018). This represents an increasing flux of carbon to the atmosphere through heterotrophic respiration of soil carbon. Global estimates of soil respiration are 68–98 PgC per year (Jian et al. 2018). Global autotrophic respiration is estimated at 64 ± 12 PgC per year (Ito 2020). The IPCC estimate total respiration and biomass burning at 118.7 PgC per year (Cubasch et al. 2013 and Figure 1.1).
Accumulation of carbon in the biosphere through NPP is offset by gaseous carbon release via mineralisation of soil carbon through heterotrophic respiration of soil detritivores. Decomposition and mineralisation of detrital organic matter is dominated by heterotrophic microbes (primarily bacteria and fungi). An excellent summary of microbial processes controlling soil respiration is given by Kirchman (2012) and the reader is referred to this source for a detailed introduction to these processes. A summary of key points is presented below.
Organic matter fixed by primary productivity is respired by microbial biota. Large detrital material is not directly utilised, but is broken down into smaller components through the release of hydrolase enzymes by microbes (Burns and Dick 2002). Organic matter in the size range < 500 Da is transported across the cellular membrane and metabolised inside bacterial and fungal cells. Aerobic respiration decomposes organic matter in the presence of oxygen releasing CO2 and H2O. Not all heterotrophic soil respiration represents the decomposition of old carbon, as at least 50% of soil respiration results from the metabolisation of root exudates by soil microbiota (Kirchman 2012). In forested systems, Högberg et al. (2001) have demonstrated that cutting the supply of photosynthates to root systems by removing tree bark results in a 54% reduction of soil respiration. This means that modern root exudates are the dominant substrate for microbial respiration (particularly by mycorrhizal fungi). Consequently, much of the carbon which is respired back to the atmosphere is rapidly turned over from the soil (through mineralisation of modern labile carbon from the root exudates) to the atmosphere. For these reasons, NPP and respiration are strongly linked at both the ecosystem and global scale (Figures 2.4a–2.4c).
Figure 2.4 (a) and (b) Relation between above ground NPP/litterfall and soil respiration for forests and grasslands. Source: After Raich and Tufekciogul 2000. Reproduced with permission of Springer. (c) NPP vs. respiration across a range of global biomes. Source: After Kirchman 2012. Reproduced with permission of Oxford University Press.
In aqueous systems, bacteria dominate microbial decomposition of organic matter because they are better able to compete for small dissolved molecules. In contrast, in terrestrial systems fungi may represent over half the microbial biomass, because fungal hyphae have the ability to span dry microsites in the soil system, and access remote sources of metabolites (Kirchman 2012). Fungi are also able to degrade recalcitrant materials in terrestrial environments such as lignin, whereas bacteria respire simpler and more labile organic compounds. However, research in arctic soils has indicated that fungi are also able to utilise available simple compounds (Rinnan and Bååth 2009). In addition, fungi are also more drought tolerant than bacteria (de Vries et al. 2012, 2018; Yuste et al. 2011), which means that they are more able to tolerate fluctuating moisture conditions and potentially play an important role in cycling carbon in disturbed systems.
Decomposition of organic matter by aerobic respiration in the presence of oxygen is an oxidation reaction where carbon is oxidised to CO2. This redox reaction generates electrons which are moved through a biochemical electron transport chain to a final electron acceptor. In the process of aerobic respiration, oxygen is the electron receptor. Under anaerobic conditions, carbon dioxide can still be produced from carbon and oxygen atoms within the organic matter, but an alternative electron acceptor is required. Nitrates and sulphates are common ionic species that perform the role of an electron acceptor. Anaerobic microorganisms (particularly bacteria and Archaea) respire in low oxygen environments using these alternative electron acceptors.
In the context of the carbon cycle, the most important mechanism of anaerobic respiration is production of methane by methanogenic microorganisms (primarily Archaea). Methanogenesis occurs in very low redox conditions once all molecular oxygen, nitrate, iron, manganese and sulphur has been reduced (Smith et al. 2003). Anaerobic decomposition of organic matter is very slow and so saturated reducing environments such as deep peats preserve organic matter and store organic carbon fixed from the atmosphere. For example, Lee et al. (2012) report rates of carbon release from thawed permafrost which were 4–10 times more rapid under aerobic conditions. Even correcting for the fact that aerobic emissions are predominantly CO2 rather than CH4, greenhouse warming potential from melted permafrost in aerobic conditions had a climate forcing 1.5–7.1 times higher than in anaerobic conditions.
Despite the rapid decomposition of organic matter under aerobic conditions, the high greenhouse gas potential of methane results in the process of methanogenesis of organic matter in reducing conditions, having the potential to produce substantial impacts on atmospheric greenhouse gas budgets. However, as a consequence of oxidation, methane has a shorter residence time in the atmosphere than CO2 (about 12 years: Myhre et al. 2013), so its relative influence on greenhouse forcing is reduced at longer timescales. For example, considering the impact of CO2 sequestration and CH4 production in peatlands, Whiting and Chanton (2001) concluded that at 20 year timescales methane production increased greenhouse warming potential, whereas over 100 years, temperate and subtropical peats provide a net cooling influence on the atmosphere, and northern wetlands are greenhouse neutral. At 500-year timescales, all peatlands were found to have a net climate cooling effect. In this study, warmer temperatures in the wetlands with longer summer growing seasons fixed more CO2 and sequestered more carbon, hence their greater cooling potential.
Controls on Heterotrophic Respiration of Soil Carbon
Soils are complex biogeochemical systems, so that the controls on rates of soil respiration are numerous and vary substantially in both time and space. For example, Bragazza et al. (2013) describe complex feedbacks between warming temperatures, increasing prevalence of Ericaceous shrubs on peatlands and changes in microbial populations, which favour enhanced decomposition of soil organic matter. Similarly, Van Der Heijden et al. (2008) argue that soil microbial populations are key drivers of plant productivity through their role in facilitating and competing for nutrient supplies (nitrogen and phosphorus).
Vegetation is an important control on rates of soil respiration, because of the linkages between plant and microbial populations, and also because of the key role of root exudates as substrates for microbial respiration. However, it has been argued that at the global scale the influence of vegetation type on respiration rates is secondary to moisture and temperature controls (Raich and Tufekciogul 2000).
Temperature is a primary control on rates of soil respiration (Davidson and Janssens 2006; Lloyd and Taylor 1994; Rustad et al. 2000). Respiration is a chemical process biologically mediated by enzyme activity, so that the temperature dependence of chemical reactions and enzyme activity drive this relation. There is a strong positive correlation between mean annual air temperature for global biomes and reported rates of soil respiration (Raich and Schlesinger 1992).
The temperature dependence of respiration is commonly reported as Q10 values, where these represent the proportional increase in respiration for a 10 degree rise in temperatures (Figure 2.5b). Kirschbaum (1995) reviewed global Q10 data and showed that values decline non-linearly from values of eight at temperatures of 0°C to less than two at 35°C. This non-linearity is potentially important as a control on changing spatial patterns of soil respiration in response to global change, since rapid warming of high latitudes (Lloyd and Taylor 1994; Pithan and Mauritsen 2014) is coincident with climate regimes where higher Q10 values are common, and additionally with extensive storage of soil carbon across the northern peatlands (Gorham 1991).
Figure 2.5 (a) Relation between air temperature and soil respiration rates from a global dataset; (b) Q10 values from a global dataset of soil respiration rates. Source: After Raich and Schlesinger 1992. (https://doi.org/10.3402/tellusb.v44i2.15428) CC-BY.
Conant et al. (2011) identify three key processes which mediate the temperature response: 1) rates of enzymatic depolymerisation of large molecules; 2) rates of microbial enzyme production; and 3) processes which limit availability of soil organic matter to microbial action, including adsorption to mineral surfaces and soil aggregate turnover (summarised in Figure 2.6). However, there is not a simple relation between temperature and respiration because of other environmental influences on the rate of reaction. Kirschbaum (2006) emphasises the importance of moisture regime and substrate availability in modifying the relation, whilst Conant et al. (2011) have modelled variable temperature dependence of soil respiration based on characterisation of either physical or chemical mechanisms protecting soil organic matter from decomposition, as described below.
Figure 2.6 Controls on soil organic matter decomposition. Source: After Conant et al. 2011. Reproduced with permission of Wiley.
After temperature, moisture is the best studied biophysical control on soil respiration rates; arguably in terms of local spatial variability in respiration rates, it is the most dominant. The principle mechanism underlying moisture control on respiration is through control on oxygen concentrations in saturated soils (since respiration requires an electron acceptor). Local accumulation of soil organic matter in wetland soils occurs because saturation limits rates of microbial decomposition. In peatlands with saturated soils, modelling of soil respiration rates is improved over simple temperature dependent approaches (Lloyd and Taylor 1994) through the addition of a water table term (Rowson et al. 2013). The response of respiration rate to moisture is however not simply bimodal between saturated and unsaturated conditions. Linn and Doran (1984) demonstrated that maximum activity by aerobic microbes occurs at around 60% pore water saturation, with activity at higher saturations limited by reduced oxygen content, and at lower saturations by water content associated limitations on microbial activity.
The combined importance of temperature and moisture as controls on respiration rates have also been explored in the context of a topographic component. Altitudinal variations in temperature, and the control of slope angle, slope form and slope position on soil moisture (Burt and Butcher 1985) mean that there is potentially strong topographic control on spatial patterns of respiration rate. This has been demonstrated at the hillslope scale in forests in Korea by Kang et al. (2003), who conclude that Q10 based models of soil respiration rates are inadequate for prediction at large spatial scales, unless topographic controls on soil moisture are considered.
Microbial Population Controls on Heterotrophic Soil Respiration
Wieder et al. (2013) argue that conventional modelling approaches to turnover of soil carbon, which are primarily driven by the nature of the organic substrate and simple kinetic models of decomposition rate, are insufficient to capture the
