Towards closing the nitrogen flow in UK agriculture: An explorative study of integrated food and bioenergy production with increased nitrogen recirculation

Abstract: Nitrogen is an essential growth factor in nature and for food production. Itexists in many forms, including reactive nitrogen compounds available for plantuptake, that can cause environmental impacts in high concentrations. Largeamounts of reactive nitrogen in fertilisers are applied to crop areas, and theanthropogenic use of reactive nitrogen has increased manifold during the lastcentury. Nitrogen in harvest residues and animal manure is recycled to crop fields ineffieciently, and the overall flow of nitrogen through agriculture is practicallylinear, with substantial losses to air and water. Policies regulate somenitrogen practices, but seem not to provide a strong incentive for recycling. Amore closed ow of reactive nitrogen within agriculture and the rest of societycould reduce the need for nitrogen fertilisers, and also reduce environmental impactscaused by nitrogen leaching, notably eutrophication of aquatic ecosystems.Pressure is also put on agriculture to meet the increased demand for bioenergy.Increased production of cellulosic bioenergy crops can, in this context, havea positive effect on the environmental performance of agriculture, and can inseveral ways be part of strategies for obtaining a more closed flow of nitrogenin the socio-agricultural system.This thesis develops and models an alternative agricultural system, with increasednitrogen recirculation. Cultivation of cellulosic bioenergy crops are integratedinto the food production to provide nitrogen leaching interception.These biomass production systems are combined with nitrogen recycling. Livestockare fed more efficiently than in the current system. Manure is collected andtreated through anaerobic digestion for production of biogas and stabilisation ofvolatile nitrogen. Analyses using a model representation of the alternative agriculturalsystem show that these measures substantially improve the recyclingof nitrogen in the system, reduce the losses and decrease the need for chemicalfertiliser input. The thesis concludes that implementation of certain bioenergyoptions that enhance nitrogen recycling can give an important contribution toclosing the nitrogen flow in agriculture. A second conclusion is that a systemsperspective on nitrogen is essential for implementation of policies promoting anincreased nitrogen recycling within the socio-agricultural system.