Nitrate leaching from the Broadbalk Wheat Experiment, Rothamsted, UK, as influenced by fertilizer and manure inputs and the weather

Nitrate leaching was measured over the eight drainage seasons spanning the nine years from 1990-1998 on the 157-year old Broadbalk Experiment at Rothamsted, UK. The weather pattern of two dry three wet and three dry years was the dominant factor controlling nitrogen (N) loss. Both the concentration of nitrate in the drainage waters and the amount of N leached increased with the amount of N applied, mostly because of long-term differential increases in soil organic matter and mineralization. On average, losses of N by leaching were 30 kg ha(-1) yr(-1) when no more than the optimum N application tvas applied and were typical of amounts leached from arable land in the UK. Losses increased significantly in both amounts and as the percentage of N applied for supra-optimal applications of N and from autumn-applied farmyard manure (FYM). Extra spring-applied fertilizer was very effective at increasing yields on plots given FYM in the autumn but at the expense of leaching losses three times those from optimum fertilizer N applications. Losses increased after potatoes because they left significant amounts of mineral N in the soil, and decreased after forage maize because it used applied N more effectively. Losses measured 120 years ago from identical treatments were 74% greater than current losses because of today's larger yields and more efficient varieties and management practices. Average concentrations of nitrate in drainage waters did not exceed the EU limit of 11.3 mg NO3-N l(-1) until supra-optimal amounts of N fertilizer (>150-200 kg ha(-1) yr(-1) ) were applied in spring or FYM was applied in autumn. However some drainage waters from all plots, even those that have not received fertilizer for >150 years, exceeded the limit when rain followed a dry summer and autumn. Nitrate leaching into waters will remain a problem for profitable arable farming in the drier parts of Eastern England and Europe despite increased N use efficiency.