Abstract
National terrestrial nitrogen budgets for many developed countries have been calculated as part of the management of impacts of N on the environment, but these rarely represent the subsurface explicitly. Using estimates of vadose zone travel time and agricultural nitrate loading, we quantify, for the first time, the total mass of nitrate contained in the vadose zone of aquifers in England and Wales. This mass peaked in 2008 at 1400 kt N (800 to >1700 kt N from sensitivity analyses) which is approximately 2.5 to 6 times greater than saturated zone estimates for this period and indicates that the subsurface is an important store of reactive nitrogen. About 70% of the nitrate mass is estimated to be in the Chalk, with the remainder split between the Permo-Triassic sandstones, the Jurassic Oolitic limestones and minor aquifers. Current controls on fertiliser application mean that the vadose zone is now a nitrate source and in 2015 we estimate the net flux from the unsaturated zone to groundwater to be 72 kt N/annum. The mass of nitrate in the vadose zone should be included in future terrestrial nitrogen budgets at national and global scales to improve ecosystem management. This article is protected by copyright. All rights reserved.
Highlights
Modern intensive agriculture has significantly affected the Earth’s nitrogen (N) cycle (Vitousek et al, 1997)
Whilst a drinking water standard for nitrate has been developed in Europe (50 mg/l; European Union, 1991), no standard currently exists for surface water
If global water resources and ecological security are to be managed and protected effectively in the future, improved approaches to quantifying the role of groundwater and the vadose zone in nitrate budgets are urgently required
Summary
Modern intensive agriculture has significantly affected the Earth’s nitrogen (N) cycle (Vitousek et al, 1997). In order to manage public water supplies and protect aquatic ecosystems, estimates of N budgets and fluxes at both national and continental scales have been developed. These N budget approaches often assume that the N cycle is in a steady state with no net change in terrestrial N storage (Galloway et al, 2004). If global water resources and ecological security are to be managed and protected effectively in the future, improved approaches to quantifying the role of groundwater and the vadose zone in nitrate budgets are urgently required. By linking estimates of nitrate leaching at the base of the soil zone with depth-togroundwater maps and vadose zone velocities, we use a simple summation approach to derive the total unsaturated zone nitrate mass at the national scale
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