The spatially-differentiated total nitrogen budget for Great Britain

Abstract

To understand Nitrogen (N) cycling, mechanisms of N transformation, and factors that influence N pathway, N budgets at different scale were constructed in this thesis. This thesis addresses N cycling in Great Britain (GB) through four discrete investigations at the national to catchment scale. At the national scale, a spatially-differentiated N budget for GB was constructed at a 1 km2 grid scale, which showed that both sink and source areas exist in GB. The spatial N budget across GB ranged from -21 (±3) to 34 (±5) tonnes N/km2/yr, with 66% of grid squares identified as source areas and 34% identified as sink areas. A spatial N budget of the Trent catchment was constructed by using local rather than country level N flux data, revealing range of -16 (±5) to 32 (±6) tonnes N/km2/yr. To test where N accumulation occurred, 24 locations were selected for soil sampling to test the C/N for different soil depths. The study demonstrates that depth profile has a significant affect upon C/N ratio between sink and source under grassland, however, this measure tested for N accumulation did prove to be significant under grassland but not under arable.
Between 1990 and 2015 the temporal changes in the N budget of the Trent catchment varied from -4.3 (±0.7)×104 to 3.5 (±0.5)×104 tonnes N. N accumulation in the Trent catchment is likely continue at a similar magnitude in the future until the soil of Trent catchment reaches saturated state. The research also updates the nitrate flux model at country scale and finds that catchment area, organic soil, each land use area and gas emission, all have a positive relationship with nitrate flux whereas N deposition and rainfall, both have a negative relationship with nitrate flux.