Fertilizer Research 42: 277-296, 1995. 277 @ 1995 KluwerAcademicPublishers. PrintedintheNetherlands. Managing nitrogen for sustainable crop production C.A. Campbell l, R.J.K. Myers 2 & D. Curtin 1 1Agriculture and Agri-Food Canada, Research Station, P.O. Box 1030, Swift Current, Saskatchewan, S9H 3X2, Canada and 21BSRAM, P.O. Box 9-109, Bangkken, Bangkok, Thailand Key words: nutrient cycling, socioeconomic constraints, sustainable agriculture, temperate/boreal ecosystems, tropical ecosystem Abstract This paper discusses the influence of N resources (fertilizer, legume, soil) on sustainable agriculture in temper- ate/boreal ecosystems (exemplified by the Canadian prairies), and in the humid, subhumid and semi-arid tropic (exemplified by southeast Asia and central and south America). A sustainable agricultural system is one that is economically viable, provides safe, nutritious food, and. conserves or enhances the environment. Consequently, we discuss the impact of N on crop yields, nitrogen use efficiency (NUE), food quality, environmental quality and on socioeconomic factors. Considerably more long-term research has been conducted in the temperate regions, consequently this was where most information was available. However, the principles governing the behaviour of N are very similar in all ecosystems. It is mainly the rates of nutrient cycling and the socioeconomic constraints that differ. Legumes and N fertilizers, used in a responsible manner, will increase crop production, provide quality food, increase net returns, reduce risk of monetary loss, improve soil quality, and reduce N loss via leaching and gaseous means. The key to sustainable management of N is to synchronize N supply with N use by the crop. Because societies in most temperate ecosystems are more affluent they are better positioned to encourage adoption of management techniques that promote sustainability. In contrast, most producers in the tropics are, subsistence farmers; consequentIy, their immediate goal is economic survival, not preservation of the environment. Introduction Sustainable land management, sustainable agriculture and sustainable cropping systems all command con- siderable attention in all developed and in many devel- oping countries. Here we discuss a microcosm of this large problem, namely, how N (sometimes in com- bination with P) influences sustainable crop manage- ment, concentrating on the general principles applica- ble to most ecosystems. We present this information in two parts: (a) agroecosystems in developed coun- tries in temperate/boreal climates where large commer- cialized production predominates [e.g. Canada, USA, Europe and most of the Organization of Econom- ic Cooperation and Development (OECD) member states], and (b) developing countries, mainly located in humid, sub-humid, and semi-arid, tropics and subtrop- ics, where agriculture is predominantly conducted on small, subsistence-type farming systems (e.g. Africa, Southeast Asia, South America, Caribbean). We use the Canadian prairies for (a), and primarily Southeast Asia for (b). Temperate/boreal ecosystems with large commercialized production systems - the Canadian prairies Background Only 7% (67 million ha) of Canada's land area is arable while another 6% is suitable for grazing [12]. Of the arable land, 79% (53 million ha) is located in the three prairie provinces of Alberta, Saskatchewan, and Man- itoba (Fig. 1). Long, cold winters; short, hot summers; low, variable precipitation; characterize the climate