ABSTRACT
Agriculture places a serious burden on the environment in the process of providing humanity with food and fibres. It is the largest consumer of water and the main source of nitrate pollution of groundwater and surface water, as well as the principal source of ammonia pollution. It is a major contributor to the phosphate pollution of waterways (OECD, 2001a) and to the release of the powerful greenhouse gases (GHGs) methane and nitrous oxide into the atmosphere (IPCC, 2001a). Increasingly, however, it is recognized that agriculture and forestry can also have positive externalities such as the provision of environmental services and amenities, for example through water storage and purification, carbon sequestration and the maintenance of rural landscapes. Moreover, research-driven intensification is saving vast areas of natural forest and grassland, which would have been developed in the absence of higher crop, meat and milk yields. But conversely, intensification has contributed to the air and water pollution mentioned above (Nelson and Mareida, 2001; Mareida and Pingali, 2001), and in some instances reduced productivity growth because of soil and water degradation (Murgai, Ali and Byerlee, 2001).
