Micucci, Gianni 
ORCID: 0000-0003-0476-4377
(2024).
Constraining in situ denitrification and its role as nitrous oxide sink in conventional and regenerative agriculture.
University of Birmingham.
Ph.D.
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Micucci2024PhD.pdf
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Abstract
The current food production system is responsible for 19 to 29% of the overall global emissions of greenhouse gases. In particular, it contributes about 75% of the global anthropogenic nitrous oxide (N2O) emissions, a greenhouse gas with 298 times greater radiative forcing than carbon dioxide (CO2). These emissions mostly result from the large quantities of synthetic nitrogen (N) fertilizer applied in arable agriculture. In the UK, intensive cultivation practices are the main contributors to the ~£1.2 billion annual costs of soil degradation, which represents 30% of the UK farm gate income. To ensure future food security whilst addressing these environmental and economic challenges, a transition to less intensive food production systems and a better understanding of the mechanisms and controls of N2O emissions are needed. However, this better understanding can only be complete by constraining the denitrification process.
Denitrification is the anaerobic microbial respiration process which transforms soil nitrate (NO3-) into gaseous dinitrogen (N2). When incomplete, it can be a predominant source of N2O in soil, while its full sequence of reaction is the only natural terrestrial sink for N2O emissions. The high sensitivity needed to distinguish small soil-N2 fluxes from the high N2 atmospheric background (~78%) unfortunately restricts its measurement to peak events. A new methodology is therefore needed to resolve the enigma of denitrification quantification in soil.
In contrast with conventional farming management, regenerative agriculture aims to restore degraded lands and maintain soil fertility using crop rotations, including 3– 4 year “ley” mixture comprising of grasses and nitrogen-fixing legumes such as clovers. These leys have the double benefit of increasing soil organic carbon and naturally fertilizing soil with nitrogen, reducing reliance on synthetic fertilizer. However, little is known about the impact of these leys on the soil nitrogen cycle and their potential contribution to greenhouse gas emission reduction.
The present work aims to investigate the importance of denitrification in conventional agriculture and long term ley rotations; in particular in response to N fertilizer application. To address the challenges of denitrification quantification, a new methodology combining 15N tracer and N2-depleted artificial atmosphere has been developed and subsequently used for a one-year field campaign. This successful campaign was the first UK attempt to characterize in situ denitrification rates under artificial atmosphere.
The newly developed method enabled the detection of soil-N2 fluxes 90% of the time, with an 8 fold greater sensitivity than conventional methods and allowing the simultaneous incubation of 24 soil cores; capturing spatial variability at farm scale. The field campaign spanned from March 2022 to May 2023 and was characterized by a large denitrification activity in the arable control following fertilizer application. Indeed, 15 kgN ha-1 were lost in this field (~8% of the 200 kgN ha-1 applied) through denitrification between April and October 2022. Our measurements indicated that 9 % of this flux occurred as N2O rather than N2. The application of fertilizer in leys also resulted in 3 times higher N2O emissions and an N2O sink efficiency dropping from 93% to 85%.
The present work improves the characterization and understanding of the denitrification process and highlights the high existing correlation between nitrous oxide sink efficiency and soil nitrate availability; calling for new strategies of fertilizer application in agricultural fields.
| Type of Work: | Thesis (Doctorates > Ph.D.) | |||||||||||||||||||||
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| Award Type: | Doctorates > Ph.D. | |||||||||||||||||||||
| Supervisor(s): |
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| Licence: | All rights reserved | |||||||||||||||||||||
| College/Faculty: | Colleges (2008 onwards) > College of Life & Environmental Sciences | |||||||||||||||||||||
| School or Department: | School of Geography, Earth and Environmental Sciences | |||||||||||||||||||||
| Funders: | Natural Environment Research Council | |||||||||||||||||||||
| Subjects: | Q Science > Q Science (General) | |||||||||||||||||||||
| URI: | http://etheses.bham.ac.uk/id/eprint/14498 |
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