Use of real-time in situ nitrogen sensors to enhance sustainability and reduce costs in livestock systems (PhD)

The efficient use of nitrogen (N) fertilisers in agriculture is of great concern as diffuse losses, where N has been applied in excess of crop demand, may lead to significant environmental pollution and contribute to global warming. There is also direct economic cost to the agricultural sector. This project investigated a range of novel and emerging techniques to better enable the real-time and in-situ determination of soil N levels, in order to increase our fundamental understanding of soil N dynamics and improve management of agricultural soils. Microdialysis is an emerging technique that has been used for in-situ and minimally-invasive sampling of soil solution solutes.

In study 1, the use of microdialysis for the assessment of soil N status was investigated. Diffusive-flux measurements of eight contrasting soils were compared to conventional soil core batch extractions (using 0.5 M K₂SO₄ or distilled H₂O).  The percentage contribution that amino acids, NH₄⁺, and NO₃^- made to total plant-available N were most similar to distilled water extractions. However, the relative magnitude of the diffusive-flux measurements did not always reflect the pool sizes as estimated by the soil extractions, which indicates the role that differing chemical and physical soil properties have in the control of plant N availability.

In study 2, microdialysis was used for the in-situ sampling of amino acids, NH₄⁺, and NO₃^- from the rhizospheres of maize (Zea mays L.) seedlings grown in soil filled rhizotubes. The results showed a significant decrease in soil solution [NO₃^-] as the root tip grew past the probe. Net amino acid exudation from root tips had been identified using direct sampling from root surfaces of seedlings grown in a sterile nutrient solution but this exudation was not evident in the microdialysis sampling, which was attributed to rapid microbial uptake. Study 3 investigated the use of commercially available NO₃^- ion-selective electrodes (ISEs) and dual-wavelength UV spectroscopy for the rapid on-farm measurement of soil N.  Our results showed that manual extraction using distilled H₂O, combined with either NO₃^- ISEs or UV spectroscopy could accurately determine the NO₃^- concentration of the extracts. As such, both of these methods have the potential to be used as on-farm quick tests.

In study 4, the use of novel NO₃^- ISEs for in-situ and real-time monitoring of an agricultural soil, both in a field trial and under controlled conditions in the laboratory, was demonstrated. Results from the ISEs were found to be statistically similar to conventional laboratory analysis of contemporaneous soil samples on 16 out of 19 occasions.  These novel NO₃^- ISEs provide a new opportunity for in-situ and real-time measurement of soil N dynamics, which represents a significant step forward for analytical soil science and environmental monitoring. Study 5 investigated the spatial variation of soil N in a grazed grassland field. It was established that at least 61% of the total accumulated variance in amino acids, NH₄⁺ and NO₃^- occurred at scales < 2 m, with significant variation occurring at the sub 1-cm scale. This data was used to demonstrate how an in-situ sensing network could be optimised on a cost-accuracy basis. Future work needs to focus on how data derived from in-situ soil N sensors can be used to improve fertiliser recommendations and the efficiency of N-use in agriculture.

Scientific literature
Shaw, R., Prysor Williams, A., Miller, A., and Jones, D.L. 2013 Assessing the Potential for Ion Selective Electrodes and Dual Wavelength UV Spectroscopy as a Rapid on-Farm Measurement of Soil Nitrate Concentration. Agriculture 2013, 3(3), 327-341; doi:10.3390/agriculture3030327

The Project

Optimising the use of nitrogen represents one of the major goals of sustainable livestock farming systems from both an economic and environmental standpoint. While there have been thousands of studies investigating different management strategies for optimising nitrogen use on farms, translating this research into practical management advice and subsequent adoption by farmers has been very patchy. Consequently, as evidenced by numerous recent reports, there is no doubt that we have a long way to go before nitrogen is used efficiently within the UK livestock sector (Wilkins, 2008; Rees and Ball, 2010; Spiertz, 2010). Why haven’t we achieved better success to date and why has previous research been a relatively blunt instrument for change? We feel this is because we have failed to provide farmers with hands-on, field-based tools. Rather we have relied on virtual (computer)-based approaches which have limited adoption and lack precision (Cuttle and Jarvis, 2005). To rectify this deficiency, we propose to use simple field-based sensors that can monitor soluble N in soil in real-time allowing active management of fertiliser and waste resources. Linked to SMS technology, this could also provide a new way for farmers to remotely monitor N concentrations. Ultimately, these sensors will enhance fertiliser use efficiency, allow better timing of waste applications, reduce greenhouse gas emissions (N2O) and reduce surface- and ground-water pollution. They should also reduce farm costs, help refine fertiliser recommendations (i.e. RB209), whilst also reducing the environmental footprint of the UK livestock sector.

The proposal addresses the following EBLEX-HCC-QMS research priority areas:

1. Develop guidelines for good environmental control alongside profitable and sustainable production
2. Develop systems to manage livestock wastes and optimise their nutrient value
3. Improve the persistence, nutritional value and utilisation of forage crops to increase grazing days, improve performance, reduce costs and add to meat quality

The Challenge

This project will aim to demonstrate how the application of new sensor technologies can enable practical management of nitrogen on livestock farms. The project objectives are as follows:

1. Demonstrate and validate the use of real-time NO3- sensors within pastures undergoing contrasting fertiliser regimes (inc. slow release fertilisers)
2. Demonstrate and validate the use of real-time NO3- sensors for fertiliser management in fodder crop systems
3. Sensor measurements will be compared with the currently available methods such as conventional soil core analysis, ion-exchange resins (Qian & Schoenau 2005) and small-scale lysimeters
4. The sensors will also be deployed around point sources of N release (e.g. waste stores) to investigate their role in the active pollution prevention control
5. Undertake an economic analysis to assess potential cost savings arising from technology adoption
6. Promote awareness of the technology within the livestock industry

Approach

This studentship will comprise five stages:

Stage 1: Literature review: The student will carry out a literature review to evaluate current technologies and strategies for monitoring nitrogen in soil and water, general aspects of farm nutrient management, relevant legislation and socioeconomic barriers to effective nutrient management.

Stage 2: Farmer recruitment: Three sheep and three cattle farms will be identified for demonstrating the technology at the farm scale, promoting active engagement and for gaining feedback for future technology enhancement.

Stage 3: Grassland N monitoring: We will install replicate N sensors in the topsoil (rooting) and subsoil zone in a range of fields at each farm. These will be equipped with dataloggers for hourly monitoring of soil NO3-. To validate the technology will we simultaneously monitor soluble N by conventional KCl extractions at biweekly intervals over a 12 month period alongside plant N and other key soil quality parameters. As part of the technology demonstration to farmers we will test the sensors against a range of conventional and slow-release N fertilisers (both inorganic and organic N forms).

Stage 4: Forage crop trial: In addition, to the grassland trials we will also undertake experiments to monitor N dynamics under fodder maize and beet again to demonstrate their usefulness in predicting where and when to apply N fertilisers. These trials will be used for demonstration open days.

Stage 5: Sensors will also be placed around six manure and slurry stores and NO3- concentrations subsequently monitored over time. This will be used to assess their potential to quickly detect excess N release.

Stage 6: Socioeconomic analysis: Finally we will undertake an economic analysis of the potential savings in fertiliser use based on the results from the six test farms.

Student

Rory Shaw