Understanding interactions of barley (Hordeum vulgare) with soil nitrogen cycling activity and links to plant nitrogen preference

Nitrogen is a crucial nutrient for plant survival and commonly applied as synthetic fertiliser in the form of NH4NO3 or urea in conventional agriculture. In soil, ammonium is rapidly converted to nitrate through the microbial process of nitrification. Much of the applied nitrogen is lost through leaching or release of N2O, a potent greenhouse gas, through denitrification, which reduces efficiency and increases costs to the farmer. Plants generate bespoke rhizosphere microbiomes with downstream effects on function, particularly on the structure and activity of nitrifier and denitrifier communities. In this thesis I have designed a tension table system to maintain soil water-filled pore space (WFPS) that facilitated the high-throughput screening of 200 varieties of the second largest UK arable crop, spring barley (Hordeum vulgare) under varying WFPS conditions, for variation in ability to manipulate nitrification and denitrification, and have observed for the first time variation in gross nitrification rate between different cultivars. I have assessed using hydroponics techniques whether plant preference for ammonium or nitrate varies across select barley cultivars and whether this is influenced by changes in light intensity or carbon dioxide (CO2). I demonstrated that when environmental constraints are removed, nitrogen preference tends to shift towards ammonium, and a shift to further increased ammonium uptake is observed in select cultivars when plants are exposed to short-term high light or growth at elevated CO2. Moreover, I have demonstrated that nitrogen preference and plant ability to inhibit nitrification may be linked. Downstream these results will be used to assess if breeding for altered preference or manipulation of nitrification and/or denitrification represent routes to improve the sustainability of conventional agriculture.