IAP-24-023

Greenhouse gas balance in an agricultural grassland system with enhanced rock weathering of basalt as a carbon dioxide removal strategy

Agricultural systems emit around 10% of all UK greenhouse gas (GHG) emissions and represent one of the most challenging sectors to transition to net zero by 2050. A crucial element for achieving this pathway is to increase the number of GHG sinks relative to sources on farms. One exciting innovation is the agronomic practice of enhanced rock weathering (ERW) that involves spreading silicate rock dust on fields. The dissolution of alkaline minerals in rock dust supplies micronutrients to boost crop and soil health, deacidifies soils, and potentially results in long-term carbon dioxide removal (CDR) from the atmosphere via storage of carbon in solid carbonate minerals or ocean alkalinity (1). Modelled projections of the viability of this practice as a CDR strategy suggest it is of global significance being scalable to the gigatonne-CO2 level by the end of the century (2). However, for this to happen it is important to demonstrate where the products of enhanced weathering reactions are transported to and stored, as well as determining any potential co-benefits relating to wider aspects of carbon and GHG cycling in agricultural systems: including soil organic carbon storage (3), and methane (CH4) and nitrous oxide (N2O) emission (4) and enhanced grassland productivity. There is limited evidence suggesting that ERW may reduce CH4 and N2O emissions, by supplying trace elements that otherwise are a limiting factor in microbial oxidation of CH4, and by raising pH to promote reduction of N2O to N2(5).
This PhD project will test the hypothesis that ERW constitutes a net negative GHG emitting practice, the effects of which are measurable above uncertainty. The project will involve directly measuring all aspects of GHG cycling within a farmed grassland trial of enhanced rock weathering with basalt at Newcastle University’s Cockle Park Farm. The unique site and sampling infrastructure (using hydrologically isolated plots, HIPs, in a replicated study design) make this a globally significant experiment into the viability of ERW.
Collaboration on this project between Newcastle University, UN-DO, and UKCEH will allow for complementarity with other ongoing projects on ERW and GHG flux. It will also provide the PhD student with a professional network of experts and a community to tap into, and the potential for comparative, cross-site studies (widening the generalisability of the research beyond this site).

This project will focus around a multi-year field trial of ERW at Newcastle University’s Cockle Park Farm, a working commercial farm that is used for a wide range of research into agricultural systems and has extensive state-of-the-art facilities including laboratory spaces and an integrated network of sensors for gathering environmental and climate data (see https://www.ncl.ac.uk/farms/virtual-tours/). Th PhD will take advantage of 9 replicated Hydrologically Isolated Plots (HIPs) with drainage that allows infiltrated water and surface run-off water to flow out separately through pipes that can be automatically sampled in response to discharge. This allows detailed measurements of solution chemistry to be made at regular intervals across rainfall events, and compared between plots that have had different treatments applied to them. In addition to water sampling, the HIPs ERW project is equipped with static chambers that can be deployed to measure fluxes of CO2, CH4, N2O and other significant gases from soil.
In 2024, a trial of ERW using basalt began at the HIPs in collaboration with the commercial CDR supplier, UN-DO. Starting in autumn 2024, baseline sampling and monitoring is being undertaken, in order to characterise background signals, and spatial and temporal variability in relevant data. Basalt rock dust, supplied by UN-DO, will be spread on a subset of the plots in 2025. Grasses will be grown on the plots, to be used as fodder/pasture for the herd of dairy cattle at Cockle Park Farm. Previous large-scale field experiments of ERW have focussed mainly on arable land (4, 6, 7), so such a trial in a grassland system is novel.
This PhD project will involve field measurements and data collection, laboratory analysis, and data analysis and synthesis across components of GHG cycling at the HIPs. This includes:
• Solution chemistry: using drainage water samples and pore water samples collected at HIPs automatically and from topsoil using rhizons/lysimeters, to measure pH, electrical conductivity, total alkalinity, dissolved inorganic carbon, major anions and cations.
• Gas fluxes: measured directly from topsoil at HIPs. This will leverage high level of expertise of CEH with automated chamber sampling techniques.
• Soil chemistry: using soil cores collected at HIPs, processed in dedicated soil chemistry laboratory space at Newcastle University to measure soil pH, soil organic carbon, cation exchange capacity, base saturation, major and trace cation concentration, and other soil textural and compositional properties.
• Plant yield and chemistry: processing grass harvested at these plots to measure yield, and major and trace cation concentration.
This data will be used to construct detailed mass budgets of GHG fluxes throughout the experiment, making use of other environmental data streams collected at the farm site. This includes a mass balance estimate of weathering of the basalt material added to fields (8), charge balance budget of solutes (9), and whole system partitioning of GHG fluxes (4).

Year 1

• Literature review to inform study focus and methodologies
• Design and test methodologies for sampling from HIPs – e.g. drainage water, pore water, soil, gas flux
• Analysis (with training) – solution chemistry, gas flux, soil chemistry
• Synthesis – baseline chemistry and variability at HIPs – Chapter 1 / manuscript writing

Year 2

• Based on year 1 findings design targeted experiments – e.g. N fertiliser application study in a mesocosm experiment – for mechanistic understanding of trace gas flux
• Sampling – drainage water, pore water, soil, gas flux
• Analysis – solution chemistry, gas flux, soil chemistry
• Synthesis – initial response to ERW of system – Chapter 2 / manuscript writing

Year 3

• Sampling – drainage water, pore water, soil, gas flux
• Analysis – solution chemistry, gas flux, soil chemistry
• Modelling – use of observational field data and experimental results to parameterise process-based models (e.g. DayCent, DNDC) to predict flux under different management scenarios.
• Synthesis – long-term evolution of system, GHG balance, implications – Chapter 3 / manuscript writing

Year 3.5

• Synthesis – finalising thesis and submission

Expertise developed over the course of this project will include:
• Field sampling techniques: statistical methods in field sampling, soil coring, water sampling (for different measurables), lysimeter pore water sampling, rhizon pore water sampling, gas flux measurements with static chambers.
• Laboratory techniques: soil property identification, soil chemical sequential extractions, chemistry lab techniques and methods (microwave digestion, centrifuging, pipetting, filtration).
• Analytical methods: pH electrode, titration, infrared spectroscopy (solutions and solids; for DOC, DOC, TOC, TIC), ion chromatography, inductively coupled plasma optical emission spectroscopy, ion-selective electrode, Fourier-transform infrared spectroscopy, inductively coupled plasma mass spectrometry.
• Data analysis and synthesis: using equipment-specific software, programming languages (e.g. Matlab, R), data presentation tools (e.g. GraphPad).
• Modelling using Daycent, DNDC or other process-based models.
• Academic literature writing, submission to peer-review, conference presentation
• Independent research skills: design, hypothesis formulation and testing.
• Grant proposal writing: budgeting, submission.
• Translatable skills: stakeholder engagement, time and project management, CV writing, interviewing.