The effect of land use on the N and P status of the Ythan, Don and Dee catchments in Northern East Scotland
The impact of land use on the N and P status of three contrasting river catchments within the north east of Scotland was studied over time and along the length of each river. Annual mean NO3-N concentrations increased significantly in the Ythan, Don (P<0.001) and Dee (P<0.05), from 1958 to 1992. Concentrations in the predominantly agricultural Ythan catchment showed the largest increase from 1.9 mg l-1 in 1958 to 7.0 mg l-1 in 1992. A significant increase in mean NO3-N concentrations in the spring from 1980 to 1992 in the Ythan appeared to reflect the increased N fertiliser usage associated with a change from winter to spring cropping which was evident over this period. Annual mean NO3-N loads over the period 1980-1992 were 1305, 2010 and 623 t yr-1 for the Ythan, Don and Dee, respectively. Annual PO4-P loads were 17.6, 40-1 and 16.8 t yr-1 over the same time period for the Ythan, Don and Dee respectively. These loads were found to have associated errors in the range 16-24, 16-17 and 18-32% for NO3-N, PO4-P and total P respectively. Annual loss coefficients, calculated for NO3-N, PO4-P total P, Si and suspended sediment were 27.2, 0.180, 0.319, 26.1 and 73.5 kg ha-1, respectively. Annual losses of N and P from forestry were calculated in the Kirkhill catchment, a small subcatchment of the Don and ranged from 0.6 to 2.5 kg ha-1 depending on flow. Annual total P losses were also related to flow and ranged from 0.029 to 0.06 kg ha-1. A nutrient budget calculated for the Kirkhill catchment found the annual loss of N from the catchment accounted for 44.5% of N inputs, in contrast, only 2.7% of P input to the catchment was detected in the output.