Bangor University DOCTOR OF PHILOSOPHY An evaluation of perennial mobile green manures for climate change mitigation in agriculture Ward, Chloe Award date: 2020 Awarding institution: Bangor University Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? 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Oct. 2021 Bangor University DOCTOR OF PHILOSOPHY An evaluation of perennial mobile green manures for climate change mitigation in agriculture Ward, Chloe Award date: 2020 Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. 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Feb. 2021 An evaluation of perennial mobile green manures for climate change mitigation in agriculture A thesis submitted to Bangor University by Chloe Ward In candidature for the degree of: Philosophiae Doctor September 2020 School of Natural Sciences Bangor University Bangor Gwynedd LL57 2UW Table of Contents Abstract ................................................................................................................................... 3 Acknowledgements……………………………………….………………………………………………..…………….…… 5 List of Abbreviations……………………….……………………………….…………………………………..………....... 7 Chapter 1: Introduction and thesis aims …………………………………………………………..…………….… 9 Chapter 2: Literature review: Climate change impact of green manure use in agriculture and a novel approach to mitigation…………………………………………………..………..…. 17 Chapter 3: Evaluating PMGMs for efficacy as N fertiliser and climate change impact: A Pot Experiment………………………………………………………………………………..…………. 67 Chapter 4: Effect of PMGMs on soil organic carbon: An incubation experiment……..…….. 111 Chapter 5: Evaluating PMGMs for efficacy as N fertiliser and climate change impact: A field experiment……………………………………….………………………………...……………. 145 Chapter 6: Effect of two years of PMGM additions on crop N fertilisation and climate change impact: A continuation of the field experiment……………………..…...…… 177 Chapter 7: Synthesis, Conclusions and Outlook……………………..………………………………………. 213 Appendix 1: Supplementary data for chapter 3………………………………………………………………. 237 Appendix 2: Supplementary data for chapter 5………………………………………………………………. 243 Appendix 3: Supplementary data for chapter 6……………………………………………….……………… 253 1 2 Abstract Much of the climate change impact of agriculture is due to the addition of nitrogen (N) to soil to increase crop yields. The production of mineral N fertiliser from atmospheric N2 causes carbon dioxide (CO2) emissions, and once added to soil, fertiliser N is easily transformed to other forms - and lost as leached nitrate (NO3 ), ammonia gas (NH3) and the greenhouse gas nitrous oxide (N2O). An alternative to fertiliser N is the use of green manures, plants grown on agricultural land, which after incorporation into soil provide N to the following crop. N fixing green manures gain atmospheric N by a symbiosis with microbes, therefore adding N without CO2 emissions, and providing beneficial organic matter to soil. Counteracting these advantages is a lower land use efficiency due to the space required for green manure cultivation. Green manure additions can result in carbon (C) sequestration, however soil C dynamics are complex and soil C can also be lost by a stimulation of respiration known as priming. N pollution from soil is generally higher when mineral N concentrations are high, which can result from poor synchrony of N supply with crop demand. The incorporation of green manures offers limited scope for effective targeting of N. This synchrony of N supply with demand could be improved by, instead of incorporating green manures, harvesting the plant material and adding it to soil to meet crop demand. Negating the requirement for incorporation, allows use of a wider range of species including shrubs and trees. Growing N fixers suited to lower quality land e.g. wet or exposed areas, could increase land use efficiency by reducing the demand for prime agricultural land. We chose three such N fixing species and refer to these as Perrenial Mobile Green Manures (PMGMs): Alnus glutinosa (Alder), Gunnera manicata (Gunnera), and Ulex europaeus (Gorse). This thesis investigated N provision to a crop, and potential for N pollution by these PMGMs, compared to the conventional green manure, Trifolium pratense (red clover) and ammonium nitrate (NH4NO3) fertiliser, in a one year pot experiment, and field experiments over two seasons. A six week incubation investigated soil C dynamics by use of 14C isotopes. In both pot and field experiments, the PMGMs supplied N to crops at a slower rate than clover or NH4NO3 but by the end of the pot experiment resulted in equal or more biomass and N uptake than from clover. Potential for N pollution from PMGMs was considerably lower than from clover - or NH4NO3, with mean NO3 concentrations in the soil solution of the pot experiment reaching -1 -1 only 25 mg N L compared to over 130 mg N L from clover and NH4NO3 additions. Emissions of N2O from PMGMs were considerably lower than those from clover and NH4NO3, in the pot trail, with applied N lost as N2O-N from gunnera and alder being 0.34 %, and 0.61 %, respectively compared to 5.3 % from clover. N2O emissions from all treatments in the field experiment were low which is likely to be due to dry weather conditions. Data from the incubation study indicated that after one year, gunnera and alder additions could result in a net loss of C due to priming. These predictions, however, do not consider factors of cropping soil e.g. roots, and meso and macro fauna and require further investigation. We conclude that PMGMs could improve N use - efficiency and reduce NO3 leaching and N2O emissions compared to incorporation of clover, and have potential for a more favourable C balance than NH4NO3. Thus, PMGMs have strong potential for inclusion into a more sustainable agricultural landscape. 3 4 Acknowledgements Many thanks to my fantastic supervisors Dr Paul Hill and Professor Dave Chadwick. I am greatly indebted to Paul, for initially taking on my ideas and giving me the opportunity to undertake a PhD, for supporting me over the years with generosity and stamina, and for believing in the validity of owls. I have benefitted hugely from Paul and Dave’s combined and complementary expertise. Thank you to our expert lab technicians Jonathan Roberts and Sarah Chesworth for holding everything together and for support during stressful times. Thanks also to the Henfaes university farm technicians, Mark Hughes and Llinos Hughes for help and advice with field experiments. I have also benefitted from helpful discussions with the panel during my annual progression reviews. I was lucky to have help from two interns, so thanks to Zac Blow and Christopher Milne, especially to Christopher for very generous amounts of time in the lab and field. Thanks also to the many staff and students on the 2nd floor of the Environment Centre Wales, for friendship, support, and musings on the eccentricities of the academic environment. Also, for advice and practical help with lab and field work, especially from Tim Peters, Joe Cotton, Antonio Sánchez-Rodríguez and Eduardo Mariano, but many more. I’ve much appreciated the many supporting services of Bangor University which make research possible and more enjoyable, for me, most notably from the fantastic information and library services, and the grounds team who maintain the beautiful wildflower plantings outside ECW, which made such a difference to a spring day. The project was funded by KESS 2, in partnership with the Centre for Alternative Technology (CAT). KESS 2 is supported by the European Social Fund through the Welsh Government. Many thanks to the staff of KESS, and to Dr Jane Fisher of the CAT Graduate School of the Environment. 5 6 List of Abbreviations BNF Biological Nitrogen Fixation CDD Cumulative Day Degree CEC Cation Exchange Capacity DM Dry Matter EF Emission Factor FE Fertiliser Equivalent DOC Dissolved Organic Carbon DON Dissolved Organic Nitrogen DW Dry Weight GHG Greenhouse Gas IPCC Intergovernmental Panel on Climate Change NRE Nitrogen Recovery Efficiency NUE Nitrogen Use Efficiency PMGM