Losses of nitrogen from the soil/plant system not only reduce soil fertility and plant yield but can also create adverse impacts on the environment. Ammonia emissions into the atmosphere contribute to acid rain and represent an indirect source of nitrous oxide greenhouse gas emissions. Nitrate leaching losses into rivers and lakes can cause eutrophication resulting in excessive growth of aquatic weeds and algae, which can reduce fish populations and the recreational value of the water. Nitrate contamination of drinking water supplies can cause health risks. Legislation that is designed to limit nitrate leaching losses from land has become a constraint on agricultural land use in many countries. Nitrous oxide emissions into the atmosphere contribute to the depletion of the ozone layer and also make a significant contribution to climate change. This review describes the nitrogen cycle in temperate soil/plant systems, the processes involved in each of the individual nitrogen loss pathways, the factors affecting the amounts of losses and the methods that are available to reduce these losses. The review has shown that careful management of temperate soil/plant systems using best management practices and newly developed technologies can increase the sustainability of agriculture and reduce its impact on the environment.

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Nitrification is a key process of the nitrogen (N) cycle in soil with major environmental implications. The recent discovery of ammonia-oxidizing archaea (AOA) questions the traditional assumption of the dominant role of ammoniaoxidizing bacteria (AOB) in nitrification. We investigated AOB and AOA growth and nitrification rate in two different layers of three grassland soils treated with animal urine substrate and a nitrification inhibitor [dicyandiamide (DCD)]. We show that AOB were more abundant in the topsoils than in the subsoils, whereas AOA were more abundant in one of the subsoils. AOB grew substantially when supplied with a high dose of urine substrate, whereas AOA only grew in the Controls without the urine-N substrate. AOB growth and the amoA gene transcription activity were significantly inhibited by DCD. Nitrification rates were much higher in the topsoils than in the subsoils and were significantly related to AOB abundance, but not to AOA abundance. These results suggest that AOB and AOA prefer different soil N conditions to grow: AOB under high ammonia (NH 3 ) substrate and AOA under low NH 3 substrate conditions.

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This paper reviews animal treading and the associated effects on soil physical properties and pasture productivity from treading-induced soil compaction and pugging. Response curve relationships between soil physical properties (e.g. macroporosity, air-filled porosity, bulk density) and pasture and crop yield are reviewed. Optimum soil macroporosity for maximum pasture and crop yield ranges from 6 to 17% v/v, but there is a paucity of yield response curves for pastoral systems, particularly critical or optimum values of soil physical properties. There is little information available on the effects of cattle treading on soil physical properties and consequently pasture yield in seasons when soil pugging and poaching is minimised. Such information is needed to provide practical and rigorously tested decision support tools for land managers during grazing seasons. Knowledge of yield response curves, and critical or optimum values of soil physical properties for field pasture-based grazing systems, is required for improved farm-system production and economic decision support.