Developing in-situ and real-time methods of soil nitrogen determination

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. This thesis investigated a range of novel and emerging techniques to better enable the real-time and in-situ determination of soil N, in order to increase our fundamental understanding of soil N dynamics and improve management of agricultural soils. Microdialysis is an emerging technique which has been used for in-situ and minimally-invasive sampling of soil solution solutes. In Article 1, the use of microdialysis for the assessment of soil N status was investigated. Diffusive-flux measurements of 8 soils along a catena sequence 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 Article II, microdialysis was used for the in-situ sampling of amino acids, NH4+, and NO3- from the rhizospheres of 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. Article III 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 Article IV, 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. Article V, investigated the spatial variation of soil N in a grazed grassland field in order to optimise the spatial and economic configuration of an in-situ sensor network. 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.